Water-Quality Trends for Selected Sampling 

Sites in the Upper Clark Fork Basin, Montana, 

Water Years 1996–2010

Scientific Investigations Report 2013–5217

U.S. Department of the Interior

U.S. Geological Survey
Cover photograph: Looking upstream from the Bear Gulch Road bridge on the Clark Fork near Drummond, 

Montana, on December 9, 2010. (Photograph by Donald A. Bischoff, U.S. Geological Survey.)
Water-Quality Trends for Selected 

Sampling Sites in the Upper Clark Fork 

Basin, Montana, Water Years 1996–2010

By Steven K. Sando, Aldo V. Vecchia, David L. Lorenz, and Elliott P. Barnhart

Scientific Investigations Report 2013–5217

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2014

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Suggested citation:

Sando, S.K., Vecchia, A.V., Lorenz, D.L., and Barnhart, E.P., 2014, Water-quality trends for selected sampling sites in 

the upper Clark Fork Basin, Montana, water years 1996–2010: U.S. Geological Survey Scientific Investigations Report 

2013–5217, 162 p., with appendixes, http://dx.doi.org/10.3133/sir20135217.

ISSN 2328-031X (print)

ISSN 2328-0328 (online)

ISBN 978-1-4113-3764-0
iii

Acknowledgments

The authors would like to recognize the valuable contributions to this report from the insightful 

technical reviews by David Anning (U.S. Geological Survey) and John Lambing (U.S. Geological 

Survey, retired). Also, the authors extend special thanks to individuals involved in the field data 

collection. Kent Dodge and Terry Heinert of the U.S. Geological Survey are noteworthy for their 

efforts.

Bill Bakeberg and Phil Sinerius (private contractors) also are noteworthy for their efforts. 
iv
v

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................2

Purpose and Scope ..............................................................................................................................2

Description of Study Area ...................................................................................................................2

Hydrographic and Hydrologic Characteristics .......................................................................2

Physiographic, Climatic, and Geologic Characteristics........................................................7

Overview of Mining and Remediation Activities ....................................................................7

Data Collection and Analytical Methods ...................................................................................................8

Quality Assurance........................................................................................................................................10

Overview of Water-Quality Characteristics for Sampling Sites in the Upper Clark Fork Basin .....11

Reach 1.................................................................................................................................................11

Reach 2.................................................................................................................................................14

Reach 3.................................................................................................................................................14

Reach 4.................................................................................................................................................15

Reach 5.................................................................................................................................................16

Reach 6.................................................................................................................................................17

Reach 7.................................................................................................................................................17

Reach 8.................................................................................................................................................18

Reach 9.................................................................................................................................................18

Data Analysis Methods...............................................................................................................................19

Trend Analysis Methods ...................................................................................................................19

Time-Series Model.....................................................................................................................20

Multiple Linear Regression on Time, Streamflow, and Season .........................................21

Selection of Trend-Analysis Time Periods.............................................................................21

Estimation of Normalized Constituent Loads .................................................................................21

Streamflow Conditions and Various Data-Related Factors that Affect Trend Analysis and 

Interpretation ..................................................................................................................................22

Streamflow Conditions.......................................................................................................................22

Various Data-Related Factors that Affect Trend Analysis and Interpretation..........................24

Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin ..........................26

Trends in Flow-Adjusted Concentrations and Estimated Normalized Loads............................26

Reach 1........................................................................................................................................38

Reach 2........................................................................................................................................38

Reach 3........................................................................................................................................41

Reach 4........................................................................................................................................42

Reach 5........................................................................................................................................47

Reach 6........................................................................................................................................50

Reach 7........................................................................................................................................53

Reach 8........................................................................................................................................56

Reach 9........................................................................................................................................59
vi

Overview of Water-Quality Trend Results for Data-Summary Reaches...........................62

Summary of Estimated Normalized Loads and Within-Reach Contributions 

of Constituents and Streamflow .......................................................................62

Overview of Temporal Changes in Flow-Adjusted Concentrations and 

Estimated Normalized Constituent Loads .......................................................67

Summary and Conclusions.........................................................................................................................68

References Cited..........................................................................................................................................71

Appendix 1—Summary Information Relating to Quality-Control, Water-Quality, and 

Continuous Streamflow Data .......................................................................................................78

Appendix 2—Summary of the Time-Series Model as Applied in this Study....................................110

Appendix 3—Summary of Multiple Linear Regression of Water-Quality Constituents on 

Time, Streamflow, and Season, as Applied in this Study.......................................................113

Appendix 4—Trend Analysis Results .....................................................................................................115

Appendix 5—Transport-Analysis Balance Calculations for Data-Summary Reaches..................152

Figures

1. Map showing location of study area, sampling sites, and data-summary reaches 

in the upper Clark Fork Basin, Montana ...................................................................................3

2. Graphs showing statistical distributions of selected constituents for sites in the 

upper Clark Fork Basin, Montana, based on data collected during water years 

2001–10 .........................................................................................................................................13

3. Graphs showing daily mean streamflow for selected sites in the upper Clark Fork 

Basin, Montana, water years 1993–2010 ................................................................................23

4. Graphs showing selected streamflow and constituent concentration information 

for Clark Fork near Galen, water years 1993–2010................................................................25

5. Graphs showing flow-adjusted fitted trends determined by using the time-series 

model for selected constituents for sites in reach 1, extending from Blacktail Creek 

to Silver Bow Creek at Butte, water years 1996–2010..........................................................36

6. Pie diagrams showing drainage area, streamflow, and estimated normalized 

constituent loads contributed from reach inflow and within-reach sources for 

reach 1, extending from Blacktail Creek to Silver Bow Creek at Butte for selected 

periods..........................................................................................................................................37

7. Graphs showing flow-adjusted fitted trends determined by using the time-series 

model for selected constituents for sites in reach 2, extending from Silver Bow 

Creek at Butte to Silver Bow Creek at Opportunity, water years 1996–2010....................39

8. Pie diagrams showing drainage area, streamflow, and estimated normalized 

constituent loads contributed from reach inflow and within-reach sources for 

reach 2, extending from Silver Bow Creek at Butte to Silver Bow Creek at 

Opportunity, for selected periods.............................................................................................40

9. Graphs showing flow-adjusted fitted trends determined by using the time-series 

model for selected constituents for sites in reach 3, extending from Silver Bow 

Creek at Opportunity to Silver Bow Creek at Warm Springs, water years 

1996–2010 .....................................................................................................................................43

10. Pie diagrams showing drainage area, streamflow, and estimated normalized 

constituent loads contributed from reach inflow and within-reach sources for 

reach 3, extending from Silver Bow Creek at Opportunity to Silver Bow Creek 

at Warm Springs for selected periods ....................................................................................44
vii

11. Graphs showing flow-adjusted fitted trends determined by using the time-series 

model for selected constituents for sites in reach 4, extending from Silver 

Bow Creek at Warm Springs to Clark Fork near Galen, based on data collected 

during water years 1985–2010 ..................................................................................................45

12. Pie diagrams showing drainage area, streamflow, and estimated normalized 

constituent loads contributed from reach inflow and within-reach sources for 

each 4, extending from Silver Bow Creek at Warm Springs to Clark Fork near 

Galen for selected periods........................................................................................................46

13. Graphs showing flow-adjusted fitted trends determined by using the time-series 

model and multiple linear regression on time, streamflow, and season for 

selected constituents for selected sites in reach 5, extending from Clark Fork 

near Galen to Clark Fork at Deer Lodge, based on data collected during water 

years 1985–2010 ..........................................................................................................................48

14. Pie diagrams showing drainage area, streamflow, and estimated normalized 

constituent loads contributed from reach inflow and within-reach sources for 

reach 5, extending from Clark Fork near Galen to Clark Fork at Deer Lodge for 

selected periods .........................................................................................................................49

15. Graphs showing flow-adjusted fitted trends determined by using the time-series 

model for selected constituents for sites in reach 6, extending from Clark 

Fork at Deer Lodge to Clark Fork at Goldcreek, based on data collected during 

water years 1985–2010...............................................................................................................51

16. Pie diagrams showing drainage area, streamflow, and estimated normalized 

constituent loads contributed from reach inflow and within-reach for reach 6, 

extending from Clark Fork at Deer Lodge to Clark Fork at Goldcreek for selected 

periods..........................................................................................................................................52

17. Graphs showing flow-adjusted fitted trends determined by using the time-series 

model for selected constituents for sites in reach 7, extending from Clark Fork at 

Goldcreek to Clark Fork near Drummond, based on data collected during water 

years 1985–2010 ..........................................................................................................................54

18. Pie diagrams showing drainage area, streamflow, and estimated normalized 

constituent loads contributed from reach inflow and within-reach for reach 7, 

extending from Clark Fork at Goldcreek to Clark Fork near Drummond............................55

19. Graphs showing flow-adjusted fitted trends determined by using the time-series 

model for selected constituents for sites in reach 8, extending from Clark 

Fork near Drummond to Clark Fork at Turah Bridge, based on data collected 

during water years 1985–2010 ..................................................................................................57

20. Pie diagrams showing drainage area, streamflow, and estimated normalized 

constituent loads contributed from reach inflow and within-reach sources for 

reach 8, extending from Clark Fork near Drummond to Clark Fork at Turah Bridge 

for selected periods ...................................................................................................................58

21. Graphs showing fitted trends determined by using the time-series model 

for selected constituents for sites in reach 9, extending from Clark Fork at Turah 

Bridge to Clark Fork above Missoula, based on data collected during water years 

1985–2010 .....................................................................................................................................60

22. Pie diagrams showing drainage area, streamflow, and estimated normalized 

constituent loads contributed from reach inflow and within-reach sources for 

reach 9, extending from Clark Fork at Turah Bridge to Clark Fork above Missoula 

for selected periods ...................................................................................................................61

23. Pie diagrams showing streamflow, and estimated normalized unfiltered-recoverable 

copper loads contributed from reach inflow and within-reach sources for datasummary reaches for selected periods ..................................................................................63
viii

24. Pie diagrams showing streamflow, and estimated normalized unfiltered-recoverable 

arsenic loads contributed from reach inflow and within-reach sources for datasummary reaches for selected periods ..................................................................................64

25. Pie diagrams showing streamflow, and estimated normalized suspended-sediment 

loads contributed from reach inflow and within-reach sources for data-summary 

reaches for selected periods....................................................................................................65

Appendix Figures

1–1. Graphs showing spike recoveries for laboratory-spiked deionized-water blank 

samples, based on data collected during water years 1993–2010.....................................84

1–2. Graphs showing spike recoveries for laboratory-spiked stream-water samples, 

based on data collected during water years 1993–2010......................................................85

 4–1. Graphs showing flow-adjusted fitted trends determined by using the time-series 

model for selected water-quality constituents and properties for Blacktail Creek, 

water years 1996–2010.............................................................................................................130

4–2. Graphs showing flow-adjusted fitted trends determined by using the time-series 

model for selected water-quality constituents and properties for Silver Bow Creek 

at Butte, water years 1996–2010 ............................................................................................131

4–3. Graphs showing flow-adjusted fitted trends determined by using the time-series 

model for selected water-quality constituents and properties for Silver Bow Creek 

at Opportunity, water years 1996–2010 .................................................................................132

4–4. Graphs showing flow-adjusted fitted trends determined by using multiple linear 

regression on time, discharge, and season for selected water-quality constituents 

and properties for Mill Creek near Anaconda, water years 2006–2010 ..........................133

4–5. Graphs showing flow-adjusted fitted trends determined by using multiple linear 

regression on time, discharge, and season for selected water-quality constituents 

and properties for Mill Creek at Opportunity, water years 2006–10.................................134

4–6. Graphs showing flow-adjusted fitted trends determined by using multiple linear 

regression on time, discharge, and season for selected water-quality constituents 

and properties for Willow Creek near Anaconda, water years 2006–10.........................135

4–7. Graphs showing flow-adjusted fitted trends determined by using multiple linear 

regression on time, discharge, and season for selected water-quality constituents 

and properties for Willow Creek at Opportunity, water years 2006–10...........................136

4–8. Graphs showing flow-adjusted fitted trends determined by using the time-series 

model for selected water-quality constituents and properties for Silver Bow Creek 

at Warm Springs, water years 1996–2010.............................................................................137

4–9. Graphs showing flow-adjusted fitted trends determined by using multiple linear 

regression on time, discharge, and season for selected water-quality constituents 

and properties for Warm Springs Creek near Anaconda, water years 2006–10 ...........138

4–10. Flow-adjusted fitted trends determined by using the time-series model for selected 

water-quality constituents and properties for Warm Springs Creek at Warm Springs, 

water years 1996–10.................................................................................................................139

4–11. Graphs showing flow-adjusted fitted trends determined by using the time-series 

model for selected water-quality constituents and properties for Clark Fork 

near Galen, water years 1996–2010.......................................................................................140

4–12. Graphs showing flow-adjusted fitted trends determined by using multiple linear 

regression on time, discharge, and season for selected water-quality constituents 

and properties for Lost Creek near Anaconda, water years 2006–10..............................141
ix

4–13. Graphs showing flow-adjusted fitted trends determined by using multiple linear 

regression on time, discharge, and season for selected water-quality constituents 

and properties for Lost Creek near Galen, water years 2006–10......................................142

4–14. Graphs showing flow-adjusted fitted trends determined by using the time-series 

model for selected water-quality constituents and properties for Clark Fork at Deer 

Lodge, water years 1996–2010................................................................................................143

4–15. Graphs showing flow-adjusted fitted trends determined by using the time-series 

model for selected water-quality constituents and properties for Little Blackfoot 

River, water years 1996–2005..................................................................................................144

4–16. Graphs showing flow-adjusted fitted trends determined by using the time-series 

model for selected water-quality constituents and properties for Clark Fork at 

Goldcreek, water years 1996–2010 ........................................................................................145

4–17. Graphs showing flow-adjusted fitted trends determined by using the time-series 

model for selected water-quality constituents and properties for Flint Creek, water 

years 1996–2005 ........................................................................................................................146

4–18. Graphs showing flow-adjusted fitted trends determined by using the time-series 

model for selected water-quality constituents and properties for Clark Fork near 

Drummond, water years 1996–2010.......................................................................................147

4–19. Graphs showing flow-adjusted fitted trends determined by using the time-series 

model for selected water-quality constituents and properties for Rock Creek, water 

years 1996–2005 ........................................................................................................................148

4–20. Graphs showing flow-adjusted fitted trends determined by using the time-series 

model for selected water-quality constituents and properties for Clark For at Turah 

Bridge, water years 1996–2010...............................................................................................149

4–21. Graphs showing flow-adjusted fitted trends determined by using the time-series 

model for selected water-quality constituents and properties for Blackfoot River 

water years 1996–2010.............................................................................................................150

4–22. Graphs showing flow-adjusted fitted trends determined by using the time-series 

model for selected water-quality constituents and properties for Clark Fork above 

Missoula, water years 1996–2010 ..........................................................................................151
x

Tables

1. Information for study sampling sites and data-summary reaches in the upper 

Clark Fork Basin, Montana..........................................................................................................4

2. Property and constituents included in the trend analysis and information relating 

to laboratory and study reporting levels...................................................................................9

3. Percent of samples with unadjusted unfiltered-recoverable concentrations 

exceeding water-quality standards for sites in the upper Clark Fork Basin, 

water years 2001–10...................................................................................................................12

4. Summary of flow-adjusted trend results determined by using the time-series 

model for selected sites and constituents, water years 1996–2010...................................27

5. Summary of flow-adjusted trend results determined by using the time-series 

model for Clark Fork above Missoula for selected constituents, water years 

1996–2010 .....................................................................................................................................31

6. Summary of flow-adjusted trend results determined by using multiple linear 

regression on time, streamflow, and season for selected sites and constituents, 

water years 2006–10...................................................................................................................32

7. Drainage area and streamflow information relevant to the transport analysis for 

data-summary reaches in the upper Clark Fork Basin, Montana, water years 

1996–2010 .....................................................................................................................................34

Appendix Tables

1–1. Summary information relating to quality-control samples collected at sites in 

the upper Clark Fork Basin, Montana, based on data collected during water 

years 1993–2010 ..........................................................................................................................79

1–2. Summary information relating to quality-control samples collected at sites in 

the upper Clark Fork Basin, Montana, based on data collected during water 

years 1993–2010 ..........................................................................................................................80

1–3. Summary information relating to quality-control samples collected at sites in the 

upper Clark Fork Basin, Montana, based on data collected during water years 

1993–2010 .....................................................................................................................................82

1–4. Summary information relating to water-quality constituents and properties in 

samples collected at sites in the upper Clark Fork Basin, Montana , based on data 

collected during water years 2001–2010 ................................................................................86

1–5. Aquatic life standards for selected sites in the upper Clark Fork Basin.........................108

1–6. Summary information relating to continuous streamflow data for sites in the upper 

Clark Fork Basin, Montana , based on data collected during water years 

2001–2010 ...................................................................................................................................109

2–1. Statistical summaries of standard errors of estimates for the time-series 

model analyses .........................................................................................................................112

3–1. Statistical summaries of standard errors of estimates for multiple linear regression 

models of water-quality constituents on time, streamflow, and season.........................114

4–1. Flow-adjusted trend results determined by using the time-series model for selected 

water-quality constituents and properties for selected sampling sites in the upper 

Clark Fork Basin, Montana, water years 1996–2010 ...........................................................115
xi

4–2. Flow-adjusted trend results determined by using the time-series model for 

selected water-quality constituents and properties for Clark Fork above 

Missoula, water years 1996–2010 ..........................................................................................125

4–3. Flow-adjusted trend results determined by using multiple linear regression on 

time, streamflow, and season for selected water-quality constituents and properties 

for selected sampling sites in the upper Clark Fork Basin, Montana, water 

years 2006–2010 ........................................................................................................................126

5–1. Transport-analysis balance calculations for sites analyzed by using the time series 

model in reach 1, extending from Blacktail Creek to Silver Bow Creek at Butte for 

selected periods, water years 1996–2010.............................................................................152

5–2. Transport-analysis balance calculations for sites analyzed by using the time series 

model in reach 2, extending from Silver Bow Creek at Butte to Silver Bow Creek at 

Opportunity for selected periods, water years 1996–2010.................................................153

5–3. Transport-analysis balance calculations for sites analyzed by using the time series 

model in reach 3, extending from Silver Bow Creek at Opportunity to Silver Bow 

Creek at Warm Springs for selected periods, water years 1996–2010............................154

5–4. Transport-analysis balance calculations for sites analyzed by using the time series 

model in reach 4, extending from Silver Bow Creek at Warm Springs to Clark Fork 

near Galen for selected periods, water years 1996–2010..................................................155

5–5. Transport-analysis balance calculations for sites analyzed by using the time series 

model in reach 5, extending from Clark Fork near Galen to Clark Fork at Deer Lodge 

for selected periods, water years 1996–2010.......................................................................157

5–6. Transport-analysis balance calculations for sites analyzed by using the time series 

model in reach 6, extending from Clark Fork at Deer Lodge to Clark Fork at Goldcreek 

for selected periods, water years 1996–2010.......................................................................158

5–7. Transport-analysis balance calculations for sites analyzed by using the time series 

model in reach 7, extending from Clark Fork at Goldcreek to Clark Fork near 

Drummond for selected periods, water years 1996–2010..................................................159

5–8. Transport-analysis balance calculations for sites analyzed by using the time series 

model in reach 8, extending from Clark Fork near Drummond to Clark Fork at Turah 

Bridge for selected periods, water years 1996–2010..........................................................160

5–9. Transport-analysis balance calculations for sites analyzed by using the time series 

model in reach 9, extending from Clark Fork at Turah Bridge to Clark Fork 

above Missoula for selected periods, water years 1996–2010 .........................................161
xii

Conversion Factors

Inch/pound to SI

Multiply By To obtain

acre 0.4047 hectare (ha)

acre-foot (acre-ft) 1,233 cubic meter (m3

)

cubic foot (ft3

) 0.02832 cubic meter (m3) 

cubic foot per second (ft3

/s) 0.02832 cubic meter per second (m3/s)

foot (ft) 0.3048 meter (m)

gallon (gal) 3.785 liter (L) 

inch (in.) 2.54 centimeter (cm)

mile (mi) 1.609 kilometer (km)

ounce, avoirdupois (oz) 28.35 gram (g) 

square mile (mi2

) 2.590 square kilometer (km2) 

tons per day (t/d) 907.1 kilograms per day (kg/d)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows: 

°F=(1.8×°C)+32

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1988 

(NGVD 88).

Horizontal coordinate information is referenced to North American Datum of 1983 (NAD 83).

Altitude, as used in this report, refers to distance above the vertical datum.

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm).

Concentrations of chemical constituents in water are given either in micrograms per liter 

(μg/L) or milligrams per liter (mg/L).

Water year is defined as the 12-month period from October 1 through September 30 of the 

following calendar year. The water year is designated by the calendar year in which it ends. 

For example, water year 2010 is the period from October 1, 2009, through September 30, 2010.
xiii

Abbreviated Units and Symbols 

ft feet

ft3

/s cubic feet per second

in inches

kg/d kilograms per day

μS/cm microsiemens per centimeter at 25 degrees Celsius

μg/L micrograms per liter

mg/L milligrams per liter

mi miles

mi2 square miles

< less than

AMC Anaconda Mining Company

ANNC annual concentration anomaly (dimensionless)

ANNQ annual streamflow anomaly (dimensionless)

ARCO Atlantic Richfield Company

C concentration, in milligrams per liter

DQO data-quality objectives 

FAC flow-adjusted concentration

HFVC high-frequency variability of concentration (dimensionless)

HFVQ high-frequency variability of streamflow (dimensionless) 

log logarithm (base 10)

LRL laboratory reporting level

MC long-term mean of the log-transformed concentration, as the base-10 logarithm 

of milligrams per liter

MQ long-term mean of the log-transformed streamflow, as the base-10 logarithm of 

cubic feet per second

NGVD National Geodetic Vertical Datum

NWQL National Water Quality Laboratory

NWIS National Water Information System

MLR Multiple linear regression of concentration on time, streamflow, and season

p-value statistical probability; that is, the probability of determining a result at least as 

extreme as the one observed

Q daily mean streamflow, in cubic feet per second

QC quality control

RPD relative percent difference

RSD relative standard deviation

SEASC seasonal concentration anomaly (dimensionless)

SEASQ seasonal streamflow anomaly (dimensionless)

SEE standard error of estimate, in percent

SRL study reporting level

TREND concentration trend (dimensionless)

TSM time-series model

USGS U.S. Geological Survey

 

Water-Quality Trends for Selected Sampling Sites in the 

Upper Clark Fork Basin, Montana, Water Years 1996–2010

By Steven K. Sando, Aldo V. Vecchia, David L. Lorenz, and Elliott P. Barnhart

Abstract

The primary purposes of this report are to (1) characterize temporal trends in flow-adjusted concentrations (filtered 

and unfiltered) of mining-related contaminants and (2) assess 

those trends in the context of source areas and transport of 

those contaminants through the upper Clark Fork Basin. A 

large-scale trend analysis was done on specific conductance, 

selected trace elements (arsenic, cadmium, copper, iron, lead, 

manganese, and zinc), and suspended-sediment data for 

22 sites for water years 1996–2010. Trend analysis was conducted by using two parametric methods: a time-series model 

(TSM) and multiple linear regression on time, streamflow, and 

season (MLR). 

The main-stem Clark Fork begins at the confluence of 

Silver Bow and Warm Springs Creeks near Warm Springs, 

Montana and flows about 485 miles through Montana and 

Idaho. The study area encompasses the upper Clark Fork 

Basin in west-central Montana upstream from Clark Fork 

above Missoula (site 22) with a drainage area of 5,999 square 

miles (mi2

). Mining in the upper Clark Fork Basin began in 

1864 when small-scale placer mining operations extracted 

gold from Silver Bow Creek and its tributaries in and near 

Butte. Large amounts of waste materials enriched with trace 

elements, including the metallic elements cadmium, copper, 

lead, and zinc, as well as the metalloid trace element arsenic, 

were generated from mining operations in the Butte area and 

the milling and smelting operations in the Anaconda area. 

Extensive deposition of mining wastes in the Silver Bow 

Creek and Clark Fork channels and floodplains had substantial 

effects on water quality. Federal Superfund remediation activities in the upper Clark Fork Basin began in 1983 and have 

included substantial remediation in the Butte area and removal 

of the former Milltown Dam near Missoula. The U.S. Geological Survey has been collecting streamflow and water-quality 

data in the upper Clark Fork Basin since 1983 to aid in identifying temporal changes in water quality.

For sites that were analyzed by using the TSM, normalized loads (hereinafter referred to as loads) were estimated to 

evaluate temporal changes in relative contributions of selected 

trace elements and suspended sediment from upstream source 

areas to reach outflows. Trend results are presented for all 

constituents investigated; however, in the discussion emphasis 

is placed on copper, arsenic, and suspended sediment. 

Trend results for 1996–2010 indicate moderate to large 

decreases in flow-adjusted concentrations (FACs) and loads of 

copper (and other metallic elements) and suspended sediment 

in Silver Bow Creek upstream from Warm Springs. Deposition 

of metallic elements and suspended sediment within Warm 

Springs Ponds substantially reduces the downstream transport 

of those constituents. However, mobilization of copper and 

suspended sediment from floodplain tailings and stream banks 

in the Clark Fork reach from Galen to Deer Lodge is a large 

source of metallic elements and suspended sediment, which 

also affects downstream transport of those constituents. Copper and suspended-sediment loads mobilized from within this 

reach accounted for about 40 and 20 percent, respectively, of 

the loads for Clark Fork at Turah Bridge (site 20); whereas, 

streamflow contributed from within this reach only accounted 

for about 8 percent of the streamflow at Turah Bridge. Minor 

changes in FACs and loads of copper and suspended sediment 

are indicated for this reach during 1996–2010.

Clark Fork reaches downstream from Deer Lodge are 

relatively smaller sources of metallic elements than the 

reach from Galen to Deer Lodge. In general, small decreases 

in loads and FACs of copper and suspended sediment are 

indicated for Clark Fork sites downstream from Deer Lodge 

during 1996–2010. Thus, although large decreases in FACs 

and loads of copper and suspended sediment are indicated for 

Silver Bow Creek upstream from Warm Springs, those large 

decreases are not translated to the more downstream reaches 

largely because of temporal stationarity in constituent transport relations in the Clark Fork reach from Galen to Deer 

Lodge.

Unlike metallic elements, arsenic (a metalloid element) 

in streams in the upper Clark Fork Basin typically is mostly 

in dissolved phase, has less variability in concentrations, and 

has weaker direct relations with suspended-sediment concentrations and streamflow. Arsenic trend results for 1996–2010 

indicate generally moderate decreases in FACs and loads in 

Silver Bow Creek upstream from Opportunity. In general, 

small temporal changes in loads and FACs of arsenic are 

indicated for Silver Bow Creek and Clark Fork reaches downstream from Opportunity during 1996–2010. Contribution of 
2 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

arsenic (from Warm Springs Ponds, the Mill-Willow bypass, 

and groundwater sources) in the Silver Bow Creek reach 

from Opportunity to Warm Springs is a relatively large source 

of arsenic. Arsenic loads originating from within this reach 

accounted for about 11 percent of the load for Clark Fork at 

Turah Bridge; whereas, streamflow contributed from within 

this reach only accounted for about 2 percent of the streamflow at Turah Bridge. 

Introduction

Mining in the upper Clark Fork Basin (upstream from 

Missoula, Montana) began in 1864 when small-scale placer 

mining operations extracted gold from Silver Bow Creek and 

its tributaries in and near Butte (Freeman, 1900; U.S. Environmental Protection Agency, 2005). Large amounts of waste 

materials enriched with metallic contaminants (cadmium, 

copper, lead, and zinc) and the metalloid trace element arsenic 

were generated from mining operations in the Butte area and 

the milling and smelting operations in the Anaconda area 

(Andrews, 1987; Gammons and others, 2006). Extensive 

deposition of mining wastes in the Silver Bow Creek and 

Clark Fork channels and floodplains had substantial effects 

on water quality. Federal Superfund remediation activities in 

the upper Clark Fork Basin began in 1983 and have included 

substantial remediation in the Butte area and removal of the 

former Milltown Dam, near Missoula (CDM, 2005; U.S. Environmental Protection Agency, 2010; U.S. Environmental 

Protection Agency, 2004; Sando and Lambing, 2011).

Water-quality data collection by the U.S. Geological 

Survey (USGS) within the upper Clark Fork Basin began 

during 1985–88 with the establishment of a small long-term 

monitoring program that has expanded through time and 

continued through present (2013). A statistical evaluation of 

flow-adjusted water-quality trends for the monitoring data 

was needed to document changes in water quality that might 

have resulted from remediation activities. The USGS, in 

cooperation with the U.S. Environmental Protection Agency, 

conducted this study to test for flow-adjusted temporal trends 

in water quality at 22 sites (study sites are located at USGS 

streamflow gaging stations, fig. 1, table 1) using two parametric trends analysis methods: a joint time-series model (TSM; 

Vecchia, 2005) for concentration and streamflow and multiple 

linear regression of concentration on time, streamflow, and 

season (MLR; Helsel and Hirsch, 2002). 

Purpose and Scope

The primary purposes of this report are to (1) characterize temporal trends in flow-adjusted concentrations (filtered 

and unfiltered) of mining-related contaminants and (2) assess 

those trends in the context of source areas and transport of 

those contaminants through the upper Clark Fork Basin. A 

large-scale trend analysis was done on 22 sites for water years 

1996–2010. This report presents the trend results and also 

background information on mining and remediation activities in the upper Clark Fork Basin, trend-analysis methods, 

streamflow conditions, and various data-related factors that 

affect trend results. This information is presented to assist in 

evaluating trend results; however, it is beyond the scope of 

this report to provide detailed explanations for all observed 

temporal changes. 

Description of Study Area

The Clark Fork drains an extensive region in western 

Montana and northern Idaho in the Columbia River Basin. The 

main-stem Clark Fork begins at the confluence of Silver Bow 

and Warm Springs Creeks near Warm Springs, Montana and 

flows about 485 miles through Montana and Idaho. The study 

area (fig. 1) encompasses the upper Clark Fork Basin in 

west-central Montana upstream from Clark Fork above Missoula (site 22, table 1), with a drainage area of 5,999 square 

miles (mi2

).

Hydrographic and Hydrologic Characteristics

Silver Bow Creek is one of two streams that join to form 

the Clark Fork. According to the National Hydrography Dataset (U.S. Geological Survey, 2012a), the Silver Bow Creek 

main-stem channel originates in mountains north of Butte, 

Montana, then is shown to flow south past the Berkeley Pit 

(not shown in fig. 1; historically, the largest single metal-ore 

mine in the upper Clark Fork Basin) and into Butte. However, streamflows in the upper reaches of Silver Bow Creek 

above the Berkeley Pit are diverted into a tailings pond. In the 

1930’s, the remnant section of Silver Bow Creek downgradient from the Berkeley Pit was channelized and replaced by 

a storm drain upstream from the confluence of Silver Bow 

Creek and Blacktail Creek (Don Booth, Atlantic Richfield 

Company, written commun., August 2013). The longest stream 

channel in the upper Silver Bow Creek Basin is Blacktail 

Creek, which originates in mountains south of Butte and 

then flows north to its confluence with Silver Bow Creek in 

Butte. Blacktail Creek (site 1, fig. 1, table 1) has a drainage 

area of 85 mi2

 and estimated mean annual streamflow (based 

on data from USGS streamflow-gaging station Blacktail Creek 

at Butte; station 12323240; water years 1989–2010) of 13 

cubic feet per second (ft3

/s; U.S. Geological Survey, 2012b). 

There have been numerous small mining operations in the 

upper reaches of the Blacktail Creek Basin, but at its mouth 

the effect of mining operations on water quality of Blacktail 

Creek is small to moderate. Silver Bow Creek at Butte (site 

2, table 1) has a drainage area of 103 mi2

 and mean annual 

streamflow (water years 1984–2010) of 22 ft3

/s (U.S. Geological Survey, 2012b). The upper reaches of the Silver Bow 

Creek Basin in and near Butte contain numerous mine shafts, 

pits, mills, smelters, and tailings piles and ponds as a result of 

mining activities that began in the 1860’s and generally ceased 
Introduction 3

Missoula

Bonner

Clinton

Drummond

Goldcreek

Garrison

Deer Lodge

Butte

2

1

22 21

20

19 18

17 16

15

14

Blackfoot River

Rock Creek

Flint Creek

Mill Creek

Lost Creek

Landers Fork

Monture Creek

Union Creek

Racetrac k Creek

Gold Creek

Browns Gulch

Bas ni Creek

North Fork Blackfoot River

Nevada Creek

Upper Willow Creek

Dunham Creek

Blacktail Creek

Ross Fork Rock Creek

Boles Creek

Douglas Creek

Placid Creek

Lower Willow Creek

Rock Creek

River

Clearwater

Cottonwood Cr

Cottonwood Creek

Little Blackfoot River

Middle Fork

Rock Creek

West Fork Rock Creek

East Fork Rock Creek

Warm Springs

Blackfoot River

Creek

Creek

Reach 1

Reach 2

Reach

 3

Reach 4

Reach 5

Reach 6

Reach 7 Reach 8

Reach 9

113°30'

113°

114°

47°

46°30'

46°

112°30'

Warm Springs Creek

Mill Creek

Lost Creek

Racetrack Creek

Willow

Creek

Warm

Springs

Pond

MillWillow

bypass

Warm Springs

Anaconda Opportunity

Galen

Reach 3

Reach 4

Reach 5

9

8

7

6

5

4

3

13

12

11

10

MONTANA

Study

area

Index map

Butte

Missoula

EXPLANATION

0 5 10 15 20 MILES

0 10 15 20 5 KILOMETERS

Base modified from U.S. Farm Services Agency National 

Agricultural Imagery Program (NAIP), 2009 

U.S. Census Bureau TIGER, 1:100,000, 1993 

Universal Transverse Mercator Projection, zone 12 

North American Datum of 1927 (NAD 27).

19

16

Study area

Data-summary reach basin boundary

Main-stem Silver Bow Creek or Clark Fork 

channel

Main-stem Silver Bow Creek or Clark Fork 

sampling site and site number (table 1)

Tributary sampling site and site number (table 1)

Figure 1. Location of study area, sampling sites, and data-summary reaches in the upper Clark Fork Basin, Montana.
Table 1. Information for study sampling sites and data-summary reaches in the upper Clark Fork Basin, Montana. [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. USGS, U.S. Geological Survey; TSM, time-series moregression on time, streamflow and season] Site number (fig. 1) USGS site identification number USGS site name Abbreviated site name Data- summary reach1 Drain- age area, square miles Period of water- quality data collection Median 

nual sam

freque

sample

year (ra

1 12323230 Blacktail Creek at Harrison Avenue, at Butte, Montana Blacktail Creek 1 85 3/1993–8/1995, 12/1996–8/2003, 12/2004–8/2010 8 (7–2 12323250 Silver Bow Creek below Black- tail Creek, at Butte, Montana Silver Bow Creek at Butte 1 and 2 103 3/1993–8/1995, 12/1996–8/2010 8 (7–3 12323600 Silver Bow Creek at Opportu- nity, Montana Silver Bow Creek at Opportunity 2 and 3 363 3/1993–8/1995, 12/1996–8/2010 8 (8–4 12323670 Mill Creek near Anaconda, Montana Mill Creek near Anaconda 3 34 12/2004–8/2010 8 (8–5 12323700 Mill Creek at Opportunity, Montana Mill Creek at Opportunity 3 43 3/2003–8/2010 8 (8–6 12323710 Willow Creek near Anaconda, Montana Willow Creek near Anaconda 3 14 12/2004–8/2010 8 (6–7 12323720 Willow Creek at Opportunity, Montana Willow Creek at Opportunity 3 31 3/2003–8/2010 8 (8–8 12323750 Silver Bow Creek at Warm Springs, Montana Silver Bow Creek at Warm Springs 3 and 4 473 3/1993–8/2010 8 (6–9 12323760 Warm Springs Creek near Ana- conda, Montana Warm Springs Creek near Ana- conda 4 157 10/2005–8/2010 6 (6–

10 12323770 Warm Springs Creek at Warm Springs, Montana Warm Springs Creek at Warm Springs 4 163 3/1993–8/2010 6 (4–

11 12323800 Clark Fork near Galen, Montana Clark Fork near Galen 4 and 5 651 7/1988–8/2010 8 (1–

12 12323840 Lost Creek near Anaconda, Montana Lost Creek near Anaconda 5 26 12/2004–8/2010 8 (7–13 12323850 Lost Creek near Galen, Montana Lost Creek near Galen 5 60 3/2003–8/2010 8 (8–14 12324200 Clark Fork at Deer Lodge, Montana Clark Fork at Deer Lodge 5 and 6 995 3/1985–8/2010 8 (4–

15 12324590 Little Blackfoot River near Gar- rison, Montana Little Blackfoot River 6 407 3/1985–8/2004 6 (2–

16 12324680 Clark Fork at Goldcreek, Mon- tana Clark Fork at Goldcreek 6 and 7 1,704 3/1993–8/2010 8 (6–
Table 1. Information for study sampling sites and data-summary reaches in the upper Clark Fork Basin, Montana.—Continued[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. USGS, U.S. Geological Survey; TSM, time-series moregression on time, streamflow and season]

Site 

number 

(fig. 1)

USGS site 

identification 

number

USGS site name Abbreviated site name

Datasummary 

reach1

Drainage area, 

square 

miles

Period of waterquality data 

collection

Median 

nual sam

freque

sample

year (ra

171819202122

123315001233180012334510123345501234000012340500

Flint Creek near Drummond, 

Montana

Clark Fork near Drummond, 

Montana

Rock Creek near Clinton, 

Montana

Clark Fork at Turah Bridge, near 

Bonner, Montana

Blackfoot River near Bonner, 

Montana

Clark Fork above Missoula, 

Montana

Flint CreekClark Fork near DrummondRock CreekClark Fork at Turah BridgeBlackfoot RiverClark Fork above Missoula

7

7 and 8

8

8 and 9

99

 490

 2,501

 885

 3,641 2,290 5,999

3/1985–8/20043/1993–8/20103/1985–8/20043/1985–8/20103/1985–8/20107/1986–8/2010

8 (1–

8 (6–6 (2–8 (6–26 (2–8 (2–

1Where two reach numbers are shown, the site is both an outflow from the upstream reach and an inflow to the downstream reach.2The numerical designations of the trend analysis periods are defined as:

 1: water years 1996–2000; 2: water years 2001–05; and 3: water years 2006–10.Because of the substantial effect of the breach and removal of Milltown Dam in 2008, for Clark Fork above Missoula (site 22), period 3 waperiod 3B (March 28, 2008–September 30, 2010). Selection of trend analysis periods is discussed in the section of this report “Selection of 

s subdivided into period 

Trend-Analysis Time Pe

3A (Octobe

riods.”
6 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

in the early 1980’s (Freeman, 1900; Gammons and others, 

2006). During the many decades of mining activities, inadequate handling of waste rock, water, and smelter emissions, 

coupled with effects of large flood events, resulted in extensive 

deposition of mining wastes in the Silver Bow Creek channel 

and floodplain with substantial effects on Silver Bow Creek 

water quality (Smith and others, 1998). Downstream from 

Butte, Silver Bow Creek flows west about 10 miles then north 

about 10 miles to the confluence of Warm Springs Creek, 

marking the start of the Clark Fork. In the reach of Silver Bow 

Creek between Butte and the confluence with Warm Springs 

Creek, large areas of the intervening basin were affected by 

production and dispersion of waste materials (rock, water, 

and smelter emissions) primarily from milling and smelting 

activities of the Anaconda Mining Company (AMC) Smelter 

(Hooper and others, 2002). 

About 5 miles upstream from the confluence of Silver 

Bow Creek and Warm Springs Creek, Silver Bow Creek enters 

the Warm Springs Ponds (fig. 1), which are a series of three 

large ponds (variously constructed during 1908–1959; CDM, 

2005) to retain and treat contaminated sediment transported 

from the upper reaches of Silver Bow Creek. Upstream from 

the Warm Springs Ponds, Silver Bow Creek at Opportunity 

(site 3, fig. 1, table 1) represents the outflow of the Silver Bow 

Creek basin above substantial retention and diversion structures. Site 3 has a drainage area of 363 mi2

 and mean annual 

streamflow (water years 1988–2010) of 52 ft3

/s (U.S. Geological Survey, 2012b). Notable tributaries to Silver Bow Creek 

in the reach below site 3 include Mill and Willow Creeks. The 

natural stream channels of those tributaries originally entered 

Silver Bow Creek near Opportunity, Montana. However, 

in about 1969, the Mill-Willow bypass was constructed to 

capture flows of Mill and Willow Creeks near their mouths 

and divert the combined flows into Silver Bow Creek between 

the outlet from Warm Springs Ponds and the confluence 

with Warm Springs Creek (CDM, 2005). Silver Bow Creek 

at Warm Springs, Montana, (site 8, fig. 1, table 1), which 

includes the combined flow from the Warm Springs Ponds 

outlet and the Mill-Willow bypass, has a drainage area of 

473 mi2

 and mean annual streamflow (water years 1994–2010) 

of 85 ft3

/s (U.S. Geological Survey, 2012b).

Warm Springs Creek joins Silver Bow Creek a short 

distance downstream from site 8 to form the Clark Fork. The 

Warm Springs Creek drainage basin also was affected by tailings, and water and smelter emissions primarily from milling and smelting activities of the AMC Smelter (Hooper and 

others, 2002). Warm Springs Creek at Warm Springs (site 10, 

fig. 1, table 1) has a drainage area of 163 mi2

 and mean annual 

streamflow (water years 1984–2010) of 54 ft3

/s (U.S. Geological Survey, 2012b).

From its start at the confluence of Silver Bow Creek and 

Warm Springs Creek, the Clark Fork flows for about 125 miles 

through the study area, draining forested mountains that flank 

the Clark Fork valley. In the reach of the Clark Fork between 

its start and Deer Lodge there are large quantities of floodplain 

tailings that originated from mining wastes generated near 

Butte and Anaconda (Smith and others, 1998). In this reach, 

the Clark Fork valley is broad (about 5 miles wide and known 

locally as the Deer Lodge Valley) and the Clark Fork channel 

is highly meandering (Lambing, 1998). A notable minor tributary that enters the Clark Fork in this reach is Lost Creek. The 

Lost Creek drainage basin has an area of about 65 mi2

 and 

was affected by deposition of contaminants in emissions 

from smelting activities of the AMC Smelter (Hooper and 

others, 2002). Clark Fork at Deer Lodge (site 14, fig. 1, 

table 1) has a drainage area of 995 mi2

 and mean annual 

streamflow (water years 1985–2010) is 228 ft3

/s (U.S. Geological Survey, 2012b). Between Deer Lodge and Garrison, 

floodplain tailings along the Clark Fork are present to a similar 

extent as in the valley upstream from Deer Lodge (Smith 

and others, 1998). The Little Blackfoot River, a major tributary with a drainage area of 407 mi2

, enters the Clark Fork 

at the lower end of this reach near Garrison. Between Garrison and Drummond where the Clark Fork valley narrows, 

floodplain tailings are less extensive than in the Deer Lodge 

Valley and meandering of the Clark Fork channel decreases 

(Smith and others, 1998; Lambing, 1998). Flint Creek, a major 

tributary with a drainage area of 490 mi2

, enters the Clark 

Fork in the reach between Goldcreek and Drummond. Clark 

Fork near Drummond (site 18, fig. 1, table 1) has a drainage 

area of 2,501 mi2

 and mean annual streamflow (water years 

1994–2010) is 701 ft3

/s (U.S. Geological Survey, 2012b). 

Downstream from Drummond, the Clark Fork valley is narrow 

(generally less than 1 mile) and meandering of the Clark Fork 

decreases further in association with the narrow valley and 

presence of highway and railroad embankments (Lambing, 

1998). Rock Creek, a major tributary with a drainage area of 

885 mi2

, enters the Clark Fork between Drummond and Turah 

Bridge, near Bonner (fig. 1). Clark Fork at Turah Bridge 

(site 20, fig. 1, table 1) has a drainage area of 3,641 mi2

and mean annual streamflow (water years 1986–2010) is 

1,220 ft3

/s (U.S. Geological Survey, 2012b). Downstream 

from Turah Bridge, the Clark Fork flows through the area 

where the former Milltown Reservoir was located. Milltown 

Dam was completed in 1907 (U.S. Environmental Protection Agency, 2004) just downstream from the confluence of 

the Clark Fork and the Blackfoot River. During the decades 

following construction of Milltown Dam, substantial amounts 

of mining wastes (about 6.6 million cubic yards; U.S. Environmental Protection Agency, 2004) generated near Butte 

and Anaconda were transported downstream and deposited 

in Milltown Reservoir. Based on a management decision, 

Milltown Dam was breached and removed in 2008 (U.S. Environmental Protection Agency, 2013) and the Clark Fork was 

returned to a free-flowing river. The Clark Fork channel in the 

former Milltown Reservoir area was artificially reconstructed 

by using designs to approximate natural geomorphologic 

characteristics. The Clark Fork was diverted from a temporary 

bypass into the reconstructed channel in 2010. The Blackfoot 

River, with a drainage area of 2,290 mi2

, is a major tributary 

to the Clark Fork between Turah Bridge and Missoula (fig. 1). 

At the downstream end of the study area, the Clark Fork above 
Introduction 7

Missoula (site 22, fig. 1, table 1) has a drainage area of 

5,999 mi2

 and mean annual streamflow (water years 1985–

2010) is 2,600 ft3

/s.

The annual hydrographs of streams in the upper Clark 

Fork Basin are dominated by snowmelt runoff, typically during April through July although early low-altitude snowmelt 

can sometimes increase streamflow in late winter (February–

March). For example, mean streamflow volume during April 

through July (water years 1985–2010) for Clark Fork above 

Missoula (site 22, fig. 1, table 1) accounts for about 64 percent 

of mean annual streamflow volume (U.S. Geological Survey, 

2012c).

Physiographic, Climatic, and Geologic 

Characteristics

The study area lies within the Middle Rockies Ecoregion (Woods and others, 2002), which is characterized by 

forested mountains and intermontane valleys. Altitudes in 

the study area range from about 3,200–10,650 feet (ft) above 

the National Geodetic Vertical Datum of 1988 (NGVD 88). 

Vegetation is predominantly Douglas fir and ponderosa pine 

conifer forests in the high-altitude mountainous areas, mixed 

conifers shrubs and grasses in the mid-altitude foothills, and 

grasses in the low-altitude valley (Woods and others, 2002). 

The predominant land uses are timber harvest in mountainous 

areas and livestock grazing and hay production in valleys.

Areally-weighted mean annual precipitation in the study 

area (1980–2010 30-year normal; PRISM Climate Group, 

2012) is about 23.9 inches (with a range in annual precipitation of 10.2–73.0 inches across the study area). About one-half 

of annual precipitation falls during May through July while 

winter typically is the driest season (Nimick, 1993). Mean 

annual precipitation is highest in mountainous areas and generally ranges from about 11–13 inches in valleys. 

Bedrock in mountainous areas of the study area generally consists of various types of consolidated Precambrian 

metasedimentary and Paleozoic and Metazoic sedimentary formations (Nimick, 1993; Woods and others, 2002). Dominant 

rock type of ore bodies targeted by the most extensive mining 

activities in the upper Clark Fork Basin near Butte is the Cretaceous Butte Quartz Monzonite of the Boulder Batholith of 

western Montana (Gammons and others, 2006). Valleys in the 

study area typically are underlain by unconsolidated Tertiary 

and Quaternary sedimentary deposits. Geology of the study is 

described in detail by Nimick (1993) and Gammons and others 

(2006).

Overview of Mining and Remediation Activities

Mining in the upper Clark Fork Basin began in 1864 

when small-scale placer mining operations extracted gold 

from floodplain and channel deposits along Silver Bow 

Creek and its tributaries in and near Butte (Freeman, 1900; 

U.S. Environmental Protection Agency, 2005). The small gold 

mining operations generally ended about 1869 when larger 

scale underground silver and copper mining began in the Butte 

area. By about 1886, there were about 10 major ore-processing 

mill and smelter operations along Silver Bow Creek (Freeman 1900; U.S. Environmental Protection Agency, 2005). In 

1884, the AMC constructed milling and smelting facilities in 

Anaconda and by about 1910 most of the ore from the 

Butte area was being processed at Anaconda (U.S. Environmental Protection Agency, 2005, 2010). By 1917, more than 

150 mines were located in and near Butte (U.S. Environmental Protection Agency, 2005). From 1910 through the 1920’s, 

AMC acquired nearly all of the mines in the Butte area and the 

AMC underground mining operations continued until the late 

1970’s (U.S. Environmental Protection Agency, 2005; Gammons and others, 2006). In 1955, the AMC mining operations 

began to transition from underground to open-pit mining with 

the opening of the Berkeley Pit north of Butte; by the late 

1960’s the Berkeley Pit operations were the major focus of 

the AMC operations (Gammons and others, 2006). In 1977, 

AMC was purchased by the Atlantic Richfield Company 

(ARCO), which continued the Berkeley Pit mining operations 

until closure in the early 1980’s. ARCO became a subsidiary 

of British Petroleum in 2000. Mining operations in the Butte 

area by Montana Resources began in the mid 1980’s and have 

continued to present (2013), with interruption of operations 

from 2001 through 2003 (Don Booth, Atlantic Richfield Company, written commun., August 2013). The mining operations 

of Montana Resources generally have been of similar scale to 

those before the closure of the Berkeley Pit; however, smelting 

is no longer done in the upper Clark Fork Basin.

Large amounts of waste materials enriched with metallic 

contaminants and arsenic were generated from mining 

operations in the Butte area. Andrews (1987) estimated that 

100 million tons of tailings were disposed of in Silver Bow 

Creek and the upper Clark Fork between 1880 and 1982. Mining activities in the Butte area severely contaminated waters 

in pit lakes, flooded underground mines, and alluvial aquifers 

(Gammons and others, 2006). The milling and smelting operations in Anaconda produced about 230 million cubic yards 

of mill tailings, 30 million cubic yards of furnace slag, and 

500,000 cubic yards of flue dust (U.S. Environmental Protection Agency, 2010). Pollution from the AMC Smelter 

was responsible for severely contaminating soil on about 

20,000 acres by airborne emissions and also contaminating 

millions of gallons of groundwater from leaching of mining 

wastes and contaminated soils (U.S. Environmental Protection 

Agency, 2010). Several large floods, especially during the late 

1800’s and early 1900’s, resulted in floodplain deposition of 

substantial amounts of mining wastes as indicated by tailings 

deposits 3–4 ft thick along Silver Bow Creek and commonly 

1 foot thick along the upper Clark Fork near Deer Lodge 

(Smith and others, 1998; Titan Environmental Corporation, 

1995; Nimick and Moore, 1994). Also, diversion of water 

from the Clark Fork for irrigation of hay fields in the upper 

Clark Fork valley resulted in accumulation of trace elements 

in soils (Axtmann and Luoma, 1991).
8 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

The need to control or remediate effects of mining wastes 

on stream environments became evident in the early 1900’s. 

From 1908 to 1917, AMC constructed dikes near the mouth 

of Silver Bow Creek to form settling ponds (later known as 

the Warm Springs Ponds) to trap sediment enriched in trace 

elements (CDM, 2005). The Warm Springs Ponds system 

was expanded during the 1950’s to provide greater sedimentcontainment capacity. In about 1967, AMC started introducing 

a lime and water suspension into Silver Bow Creek upstream 

from Warm Springs Ponds to raise pH and encourage precipitation and deposition of metals within Warm Springs Ponds 

(U.S. Environmental Protection Agency, 2000). In about 

1969, the Mill-Willow bypass was constructed to capture 

flows of Mill and Willow Creeks near their mouths and divert 

the combined flows (believed to be “relatively clean water”; 

U.S. Environmental Protection Agency, 2000) around Warm 

Springs Ponds and into Silver Bow Creek between the outlet 

from the Warm Springs Ponds and the confluence with Warm 

Springs Creek (CDM, 2005). However, water from Silver 

Bow Creek, enriched in trace elements, overflowed into the 

Mill-Willow bypass on several occasions when channels in 

the Warm Springs Ponds became plugged with debris during 

runoff events (U.S. Environmental Protection Agency, 2000). 

By 1989 the channel banks of the bypass had accumulated 

mine tailings from Silver Bow Creek overflow, which contributed to several fish kills that resulted from exposure to 

contaminants along the lower bypass and upper Clark Fork 

(U.S. Environmental Protection Agency, 2000). The potential 

toxicity issues associated with Warm Springs Ponds and the 

Mill-Willow bypass were a primary focus of early Federal 

Superfund activities.

Early Federal Superfund activities in the upper Clark 

Fork Basin involved designation of three areas as Superfund 

sites in 1983: Silver Bow Creek Site; Anaconda Smelter Site; 

and Milltown Reservoir Site. Silver Bow Creek Site was 

redesignated as Silver Bow Creek/Butte Area Site in 1987 and 

includes remnants from mining operations in Butte and about 

26 miles of Silver Bow Creek extending from near Butte to the 

outlet of Warm Springs Ponds (U.S. Environmental Protection Agency, 2000; CDM, 2005). Remediation activities in 

the Silver Bow Creek/Butte Area Site have targeted reducing 

environmental effects of (1) mining wastes deposited along 

upper Silver Bow Creek and its tributaries, (2) acidic water, 

with high concentrations of trace elements, from the Berkeley 

Pit and numerous underground mine workings, and (3) large 

amounts of sediments enriched in trace elements deposited in 

Warm Springs Ponds. 

Anaconda Smelter Site includes about 300 mi2

, primarily 

in the Mill, Willow, Warm Springs, and Lost Creek drainage 

basins near Anaconda (U.S. Environmental Protection Agency, 

2010). Remediation activities in the Anaconda Smelter Site 

have targeted reducing environmental effects of the extensive 

soil and water contamination caused by the operations, including airborne emissions, of the AMC Smelter. 

Milltown Reservoir Site was redesignated as the Milltown Reservoir/Clark Fork River Site in 1992. Milltown 

Reservoir/Clark Fork River Site includes two primary operable units: Milltown Reservoir Sediments Operable Unit and 

Clark Fork River Operable Unit. The Milltown Reservoir 

Sediments Operable Unit includes about 540 acres defined by 

the area inundated by maximum pool elevation of the former 

Milltown Reservoir (U.S. Environmental Protection Agency, 

2004). Remediation activities in the Milltown Reservoir Sediments Operable Unit have targeted reducing environmental 

effects of large amounts of sediments enriched in trace elements that were deposited in the former Milltown Reservoir. 

Those activities have included removal of the Milltown Dam 

in 2008 and excavation of millions of cubic yards of contaminated sediment (U.S. Environmental Protection Agency, 

2004). Clark Fork River Operable Unit includes streamside 

areas of the 115-mile reach of the Clark Fork extending from 

Warm Springs Ponds outlet to the start of Milltown Reservoir 

Sediments Operable Unit (U.S. Environmental Protection 

Agency, 2012). Remediation activities of Clark Fork River 

Operable Unit have targeted reducing environmental effects 

of mining wastes deposited along the Clark Fork and in its 

floodplain. 

Data Collection and Analytical 

Methods

Concerns about effects of mining wastes in the upper 

Clark Fork Basin on stream environments and human health 

prompted extensive data-collection efforts by various State, 

Federal, university, and private entities that began in earnest 

in the late 1970’s and early 1980’s. Need for a long-term 

consistent monitoring effort was recognized to evaluate 

temporal changes in water quality. Water-quality data collection by USGS began during 1985 to 1988 at four main-stem 

Clark Fork sites (sites 11, 14, 20, and 22; fig. 1, table 1) and 

four sites on major tributaries to the Clark Fork (sites 15, 17, 

19, and 21). In 1993, the monitoring program was expanded to 

include three sites on Silver Bow Creek (sites 2, 3, and 8), one 

site on Blacktail Creek (site 1), which is a tributary to Silver 

Bow Creek, one site on Warm Springs Creek (site 10), which 

is a tributary to the Clark Fork, and two additional main-stem 

Clark Fork sites (sites 16, and 18; fig.1, table 1). In 2003 to 

2005, the monitoring program was expanded again to include 

additional sites on tributaries to Silver Bow Creek and the 

Clark Fork that drain areas affected by milling and smelting 

activities near Anaconda. Those sites included sites 4 and 5 

on Mill Creek, sites 6 and 7 on Willow Creek, site 9 on Warm 

Springs Creek, and sites 12 and 13 on Lost Creek. At the end 

of water year 2004, data collection was discontinued at three 

of the major tributary sites (sites 15, 17, and 19). Thus, there 

have been a total of 22 sites in the monitoring program with 

variable periods of record (fig. 1, table 1).

Sampling frequencies have been variable, especially during the early period of data collection. However, all main-stem 

Silver Bow Creek and Clark Fork sites and most tributary sites 
Data Collection and Analytical Methods 9

have median annual sampling frequencies of eight samples 

per year (table 1). Some tributary sites have a median annual 

sampling frequency of six samples per year. Two sites (sites 

2 and 3) have single 1-year data gaps and one site (site 1) has 

two nonconsecutive 1-year gaps when no data were collected. 

Some other sites have a small number of years (no more than 

3) when the sampling frequency was less than four samples 

per year. Since 1993, the sampling frequency of the monitoring program has been consistent, with most sites being 

sampled eight times per year, except for the tributary sites 9, 

10, 15, 19, and 21, which generally were sampled six times 

per year. In the monitoring program, the seasonal timing of 

sample collection placed greater emphasis on the snowmelt 

runoff period (typically April through July) when streamflow 

conditions are high and variable and constituent transport is 

large. About 75 percent of samples were collected during April 

through July. In general, throughout the period of data collection there is reasonable consistency in the frequency and timing of sample collection among the sites to provide reasonable 

consistency in trend analysis results.

Data collected in the monitoring program are published 

(typically on an annual basis) in data reports that present the 

methods of data collection, water-quality data, quality-assurance data, and statistical summaries of the data (Lambing, 

1987 through 1991; Lambing and others, 1994, 1995; Dodge 

and others, 1996 through 2012). Those data reports provide 

detailed descriptions of the specific field-collection and laboratory-analytical methods (with associated literature references) 

used in each year of data collection. General information on 

field-collection and laboratory-analytical methods used during 

the study period is presented in the following paragraph. Specific issues related to changes in field and laboratory analytical 

methods during the study period that are relevant to longterm trend analysis are discussed in more detail in following 

paragraphs.

In the monitoring program, water samples were collected 

from vertical transits throughout the entire stream depth at 

multiple locations across the stream by using standard USGS 

depth- and width-integration methods (U.S. Geological Survey, variously dated). Those methods provide a vertically and 

laterally discharge-weighted composite sample that is intended 

to be representative of the entire flow passing through the 

cross section of a stream (Dodge and others, 2012). Specific 

conductance was measured on site in subsamples from the 

composite water samples. Subsamples of the composite water 

samples also were analyzed at the USGS National Water 

Quality Laboratory (NWQL) in Denver, Colorado for filtered (0.45-micrometer pore size) and unfiltered-recoverable 

concentrations of the trace element constituents included in 

the trend analysis (table 2). Water samples also were analyzed 

for suspended-sediment concentrations by the USGS sediment 

laboratory in Helena, Montana.

Table 2. Property and constituents included in the trend analysis and information relating to laboratory and study reporting levels.

[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. NWQL, U.S. Geological Survey 

National Water Quality Laboratory; μS/cm, microsiemens per centimeter at 25 degrees Celsius; NA, not applicable; μg/L, micrograms per liter; mg/L, milligrams per liter]

Property or constituent Units of measurement

Number of NWQL laboratory reporting levels 

during water years 

1993–2010

Range in NWQL 

laboratory reporting 

levels

Study reporting level 

used in application of 

the time-series model1

Specific conductance μS/cm NA NA 8

Cadmium, filtered μg/L 7 0.02–1.4 0.05

Cadmium, unfiltered-recoverable μg/L 10 0.014–1.3 0.09

Copper, filtered μg/L 4 0.4–3 1

Copper, unfiltered-recoverable μg/L 5 0.6–4 1

Iron, filtered μg/L 7 3–10 6

Iron, unfiltered-recoverable μg/L 8 6–21 20

Lead, filtered μg/L 9 0.03–5 0.5

Lead, unfiltered-recoverable μg/L 5 0.06–5 0.5

Manganese, filtered μg/L 9 0.18–10 1

Manganese, unfiltered-recoverable μg/L 7 0.2–10 1

Zinc, filtered μg/L 11 0.6–20 5

Zinc, unfiltered-recoverable μg/L 7 1–40 2

Arsenic, filtered μg/L 5 0.044–2 1

Arsenic, unfiltered-recoverable μg/L 7 0.12–3 1

Suspended sediment mg/L NA NA 1

1

Procedures for determining and applying the study reporting level used in the application of the time-series model are discussed in the section of this report 

“Time-Series Model.”
10 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

Consistent field-collection and laboratory-analytical 

methods are important in trend analysis to be confident that 

observed trends represent real environmental changes and not 

methodology changes. Primary issues with respect to changes 

in field-collection and laboratory-analytical methods during the period of data collection include the following: (1) a 

change in April 1992 by NWQL from open-beaker to in-bottle 

digestion for analysis of unfiltered-recoverable concentrations 

of trace elements (Hoffman and others, 1996), (2) implementation by USGS in water year 1993 of ultra-clean field sample 

collection and processing methods (Lambing and others, 

1994), and (3) a change in about water year 2000 by NWQL 

from analysis of most metallic elements by graphite furnace 

atomic absorption spectrophotometry (Fishman, 1993) to 

inductively coupled plasma-mass spectrometry (Garbarino and 

Struzeski, 1998; Garbarino and others, 2006). The most substantial changes in field and laboratory methods were before 

water year 1993. Also, many of the study sites were not sampled before water year 1993. Thus, to maintain consistency in 

trend analysis among sites, trend analysis was restricted to the 

period of water years 1996–2010 (as discussed in the section 

of this report “Selection of Trend-Analysis Time Periods”). 

However, potential effects of the change in about water year 

2000 in analytical methods for most metallic elements warrant 

further discussion.

Before about water year 2000, NWQL analyzed for most 

metallic elements by using graphite-furnace atomic absorption 

spectrophotometry (Fishman, 1993). In about water year 2000, 

NWQL began analyzing for most metallic elements in water 

samples for the Clark Fork long-term monitoring program by 

using inductively coupled plasma-mass spectrometry (Garbarino and Struzeski, 1998; Garbarino and others, 2006). The 

change in analytical methods resulted in small changes in 

recoveries of metallic elements in laboratory-spiked deionized-water blank samples and laboratory-spiked stream-water 

samples. Because of importance of temporal consistency in 

analytical methods in trend analysis, potential effect of the 

change in analytical methods was evaluated and is discussed 

in detail in the section of this report “Appendix 1: Summary 

Information Relating to Quality-Control, Water-Quality, and 

Continuous Streamflow Data.” It was determined that the 

change in analytical methods had minor effect on trend results. 

Quality Assurance

Quality-assurance procedures (with associated literature 

references) and quality-assurance data for each year of data 

collection are presented in the data reports for the monitoring 

program (Lambing, 1987 through 1991; Lambing and others, 

1994, 1995; Dodge and others, 1996 through 2012). General 

information on quality-assurance procedures used during the 

study period is provided in the following paragraph. An overview of quality-assurance results relevant to long-term trend 

analysis also is provided in this section of the report.

From its inception, the Clark Fork monitoring program 

used quality-assurance procedures that included chain-ofcustody procedures and collection of field blank and replicate samples at a frequency of about 15 percent of the total 

number of samples (Lambing, 1991). Coincident with the 

1993 implementation of ultra-clean sampling methods, the 

following additional quality-assurance procedures were 

included in the monitoring program (Lambing and others, 

1994): (1) establishment of data-quality objectives (DQOs) 

to ensure that the field-collection and laboratory-analytical 

methods are producing environmentally representative data in 

a consistent manner, (2) establishment of specific guidelines 

for evaluating analytical results for field blank samples and 

resolving persistent contamination issues, (3) collection of 

laboratory replicate samples used to evaluate data precision 

for meeting specific DQOs, (4) collection of laboratory spike 

samples used to evaluate recoveries of trace elements for 

meeting specific DQOs, and (5) systematic electronic storage 

of quality-assurance data in the USGS water-quality database 

[National Water Information System (NWIS); U.S. Geological 

Survey, 2012c]. DQOs established in 1993 identify analytical 

requirements of detectability, and serve as a guide for identifying questionable data by establishing limits for precision 

and bias of laboratory results (Lambing and others, 1994). In 

addition to analysis of analytical results for quality-assurance 

samples with respect to DQOs, analytical results for individual 

environmental samples were carefully reviewed based on the 

following: (1) comparisons with associated quality-assurance 

sample results, (2) comparisons with results for previously 

collected samples at the site, (3) relations between filtered and 

unfiltered-recoverable concentrations, (4) relations between 

unfiltered-recoverable concentrations and suspended-sediment 

concentrations, and (5) relations between concentrations and 

streamflow conditions. When one or more of those review 

criteria indicated problematic results for a given trace-element 

constituent, laboratory re-analysis was done. If the re-analysis 

did not resolve the problematic results, the analytical results 

were excluded from the trend analysis. For the entire study 

period, exclusion of analytical results that resulted from 

review of analytical results for field blank samples or review 

of analytical results for individual environmental samples 

affected a small proportion of the study database (about 

0.4 percent of all trace-element analyses). Excluded sample 

results generally were sporadic or limited to short time periods 

and are considered to have negligible effects on trend analysis.

Analytical results for field quality-assurance samples 

(including field blank and replicate samples) that were collected during water years 1993–2010 were compiled and 

statistically summarized (table 1–1 in appendix 1 at the back 

of the report). Those data provide information on the consistency and environmental representativeness of data collection. Representative sampling for trace elements in streams is 

particularly difficult because of low concentrations in stream 

waters and ubiquitous presence in the sampling environment 

that produce an associated large potential for contamination. 

Analysis of analytical results for field blank samples provides 
Overview of Water-Quality Characteristics for Sampling Sites in the Upper Clark Fork Basin 11

information on potential effects of contamination during the 

sampling process on trend analysis results. For field blank 

samples, the frequency of detection at concentrations greater 

than the laboratory reporting level (LRL) at the time of 

analysis was less than about 5 percent for all trace-element 

constituents except for filtered and unfiltered-recoverable 

zinc, which had detection frequencies of 14.7 and 6.6 

percent, respectively (table 1–1). Precise statistical analysis 

of the analytical results of field blank samples is difficult 

because of the multiple LRLs used by NWQL during the 

study period (table 2). Also, it is difficult to precisely quantify the field blank sample results with respect to the study 

database because contamination indicated by field blank 

samples was monitored routinely in the Clark Fork monitoring program and stream-sample data judged to be affected 

by persistent contamination issues were deleted. However, 

it is important that trend-analysis procedures are structured 

to minimize potential effects of sampling contamination on 

low-concentration data included in the trend analysis. Specific procedures used in application of the two trend-analysis 

methods used in this report with respect to handling of lowconcentration and censored data (that is, analytical results 

reported as less than the LRL; Helsel, 2005) are discussed 

in the section of this report “Trend Analysis Methods.” For 

the entire study period, relative standard deviations (RSDs; a 

measure of overall precision) for field replicate sample pairs 

(table 1–1) indicate that for all constituents the RSDs were 

within 20 percent (the DQO for laboratory replicate samples) 

indicating reasonable precision (Dodge and others, 2012). 

Overview of Water-Quality 

Characteristics for Sampling Sites in 

the Upper Clark Fork Basin

Statistically summarizing water-quality characteristics 

of the sites is useful for generally describing water quality of 

the study sites and in providing comparative information relevant for interpretation of trend results. Statistical summaries 

of water-quality data (water years 2001–10) for sites in 

the upper Clark Fork Basin are presented in table 1–4 in 

appendix 1 (at the back of this report). Unadjusted (not flow 

adjusted) concentrations are presented in table 1-4; flow 

adjustment, discussed in the “Trend Analysis Methods” 

section of this report, is relevant when interpreting trends in 

concentrations of water-quality constituents that are strongly 

dependent on streamflow conditions. For trace elements, in 

addition to statistical summaries of unadjusted concentrations, ratios of median filtered to unfiltered-recoverable concentrations also are reported in table 1–4 to provide general 

information on the predominant phase (that is, dissolved or 

particulate) of transport. Aquatic life standards (Montana 

Department of Environmental Quality, 2012a; based on 

median hardness for each site for water years 2001–10) are 

presented in table 1–5. The arsenic human health standard 

is 10 μg/L (Montana Department of Environmental Quality, 2012a). Percents of samples (water years 2001–10) with 

unadjusted unfiltered-recoverable concentrations exceeding 

water-quality standards for each site are presented in table 

3. To aid in interpreting differences in water-quality characteristics between sites, statistical summaries of continuous 

streamflow data are presented in table 1–6. Data are summarized for water years 2001–10, which was selected as the 

summary period because all sites have available data for this 

time period, a large range in streamflow conditions is represented, and recent water-quality conditions are represented. 

Water-quality characteristics of the study sites are 

illustrated by using boxplots of selected example constituents (specific conductance, and unadjusted concentrations 

of copper, arsenic, and suspended-sediment) presented in 

figure 2. Also shown in figure 2 are applicable water-quality 

standards. Specific conductance is presented as an example 

because it is an index of ionic strength and provides information on extent of water contact with geologic materials and 

types of geologic materials present in the site basins. Copper 

and arsenic are presented as examples of trace elements 

because they have much different geochemical characteristics, but are constituents of concern with respect to potential 

toxicity issues. Further, spatial and temporal variability in 

copper concentrations generally is similar to variability in 

other metallic contaminants, and is considered generally 

representative of those constituents. Suspended sediment 

is presented because it provides information on transport 

of particulate materials, which is a factor that can strongly 

affect transport of metallic contaminants.

To assist in the presentation of results, Silver Bow 

Creek and the Clark Fork were divided into nine data-summary reaches based on the location of sites along the mainstems of those streams. Water-quality characteristics within 

the reaches are affected by environmental characteristics 

within the delineated reach basin boundaries (fig. 1). Waterquality characteristics of the sites are discussed for each of 

the data-summary reaches. Emphasis is placed on describing 

spatial differences in water quality in the upper Clark Fork 

Basin.

Reach 1

Sites in reach 1 include the reach inflow [Blacktail 

Creek (site 1, fig. 1, table 1)] and the reach outflow [Silver 

Bow Creek at Butte (site 2, fig. 1, table 1)]. Reach 1 is 

3.5-mi long and flows through an area substantially affected 

by large-scale mining activities in the Butte area, including 

the Berkeley Pit. In reach 1, mean annual streamflow of Silver 

Bow Creek increases from 13 ft3

/s (site 1) to 21 ft3/s (site 2, 

table 1–6) because of base streamflow increases from groundwater inflow, discharges from the Butte wastewater treatment 

facility, and contributions from ephemeral sources (including 

stormwater runoff from Butte).
12 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

Blacktail Creek (site 1) is on a tributary of Silver Bow 

Creek, but the Blacktail Creek channel is the longest stream 

channel in the upper Silver Bow Creek Basin, thus, site 1 is 

considered a main-stem Silver Bow Creek site for the purpose of discussion. There have been numerous small mining 

operations in the upper reaches of the Blacktail Creek Basin; 

however, at site 1 the effect of mining operations on water 

quality is small to moderate. Thus, site 1 is considered an 

“index site” and is used as a reference for comparing concentrations among sites. Median values of specific conductance, 

copper, arsenic, and suspended sediment for site 1 generally 

are lower than median values for most main-stem Silver Bow 

Creek and Clark Fork sites (fig. 2, table 1–4). Exceedances of 

water-quality standards for site 1 were relatively infrequent 

(that is, less than or equal to 25 percent of samples; table 3).

There are large spatial changes in water quality between 

site 1 and site 2 (the first study site downstream from the 

substantial mining activities in the Butte area). Median specific conductance increases from 265 to 484 microsiemens per 

centimeter at 25 degrees Celsius (μS/cm) and median concentrations of metallic contaminants increase substantially (fig. 2, 

table 1–4). For example, median unfiltered-recoverable copper 

concentration increases from 5.0 to 20 μg/L. Increases in metallic contaminants in reach 1 are affected by groundwater inflow 

from contaminated alluvial aquifers (Gammons and others, 

2006) and mobilization of contaminated sediments during storm 

and snowmelt runoff events (Gammons and others, 2005). 

Table 3. Percent of samples with unadjusted (not flow adjusted) unfiltered-recoverable concentrations exceeding water-quality 

standards for sites in the upper Clark Fork Basin, water years 2001–10.

Site 

number 

(fig.1, 

table 1)

Site name 

(fig. 1, table 1)

Percent of samples exceeding indicated standard

Arsenic 

human 

health 

standard

Aquatic life standards

Cadmium Copper Lead Zinc

Acute Chronic Acute Chronic Acute Chronic Acute Chronic

1 Blacktail Creek 7 0 0 3 13 0 3 0 0

2 Silver Bow Creek at Butte 13 0 29 48 91 0 9 25 25

3 Silver Bow Creek at 

Opportunity

95 6 99 100 100 4 95 53 53

4 Mill Creek near Anaconda 94 0 2 4 13 0 6 0 0

5 Mill Creek at Opportunity 100 0 17 19 33 0 17 0 0

6 Willow Creek near Anaconda98 0 5 9 21 0 25 0 0

7 Willow Creek at Opportunity100 0 3 16 39 0 9 0 0

8 Silver Bow Creek at Warm 

Springs

100 0 3 4 8 0 3 0 0

9 Warm Springs Creek near 

Anaconda

0 0 0 4 7 0 0 0 0

10 Warm Springs Creek at 

Warm Springs

13 0 4 17 27 0 9 0 0

11 Clark Fork near Galen 89 0 4 20 26 0 6 0 0

12 Lost Creek near Anaconda 13 2 2 11 20 2 4 2 2

13 Lost Creek near Galen 83 0 0 0 0 0 0 0 0

14 Clark Fork at Deer Lodge 96 0 12 47 77 0 23 1 1

15 Little Blackfoot River 0 0 0 0 0 0 0 0 0

16 Clark Fork at Goldcreek 79 0 12 40 63 0 23 0 0

17 Flint Creek near Drummond 83 0 0 0 0 0 24 0 0

18 Clark Fork near Drummond 81 0 14 40 56 0 33 5 5

19 Rock Creek 0 0 0 0 0 0 0 0 0

20 Clark Fork at Turah Bridge 23 0 14 39 50 0 26 1 1

21 Blackfoot River 0 0 0 0 0 0 2 0 0

22 Clark Fork above Missoula 6 0 14 31 45 0 26 5 5
Overview of Water-Quality Characteristics for Sampling Sites in the Upper Clark Fork Basin 13 Parts of distributions less than study reporting level not shown 

Blacktail Creek

Silver Bow Creek at Butte

Silver Bow Creek at Opportunity

Silver Bow Creek at Warm Springs

Warm Springs Creek at Warm Springs

Warm Springs Creek at Anaconda

Mill Creek near Anaconda

Clark Fork near Galen

Little Blackfoot River

Clark Fork at Deer Lodge

Clark Fork at Goldcreek

Blackfoot River

Clark Fork above Missoula

Flint Creek

Clark Fork near Drummond

Rock Creek

Clark Fork at Turah Bridge

Lost Creek near Galen

Lost Creek near Anaconda

Willow Creek near Anaconda

Mill Creek at Opportunity

Willow Creek at Opportunity

Study reporting level for copper used in application of the time-series model (TSM)Study reporting level for arsenic used in application of the time-series model (TSM)

A.

C.

D.

reach

1

reach

2

reach

3

reach

4

reach

5

reach

6

reach

7

reach

8

reach

9

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

B.

Specific 

 Copper

 conductance

ArsenicSuspended sediment

distribution less than study reporting level distribution less than study reporting level

distribution less than study reporting level

Parts of distributions less than studyreporting level not shown 

10

100

1,000

1

10

0.10.1

100

1,000

1

10

100

1,000

1

10

0.5

100

1,000

EXPLANATION

75th percentilemedian

25th percentile

90th percentile10th percentileAcute aquatic life standard  for copper based on median  hardness for given site for water years 2001–10 (Montana Department of Environmental Quality, 2012a)

Human health standard for  arsenic (Montana Department  of Environmental Quality, 2012a)

22

For copper and arsenic,green boxplots denote filteredconcentrationsFor copper and arsenic,magenta boxplots denoteunfiltered-recoverable concentrations

[Water year is defined as the 12-month period from October 1 through September 30 and is designated by the year in which it ends]

Site number (fig. 1, table 1)— 

Site numbers that lie on top of reach dividing lines indicate the site is both an outflow from the upstream reach and an inflow to the downstream reach 

Unadjusted (not flow adjusted) 

specific conductance, in 

microsiemens per centimeter 

at 25 degrees Celsius

Unadjusted (not flow adusted) 

filtered and unfilteredrecoverable copper concentration, in micrograms per liter

Unadjusted (not flow adjusted) 

filtered and unfilteredrecoverable arsenic concentration, 

in micrograms per liter

Unadjusted (not flow adjusted) 

suspended- sediment 

concentration, in milligrams 

per liter

Interquartile

range

Figure 2. Statistical distributions of selected constituents for sites in the upper Clark Fork Basin, Montana (fig. 1, table 1), based on data collected during water years 2001–10.
14 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

Cessation of most of the mining activities near Butte in the 

early 1980’s and subsequent remediation efforts have substantially decreased mining effects on water quality at 

site 2 (as discussed in the section of this report “Water-Quality 

Trends for Selected Sampling Sites in the upper Clark Fork 

Basin”), but mining effects on water quality are still evident. 

Discharge of effluent from the Butte wastewater treatment 

facility just upstream from site 2 also affects water quality 

at site 2. The effluent contributes elevated concentrations of 

nutrients, strongly affecting the biogeochemistry of Silver 

Bow Creek (Gammons and others, 2011). Also, filtered and 

unfiltered-recoverable copper concentrations in Silver Bow 

Creek substantially increase immediately downstream from 

the effluent discharge (Naughton, 2013), probably affected by 

direct contributions of copper in the effluent discharge. Contribution of complex organic compounds from the wastewater 

effluent to Silver Bow Creek also might affect metallic-element chemistry and transport at site 2 by complexation. Ratios 

of median filtered to unfiltered-recoverable concentrations 

of metallic elements for site 2 are higher than for most 

other main-stem Silver Bow Creek and Clark Fork sites 

(table 1–4). The ratio of median filtered to unfiltered-recoverable zinc for site 2 (80 percent) is higher than the zinc ratio 

for any other study site (except for site 19, which has low zinc 

concentrations). Relatively high ratios of median filtered to 

unfiltered-recoverable concentrations of metallic elements for 

site 2 might be affected by several factors, including inflow 

of groundwater with high filtered concentrations of metallic 

elements and effects of the Butte wastewater effluent. Exceedances of most water-quality standards were relatively infrequent for site 2 (table 3). However, exceedances of aquatic life 

standards for copper were relatively frequent for site 2, with 

48 and 91 percent of samples exceeding the acute and chronic 

standards, respectively. The chronic aquatic life standard for 

cadmium was exceeded in 29 percent of samples.

Reach 2

Sites in reach 2 include the reach inflow [Silver Bow 

Creek at Butte (site 2, fig. 1, table 1)] and the reach outflow 

[Silver Bow Creek at Opportunity (site 3, fig. 1, table 1)]. 

Reach 2 is about 15-mi long and meanders through a floodplain with extensive deposits of mining wastes. In reach 2, 

mean annual streamflow of Silver Bow Creek increases from 

21 ft3/s (site 2) to 44 ft3/s (site 3, table 1–6) largely because of 

contributions from Browns Gulch, other perennial tributaries, 

ephemeral gulches, and groundwater inflow.

There are large spatial changes in water quality between 

sites 2 and 3. Median suspended-sediment concentration 

increases from 8 to 18 mg/L (fig. 2 and table 1–4) and median 

pH increases from 7.6 to 8.4 (table 1–4). Concentrations 

of metallic contaminants and arsenic increase substantially 

between sites 2 and 3 (fig. 2, table 1–4). For example, median 

unfiltered-recoverable copper increases from 20 to 79 μg/L, 

and unfiltered-recoverable arsenic increases from 6.3 to 

16 μg/L. Median unfiltered-recoverable concentrations of 

cadmium, copper, and zinc for site 3 (0.80, 79, and 197 μg/L, 

respectively) are substantially higher than concentrations for 

any other study site. Ratios of filtered to unfiltered-recoverable 

concentrations of metallic elements are lower for site 3 than 

for site 2, probably affected by increases in suspended sediment and pH between the sites. Exceedances of most waterquality standards were relatively frequent for site 3 (table 3). 

The arsenic human health standard and chronic aquatic life 

standards for cadmium, copper, and lead were exceeded in 

greater than or equal to 95 percent of samples. It is notable 

that the most extensive remediation activities in reach 2 began 

in 2004 and have included removal of tailings deposits and 

channel reconstruction (Montana Department of Environmental Quality, 2012b). Cessation of most of the mining activities 

near Butte in the early 1980’s and subsequent remediation 

efforts have substantially decreased mining effects on water 

quality at site 3 (as discussed in the section of this report 

“Water-Quality Trends for Selected Sampling Sites in the 

upper Clark Fork Basin”), but mining effects on water quality 

are still evident.

Reach 3

Sites in reach 3 include the reach inflow [Silver Bow 

Creek at Opportunity (site 3, fig. 1, table 1)] and the reach 

outflow [Silver Bow Creek at Warm Springs (site 8, fig. 1, 

table 1)]. Also in reach 3 are the monitored tributary sites Mill 

Creek near Anaconda (site 4, fig. 1, table 1), Mill Creek at 

Opportunity (site 5, fig. 1, table 1), Willow Creek near Anaconda (site 6, fig. 1, table 1), and Willow Creek at Opportunity 

(site 7, fig. 1, table 1). Reach 3 is about 6-mi long, meanders through a floodplain with extensive deposits of mining 

wastes, and also passes through Warm Springs Ponds where 

deposition of particulate materials and treatment (liming) to 

remove metallic elements is conducted. Mill Creek and Willow Creek originate in mountains southwest from the former 

AMC Smelter and flow generally northeast to their confluence 

with the Mill-Willow bypass near Opportunity. The Mill and 

Willow Creek Basins are affected by pollution from milling 

and smelting operations of the former AMC Smelter, which 

primarily resulted in deposition of arsenic from flue dust and 

smelter emissions with resultant large-scale soil and groundwater contamination (U.S. Environmental Protection Agency, 

2010). The Mill-Willow bypass diverts the combined flows of 

Mill Creek and Willow Creek into Silver Bow Creek between 

the outlet from Warm Springs Ponds and site 8. In reach 3, 

mean annual streamflow of Silver Bow Creek increases from 

44 ft3

/s (site 3) to 70 ft3/s (site 8, table 1–6), primarily because 

of contributions from the Mill-Willow bypass and groundwater inflow. 

There are large spatial changes in water quality between 

sites 3 and 8 that are strongly affected by Silver Bow Creek 

passing through Warm Springs Ponds. Warm Springs Ponds 

were designed to trap suspended sediment and metallic 
Overview of Water-Quality Characteristics for Sampling Sites in the Upper Clark Fork Basin 15

contaminants by physical deposition and treatment (liming). Warm Springs Ponds are eutrophic and have substantial 

growth of algae and rooted macrophytes that results from large 

inputs of nutrients (Chatham, 2012; Gammons and others, 

2007). Highly complex biogeochemical processes in Warm 

Springs Ponds, relating to metals chemistry and nutrient processing, result in substantial chemical changes as Silver Bow 

Creek passes through Warm Springs Ponds (Chatham, 2012; 

Gammons and others, 2007). Also, contributions from the 

Mill-Willow bypass substantially affect water-quality in reach 

3. Median suspended-sediment concentration decreases from 

18 to 5 mg/L (fig. 2, table 1–4) and median pH increases from 

8.4 to 8.8 (table 1–4). The median pH for site 8 is higher than 

the median pH for any other study site. Similar to suspended 

sediment, concentrations of metallic elements decrease substantially between site 3 and site 8. For example, median unfiltered-recoverable copper decreases from 79 to 8.0 μg/L (fig. 2, 

table 1–4). In contrast, median unfiltered-recoverable arsenic 

concentration increases substantially between site 3 (16 μg/L ) 

and site 8 (29 μg/L; fig. 2, table 1–4). The increase in arsenic 

concentration is affected by contributions of water with high 

arsenic concentrations from the Mill-Willow bypass. The 

increase in arsenic concentration in reach 3 also is affected by 

complex hydrologic and limnologic factors that affect arsenic 

biogeochemical processing in Warm Springs Ponds (Chatham, 

2012). High pH in Warm Springs Ponds (a result of a combination factors, including liming and nutrient processing by 

aquatic vegetation; Chatham, 2012), promotes arsenic solubility and mobilization (Stumm and Morgan, 1970). Filtered and 

unfiltered-recoverable arsenic concentrations for site 8 (26 and 

29 μg/L, respectively) are higher than any other main-stem 

Silver Bow Creek or Clark Fork study site. Exceedances of 

aquatic life standards for metals were relatively infrequent for 

site 8 (table 3). However, the arsenic human health standard 

was exceeded in all samples.

For the two Mill Creek sites [Mill Creek near Anaconda 

(site 4, fig. 1, table 1) and Mill Creek at Opportunity 

(site 5, fig. 1, table 1)], specific conductance, and concentrations of metallic elements and suspended sediment are 

relatively low (fig. 2, table 1–4), with median values less than 

those for Blacktail Creek (site 1, fig.1, table 1; an index site 

representative of small to moderate mining effect). However, 

arsenic concentrations are high in Mill Creek (median unfiltered-recoverable arsenic concentrations of 17 and 25 μg/L 

for site 4 and site 5, respectively). Mill Creek mean annual 

streamflow decreases by about 50 percent from 33 to 15 ft3

/s 

(table 1–6) in the 6-mi reach between site 4 and site 5 primarily because of irrigation diversions, year-round diversions to 

ditches that flow into tailings ponds and ultimately discharge 

to the Mill-Willow bypass, and losing stream reaches. However, the factors affecting streamflow decreases between 

sites 4 and 5 do not result in substantial spatial changes in 

water quality between the sites. Exceedances of most aquatic 

life standards for metals were relatively infrequent for the 

Mill Creek sites (table 3). However, the arsenic human health 

standard was exceeded in 94 percent of samples for site 4, and 

in all samples for site 5.

In contrast to the two Mill Creek sites (site 4 and site 5), 

there are large spatial changes in water-quality between the 

two Willow Creek sites [Willow Creek near Anaconda (site 6) 

and Willow Creek at Opportunity (site 7)]. For site 6, specific 

conductance, and concentrations of metallic elements and 

suspended sediment are relatively low (fig. 2, table 1–4), with 

median values less than median values for Blacktail Creek 

(site 1) and similar to median values for the upstream Mill 

Creek site (site 4). Similar to site 4, arsenic concentrations are 

high for site 6 (median unfiltered-recoverable arsenic concentration of 15 μg/L). Willow Creek mean annual streamflow 

increases from 7.0 to 10 ft3

/s (table 1–4) in the 5-mi reach 

between sites 6 and site 7. However, hydrologic characteristics 

of the reach are complex, with streamflow variably (spatially 

and seasonally) affected by irrigation diversions and return 

flows, contributions from tile drains, and losses to and gains 

from groundwater. In the reach, Willow Creek receives runoff 

from ephemeral gulches, irrigation return flows, and inflow 

from groundwater moving downgradient towards Silver Bow 

Creek. Specific conductance, pH, and concentrations of unfiltered-recoverable copper and arsenic are substantially higher 

for site 7, than for site 6 (fig. 2, table 1–4). Median filtered 

and unfiltered-recoverable arsenic concentrations for 

site 7 (32 and 33 μg/L, respectively) are moderately 

higher than concentrations for the downstream Mill Creek 

site (site 5) and also higher than concentrations for any other 

study site. Exceedances of most water-quality standards were 

relatively infrequent for the Willow Creek sites (table 3). 

However, the arsenic human health standard was exceeded 

in 98 percent of samples for site 6, and in all samples for 

site 7. Also, the chronic aquatic life standard for copper was 

exceeded in 39 percent of samples for site 7.

Reach 4

Sites in reach 4 include the reach inflow [Silver Bow 

Creek at Warm Springs (site 8, fig. 1, table 1)] and the reach 

outflow [Clark Fork near Galen (site 11, fig. 1, table 1)]. Also 

in reach 4 are the monitored tributary sites Warm Springs 

Creek near Anaconda (site 9, fig. 1, table 1) and Warm Springs 

Creek at Warm Springs (site 10, fig. 1, table 1). Reach 4 is 

short (about 2-mi long) but environmentally complex. Within 

the reach, water from Warm Springs Ponds, which is affected 

by treatment (liming), mixes with water contributed from the 

Mill-Willow bypass and Warm Springs Creek. Thick tailings 

deposits are extensive in the floodplain near Warm Springs 

(Smith and others, 1998) and provide a source of sediment 

enriched with metallic contaminants. Thus, there is potential 

for complex water-quality processes in the short reach. Warm 

Springs Creek originates in the mountains west of the former 

AMC Smelter, flows generally east through areas adjacent to 

the former AMC Smelter and various tailings piles and ponds, 

and joins Silver Bow Creek to form the Clark Fork near 
16 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

Warm Springs. The Warm Springs Creek Basin is affected by 

pollution from milling and smelting operations of the former 

AMC Smelter. In reach 4, mean annual streamflow increases 

from 70 ft3

/s (site 8) to 130 ft3/s (site 11, table 1–6) primarily 

because of contributions from Warm Springs Creek (site 10 

mean annual streamflow of 58 ft3

/s), ephemeral gulches, and 

groundwater inflow. Near the end of reach 4, Warm Springs 

Creek and Silver Bow Creek join to form the Clark Fork. 

Spatial changes in water quality between sites 8 and 

11 generally are not large. However, unfiltered-recoverable 

arsenic decreases from 29 to 18 μg/L. Site 11 is about 2 mi 

downstream from site 8 and about 1 mi downstream from the 

start of the Clark Fork at the confluence of Silver Bow Creek 

and Warm Springs Creek. There is little inflow between sites 

8 and 11 other than the monitored contribution from Warm 

Springs Creek (site 10). On a median basis, water-quality 

characteristics for site 11 generally reflect proportional mixing 

of waters from site 8 and site 10. However, median concentrations of unfiltered-recoverable copper and zinc and suspended 

sediment are somewhat higher for site 11 relative to proportional mixing of sites 8 and 10, which might provide evidence 

of mobilization of materials from floodplain tailings deposits 

near Warm Springs. Also, median concentration of unfilteredrecoverable arsenic is somewhat lower than proportional mixing of sites 8 and 10, which might provide evidence that arsenic can at times precipitate from solution in reach 4. Ratios 

of filtered to unfiltered-recoverable concentrations of metallic 

elements are higher for site 11 than for other main-stem Clark 

Fork sites (table 1–4). Exceedances of most water-quality standards were relatively infrequent for site 11 (table 3). However, 

the arsenic human health standard was exceeded in 89 percent 

of samples. Also, the chronic aquatic life standard for copper 

was exceeded in 26 percent of samples.

There are large spatial changes in water-quality between 

the two Warm Springs Creek sites [Warm Springs Creek near 

Anaconda (site 9, fig. 1, table 1) and Warm Springs Creek at 

Warm Springs (site 10, fig. 1, table 1)]. For site 9, specific 

conductance, and concentrations of metallic elements, arsenic, 

and suspended sediment are relatively low (fig. 2, table 1–4), 

with median values less than median values for Blacktail 

Creek (site 1, fig.1, table 1; an index site representative of 

small to moderate mining effect). Warm Springs Creek mean 

annual streamflow decreases from 84 to 58 ft3

/s (table 1–6) 

in the 9-mi reach between sites 9 and 10, largely because of 

irrigation and various drainage-control diversions. Hydrologic 

characteristics of the reach are complex, with streamflow 

variably (spatially and seasonally) affected by instream-flow 

releases, irrigation diversions and return flows, drainagecontrol diversions, and groundwater inflow. In the reach, 

Warms Springs Creek receives runoff from ephemeral gulches, 

irrigation return flows, and inflow from groundwater. Median 

specific conductance, and concentrations of trace elements are 

higher for site 10 than medians for site 9 (fig. 2, table 1–4). 

For example, median unfiltered-recoverable copper increases 

from 2.1 to 8.1 μg/L between sites 9 and 10. Exceedances of 

water-quality standards for the Warm Springs Creek sites were 

relatively infrequent (table 3). However, the chronic aquatic 

life standard for copper was exceeded in 27 percent of samples 

for site 10.

Reach 5

Sites in reach 5 include the reach inflow [Clark Fork at 

Galen (site 11, fig. 1, table 1)] and the reach outflow [Clark 

Fork at Deer Lodge (site 14, fig. 1, table 1)]. Also in reach 5 

are the monitored tributary sites Lost Creek near Anaconda 

(site 12, fig. 1, table 1) and Lost Creek near Galen (site 13, 

fig. 1, table 1). Reach 5 is about 21-mi long and meanders 

through a broad valley with extensive floodplain tailings 

deposits. Lost Creek originates in the mountains northwest 

of the former AMC Smelter and flows generally east to its 

confluence with the Clark Fork near Galen. The Lost Creek 

Basin is affected by pollution from milling and smelting 

operations of the former AMC Smelter (U.S. Environmental 

Protection Agency, 2010). In reach 5, mean-annual streamflow 

of the Clark Fork increases from 130 ft3

/s (site 11) to 216 ft3/s 

(site 14, table 1–6) partly because of contributions from Lost 

Creek (site 13 mean annual streamflow of 29 ft3

/s), numerous ungaged tributaries, ephemeral gulches, and groundwater 

inflow. 

There are large spatial increases in concentrations of 

suspended sediment and metallic contaminants between sites 

11 and 14 that largely result from mobilization of mining 

wastes from floodplain tailings deposits and stream banks. 

Median suspended-sediment concentration increases from 7 to 

18 mg/L and median unfiltered-recoverable copper concentration increases from 12 to 27 μg/L (fig. 2, table 1–4). Median 

concentrations of most metallic contaminants for site 14 are 

higher than concentrations for any other main-stem Clark Fork 

site. Median concentration of unfiltered-recoverable arsenic 

for site 14 (17 μg/L) is nearly equal to the concentration of site 

11 (18 μg/L), but higher than concentrations for downstream 

main-stem Clark Fork sites. Exceedances of most water-quality standards were relatively infrequent for site 14 (table 3). 

However, exceedances of aquatic life standards for copper 

were relatively frequent, with 47 and 77 percent of samples 

exceeding the acute and chronic standards, respectively. Also, 

the arsenic human health standard was exceeded in 96 percent 

of samples.

There are some large spatial changes in water-quality 

between the two Lost Creek sites [Lost Creek near Anaconda 

(site 12, fig. 1, table 1) and Lost Creek near Galen (site 13, 

fig. 1, table 1)], but changes are variable among constituents. 

For site 12, specific conductance and concentrations of metallic elements, arsenic, and suspended sediment are relatively 

low (fig. 2, table 1–4), with median values less than median 

values for Blacktail Creek (site 1, fig.1, table 1; an index site 

representative of small to moderate mining effect). Lost 

Creek mean annual streamflow increases from 6.5 to 29 ft3

/s 

(table 1–6) in the 8-mi reach between sites 12 and 13. Hydrologic characteristics of the reach are complex, with streamflow 
Overview of Water-Quality Characteristics for Sampling Sites in the Upper Clark Fork Basin 17

variably (spatially and seasonally) affected by interbasin transfers, irrigation diversions and return flows, drainage-control 

diversions, and groundwater inflow. In the reach, Lost Creek 

receives runoff from ephemeral gulches, irrigation return 

flows, and groundwater inflow. Median specific 

conductance, and concentrations of arsenic and suspended 

sediment are higher for site 13 than medians for site 12 (fig. 2, 

table 1–4). Between sites 12 and 13, median specific conductance increases from 211 to 631 μS/cm, median concentration of unfiltered-recoverable arsenic increases from 3.7 to 

14 μg/L, and median concentration of suspended sediment 

increases from 5 to 15 mg/L. Median concentrations of metallic contaminants are similar among sites 12 and 13. Exceedances of water-quality standards were relatively infrequent 

for site 12 (table 3). Exceedances of aquatic life standards for 

metals were relatively infrequent for site 13 (table 3). However, the arsenic human health standard was exceeded in 

83 percent of samples for site 13.

Reach 6

Sites in reach 6 include the reach inflow [Clark Fork at 

Deer Lodge (site 14, fig. 1, table 1)] and the reach outflow 

[Clark Fork at Goldcreek (site 16, fig. 1, table 1)]. Also in 

reach 6 is the monitored tributary site Little Blackfoot River 

(site 15, fig. 1, table 1). Reach 6 is about 26-mi long and 

meanders through a broad valley from Deer Lodge to Garrison. Between Deer Lodge and Garrison, floodplain tailings 

along the Clark Fork are present to a similar extent as in the 

valley upstream from Deer Lodge (Smith and others, 1998). 

The Little Blackfoot River, which drains a largely forested 

basin that historically has had numerous small-scale mining 

operations, discharges into reach 6 near Garrison where the 

Clark Fork valley begins to narrow. Downstream from Garrison, floodplain tailings are less extensive than in the 

valley upstream. In reach 6, mean-annual streamflow of the 

Clark Fork increases from 216 ft3

/s (site 14) to 460 ft3/s 

(site 16, table 1–6) partly because of contributions from the 

Little Blackfoot River (site 15 mean annual streamflow of 

145 ft3

/s), numerous ungaged tributaries, ephemeral gulches, 

and groundwater inflow. 

There are moderate spatial decreases in median 

concentrations of suspended sediment, metallic contaminants, 

and arsenic between sites 14 and 16. Median concentration

 of unfiltered-recoverable copper decreases from 27 to 

20 μg/L, median concentration of unfiltered-recoverable arsenic decreases from 17 to 12 μg/L, and median concentration 

of suspended sediment slightly decreases from 18 to 17 mg/L 

(fig.2, table 1–4). Exceedances of most water-quality standards were relatively infrequent for site 16 (table 3). However, 

exceedances of aquatic life standards for copper were relatively frequent, with 40 and 63 percent of samples exceeding 

the acute and chronic standards, respectively. Also, the arsenic 

human health standard was exceeded in 79 percent of samples. 

Water-quality changes in reach 6 are affected by transport of 

mining wastes from upstream source areas in combination 

with streamflow inputs from areas with less mining effects 

(including the Little Blackfoot River). Dispersion and dilution 

of mining wastes generally results in decreasing water-quality 

effects with distance downstream from primary source areas.

For Little Blackfoot River (site 15; fig. 1, table 1), 

specific conductance and concentrations of metallic elements, 

arsenic, and suspended sediment are relatively low (fig. 2, 

table 1–4), with median values less than or similar to median 

values for Blacktail Creek (site 1, fig. 1, table 1; an index site 

representative of small to moderate mining effect). Historical 

mining activities in the Little Blackfoot River have had relatively small effect on water quality of site 15. There were no 

exceedances of water-quality standards for site 15 (table 3).

Reach 7

Sites in reach 7 include the reach inflow [Clark Fork 

at Goldcreek (site 16, fig. 1, table 1)] and the reach outflow 

[Clark Fork near Drummond (site 18, fig. 1, table 1)]. Also in 

reach 7 is the monitored tributary site Flint Creek (site 17, 

fig. 1, table 1). Reach 7 is about 31-mi long; channel meandering and exposed floodplain tailings are less extensive in 

reach 7 than in upstream reaches (Smith and others, 1998; 

Lambing, 1998). Flint Creek, which drains a basin with substantial irrigation activities and also has had numerous smallscale mining operations, discharges into reach 7 near Drummond. Downstream from Drummond, the Clark Fork valley 

narrows further and meandering of the Clark Fork decreases 

further in association with the narrow valley and presence 

of highway and railroad embankments (Smith and others, 

1998; Lambing, 1998). In reach 7, mean annual streamflow 

of the Clark Fork increases from 460 ft3

/s (site 16) to 648 ft3/s 

(site 18, table 1–6) partly because of contributions from Flint 

Creek (site 17 mean annual streamflow of 123 ft3

/s), numerous ungaged tributaries, ephemeral gulches, and groundwater 

inflow. 

Spatial changes in water quality between sites 16 and 

18 generally are not large. Median specific conductance 

and concentrations of most trace elements for site 18 are 

similar to the medians for site 16. However, between sites 

16 and 18, median concentration of unfiltered-recoverable 

lead increases from 2.4 to 3.2 μg/L, unfiltered-recoverable 

zinc increases from 20 to 25 μg/L, and suspended sediment 

increases from 17 to 23 mg/L (fig. 2, table 1–4). Although the 

increases in lead, zinc, and suspended-sediment concentrations are not large, they contrast with the pattern of decreasing 

water-quality effects with distance downstream from primary 

mining-waste source areas in the upper Clark Fork Basin. The 

spatial changes in water quality between sites 16 and 18 probably are affected by streamflow contributions from the Flint 

Creek Basin, which has a combination of relatively extensive 

historical mining operations (in comparison to other tributaries downstream from Deer Lodge) and irrigation activities. 

The Clark Fork floodplain and stream banks downstream 
18 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

from Flint Creek probably also contain mining-waste deposits 

sourced from the Flint Creek Basin. The median unfilteredrecoverable lead concentration for site 18 (3.2 μg/L ) is higher 

than the median concentration for any other main-stem Clark 

Fork site. Exceedances of most water-quality standards were 

relatively infrequent for site 18 (table 3). However, exceedances of aquatic life standards for copper were relatively 

frequent, with 40 and 56 percent of samples exceeding the 

acute and chronic standards, respectively. The chronic aquatic 

life standard for lead was exceeded in 33 percent of samples. 

Also, the arsenic human health standard was exceeded in 81 

percent of samples.

For Flint Creek (site 17, fig. 1, table 1), median specific 

conductance and concentrations of metallic contaminants, 

arsenic, and suspended sediment are higher than medians for 

Blacktail Creek (site 1, fig. 1, table 1; an index site representative of small to moderate mining effect) and also higher 

than medians for other major tributary sites downstream from 

Deer Lodge [Little Blackfoot River (site 15, fig. 1, table 1), 

Rock Creek (site 19, fig. 1, table 1), and Blackfoot River 

(site 21, fig. 1, table 1)]. Median unfiltered-recoverable lead 

concentration for site 17 (3.1 μg/L) exceeds the medians of 

all study sites except Silver Bow Creek at Opportunity (site 3, 

fig. 1, table 1) and Clark Fork near Drummond (site 18, fig. 1, 

table 1). Water-quality characteristics for site 17 probably are 

affected by the combination of relatively extensive historical 

mining operations (in comparison to other major tributaries 

downstream from Deer Lodge) and irrigation activities in 

the basin. Exceedances of most water-quality standards were 

relatively infrequent for site 17 (table 3). However the arsenic 

human health standard was exceeded in 83 percent of samples.

Reach 8

Sites in reach 8 include the reach inflow [Clark Fork near 

Drummond (site 18, fig. 1, table 1)] and the reach outflow 

[Clark Fork at Turah Bridge (site 20, fig. 1, table 1)]. Also in 

reach 8 is the monitored tributary site Rock Creek (site 19, 

fig. 1, table 1). Reach 8 is about 34-mi long and flows through 

a narrow floodplain with little or no visible mining tailings. In 

reach 8, the Clark Fork Valley is narrow (generally less than 

1 mile). Rock Creek, which drains a heavily forested basin 

with few historical mining activities, discharges into reach 8 

near Clinton. In reach 8, mean annual streamflow of the Clark 

Fork increases from 648 ft3

/s (site 18) to 1,200 ft3/s (site 20, 

table 1–6) primarily because of contributions from Rock Creek 

(mean annual streamflow of 471 ft3/s), numerous ungaged 

tributaries, ephemeral gulches, and groundwater inflow.

There are large spatial decreases in median concentrations of suspended sediment, metallic contaminants, and 

arsenic between sites 18 and 20. Median concentration of 

unfiltered-recoverable copper decreases from 20 to 12 μg/L, 

median concentration of unfiltered-recoverable arsenic 

decreases from 13 to 7.0 μg/L, and median concentration of 

suspended sediment decreases from 23 to 17 mg/L (fig.2, 

table 1–4). Water-quality changes in reach 8 are affected by 

dilution from Rock Creek. Exceedances of most water-quality 

standards were relatively infrequent for site 20 (table 3). However, exceedances of aquatic life standards for copper were 

relatively frequent, with 39 and 50 percent of samples exceeding the acute and chronic standards, respectively. The chronic 

aquatic life standard for lead was exceeded in 26 percent of 

samples.

For Rock Creek (site 19), median specific conductance 

and concentrations of metallic elements, arsenic, and suspended sediment are low. Those medians are less than medians 

for Blacktail Creek (site 1, fig. 1, table 1; an index site representative of small to moderate mining effect), and also less 

than median values for other major tributary sites downstream 

from Deer Lodge [Little Blackfoot River (site 15, fig. 1, 

table 1), Flint Creek (site 17, fig. 1, table 1), and Blackfoot 

River (site 21, fig. 1, table 1)]. There were no exceedances of 

water-quality standards for site 19 (table 3).

Reach 9

Sites in reach 9 include the reach inflow [Clark Fork at 

Turah Bridge (site 20, fig. 1, table 1)] and the reach outflow 

[Clark Fork above Missoula (site 22, fig. 1, table 1)]. Also in 

reach 9 is the monitored tributary site Blackfoot River (site 21, 

fig. 1, table 1). Reach 9 is about 9-mi long and includes the 

former Milltown Reservoir where substantial amounts of mining wastes had been deposited. The former Milltown Dam was 

removed in 2008. The Blackfoot River, which drains a largely 

forested basin that historically has had numerous small-scale 

mining operations, discharges into reach 9 near Bonner. In 

reach 9, mean annual streamflow of the Clark Fork more 

than doubles from 1,200 ft3/s (site 20) to 2,500 ft3/s (site 22, 

table 1–6) primarily because of contributions from the Blackfoot River (mean annual streamflow of 1,330 ft3

/s). 

Assessment of spatial water-quality changes in reach 

9 is complicated by effects of activities associated with the 

removal of the former Milltown Dam. The former Milltown 

Dam was breached in March 2008; however, activities in 

preparation for the dam removal (including construction of 

access roads and operational facilities, and preliminary reservoir drawdowns) began several years earlier (at least as early 

as water year 2004) and potentially affected water quality at 

site 22. Thus the data-summary period (water years 2001–10) 

encompasses periods before, during, and after the removal 

of the former Milltown Dam. The variable conditions during 

the data-summary period do not represent typical conditions 

within the reach.

Spatial changes in water quality between sites 20 and 

22 generally are not large. The most substantial change is a 

decrease in median unfiltered-recoverable arsenic from 7.0 

to 4.3 μg/L. Water-quality changes in reach 9 are affected 

by dilution from Blackfoot River and activities associated 

with the removal of the former Milltown Dam. Exceedances 

of most water-quality standards were relatively infrequent 
Data Analysis Methods 19

for site 22 (table 3). However, exceedances of aquatic life 

standards for copper were relatively frequent, with 31 and 45 

percent of samples exceeding the acute and chronic standards, 

respectively. The chronic aquatic life standard for lead was 

exceeded in 26 percent of samples.

For Blackfoot River (site 21) median specific conductance and concentrations of metallic elements, arsenic, and 

suspended sediment are low. Those medians are less than 

medians for Blacktail Creek (site 1, fig. 1, table 1; an index 

site representative of small to moderate mining effect). 

Exceedances of water-quality standards were relatively infrequent for site 21 (table 3).

Data Analysis Methods

This section of the report describes methods used to 

analyze trends in flow-adjusted concentrations of water-quality 

constituents. For sites that were analyzed by using the TSM 

(Vecchia, 2005), normalized loads (as defined in the section 

of this report “Estimation of Normalized Constituent Loads”) 

were estimated to evaluate temporal changes in relative contributions of selected trace elements and suspended sediment 

from upstream source areas to the outflows of each datasummary reach. Methods used for estimation of normalized 

constituent loads also are described.

Trend Analysis Methods 

A variety of methods are available for analysis of waterquality trends, including nonparametric and parametric procedures (Hirsch and Slack, 1984; Helsel and Hirsch, 2002). Two 

parametric trend-analysis methods were used in this study: the 

TSM (Vecchia, 2005) and MLR (Helsel and Hirsch, 2002). 

Both of the trend-analysis methods analyze trends in flowadjusted concentrations (FACs); that is, the methods compute 

FACs, estimate best-fit trend lines that represent temporal 

changes in FACs, and determine statistical significance of 

changes. Flow adjustment is necessary because concentrations 

of many water-quality constituents are strongly dependent 

on streamflow conditions, which primarily are affected by 

climatic variability within the study area. The intent of flowadjustment is to identify and remove streamflow-related variability in concentration and thereby enhance the capability to 

detect trends independent from effects of climatic variability. 

Flow-adjustment procedures produce FACs that are estimates 

of constituent concentrations after removing effects of streamflow variability. 

Flow-adjustment procedures differ between the TSM and 

MLR, which are discussed in more detail in appendixes 2 and 

3, respectively. In general, the primary difference between 

the two approaches is the TSM uses multiple flow-related 

variables computed from concurrent (same day as the concentration sample) and antecedent (days before the concentration sample) daily mean streamflow in the flow-adjustment 

process, whereas MLR uses only streamflow at the time of 

sampling. Thus, FACs determined by the TSM are analogous 

to FACs determined by MLR, in that FACs of both methods 

account for streamflow effects, but TSM FACs provide more 

detailed accounting by incorporating interannual, seasonal, 

and short-term streamflow variability (Vecchia, 2005).

The TSM was selected as the preferred trend-analysis 

method to MLR primarily because the TSM incorporates 

continuous streamflow data instead of just concurrently determined streamflow at the time of sampling. Detailed analysis 

of continuous streamflow data provides better definition of 

concentration and streamflow relations through time, better 

handling of temporal variability in sampling frequency, and 

interpolation of trend patterns to periods when water-quality 

data are sparse or absent. The TSM inherently accounts for 

effects of serial correlation. Further, the TSM incorporates 

interannual, seasonal, and short-term information in flowadjustment procedures. The MLR method used for selected 

sites in this study incorporates only streamflow at the time of 

sampling and fixed seasonal functions; thus, the concentration and streamflow relation at a given time of sampling is 

assumed to depend only on streamflow magnitude and season 

with no accounting for streamflow conditions before sampling. 

For example, if two water-quality samples were collected at 

similar streamflow magnitudes at the same time of year, the 

flow-adjustment applied using MLR to the samples would be 

identical regardless of differences in streamflow conditions 

before sampling. If one sample was collected during increasing streamflow (for example, on the rising limb of snowmelt 

runoff) in a dry year and the other sample was collected at a 

similar streamflow during decreasing streamflow (for example, 

on the receding limb of snowmelt runoff) in a wet year, the 

same flow-adjustment would be applied to concentrations 

of both samples; thus, there is no accounting for interannual 

or short-term hysteresis factors that affect concentration and 

streamflow relations (Vecchia, 2005; Colby, 1956; Chanat 

and others, 2002). The TSM, however, analyzes continuous 

streamflow data to determine the context of streamflow conditions associated with a given time of sampling and account for 

interannual, seasonal, and short-term streamflow variability in 

flow-adjustment procedures. 

As applied in this study, the TSM required at least 

15 years of continuous streamflow data and at least 11 years 

of water-quality data with at least 60 total samples and at least 

10 samples (for all years) in each 3-month season. The TSM 

was used to analyze trends for 15 of the study sites that met 

these intensive data requirements (table 1). For those 15 sites, 

only the TSM results are presented to simplify and condense 

presentation of results. For seven sites, data requirements of 

the TSM were not met (table 1). In those cases, MLR was 

used to analyze trends. As applied in this study, MLR required 

at least 5 sequential years of water-quality data with six or 

more samples per year (temporally distributed consistently 

among years) and associated concurrent streamflow at times of 

sample collection. 
20 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

The TSM and MLR methods incorporate base-10 logarithm (hereinafter referred to as log) transformation of the 

concentration and streamflow data. As such, the fitted trends in 

FACs quantify temporal changes in central tendency represented by the geometric mean of concentration in reference 

to log-transformed streamflow. The geometric mean is the 

mean of the logs transformed back into their original units. 

Overviews of the TSM and MLR methods are presented in the 

following sections of this report.

Time-Series Model (TSM)

A statistical time-series model for streamflow and 

constituent concentration (Vecchia, 2005) was used in this 

report to detect water-quality trends. Details on theory and 

parameter estimation for the model are presented in Vecchia 

(2005) and the model is summarized in appendix 2 (at the end 

of this report). Specific information concerning suitability of 

application of the TSM to the study datasets, and procedures 

for determination of statistical significance and magnitude of 

trends also is presented in appendix 2.

Included in appendix 2 are definitions of anomaly terms 

that are used in the TSM and are important in contributing to 

the rigor of the TSM. In analysis of concentration and streamflow relations, the TSM partitions effects of streamflow variability into separate components for interannual, seasonal, and 

short-term (day-to-day) variability, and relative importance 

of each of those components is quantified. The annual concentration anomaly (ANNC; appendix 2, equation 2) quantifies interannual variability in concentration that is related to 

interannual variability in streamflow [as determined by the 

annual streamflow anomaly (ANNQ; appendix 2, equation 5)]. 

The seasonal concentration anomaly (SEASC; appendix 2, 

equation 2) quantifies seasonal variability in concentration that 

is related to seasonal variability in streamflow [as determined 

by the seasonal streamflow anomaly (SEASQ; appendix 2, 

equation 5)]. For unfiltered-recoverable constituent concentrations, ANNC and SEASC typically are positive and indicate direct relations between concentration and streamflow. 

However, the strength of ANNC relative to SEASC might differ 

substantially among sites and constituents. For some site and 

constituent combinations, constituent concentration might be 

more sensitive to annual streamflow variability than seasonal 

streamflow variability and for other combinations, the reverse 

situation might hold. Short-term variability in concentration, 

also referred to as high-frequency variability (HFVC; 

appendix 2, equation 2), is variability remaining after removing ANNC and SEASC. Similarly, high-frequency streamflow 

variability (HFVQ; appendix 2, equation 5) is variability 

remaining after removing ANNQ and SEASQ. Relations 

between HFVC and HFVQ generally are more complex than 

relations for ANNC and ANNQ and for SEASC and SEASQ. In 

accounting for relations between HFVC and HFVQ, the TSM 

can account for effects of short-term streamflow variability 

(for example, hysteresis) and also potential serial correlation. 

A limitation of the TSM is that it does not handle censored data in a rigorous manner. In the TSM, a single value 

is substituted for all censored data for a given constituent. 

Thus, criteria must be set to specify the allowable amount 

of censored data and a consistent substitution value for each 

constituent. Based on analysis of trial datasets with artificiallyimposed variable levels of censoring, the TSM generally can 

be applied to datasets with about 10 percent or less censored 

data without substantial effects on trend results (Vecchia, 

2003). Multiple LRLs in the datasets of the Clark Fork 

monitoring program complicates the task of setting consistent 

substitution values. In applying the TSM to the study database, 

study reporting levels (SRLs; table 2) for setting consistent 

substitution values were established for each trace-element 

constituent based on investigation of the time frame during which various NWQL LRLs were used, the frequency 

of censoring that resulted from each LRL, and field blank 

sample data providing information on potential contamination bias of low concentrations. The SRLs were applied to 

the study database by (1) substituting one-half the SRL for all 

censored observations with LRLs equal or close to the SRL, 

(2) substituting one-half the SRL for all reported uncensored 

concentrations (analyzed during times when the LRL was less 

than the SRL) that were less than the SRL, and (3) excluding 

censored data with LRLs substantially larger than the SRL. 

Any analytical result that was revised by either substitution 

or exclusion was considered to be affected by the recensoring 

procedures used in applying the SRL. Application of the TSM 

generally was restricted to site and constituent combinations 

that had less than or equal to 6 percent of analytical results 

affected by the recensoring procedures. This conservative 

criterion was selected to minimize the potential effects of 

greater uncertainty (because of LRL and censoring issues, 

and potential contamination bias) in low-concentration data 

on trend results. The criterion was relaxed allowing as much 

as 10 percent of analytical results affected by the recensoring 

procedures for three site and constituent combinations (site 15, 

unfiltered-recoverable copper; site 17, unfiltered-recoverable 

lead and zinc) as discussed in the section of this report “WaterQuality Trends for Selected Sampling Sites in the Upper Clark 

Fork Basin.” For many site and constituent combinations, the 

censored-data limitations of the TSM did not allow reporting 

of trend results. However, in this report, particular emphasis 

is placed on copper, arsenic, and suspended-sediment trend 

results. Copper and arsenic have much different geochemical 

characteristics and are constituents of concern with respect to 

potential toxicity issues. Further, trend patterns for copper generally are similar to other metallic contaminants. Suspendedsediment data provide information on transport of particulate 

materials, which is a factor that can strongly affect transport of 

metallic contaminants. For most sites with sufficient periods of 

data collection, copper, arsenic, and suspended-sediment data 

met all requirements for application of the TSM.

The TSM accounts for many hydrological factors that 

contribute to complexity in concentration and streamflow 

relations. In this study, the TSM was applied as consistently 
Data Analysis Methods 21

as possible among sites, and is considered to be a useful tool 

for simplifying the environmental complexity in the upper 

Clark Fork Basin to provide a large-scale evaluation of general 

temporal changes in FACs and constituent transport independent from streamflow variability. The TSM best-fit trend lines 

are considered to provide important information beyond the 

strict statistical characteristics of the trend results (in terms of 

p-values and levels of significance) because they aid in comparing and summarizing large-scale patterns among sites. 

Multiple Linear Regression on Time, Streamflow, 

and Season (MLR)

MLR generally is regarded as a preferred alternative 

trend-analysis method relative to nonparametric methods 

when data distributions are approximately normal (Helsel and 

Hirsch, 2002). MLR for trend analysis was applied following 

guidelines presented in Helsel and Hirsch (2002) and specific 

information concerning application of MLR in this study is 

presented in appendix 3. 

A consistent (in terms of dependent and explanatory 

variables) MLR model was used to provide general application 

for the numerous site and constituent combinations. Constituent concentrations were regressed on streamflow, decimal 

time, and periodic functions to represent seasonal variability 

in concentration and streamflow relations. MLR models were 

developed by using ordinary least squares if the concentration 

data contained no censored observations or by using adjusted 

maximum-likelihood estimation (Cohn, 1988, 2005) if censored observations were present. Specific information concerning suitability of application of MLR to the study datasets, 

and procedures for determination of the statistical significance 

and magnitude of trends is presented in appendix 3.

Selection of Trend-Analysis Time Periods

Appropriate selection of trend analysis time periods is 

important because the results of trend analyses are dependent 

on how the time periods are structured. Factors considered in 

selection of trend analysis time periods included providing 

capability to (1) compare trend results among sites with different periods of data collection; (2) distinguish the short-term 

timing of changes in concentration and streamflow relations 

within the long study period; and (3) allow periodic future 

updates of trend analyses for evaluation of effects of remediation activities. Based primarily on those factors, trendanalysis periods were defined as sequential 5-year periods 

that extended from near the start of long-term data-collection 

activities for most sites in the upper Clark Fork Basin to the 

end of water year 2010. Thus, three trend-analysis time periods were defined: period 1 (water years 1996–2000); period 2 

(water years 2001–05); and period 3 (water years 2006–10). 

Trend analysis periods that were applicable for each site are 

presented in table 1. For recently-established sites that did 

not satisfy data requirements for the TSM, trends for period 3 

were analyzed by using MLR. 

For the TSM, fitted trends in FACs during a defined 

trend-analysis period are monotonic trends that are smoothed 

to produce generally consistent slopes across the middle section of the trend-analysis period that become flatter near the 

ends. The flatter slopes near the ends provide gradual transition between adjacent trend-analysis periods. For MLR, fitted 

trends in FACs during a defined trend-analysis period are 

straight-line monotonic trends. In some cases, the fitted trends 

within a defined trend-analysis time period do not precisely 

follow the patterns in FACs and there are short-term (about 

1–2 years) trend patterns in the FACs that are unresolved in 

the fitted trends. In those cases, better temporal resolution 

might have been attained by defining two or more additional 

trend-analysis periods within one of the defined 5-year trendanalysis periods. This approach generally was avoided because 

it would have required detailed site-by-site trend analysis 

for potentially inconsistent time periods among the 22 sites 

in this study. An important consideration in the design of the 

trend-analysis structure of this study was the capability to 

make general comparisons among the 22 sites with respect 

to evaluating potential effects of mining and remediation 

activities on a large-scale basis throughout consistent time 

periods. In general, when unresolved trending was apparent, 

more complicated trend models (with additional trend-analysis 

periods) were tested and the more complicated models did not 

change the general findings and conclusions of this report; that 

is, the overall fitted trends during the affected trend-analysis 

periods were consistent with overall patterns in FACs during 

the period. However, because of the substantial effect of the 

breach of Milltown Dam on March 28, 2008, an exception to 

consistent trend-analysis periods was made. For Clark Fork 

above Missoula (site 22), period 3 was subdivided into period 

3A (October 1, 2005–March 27, 2008) and period 3B (March 

28, 2008–September 30, 2010).

Estimation of Normalized Constituent Loads

For sites that were analyzed by using the TSM, normalized constituent loads were estimated to assess the temporal 

trends in FACs of mining-related contaminants in the context 

of sources and transport. Sites analyzed by using MLR were 

not included in this transport analysis because of factors that 

complicate directly combining the TSM and MLR results 

within a single analysis, including: (1) MLR uses different 

flow-adjustment procedures than the TSM, and (2) the sites 

analyzed by using MLR had substantially shorter periods of 

data collection than the sites analyzed by using the TSM.

The fitted trends in concentration are best-fit lines 

through the FACs, which are independent of streamflow conditions. FAC trends at individual sites are important descriptors 

of water-quality changes in the upper Clark Fork Basin, but 

without consideration of differences in streamflow magnitudes 

between different sites, the FAC trends do not provide direct 
22 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

information on resultant changes in contaminant source-area 

contributions and transport characteristics. Combining the 

FAC trends with a stationary streamflow index (that maintains 

relative differences in streamflow magnitudes between sites 

but normalizes streamflow for a given site to a constant value 

through time) allows assessment of how the temporal changes 

in FACs translate into relative temporal changes in source and 

transport of mining-related contaminants in the upper Clark 

Fork Basin. Thus, normalized loads were estimated to conduct 

a transport analysis. 

Normalized loads were estimated for each of the three 

5-year trend-analysis periods. The stationary streamflow index 

used in estimating normalized loads was the geometric mean 

streamflow for each site for water years 1996–2010. The 

geometric mean was selected as a measure of central tendency 

in streamflow to maintain consistency with the TSM analysis, 

which is conducted on log-transformed data. 

For each site and constituent combination and each of the 

5-year periods, the normalized load was estimated by multiplying the mean annual fitted trend FAC during the period 

times the geometric mean streamflow for water years 1996–

2010 and a units conversion factor, according to the following 

equation:

LOAD = MAC * GMQ * K (1)

where

LOAD is the estimated normalized constituent load 

(in kilograms per day) for the indicated 

5-year period;

MAC is the mean annual fitted trend FAC (in 

micrograms per liter for trace-elements 

or milligrams per liter for suspended 

sediment) for the indicated 5-year period;

GMQ is the geometric mean of daily mean 

streamflow for water years 1996–2010, in 

cubic feet per second; and

K is a units conversion constant (0.00245 for 

concentrations in micrograms per liter or 

2.45 for concentrations in milligrams per 

liter) to convert instantaneous constituent 

discharge (in mass units per second) to 

an equivalent daily constituent load (in 

kilograms per day).

The MAC is calculated by temporally averaging (within 

each of the three 5-year periods) the fitted trend FACs that 

quantify temporal changes in central tendency based on the 

geometric mean. It is notable that the MAC is referred to as a 

mean annual value, and this terminology indicates temporal 

averaging of geometric mean concentrations. For data that are 

approximately log-normally distributed, the geometric mean 

generally is closely associated with the median of the original 

untransformed units. Thus, because of effects of analysis of 

log-transformed data the estimated normalized loads generally represent quantification with respect to near-median 

conditions. As such, the estimated normalized loads do not 

represent actual magnitudes of total mass transport. Rather, the 

estimated normalized loads provide information on relative 

temporal changes in constituent transport characteristics of the 

study sites quantified with respect to near-median conditions.

Streamflow Conditions and Various 

Data-Related Factors that Affect Trend 

Analysis and Interpretation

Several factors affect temporal trends in water quality. 

Climatic variability (interannual and seasonal) is indicated 

in variability in streamflow conditions, which strongly affect 

concentration and streamflow relations. Investigating streamflow conditions during the study period is relevant to interpreting trend results. Other factors relating to data assessment or 

treatment that also are relevant to understanding trend-analysis 

procedures and interpreting trend results include relations 

between unadjusted concentrations and FACs, and data transformation. The TSM is emphasized in this section because it 

is the method used for most sites in this study and it provides 

convenient access to relevant intermediate results to indicate 

trend-analysis concepts. 

Streamflow Conditions

Daily mean streamflows for water years 1993–2010 for 

selected sites in the upper Clark Fork Basin are presented in 

figure 3. Locally weighted scatter plot smooth (LOWESS; 

Cleveland, 1985; Cleveland and McGill, 1984) lines through 

the daily mean streamflows also are presented in figure 3 to 

represent temporal variability in the moving central tendency 

of streamflow. The geometric mean streamflows for water 

years 1996–2010 are presented to represent overall central tendency of streamflow during the period of trend analysis. Silver 

Bow Creek at Opportunity (site 3, fig. 1, table 1), Clark Fork 

at Deer Lodge (site 14, fig. 1, table 1), and Clark Fork at Turah 

Bridge (site 20, fig. 1, table 1) were selected as examples for 

showing hydrologic patterns that generally apply to most other 

sites. 

Temporal variability in streamflow conditions during the 

study period generally is similar among sites. In about water 

year 1993, streamflow conditions generally increased to above 

the geometric mean streamflows during a period of several 

years. Streamflows were high during water years 1996–97, 

near the start of period 1 (water years 1996–2000). During 

period 1, streamflow conditions above the geometric mean 

streamflows generally persisted through water year 1999, and 

then decreased substantially to below the geometric mean 

streamflows during water year 2000. High streamflow conditions prevalent through most of period 1 are evident in annual 

maximum streamflows being higher than maximums of most 

other years and also in annual minimum streamflows being 

higher than minimums of most other years. Streamflow during 
Streamflow Conditions and Various Data-Related Factors that Affect Trend Analysis and Interpretation 23

10

100

1,000

10,000

10

100

1,000

10,000

10

100

1,000

10,000

EXPLANATION

[Water year is defined as the 12-month period 

from October 1 through September 30 and is 

designated by the year in which it ends]

Daily mean streamflow

Geometric mean streamflow for 

water years 1996–2010

Locally weighted scatter plot 

smooth (LOWESS; Cleveland, 

1985; Cleveland and McGill, 

1984) line for daily mean 

streamflow

 Water year (October–September)

1990 1995 2000 2005 2010

1

Period

2 3

C. Clark Fork at Turah Bridge (site 20, fig. 1, table 1)

B. Clark Fork at Deer Lodge (site 14, fig. 1, table 1)

A. Silver Bow Creek at Opportunity (site 3, fig. 1, table 1)

Streamflow, in cubic feet per second

Figure 3. Daily mean streamflow for selected sites in the upper Clark Fork Basin, Montana, water years 1993–2010.
24 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

water year 1997 was particularly unusual in that the receding 

limb of snowmelt runoff was less abrupt and less variable than 

most years, and post-runoff base streamflows generally were 

above or near the geometric mean streamflow. Further, the 

post-runoff base streamflows in water year 1997 at site 14 

(fig. 3B) sometimes exceeded annual maximum streamflows 

during the low streamflow years 2000–02. During period 2 

(water years 2001–05), streamflow conditions generally were 

below the geometric mean streamflows and represented a 

period of prolonged drought in the upper Clark Fork Basin. 

During period 3 (water years 2006–10), streamflow conditions 

generally were near the geometric mean streamflows during 

water years 2005–07, and increased to above the geometric 

mean streamflows during water years 2008–10.

Various Data-Related Factors that Affect Trend 

Analysis and Interpretation 

Factors relating to data requirements, treatments, and 

assessment that affect trend analysis and interpretation of 

results include relations between unadjusted concentrations 

and FACs, and data transformation. Those factors are discussed in this section of the report.

FACs are estimates of constituent concentrations after 

removing effects of streamflow variability. Thus, FACs typically have less variability than unadjusted concentrations, 

although the strength of this pattern is variable among sites 

and constituents, and also can be variable through time for a 

given site. Data for Clark Fork near Galen (site 11) are presented in figure 4 to provide examples for discussion of relations between unadjusted and flow-adjusted concentrations. 

For suspended sediment (fig. 4D), unadjusted concentrations tend to be higher during high streamflow conditions than 

during low-streamflow conditions. During high-streamflow 

conditions, with associated high hydraulic energy, particulate 

material is mobilized and transported in the stream. During low streamflow conditions, streams have less capacity 

for transporting particulate materials. Similarities among the 

LOWESS smooth line for unadjusted suspended-sediment 

concentrations (fig 4D) and streamflow (fig. 4A) illustrate the 

direct relations between streamflow and unadjusted suspended-sediment concentrations. Flow-adjustment procedures 

account for the response of suspended-sediment concentrations to variations in streamflow and produce FACs that 

represent temporal variability within consistent streamflow 

conditions. In the Clark Fork for high-streamflow conditions, 

FACs of suspended sediment are less variable and lower than 

unadjusted concentrations (for example, fig. 4D, water years 

1996–99). For low-streamflow conditions, FACs of suspended 

sediment are less variable and generally are centered within 

unadjusted concentrations (for example, fig. 4D, water years 

2000–01).

Unfiltered-recoverable copper (and other metallic elements) has concentration and streamflow relations that are 

similar to suspended sediment because of adsorption on 

inorganic and organic particulate materials. As a result, patterns in unadjusted concentrations and FACs for unfilteredrecoverable copper (fig. 4B) are similar to those of suspended 

sediment (fig. 4D). 

Arsenic in streams in the upper Clark Fork Basin typically is mostly in dissolved phase, and has less variability 

and weaker direct relation with streamflow than is the case 

for metallic elements. Arsenic has been widely dispersed in 

the upper Clark Fork Basin as a result of deposition of flue 

dust and smelter emissions with resultant large-scale soil and 

groundwater contamination (U.S. Environmental Protection 

Agency, 2010). Further, arsenic generally is more soluble than 

metallic elements within the geochemical conditions that are 

prevalent in the upper Clark Fork Basin. These factors result 

in high arsenic concentrations in groundwater in some areas 

and also mobilization of arsenic to stream channels for a large 

range of streamflow conditions. Thus, patterns in unadjusted 

concentrations and FACs for unfiltered-recoverable arsenic 

(fig. 4C) generally are less variable than for unfiltered-recoverable copper (fig. 4B) and suspended sediment (fig. 4D). Also 

unadjusted concentrations of unfiltered-recoverable arsenic 

have less correspondence with streamflow than unfilteredrecoverable copper and suspended sediment. 

For the period 2004–10, during transition from lowstreamflow to high-streamflow conditions, LOWESS smooth 

lines for unadjusted concentrations of unfiltered-recoverable 

copper and suspended sediment show substantial increases. 

However, the TSM flow-adjustment procedure accounts for 

the streamflow variability and fitted trends indicate consistent 

but small decreases in FACs of those constituents. Another 

period of substantial deviation between unadjusted concentrations and FACs is from 1994–2000. During both periods 

of deviation, the FACs are unaffected by large variations in 

streamflow. The dissimilar patterns between unadjusted concentrations and FACs indicate the importance of flow-adjusted 

trend analysis for identifying actual patterns in constituent 

concentrations independent from variability in streamflow 

conditions.

An important consideration in interpreting trend results 

relates to the trend-analysis methods incorporating log-transformation of constituent concentrations. Thus, the methods evaluate changes in geometric mean concentrations in 

reference to log-transformed streamflow. Log-transformation 

results in datasets that are approximately normally distributed 

and allow analysis using rigorous parametric procedures. 

However, log-transformation decreases variability in the data 

relative to the original untransformed units representative 

of actual environmental variability. In general, the statistical 

distributions of constituent concentrations and streamflow 

(in original untransformed units) for sites in the upper Clark 

Fork Basin are right skewed, indicating that the extent of data 

higher than the median is greater than the extent of data lower 

than the median. Log transformation results in expansion of 

the lower end of the distribution and compression of the higher 

end of the distribution. Compression of the higher end of the 

distribution has relatively larger effect than expansion of the 
Streamflow Conditions and Various Data-Related Factors that Affect Trend Analysis and Interpretation 25

Water year (October–September)

19901995200020052010

A. Streamflow

B. 

C.

D. Suspended sediment

Unfiltered-recoverable arsenic

Unfiltered-recoverable copper

1

Period

2 3

10

10,000

1,000

100

1

10

1,000

100

1

10

1,000

100

1

10

1,000

100

Unfiltered-recoverable arsenic

concentration, in micrograms per liter

Unfiltered-recoverable copper 

concentration, in micrograms per liter

Suspended-sediment concentration,

in milligrams per liter Streamflow, in cubic feet per second

EXPLANATION

[Water year is defined as the 12-month 

period from October 1 through 

September 30 and is designated by the 

year in which it ends]

Daily mean streamflow

Geometric mean streamflow 

for water years 1996–2010

Locally weighted scatter 

plot smooth (LOWESS; 

Cleveland, 1985; Cleveland 

and McGill, 1984) line for 

daily mean streamflow

Flow-adjusted concentraton 

determined by using the 

time-series model (TSM)

Flow-adjusted fitted trend 

determined by using the 

TSM

Unadjusted (not flow 

adjusted) concentration

LOWESS line for unadjusted 

concentrations

Figure 4. Selected streamflow and constituent concentration information for Clark Fork near Galen (site 11, fig. 1, table 1), water 

years 1993–2010.
26 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

lower end of the distribution. This factor is important in interpreting trend results with respect to various regulatory issues, 

including compliance with drinking water standards or aquatic 

life criteria. Trends in FACs represent changes in central 

tendency quantified as changes in the geometric mean in reference to log-transformed streamflow. Thus, the trends in FACs 

provide general information on overall temporal changes (in 

terms of directions and relative magnitudes) in concentrations, 

but lack the specificity to indicate compliance or noncompliance with various regulatory standards. However, effects of 

data transformation do not negatively affect the primary purpose of this study in determining temporal water-quality trends 

through time and using the trend results to evaluate relative 

changes in constituent transport characteristics among sites. In 

the trend analyses, all data (high as well as low values) affect 

changes in the FAC geometric means. Thus, the fitted trends 

truly represent unbiased estimates of overall changes in central 

tendency. 

Water-Quality Trends for Selected 

Sampling Sites in the Upper Clark Fork 

Basin

For all constituents investigated, detailed results for trend 

magnitudes, computed as the total percent changes in FAC 

geometric means from the beginning to the end of each 5-year 

period, are presented in table 4–1 (for most sites analyzed by 

using the TSM), table 4–2 [for Clark Fork above Missoula 

(site 22, fig. 1, table 1) analyzed by using the TSM], and 

table 4–3 (for sites analyzed by using MLR). Fitted trend 

values (that quantify the temporal changes in FAC geometric 

means in terms of concentration units) for selected constituents are summarized in tables 4–6 and graphically presented 

for all sites in figures 4–1 through 4–22. Although, trend 

magnitude results are presented for all constituents investigated, emphasis is placed on copper, arsenic, and suspended 

sediment. 

In the discussion, qualitative observations on trend 

magnitude (percent change) are made. Trend magnitude was 

considered to be (1) large, if the deviation from zero was 

greater than about 60 percent; (2) moderate, if the deviation 

from zero was within the range of about 40–60 percent; 

(3) small, if the deviation from zero was within the range of 

about 20–40 percent; and (4) minor, if the deviation from zero 

was within the range of about 0–20 percent. In some cases, 

when trending was within a small range at low concentrations, 

large trend magnitudes (on a percent basis) also were considered to be minor. 

Trend-magnitude and fitted trend values are considered 

semiquantitative estimates determined by statistical analysis. Throughout this report, trend-magnitude and fitted trend 

values frequently are referred to (reported to two significant 

figures) in discussion of temporal and spatial changes in water 

quality. Reference to specific trend-magnitude and fitted trend 

values is intended to facilitate discussion of relative spatial 

and temporal differences between values, but is not intended 

to represent absolute accuracy at two significant figures. The 

discussion on trend results focuses on the trend-magnitude 

and fitted trend values. The p-values and levels of significance 

associated with the trend results are indicated in the tables and 

figures that present trend results, but not emphasized in the 

discussion. In this study, the TSM is considered to be a useful 

tool for simplifying the environmental complexity in the upper 

Clark Fork Basin to provide a large-scale evaluation of general 

temporal changes in FACs and constituent transport independent from streamflow variability. Thus, the TSM best-fit trend 

lines are considered to provide important information beyond 

the strict statistical characteristics of the trend results (in terms 

of p-values and levels of significance) because they aid in 

comparing and summarizing large-scale patterns among sites. 

Trends in Flow-Adjusted Concentrations (FACs) 

and Estimated Normalized Loads

Fitted trends and estimated normalized loads of the study 

sites are presented and discussed for each of the data-summary 

reaches. Fitted trends for unfiltered-recoverable copper and 

arsenic, and suspended-sediment concentrations for sites in 

the reach are graphically presented (for example, fig. 5, which 

shows fitted trends for sites in reach 1). Estimated normalized 

loads are presented within the framework of a transport analysis to assess the temporal trends in FACs in the context of 

sources and transport. Drainage area and streamflow information relevant to the transport analysis is presented in table 7. 

Balance calculations for the transport analysis (that is, 

differences between reach inflows, reach outflows, and 

monitored within-reach contributions) are presented in 

tables 5–1 through 5–9 for reaches 1–9, respectively. The 

transport balance calculations indicate within-reach changes 

in loads and allow assessment of relative contributions from 

upstream source areas to loads transported past each reach 

outflow. 

Characteristics of the source areas (including drainage 

area and geometric mean streamflow; table 7) and balance 

results for the transport analysis are illustrated by using pie 

charts that show source-area information and load contributions to reach outflow. For example, in figure 6, pie charts 

illustrating drainage area and geometric streamflow characteristics of the upstream source areas that contribute to the 

reach 1 outflow are shown on the left-hand side. Pie charts 

illustrating balance results for estimated normalized constituent loads are shown on the right hand side. In figure 6, the 

reach 1 inflow is the estimated normalized load for Blacktail 

Creek (site 1, fig. 1, table 1). The reach 1 outflow is the estimated normalized load for Silver Bow Creek at Butte 

(site 2, fig. 1, table 1). The difference between the reach 

outflow and the reach inflow indicates the within-reach change 

in load; that is, the incremental change in the load transported 
Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin 27

Table 4. Summary of flow-adjusted trend results determined by using the time-series model (TSM) for selected sites and 

constituents, water years 1996–2010.

[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. Bold values indicate statistically 

significant (p-value less than 0.01) trend for trend period before the value presented in bold. p-value, statistical probability level; μS/cm, microsiemens per 

centimeter at 25 degrees Celsius; NR, not reported; μg/L, micrograms per liter; mg/L, milligrams per liter]

Constituent or property, 

flow-adjusted units of 

measurement

Fitted trend values Percent change 

from start of 

water year 1996 

to end of water 

year 2010

Start of water year 

1996 (start of 

period 1)

Start of water year 

2001 (start of 

period 2)

Start of water year 

2006 (start of 

period 3)

End of water year 

2010 (end of 

period 3)

Blacktail Creek (site 1, fig. 1, table 1)

Specific conductance, μS/cm 279 297 295 289 4

Copper, filtered, μg/L 2.3 2.0 2.0 1.9 -17

Copper, unfiltered-recoverable, μg/L 5.2 4.3 4.3 3.8 -27

Zinc, filtered, μg/L NR1 NR1 NR1 NR1 NR1

Zinc, unfiltered-recoverable, 

μg/L

NR1 NR1 NR1 NR1 NR1

Arsenic, filtered, μg/L 2.9 2.1 2.5 2.6 -10

Arsenic, unfiltered-recoverable, μg/L 3.8 2.8 3.1 3.3 -13

Suspended sediment, mg/L 6.7 7.2 5.6 5.1 -24

Silver Bow Creek at Butte (site 2, fig. 1, table 1)

Specific conductance, μS/cm 522 493 493 476 -9

Copper, filtered, μg/L 60 13 13 6.0 -90

Copper, unfiltered-recoverable, μg/L 180 44 31 14 -92

Zinc, filtered, μg/L 890 280 77 32 -96

Zinc, unfiltered-recoverable, 

μg/L

 1,100 350 98 43 -96

Arsenic, filtered, μg/L 6.5 6.8 4.0 4.3 -34

Arsenic, unfiltered-recoverable, μg/L 13 10 5.6 6.0 -54

Suspended sediment, mg/L 15 14 7.5 7.3 -51

Silver Bow Creek at Opportunity (site 3, fig. 1, table 1)

Specific conductance, μS/cm 424 463 470 440 4

Copper, filtered, μg/L 48 42 29 17 -65

Copper, unfiltered-recoverable, μg/L 130 120 94 44 -66

Zinc, filtered, μg/L 290 220 130 53 -82

Zinc, unfiltered-recoverable, 

μg/L

 540 470 270 130 -76

Arsenic, filtered, μg/L 9.0 11 12 7.4 -18

Arsenic, unfiltered-recoverable, μg/L 16 17 18 8.8 -45

Suspended sediment, mg/L 24 18 21 19 -21

Silver Bow Creek at Warm Springs (site 8, fig. 1, table 1)

Specific conductance, μS/cm 527 526 506 530 0

Copper, filtered, μg/L 11 4.8 4.2 4.0 -64

Copper, unfiltered-recoverable, μg/L 18 8.8 8.0 7.0 -61
28 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

Table 4. Summary of flow-adjusted trend results determined by using the time-series model (TSM) for selected sites and 

constituents, water years 1996–2010.—Continued

[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. Bold values indicate statistically 

significant (p-value less than 0.01) trend for trend period before the value presented in bold. p-value, statistical probability level; μS/cm, microsiemens per 

centimeter at 25 degrees Celsius; NR, not reported; μg/L, micrograms per liter; mg/L, milligrams per liter]

Constituent or property, 

flow-adjusted units of 

measurement

Fitted trend values Percent change 

from start of 

water year 1996 

to end of water 

year 2010

Start of water year 

1996 (start of 

period 1)

Start of water year 

2001 (start of 

period 2)

Start of water year 

2006 (start of 

period 3)

End of water year 

2010 (end of 

period 3)

Silver Bow Creek at Warm Springs (site 8, fig. 1, table 1)—Continued

Zinc, filtered, μg/L NR1 NR1 NR1 NR1 NR1

Zinc, unfiltered-recoverable, 

μg/L

 48 17 9.1 9.2 -81

Arsenic, filtered, μg/L 17 19 19 20 18

Arsenic, unfiltered-recoverable, μg/L 20 22 21 23 15

Suspended sediment, mg/L 5.4 5.8 4.1 1.9 -65

Warm Springs Creek at Warm Springs (site 10, fig. 1, table 1)

Specific conductance, μS/cm 426 376 377 351 -18

Copper, filtered, μg/L 2.4 2.4 2.2 1.5 -38

Copper, unfiltered-recoverable, μg/L 9.9 7.4 7.7 9.8 -1

Zinc, filtered, μg/L NR1 NR1 NR1 NR1 NR1

Zinc, unfiltered-recoverable, 

μg/L

NR1 NR1 NR1 NR1 NR1

Arsenic, filtered, μg/L 4.7 5.3 5.2 4.0 -15

Arsenic, unfiltered-recoverable, μg/L 6.2 5.6 5.9 5.2 -16

Suspended sediment, mg/L 8.7 4.7 3.1 7.7 -11

Clark Fork near Galen (site 11, fig. 1, table 1)

Specific conductance, μS/cm 459 450 440 431 -6

Copper, filtered, μg/L 8.0 4.1 3.8 3.3 -59

Copper, unfiltered-recoverable, μg/L 16 10 11 10 -38

Zinc, filtered, μg/L NR1 NR1 NR1 NR1 NR1

Zinc, unfiltered-recoverable, 

μg/L

 32 13 8.8 10 -69

Arsenic, filtered, μg/L 12 12 13 10 -17

Arsenic, unfiltered-recoverable, μg/L 16 14 16 12 -25

Suspended sediment, mg/L 6.0 5.8 4.8 4.8 -20

Clark Fork at Deer Lodge (site 14, fig. 1, table 1)

Specific conductance, μS/cm 495 488 481 449 -9

Copper, filtered, μg/L 7.1 5.8 6.0 5.0 -30

Copper, unfiltered-recoverable, μg/L 26 23 22 23 -12

Zinc, filtered, μg/L NR1 NR1 NR1 NR1 NR1

Zinc, unfiltered-recoverable, 

μg/L

 36 23 21 20 -44

Arsenic, filtered, μg/L 11 10 12 11 0
Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin 29

Table 4. Summary of flow-adjusted trend results determined by using the time-series model (TSM) for selected sites and 

constituents, water years 1996–2010.—Continued

[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. Bold values indicate statistically 

significant (p-value less than 0.01) trend for trend period before the value presented in bold. p-value, statistical probability level; μS/cm, microsiemens per 

centimeter at 25 degrees Celsius; NR, not reported; μg/L, micrograms per liter; mg/L, milligrams per liter]

Constituent or property, 

flow-adjusted units of 

measurement

Fitted trend values Percent change 

from start of 

water year 1996 

to end of water 

year 2010

Start of water year 

1996 (start of 

period 1)

Start of water year 

2001 (start of 

period 2)

Start of water year 

2006 (start of 

period 3)

End of water year 

2010 (end of 

period 3)

Clark Fork at Deer Lodge (site 14, fig. 1, table 1)—Continued

Arsenic, unfiltered-recoverable, μg/L 14 14 15 14 0

Suspended sediment, mg/L 17 14 13 13 -24

Little Blackfoot River (site 15, fig. 1, table 1)

Specific conductance, μS/cm 253 261 264 NR2 NR2

Copper, filtered, μg/L NR1 NR1 NR1 NR2 NR2

Copper, unfiltered-recoverable, μg/L 1.1 0.94 1.0 NR2 NR2

Zinc, filtered, μg/L NR1 NR1 NR1 NR2 NR2

Zinc, unfiltered-recoverable, 

μg/L

NR1 NR1 NR1 NR2 NR2

Arsenic, filtered, μg/L 4.9 4.7 5.3 NR2 NR2

Arsenic, unfiltered-recoverable, μg/L 6.5 5.0 4.7 NR2 NR2

Suspended sediment, mg/L 5.4 4.0 3.0 NR2 NR2

Clark Fork at Goldcreek (site 16, fig. 1, table 1)

Specific conductance, μS/cm 422 422 411 391 -7

Copper, filtered, μg/L 4.9 3.9 4.3 4.0 -18

Copper, unfiltered-recoverable, μg/L 22 19 15 16 -27

Zinc, filtered, μg/L NR1 NR1 NR1 NR1 NR1

Zinc, unfiltered-recoverable, 

μg/L

 28 19 12 17 -39

Arsenic, filtered, μg/L 9.0 8.2 9.1 8.9 -1

Arsenic, unfiltered-recoverable, μg/L 11 9.8 10 11 0

Suspended sediment, mg/L 15 17 8.1 14 -7

Flint Creek (site 17, fig. 1, table 1)

Specific conductance, μS/cm 324 346 376 NR2 NR2

Copper, filtered, μg/L NR1 NR1 NR1 NR2 NR2

Copper, unfiltered-recoverable, μg/L 3.4 2.6 2.4 NR2 NR2

Zinc, filtered, μg/L NR1 NR1 NR1 NR2 NR2

Zinc, unfiltered-recoverable, 

μg/L

 18 13 8.0 NR2 NR2

Arsenic, filtered, μg/L 8.7 8.1 7.8 NR2 NR2

Arsenic, unfiltered-recoverable, μg/L 13 11 10 NR2 NR2

Suspended sediment, mg/L 18 17 10 NR2 NR2
30 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

Clark Fork near Drummond (site 18, fig. 1, table 1)

Specific conductance, μS/cm

Copper, filtered, μg/L

Copper, unfiltered-recoverable, μg/L

Zinc, filtered, μg/L

Zinc, unfiltered-recoverable, 

μg/L

Arsenic, filtered, μg/L

Arsenic, unfiltered-recoverable, μg/L

Suspended sediment, mg/L

 486

 3.7

 19

NR1

 40

 9.3

 12

 21

 472

 3.7

 15

NR1

 18

 8.6

 10

 13

 458

 4.3

 13

NR1

 15

 9.3

 10

 11

 445

 3.3

 13

NR1

 17

 8.3

 10

 12

-8

-11

-32

NR1

-58

-11

-17

-43

Rock Creek (site 19, fig. 1, table 1)

Specific conductance, μS/cm

Copper, filtered, μg/L

Copper, unfiltered-recoverable, μg/L

Zinc, filtered, μg/L

Zinc, unfiltered-recoverable, 

μg/L

Arsenic, filtered, μg/L

Arsenic, unfiltered-recoverable, μg/L

Suspended sediment, mg/L

 121

NR1

NR1

NR1

NR1

NR1

NR1

 4.7

 123

NR1

NR1

NR1

NR1

NR1

NR1

 7.1

 113

NR1

NR1

NR1

NR1

NR1

NR1

 3.2

NR2

NR2

NR2

NR2

NR2

NR2

NR2

NR2

NR2

NR2

NR2

NR2

NR2

NR2

NR2

NR2

Clark Fork at Turah Bridge (site 20, fig. 1, table 1)

Specific conductance, μS/cm

Copper, filtered, μg/L

Copper, unfiltered-recoverable, μg/L

Zinc, filtered, μg/L

Zinc, unfiltered-recoverable, 

μg/L

Arsenic, filtered, μg/L

Arsenic, unfiltered-recoverable, μg/L

Suspended sediment, mg/L

 344

 3.3

 9.4

NR1

 22

 5.3

 6.8

 13

 327

 2.7

 8.9

NR1

 15

 5.1

 6.3

 12

 324

 2.9

 7.4

NR1

 9.0

 5.4

 5.9

 8.4

 325

 2.6

 9.5

NR1

 16

 5.5

 7.2

 13

-6

-21

1

NR1

-27

4

6

0

Blackfoot River (site 21, fig. 1, table 1)

Specific conductance, μS/cm

Copper, filtered, μg/L

 237

NR1

 238

NR1

 229

NR1

 243

NR1

3

NR1

Table 4. Summary of flow-adjusted trend results determined by using the time-series model (TSM) for selected sites and 

constituents, water years 1996–2010.—Continued

[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. Bold values indicate statistically 

significant (p-value less than 0.01) trend for trend period before the value presented in bold. p-value, statistical probability level; μS/cm, microsiemens per 

centimeter at 25 degrees Celsius; NR, not reported; μg/L, micrograms per liter; mg/L, milligrams per liter]

Constituent or property, 

flow-adjusted units of 

measurement

Fitted trend values Percent change 

from start of 

water year 1996 

to end of water 

year 2010

Start of water year 

1996 (start of 

period 1)

Start of water year 

2001 (start of 

period 2)

Start of water year 

2006 (start of 

period 3)

End of water year 

2010 (end of 

period 3)
Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin 31

Table 4. Summary of flow-adjusted trend results determined by using the time-series model (TSM) for selected sites and 

constituents, water years 1996–2010.—Continued

[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. Bold values indicate statistically 

significant (p-value less than 0.01) trend for trend period before the value presented in bold. p-value, statistical probability level; μS/cm, microsiemens per 

centimeter at 25 degrees Celsius; NR, not reported; μg/L, micrograms per liter; mg/L, milligrams per liter]

Constituent or property, 

flow-adjusted units of 

measurement

Fitted trend values Percent change 

from start of 

water year 1996 

to end of water 

year 2010

Start of water year 

1996 (start of 

period 1)

Start of water year 

2001 (start of 

period 2)

Start of water year 

2006 (start of 

period 3)

End of water year 

2010 (end of 

period 3)

Blackfoot River (site 21, fig. 1, table 1)—Continued

Copper, unfiltered-recoverable, μg/LNR1 NR1 NR1 NR1 NR1

Zinc, filtered, μg/L NR1 NR1 NR1 NR1 NR1

Zinc, unfiltered-recoverable, 

μg/L

NR1 NR1 NR1 NR1 NR1

Arsenic, filtered, μg/L NR1 NR1 NR1 NR1 NR1

Arsenic, unfiltered-recoverable, μg/LNR1 NR1 NR1 NR1 NR1

Suspended sediment, mg/L 4.4 3.9 4.0 3.1 -30

1

Results not reported because greater than 6 percent of values were affected by recensoring at study reporting level, as discussed in the section of this report 

“Time-Series Model.”

2

Results not reported because of no or insufficient data for application of the time-series model (TSM) during indicated trend-analysis period.

Table 5. Summary of flow-adjusted trend results determined by using the time-series model (TSM) for Clark Fork above Missoula 

(site 22, fig. 1, table 1) for selected constituents, water years 1996–2010.

[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. Bold values indicate statistically 

significant (p-value less than 0.01) trend for trend period before the value presented in bold. p-value, statistical probability level; μS/cm, microsiemens per 

centimeter at 25 degrees Celsius; NR, not reported; μg/L, micrograms per liter; mg/L, milligrams per liter]

Constituent or property, flowadjusted units of measurement

Fitted trend values Percent change 

from start of 

water year 1996 

to end of water 

year 2010

Start of water 

year 1996 (start 

of period 1)

Start of water 

year 2001 (start 

of period 2)

Start of water 

year 2006 (start 

of period 3A)

March 28, 2008 

(start of period 

3B)

End of water 

year 2010 (end 

of period 3B)

Clark Fork above Missoula (site 22, fig.1, table 1)

Specific conductance, μS/cm 274 274 270 272 286 4

Copper, filtered, μg/L 2.2 1.7 2.1 2.2 2.5 14

Copper, unfiltered-recoverable, μg/L 5.6 4.6 7.0 14 5.9 5

Zinc, filtered, μg/L NR1 NR1 NR1 NR1 NR1 NR1

Zinc, unfiltered-recoverable, 

μg/L

 15 8.6 12 31 13 -13

Arsenic, filtered, μg/L 2.9 2.9 3.2 3.4 3.9 34

Arsenic, unfiltered-recoverable, μg/L 3.8 3.6 4.1 5.1 5.0 32

Suspended sediment, mg/L 8.5 7.2 9.4 23 9.3 9

1

Results not reported because greater than 6 percent of values were affected by recensoring at study reporting level, as discussed in the section of this report 

“Time-Series Model.”
32 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

Table 6. Summary of flow-adjusted trend results determined by using multiple linear regression on time, streamflow, and season 

(MLR) for selected sites and constituents, water years 2006–10.

[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. Bold values indicate statistically 

significant (p-value less than 0.01) trend for trend period 3. p-value, statistical probability level; μS/cm, microsiemens per centimeter at 25 degrees Celsius; 

μg/L, micrograms per liter; mg/L, milligrams per liter; NR, not reported]

Constituent or property, flow-adjusted 

units of measurement

Fitted trend values Percent change from start 

of water year 2006 to end of 

water year 2010

Start of water year 2006 

(start of period 3)

End of water year 2010 (end 

of period 3)

Mill Creek near Anaconda (site 4, fig. 1, table 1)

Specific conductance, μS/cm 120 128 7

Copper, filtered, μg/L 2.1 1.8 -15

Copper, unfiltered-recoverable, μg/L 3.6 2.8 -21

Zinc, filtered, μg/L 1.3 1.6 16

Zinc, unfiltered-recoverable, μg/L 2.5 2.5 0

Arsenic, filtered, μg/L 17 15 -9

Arsenic, unfiltered-recoverable, μg/L 18 16 -13

Suspended sediment, mg/L 4.1 3.3 -20

Mill Creek at Opportunity (site 5, fig. 1, table 1)

Specific conductance, μS/cm 134 148 11

Copper, filtered, μg/L 2.9 1.9 -36

Copper, unfiltered, μg/L 5.1 3.4 -35

Zinc, filtered, μg/L 2.9 1.7 -42

Zinc, unfiltered, μg/L 5.0 3.3 -33

Arsenic, filtered, μg/L 24 16 -33

Arsenic, unfiltered, μg/L 27 18 -34

Suspended sediment, mg/L 4.0 3.4 -15

Willow Creek near Anaconda (site 6, fig. 1, table 1)

Specific conductance, μS/cm 90 111 24

Copper, filtered, μg/L 2.0 1.6 -23

Copper, unfiltered-recoverable,μg/L 3.5 2.4 -32

Zinc, filtered, μg/L 1.7 1.3 -26

Zinc, unfiltered-recoverable, μg/L 2.7 2.2 -18

Arsenic, filtered, μg/L 16 14 -16

Arsenic, unfiltered-recoverable, μg/L 17 14 -18

Suspended sediment, mg/L 8.0 3.6 -54

Willow Creek at Opportunity (site 7, fig. 1, table 1)

Specific conductance, μS/cm 274 238 -13

Copper, filtered, μg/L 4.7 3.5 -26

Copper, unfiltered-recoverable, μg/L 9.3 7.9 -15

Zinc, filtered, μg/L 3.9 3.1 -21

Zinc, unfiltered-recoverable, μg/L 8.4 8.9 6

Arsenic, filtered, μg/L 36 27 -23

Arsenic, unfiltered-recoverable, μg/L 38 31 -19

Suspended sediment, mg/L 6.3 6.9 10

Warm Springs Creek near Anaconda (site 9, fig. 1, table 1)

Specific conductance, μS/cm 194 225 16

Copper, filtered, μg/L 1.0 0.88 -15

Copper, unfiltered-recoverable, μg/L 2.4 3.0 24
Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin 33

Table 6. Summary of flow-adjusted trend results determined by using multiple linear regression on time, streamflow, and season 

(MLR) for selected sites and constituents, water years 2006–10.—Continued

[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. Bold values indicate statistically 

significant (p-value less than 0.01) trend for trend period 3. p-value, statistical probability level; μS/cm, microsiemens per centimeter at 25 degrees Celsius; 

μg/L, micrograms per liter; mg/L, milligrams per liter; NR, not reported]

Constituent or property, flow-adjusted 

units of measurement

Fitted trend values Percent change from start 

of water year 2006 to end of 

water year 2010

Start of water year 2006 

(start of period 3)

End of water year 2010 (end 

of period 3)

Warm Springs Creek near Anaconda (site 9, fig. 1, table 1)—Continued

Zinc, filtered, μg/L NR1 NR1 NR1

Zinc, unfiltered-recoverable, μg/L 2.1 2.6 22

Arsenic, filtered, μg/L 2.3 2.1 -9

Arsenic, unfiltered-recoverable,μg/L 2.4 2.8 18

Suspended sediment, mg/L 5.6 4.3 -24

Lost Creek near Anaconda (site 12, fig. 1, table 1)

Specific conductance, μS/cm 200 192 -4

Copper, filtered, μg/L 2.2 1.2 -46

Copper, unfiltered-recoverable, μg/L 6.0 3.5 -42

Zinc, filtered, μg/L 1.5 1.0 -32

Zinc, unfiltered-recoverable, μg/L 3.3 1.9 -43

Arsenic, filtered, μg/L 4.9 2.4 -51

Arsenic, unfiltered-recoverable, μg/L 5.4 3.1 -44

Suspended sediment, mg/L 7.4 3.3 -56

Lost Creek near Galen (site 13, fig. 1, table 1)

Specific conductance, μS/cm 632 659 4

Copper, filtered, μg/L 2.3 1.1 -52

Copper, unfiltered-recoverable, μg/L 5.1 2.6 -49

Zinc, filtered, μg/L 1.3 1.5 13

Zinc, unfiltered-recoverable, μg/L 2.4 1.8 -24

Arsenic, filtered, μg/L 13 13 -3

Arsenic, unfiltered-recoverable, μg/L 14 15 6

Suspended sediment, mg/L 11 13 16

1

Results not reported because greater than 45 percent of values were censored (that is, concentrations reported as less than the laboratory reporting level).

in the main-stem stream between the reach inflow and the 

reach outflow. When the reach outflow is greater than the 

reach inflow, the within-reach change in load is positive and 

indicates net mobilization of the constituent from within-reach 

sources, including groundwater inflow, unmonitored tributaries, and the main-stem channel and floodplain. When the reach 

outflow is less than the reach inflow, the within-reach change 

in load is negative and indicates net accumulation of the constituent in the main-stem channel.

For the pie charts illustrating estimated normalized loads 

(hereinafter referred to as loads) in figure 6, the size (area) of 

each pie chart represents the total outflow from reach 1, with 

colored areas indicating relative contributions from each of 

the two source areas [that is, (1) the reach 1 inflow and (2) the 

intervening drainage between the reach 1 inflow and outflow 

(or within-reach sources)]. For each constituent column on 

the right side of fig. 6 (that is, copper, arsenic, and suspended 

sediment), the size of the pie chart representing a given timeperiod load is sized proportionally to the largest load of all 

three time periods within the constituent group. For example, 

in the unfiltered-recoverable copper column, the largest load 

was 5.0 kilograms per day (kg/d) for period 1; thus, the size 

of the period 1 pie chart for unfiltered-recoverable copper is 

the largest and serves as the reference for scaling the other 

copper pie charts. The unfiltered-recoverable copper load 

for period 3 was 1.1 kg/d, which is 22 percent of the load for 

period 3. Thus, the size of the period 3 pie chart for unfilteredrecoverable copper is 22 percent of the size of the period 1 

pie chart for unfiltered-recoverable copper. Reach 1 is simple 

with respect to balance accounting because it consists only of 
34 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

Table 7. Drainage area and streamflow information relevant to the transport analysis for data-summary reaches in the upper Clark 

Fork Basin, Montana, water years 1996–2010.

[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. ft3/s, cubic feet per second]

Site name and number or summation category Drainage area 

(square miles)

Geometric mean 

streamflow, water 

years 1996–2010 

(ft3

/s)

Reach 1 [extending from Blacktail Creek (site 1, fig. 1, table 1) to Silver Bow Creek at Butte (site 2, fig. 1, table 1)]

Inflow Blacktail Creek (site 1) 85 12

Outflow Silver Bow Creek at Butte, Montana (site 2) 103 21

Within-reach change—outflow (site 2) minus inflow (site 1) (contributions from all within-reach 

sources, including unmonitored tributaries and the main-stem channel and floodplain)

 18 9.1

Reach 2 [extending from Silver Bow Creek at Butte (site 2, fig. 1, table 1) to Silver Bow Creek at Opportunity (site 3, fig. 1, table 1)]

Inflow Silver Bow Creek at Butte (site 2) 103 21

Outflow Silver Bow Creek at Opportunity (site 3) 363 40

Within-reach change—outflow (site 3) minus inflow (site 2) (contributions from all within-reach 

sources, including unmonitored tributaries and the main-stem channel and floodplain)

 260 19

Reach 3 [extending from Silver Bow Creek at Opportunity (site 3, fig. 1, table 1) to Silver Bow Creek at Warm Springs (site 8, fig. 1, table 1)]

Inflow Silver Bow Creek at Opportunity (site 3) 363 40

 Monitored tributary sites within reach1

 Mill Creek near Anaconda (site 4, fig. 1, table 1) 34 2.4

 Mill Creek at Opportunity (site 5, fig. 1, table 1) 43 5.2

 Willow Creek near Anaconda (site 6, fig. 1, table 1) 14 3.3

 Willow Creek near Anaconda (site 7, fig. 1, table 1) 31 8.1

Outflow Silver Bow Creek at Warm Springs (site 8) 473 62

Within-reach change—outflow (site 8) minus inflow (site 3) (contributions from all within-reach 

sources, including the monitored tributaries, unmonitored tributaries, and the main-stem channel 

and floodplain)

 110 22

Reach 4 [extending from Silver Bow Creek at Warm Springs (site 8, fig. 1, table 1) to Clark Fork near Galen (site 11, fig. 1, table 1)]

Inflow Silver Bow Creek at Warm Springs (site 8) 473 62

 Monitored tributary sites within reach

 Warm Springs Creek near Anaconda (site 9, fig. 1, table 1) 157 71

 Warm Springs Creek at Warm Springs (site 10, fig. 1, table 1) 163 48

Combined inflow (sum of sites 8 and 10) 636 110

Outflow Clark Fork near Galen (site 11) 651 113

Within-reach change—outflow (site 11) minus inflow (site 8) (contributions from all withinreach sources, including the monitored tributary, unmonitored tributaries, and the main-stem 

channel and floodplain)

 178 52

Within-reach sources other than the monitored tributary—outflow (site 11) minus combined 

inflow (sum of sites 8 and 10) (contributions from other within-reach sources, including unmonitored tributaries and the main-stem channel and floodplain)

 15 3.3

Reach 5 [extending from Clark Fork near Galen (site 11, fig. 1, table 1) to Clark Fork at Deer Lodge (site 14, fig. 1, table 1)]

Inflow Clark Fork near Galen (site 11) 651 113

 Monitored tributary sites within reach1

 Lost Creek near Anaconda (site 12, fig. 1, table 1) 26 3.9

 Lost Creek near Galen (site 13, fig. 1, table 1) 60 18

Outflow Clark Fork at Deer Lodge (site 14) 995 200

Within-reach change—outflow (site 14) minus inflow (site 11) (contributions from all withinreach sources, including the monitored tributary, unmonitored tributaries, and the main-stem 

channel and floodplain)

 344 87
Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin 35

Table 7. Drainage area and streamflow information relevant to the transport analysis for data-summary reaches in the upper Clark 

Fork Basin, Montana, water years 1996–2010.—Continued

[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. ft3/s, cubic feet per second]

Site name and number or summation category Drainage area 

(square miles)

Geometric mean 

streamflow, water 

years 1996–2010 

(ft3

/s)

Reach 6 [extending from Clark Fork at Deer Lodge (site 14, fig. 1, table 1) to Clark Fork at Goldcreek (site 16, fig. 1, table 1)]

Inflow Clark Fork at Deer Lodge (site 14) 995 200

 Monitored tributary site within reach

 Little Blackfoot River (site 15, fig. 1, table 1) 407 95

Combined inflow (sum of sites 14 and 15) 1,402 296

Outflow Clark Fork at Goldcreek (site 16) 1,704 394

Within-reach change—outflow (site 16) minus inflow (site 14) (contributions from all withinreach sources, including the monitored tributary, unmonitored tributaries, and the main-stem 

channel and floodplain)

 709 194

Within-reach sources other than the monitored tributary—outflow (site 16) minus combined 

inflow (sum of sites 14 and 15) (contributions from other within-reach sources, including unmonitored tributaries and the main-stem channel and floodplain)

 302 99

Reach 7 [extending from Clark Fork at Goldcreek (site 16, fig. 1, table 1) to Clark Fork near Drummond (site 18, fig. 1, table 1)]

Inflow Clark Fork at Goldcreek (site 16) 1,704 394

Monitored tributary site within reach Flint Creek (site 17, fig. 1, table 1) 490 95

Combined inflow (sum of sites 16 and 17) 2,194 489

Outflow Clark Fork near Drummond (site 18) 2,501 577

Within-reach change—outflow (site 18) minus inflow (site 16) (contributions from all withinreach sources, including the monitored and unmonitored tributaries and the main-stem channel 

and floodplain)

 797 182

Within-reach sources other than the monitored tributary—outflow (site 18) minus combined 

inflow (sum of sites 16 and 17) (contributions from other within-reach sources, including unmonitored tributaries and the main-stem channel and floodplain)

 307 87

Reach 8 [extending from Clark Fork near Drummond (site 18, fig. 1, table 1) to Clark Fork at Turah Bridge (site 20, fig. 1, table 1)]

Inflow Clark Fork near Drummond (site 18) 2,501 577

Monitored tributary site within reach Rock Creek (site 19, fig. 1, table 1) 885 342

Combined inflow (sum of sites 18 and 19) 3,386 919

Outflow Clark Fork at Turah Bridge (site 20) 3,641 1,030

Within-reach change—outflow (site 20) minus inflow (site 18) (contributions from all withinreach sources, including the monitored and unmonitored tributaries and the main-stem channel 

and floodplain)

 1,140 456

Within-reach sources other than the monitored tributary—outflow (site 20) minus combined 

inflow (sum of sites 18 and 19) (contributions from other within-reach sources, including unmonitored tributaries and the main-stem channel and floodplain)

 255 113

Reach 9 [extending from Clark Fork at Turah Bridge (site 20, fig. 1, table 1) to Clark Fork above Missoula (site 22, fig. 1, table 1)]

Inflow Clark Fork at Turah Bridge (site 20) 3,641 1,030

Monitored tributary site within reach Blackfoot River (site 21, fig. 1, table 1) 2,290 972

Combined inflow (sum of sites 20 and 21) 5,931 2,000

Outflow Clark Fork above Missoula (site 22) 5,999 2,030

Within-reach change—outflow (site 22) minus inflow (site 20) (contributions from all withinreach sources, including the monitored and unmonitored tributaries and the main-stem channel 

and floodplain)

 2,358 998

Within-reach sources other than the monitored tributary—outflow (site 22) minus combined 

inflow (sum of sites 20 and 21) (contributions from other within-reach sources, including unmonitored tributaries and the main-stem channel and floodplain)

 68 26

1

Trends for the monitored tributaries in reaches 3 and 5 were analyzed by using multiple linear regression on time, streamflow, and season (MLR). Trend 

results for the monitored tributaries were not included in the transport analysis because of factors that complicate directly combining the time-series model 

(TSM) results and MLR results within a single analysis. Thus, combined inflow calculations and the associated calculation of contributions from other 

within-reach sources are not presented for reaches 3 and 5. Drainage area and geometric mean streamflow for the monitored tributaries in reaches 3 and 5 are 

presented for informational purposes.
36 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

1

10

100

1,000

1

10

100

1,000

1

10

100

1,000

A

B.

C.

Unfiltered-recoverable copper

Unfiltered-recoverable arsenic

Suspended sediment

EXPLANATION

Reach inflow—Blacktail 

Creek (site 1, fig. 1, table 1)

Reach outflow—Silver Bow 

Creek at Butte (site 2, 

fig. 1, table 1)

Fitted trend value at start or 

end of period

Bold values indicate 

statistical significance 

(p-value less than 0.01) for 

period before value 

presented in bold

[Water year is defined as the 12-month 

period from October 1 through 

September 30 and is designated by the 

year in which it ends. p-value, statistical 

probability level]

Flow-adjusted suspended-sediment

concentration, in milligrams per liter

Flow-adjusted unfiltered-recoverable copper

concentration, in micrograms per liter

Flow-adjusted ufniltered-recoverable arsenic

concentration, in micrograms per liter

15 14

7.5 7.3

6.7 7.2

5.6 5.1

13

10

5.6 6.0

3.8

2.8 3.1 3.3

180

44

31

31

14

5.2

4.3 4.3

3.8

3.8

1

Period

2 3

.

 Water year (October–September)

1995 2000 2005 2010

Figure 5. Flow-adjusted fitted trends determined by using the time-series model (TSM) for selected constituents for sites in reach 1, 

extending from Blacktail Creek (site 1, fig. 1, table 1) to Silver Bow Creek at Butte (site 2, fig. 1, table 1), water years 1996–2010.
Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin 37Figure 6. Drainage area, streamflow, and estimated normalized constituent loads contributed from reach inflow and within-reach sources for reach 1, extending from Blacktail 

Creek (site 1, fig. 1, table 1) to Silver Bow Creek at Butte (site 2, fig. 1, table 1) for selected periods.

0.13 0.093

0.50

0.083

0.30

0.090

0.21

200

540

180

350

150

220

4.9

1.7

0.12

1.0

85

18

12

9.1

0.11

5.0

1.9

1.1

0.590.39

0.30

740

530

380

103

21

Unfiltered-recoverable arsenic Suspended sediment Unfiltered-recoverable copper

Drainage area and streamflow information

Drainage area,

in square miles

Geometric mean streamflow,

in cubic feet per second

(water years 1996–2010)

Estimated normalized load1, in kilograms per day

Water years1996–2000(period 1)Water years2001–05(period 2)Water years2006–10(period 3)

Value

Circular pie chartsrepresent loadstransported pastreach outflow[Silver Bow Creek atButte (site 2, fig. 1, table 1)]Size (area) of circular pie charts are proportional to largest estimatednormalized load (for given constituent)

Load transported pastreach outflow [Silver BowCreek at Butte (site 2, fig. 1, table 1)]

Value

Drainage area, in squaremiles,or geometric meanstreamflow, in cubicfeet per second

Drainage area orgeometric mean streamflowat reach outflow[Silver Bow Creek atButte (site 2, fig. 1, table 1)]

EXPLANATION, estimated normalized load pie charts

EXPLANATION, drainage area and streamflow pie charts

Value

Reach inflow[ Blacktail Creek(site 1, fig. 1, table 1)]

Reach inflow[Blacktail Creek (site 1, fig. 1, table 1)]

Value

All within-reach sources (including groundwater inflow, unmonitored tributaries, and the mainstem channel and floodplain)

Total within-reach change in load(net mobilization from within-reachsources, including groundwater inflow,unmonitored tributaries, and the mainstem channel and floodplain)

Estimated flownormalized load,in kilograms per day

1Estimated normalized load calculated by multiplying the mean annual fitted trend concentration 

(determined by using the time-series model) for the indicated period times the geometric mean streamflow for water years 1996–2010 and a units conversion factor. Calculation of the estimated normalized load is described in detail in the section of this report "Estimation of Normalized Constituent Loads."
38 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

the reach inflow (site 1) and the reach outflow (site 2), with 

no monitored tributary inflows within the reach. Also, for all 

time periods and all constituents, reach 1 outflow loads are 

greater than reach 1 inflow loads, which indicates consistent 

net mobilization from within-reach sources. Other reaches are 

more complex than reach 1. Some reaches have monitored 

tributary inflows within the reach. Also, for some reaches, the 

reach outflow load for some time periods is less than the combined reach inflow load (sum of reach inflow plus within-reach 

monitored tributary inflows), which indicates net accumulation 

within the reach channel. For reaches that are more complex 

than reach 1, the formats of the pie charts illustrating estimated normalized loads are modified from figure 6 to represent the more complex characteristics. Modifications to the pie 

chart formats for reaches that are more complex than reach 1 

are discussed on a case by case basis in following paragraphs. 

In discussion of transport analysis results, observations 

are made comparing the relative proportions of within-reach 

contributions of constituent loads and within-reach contributions of streamflow. Those proportional comparisons indicate 

the importance of a given reach as a source of constituent 

loading to Silver Bow Creek or the Clark Fork. If the contribution of a constituent from within reach is proportionally larger 

than the contribution of streamflow from within reach, the 

given reach is indicated to be an important disproportionate 

source of constituent loading. Conversely, if the contribution 

of a constituent from within reach is proportionally smaller 

than the contribution of streamflow from within reach, the 

given reach is not indicated to be an important disproportionate source of constituent loading and acts either as an accumulation reach (sink) or flow-through reach.

Factors affecting temporal variability in water quality in 

the upper Clark Fork Basin are complex. Much information 

on observed changes in water quality is presented, but it is 

beyond the scope of this report to provide detailed explanations for all of the observed changes or to link specific trends 

with specific remediation activities. The primary focus is on 

describing flow-adjusted trends and patterns of constituent 

transport within each reach that will allow evaluation of temporal changes within the upper Clark Fork Basin as a whole.

Reach 1

Sites in reach 1 include the inflow [Blacktail Creek 

(site 1, fig. 1, table 1)], and the outflow [Silver Bow Creek at 

Butte (site 2, fig. 1, table 1)]. Trend results for both sites in 

reach 1 were determined by using the TSM and are presented 

in tables 4 and 4–1, figure 5, and also figures 4–1, and 4–2 for 

sites 1 and 2, respectively. Transport-analysis balance accounting for reach 1 (table 5–1) is simple and consists of calculation of the total within-reach change in load (outflow minus 

inflow). Transport analysis results for reach 1 are graphically 

presented in figure 6.

Trend results for site 1 generally indicate minor changes 

in water quality during periods 1 through 3 (figs. 5 and 4–1; 

tables 4 and 4–1). Decreases in arsenic, copper, and suspended 

sediment are within a small range at low concentrations. 

In contrast to site 1, trend results for site 2 indicate moderate to large decreases in FACs of metallic contaminants and 

suspended sediment (figs. 5 and 4–2; tables 4 and 4–1). The 

geometric mean FAC for unfiltered-recoverable copper for 

site 2 decreases by 92 percent from 180 μg/L at the start of 

period 1 to 14 μg/L at the end of period 3 (fig. 5 and table 4). 

Trend results for unfiltered-recoverable arsenic and suspended 

sediment for site 2 indicate moderate decreases of about 

50 percent mostly during periods 1 and 2 (fig. 5, table 4). 

Effects of moderate to large decreases in FACs of metallic contaminants, arsenic, and suspended sediment for site 2 

are reflected in temporal changes in relative contributions 

from upstream source areas to reach 1 outflow loads (fig. 6, 

table 5–1). For unfiltered-recoverable copper, contributions 

(net mobilization) from within-reach sources decrease 

by about 80 percent from period 1 (4.9 kg/d) to period 3 

(1.0 kg/d). For unfiltered-recoverable arsenic and suspended 

sediment, contributions from within-reach sources decrease by 

about 60 percent from period 1 (0.50 and 540 kg/d, respectively) to period 3 (0.21 and 220 kg/d, respectively). However, 

for all periods, contributions of metallic-contaminant, arsenic, 

and suspended-sediment loads from within-reach sources are 

proportionally much larger than streamflow contributed from 

within reach 1, which indicates that reach 1 is a disproportionate source of constituent loading. For example, based on 

geometric mean streamflow for water years 1996–2010, the 

within-reach increase in streamflow (9.1 ft3

/s, table 7) accounts 

for about 43 percent of the streamflow at the reach 1 outflow 

(21 ft3/s, table 7). In contrast, even though FACs of metallic 

contaminants have declined sharply, in period 3 the withinreach increase in unfiltered-recoverable copper load (1.0 kg/d, 

table 5–1) accounts for about 91 percent of the copper load at 

the reach 1 outflow (1.1 kg/d, table 5–1).

Reach 2

Sites in reach 2 include the inflow [Silver Bow Creek 

at Butte (site 2, fig. 1, table 1)] and the outflow [Silver Bow 

Creek at Opportunity (site 3, fig. 1, table 1)]. Trend results for 

both sites in reach 2 were determined by using the TSM and 

are presented in tables 4 and 4–1, figure 7, and also figures 

4–2, and 4–3 for sites 2 and 3, respectively. Transport-analysis 

balance accounting for reach 2 (table 5–2) is simple and 

consists of calculation of the total within-reach change in load 

(outflow minus inflow). Transport analysis results for reach 2 

are graphically presented in figure 8.

Similar to site 2, trend results for site 3 indicate moderate to large decreases in FACs of metallic contaminants and 

arsenic (figs. 7 and 4–3; tables 4 and 4–1); however, decreases 

are smaller for site 3 than for site 2. The geometric mean 

FAC for unfiltered-recoverable copper for site 3 decreases by 

66 percent from 130 μg/L at the start of period 1 to 44 μg/L 

at the end of period 3. The geometric mean FAC for unfiltered-recoverable arsenic for site 3 decreases by 45 percent 

from 16 μg/L at the start of period 1 to 8.8 μg/L at the end 
Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin 39

1

10

100

1,000

1

10

100

1,000

1

10

100

1,000

A.

B.

C.

Unfiltered-recoverable copper

Unfiltered-recoverable arsenic

Suspended sediment

120

EXPLANATION

Reach inflow—Silver Bow 

Creek at Butte (site 2, 

fig. 1, table 1)

Reach outflow—Silver Bow 

Creek at Opportunity 

(site 3, fig. 1, table 1)

Fitted trend value at start or 

end of period

Bold values indicate 

statistical significance 

(p-value less than 0.01) for 

period before value 

presented in bold

[Water year is defined as the 12-month 

period from October 1 through 

September 30 and is designated by the 

year in which it ends. p-value, statistical 

probability level]

Flow-adjusted suspended-sediment

concentration, in milligrams per liter

Flow-adjusted unfiltered-recoverable copper

concentration, in micrograms per liter

Flow-adjusted ufniltered-recoverable arsenic

concentration, in micrograms per liter

15 14

7.5 7.3

24

18 21 19

13

10

5.6 6.0

16 17 18

8.8

8.8

180

44

31

14

130

120

94

44

1

Period

2 3

 Water year (October–September)

1995 2000 2005 2010

Figure 7. Flow-adjusted fitted trends determined by using the time-series model (TSM) for selected constituents for sites in reach 2, 

extending from Silver Bow Creek at Butte (site 2, fig. 1, table 1) to Silver Bow Creek at Opportunity, (site 3, fig. 1, table 1), water years 

1996–2010.
40 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

Figure 8. Drainage area, streamflow, and estimated normalized constituent loads contributed from reach inflow and within-reach sources for reach 2, extending from Silver Bow Creek at Butte (site 2, fig. 1, table 1) to Silver Bow Creek at Opportunity (site 3, fig. 1, table 1), for selected periods.

7.3

0.59

1.0

0.39

1.3

0.29

1.0

740

1,300

530

1,400

380

1,600

5.0

8.6

1.9

5.4

260

103

21

19

1.1

12

106.5

1.6

1.71.3

2,000

1,900

2,000

363

40

Drainage area and streamflow information Estimated normalized load1, in kilograms per day

Drainage area,

in square miles

Unfiltered-recoverable arsenic Suspended sediment Unfiltered-recoverable copper

Geometric mean streamflow,

in cubic feet per second

(water years 1996–2010)

Water years1996–2000(period 1)Water years2001–05(period 2)Water years2006–10(period 3)

Value

Estimated flownormalized load, in kilograms per day

Drainage area, in squaremiles,or geometric meanstreamflow, in cubicfeet per second

EXPLANATION, estimated normalized load pie charts

EXPLANATION, drainage area and streamflow pie charts

Value

Value

Drainage area orgeometric mean streamflowat reach outflow [Silver Bow Creek at Opportunity (site 3, fig. 1, table 1)]

Load transported pastreach outflow [Silver Bow Creek at Opportunity(site 3, fig. 1, table 1)]

Reach inflow [Silver Bow Creek at Butte (site 2, fig. 1, table 1)]

Reach inflow [Silver Bow Creek at Butte, (site 2, fig. 1, table 1)]

Value

All within-reach sources (including groundwater inflow, unmonitored tributaries, and the mainstem channel and floodplain) Total within-reach change in load (net mobilization from within-reach sources, including groundwater inflow,

unmonitored tributaries, and the mainstem channel and floodplain) 1Estimated normalized load calculated by multiplying the mean annual fitted trend 

concentration (determined by using the time-series model) for the indicated period times the geometric mean streamflow for water years 1996–2010 and a units conversion factor. Calculation of the estimated normalized load is described in detail in the section of this report "Estimation of Normalized Constituent Loads."

Circular pie charts represent loads transported past reach outflow [Silver Bow Creek at Opportunity (site 3, fig. 1, table 1)]Size (area) of circular pie charts are proportional to largest estimated normalized load (for given constituent)
Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin 41

of period 3. Smaller decreases in FACs of metallic contaminant and arsenic for site 3 than for site 2 might reflect more 

intensive remediation efforts upstream from site 2 than in the 

reach between site 2 and site 3. In contrast to site 2, decreases 

in FACs of metallic contaminants and arsenic for site 3 were 

mostly during period 3. Also, a small decrease in FAC of 

suspended sediment is indicated for site 3, whereas a moderate 

decrease is indicated for site 2. The inconsistency between the 

large decrease in copper but the small decrease in suspended 

sediment for site 3 is difficult to interpret. A possible explanation for this pattern might relate to floodplain disturbance and 

placement of uncontaminated fill in the floodplain associated 

with remediation activities. As particulate materials derived 

from mining wastes are removed and transported from the 

reach through time, less-contaminated particulate materials 

might be proportionally increasing in suspended-sediment 

concentrations. The moderate decrease in suspended sediment for site 2 also might contribute to the capacity for the 

stream to transport the less-contaminated materials from 

reach 2. Because trend results for site 3 affect interpretation of 

trend results for Silver Bow Creek at Warm Springs (site 8), 

several TSM and MLR trend models for copper and suspended 

sediment were investigated, and all trend models produced 

similar results of large decreases in copper and minor to small 

changes in suspended sediment during water years 1996–2010. 

Further, the 95-percent confidence intervals about the trend 

magnitudes for unfiltered-recoverable copper and suspended 

sediment for period 3 (which had the largest decrease in 

copper) were examined and indicated no overlap. Thus, the 

TSM results for copper and suspended sediment for site 3 

are considered to reasonably represent general trend patterns 

and there is strong indication that metallic contaminant FACs 

substantially decreased during water years 1996–2010 in the 

absence of substantial trending in suspended sediment.

Results of the transport analysis indicate how temporal 

changes in geometric mean FACs for sites 2 and 3 translate 

into temporal changes in relative contributions from upstream 

source areas to the reach 2 outflow loads (fig. 8, table 5–2). 

For unfiltered-recoverable copper, reach 2 outflow load 

decreases by about 50 percent from period 1 (12 kg/d) to 

period 3 (6.5 kg/d). Contributions (net mobilization) from 

sources within reach 2 decrease by about 25 percent from 

period 1 (7.3 kg/d) to period 3 (5.4 kg/d). However, because 

of a larger decrease in copper load for site 2 than for site 3, 

contributions of copper from sources within reach 2 proportionally increase in relative contribution to the reach outflow 

between period 1 and period 3. Thus, most of the decrease in 

copper load at the reach 2 outflow is because of a decrease 

in load at site 2. For unfiltered-recoverable arsenic, there is a 

minor decrease in the reach 2 outflow load of about 20 percent 

from period 1 (1.6 kg/d) to period 3 (1.3 kg/d). Contributions 

from sources within reach 2 are near constant. For suspended 

sediment, there is no change in the reach 2 outflow load from 

period 1 to period 3, with both values equal to 2,000 kg/d. 

Contributions from sources within reach 2 increase by about 

23 percent from period 1 (1,300 kg/d) to period 3 (1,600 kg/d). 

For all periods, contributions of metallic-contaminant, arsenic, 

and suspended-sediment loads from within-reach sources are 

proportionally larger than streamflow contributed from within 

reach 2, which indicates that reach 2 is a disproportionate 

source of constituent loading.

Reach 3

Sites in reach 3 include the inflow [Silver Bow Creek at 

Opportunity (site 3)], monitored tributary sites [Mill Creek 

near Anaconda (site 4), Mill Creek at Opportunity (site 5), 

Willow Creek near Anaconda (site 6), and Willow Creek at 

Opportunity (site 7)], and the outflow [Silver Bow Creek at 

Warm Springs (site 8)]. Trend results for the reach inflow and 

outflow sites (sites 3 and 8) were determined by using the 

TSM and are presented in tables 4 and 4–1, figure 9, and also 

in figures 4–3, and 4–8 for sites 3 and 8, respectively. Trend 

results for monitored tributary sites [sites on Mill Creek 

(site 4 and site 5) and Willow Creek (site 6 and site 7), which 

had short periods of data collection] were determined by using 

MLR. Transport analysis results for reach 3 are graphically 

presented in figure 10.

Detailed trend results for the monitored tributary sites on 

Mill Creek and Willow Creek are presented in table 4–3 and 

figures 4–4 through 4–7 for sites 4 through 7, respectively. 

Trend results are summarized in table 6. Trend results for the 

downstream site on Mill Creek (site 5) and the downstream 

site on Willow Creek (site 7) also are shown in figure 9, in 

reference to trend results for the main-stem Silver Bow Creek 

inflow and outflow sites that bound the reach (site 3 and 

site 8). Because MLR and the TSM use different flow-adjustment procedures, caution should be used in directly comparing results of the two methods in terms of the estimated fitted 

trend values presented in figure 9. Magnitudes and directions 

of trends probably are reasonably comparable between the two 

methods within the short time frame of period 3. Trend results 

for both Mill Creek sites (sites 4 and 5) indicate decreases 

in geometric mean FACs of copper, arsenic, and suspended 

sediment for period 3 (figs. 4–4 and 4–5, tables 5 and 4–3). 

Decreases generally are minor for site 4, but small for site 5. 

For example, unfiltered-recoverable copper results for period 3 

for site 4 indicate a minor decrease of 21 percent (from 3.6 to 

2.8 μg/L) and for site 5 a small decrease of 35 percent (from 

5.1 to 3.4 μg/L; table 6). Unfiltered-recoverable arsenic results 

for period 3 for site 4 indicate a minor decrease of 13 percent (from 18 to 16 μg/L) and for site 5 a small decrease of 

34 percent (from 27 to 18 μg/L; table 6). Trend results for both 

Willow Creek sites (site 6 and site 7) indicate minor to small 

decreases in FACs of copper and arsenic (figs. 4–6 and 4–7, 

tables 5 and 4–3) that generally were near or within the ranges 

of those for the Mill Creek sites.

Trend results for the reach 3 outflow [Silver Bow Creek 

at Warm Springs (site 8)] indicate large decreases in geometric 

mean FACs of metallic contaminants (figs. 9 and 4–8; tables 4 

and 4–1) that are similar in magnitude to decreases in FACs for 

the reach 3 inflow [Silver Bow Creek at Opportunity (site 3)]. 
42 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

Unfiltered-recoverable copper for site 8 decreases by 61 percent from 18 μg/L at the start of period 1 to 7.0 μg/L at the 

end of period 3. Trend results for site 8 differ from results for 

site 3 with respect to arsenic and suspended sediment. Trend 

results for unfiltered-recoverable arsenic for site 8 indicate 

a minor increase of 15 percent from 20 μg/L at the start of 

period 1 to 23 μg/L at the end of period 3; whereas trend 

results for site 3 indicate a moderate decrease of 45 percent. 

Trend results for suspended sediment for site 8 indicate a large 

decrease of 65 percent from 5.4 mg/L at the start of period 1 to 

1.9 mg/L at the end of period 3, with the largest decrease during period 3; whereas trend results for site 3 indicate a small 

decrease. These differences in trend results between site 8 and 

site 3 illustrate effects of particulate deposition and complex 

geochemical processes in the Warm Springs Ponds, coupled 

with mixing of tributary contributions from the Mill-Willow 

bypass.

The transport analysis for reach 3 is complicated by 

several factors including effects of Warm Springs Ponds on 

constituent transport and the inability to distinguish between 

the within-reach contributions from Warm Springs Ponds 

and the Mill-Willow bypass. Data for the monitored tributary 

sites on Mill Creek and Willow Creek were not included in 

transport analysis because of factors that complicate directly 

combining the TSM and MLR results within a single analysis. 

The combined unquantified contributions from Mill Creek and 

Willow Creek in the Mill-Willow bypass are part of the total 

undifferentiated contributions from within-reach sources. 

Because of the complexity of reach 3, the transport-analysis pie charts were modified to accommodate the characteristics of reach 3 (fig. 10). Most particulate material transported 

in Silver Bow Creek from site 3 is presumed to be deposited 

in Warm Springs Ponds. Thus, for suspended sediment and 

metallic elements, which entirely or predominantly are transported in particulate phase, the load at the reach 3 outflow 

(site 8) typically is less than the load at the reach 3 inflow 

(site 3). This consistently results in negative within-reach 

change in load and indicates net accumulation of sediment 

and metallic elements in Warm Springs Ponds. In figure 10, 

squares are used to represent net accumulation of copper and 

suspended sediment in Warm Springs Ponds. The squares 

represent the reach inflow load (site 3), all of which is presumed to be deposited in Warm Springs Ponds. Presumption 

that all of the reach inflow load (site 3) is deposited in Warm 

Springs Ponds results in the reach outflow load (site 8) entirely 

consisting of undifferentiated contributions (net mobilization) 

from within-reach sources including groundwater inflow, the 

Mill-Willow bypass, the Warm Springs Ponds outflow, and the 

main-stem channel and floodplain downstream from Warm 

Springs Ponds. Intuitively [and also based on similarities in 

statistical distributions of copper and suspended-sediment 

concentrations among sites 5, 7, and 8 (fig. 2)], contributions 

from the Mill-Willow bypass probably account for a large part 

of the reach 3 outflow loads of unfiltered-recoverable copper 

and suspended sediment at site 8.

In contrast to copper, arsenic in the upper Clark Fork 

Basin typically is in dissolved phase (fig. 2, table 1–4) and a 

smaller fraction of the unfiltered-recoverable arsenic load is 

deposited in Warm Spring Ponds. As a result, the load at the 

reach 3 outflow (site 8) typically is greater than the load at the 

reach 3 inflow (site 3), indicating positive within-reach change 

in load and net mobilization from within-reach sources. Thus, 

pie charts for unfiltered-recoverable arsenic in figure 10 are 

similar in format to those of figures 6 and 8.

For unfiltered-recoverable copper, reach 3 outflow load 

decreases by about 40 percent from period 1 (2.0 kg/d) to 

period 3 (1.1 kg/d; fig. 10, table 5–3). The estimated copper 

load deposited in Warm Springs Ponds decreases by about 

45 percent from period 1 (12 kg/d) to period 3 (6.5 kg/d). For 

unfiltered-recoverable arsenic, a minor increase (from 3.1 

to 3.4 kg/d) in reach 3 outflow load is indicated during 

periods 1 through 3, but contributions (net mobilization) from 

within-reach sources increase by about 30 percent from 

period 1 (1.6 kg/d) to period 3 (2.1 kg/d). For suspended 

sediment, reach 3 outflow loads decrease by about 50 percent 

from period 1 (850 kg/d) to period 3 (430 kg/d). The estimated 

suspended-sediment load deposited in Warm Springs Ponds 

has no change from period 1 to period 3, with both values 

equal to 2,000 kg/d.

Reach 4

Sites in reach 4 include the inflow [Silver Bow Creek 

at Warm Springs (site 8)], monitored tributary sites [Warm 

Springs near Anaconda (site 9) and Warm Springs Creek at 

Warm Springs (site 10)], and the outflow [Clark Fork near 

Galen (site 11)]. Trend results for the reach inflow and outflow sites (sites 8 and 11) and the downstream site on Warm 

Springs Creek (site 10) were determined by using the 

TSM and are presented in tables 4 and 4–1, figure 11, and 

figures 4–8, 4–11, and 4–10, for sites 8, 11, and 10, respectively. Trend results for the upstream site on Warm Springs 

Creek [site 9, which had a short period of data collection] were 

determined by using MLR and are presented in tables 5 and 

4–3, and figure 4–9. Transport analysis results for reach 4 are 

graphically presented in figure 12.

The MLR trend results for the upstream site on Warm 

Springs Creek (site 9) indicate minor to small increases in 

geometric mean FACs of unfiltered-recoverable copper and 

arsenic, and a small decrease in geometric mean FAC of 

suspended sediment for period 3 (fig. 4–9, tables 5 and 4–3). 

The TSM trend results for the downstream site on Warm 

Springs Creek (site 10) indicate small to moderate decreases 

in geometric mean FACs of unfiltered-recoverable copper and 

suspended sediment for period 1, but small to large increases 

for period 3 (figs. 11 and 4–10; tables 4 and 4–1). The large 

increase in geometric mean FAC of suspended sediment from 

3.1 to 7.7 mg/L for site 10 for period 3 is notable. Several 

TSM and MLR trend models for suspended sediment for site 

10 were investigated, and all trend models produced similar 

results of large increases in suspended sediment FAC during 
Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin 43

1

10

100

1,000

1

100

1,000

1

10

10

100

1,000

23

EXPLANATION

[Water year is defined as the 12-month 

period from October 1 through 

September 30 and is designated by the 

year in which it ends. p-value, statistical 

probability level]

Reach inflow—Silver Bow 

Creek at Opportunity (site 3, 

fig. 1, table 1). Fitted trend 

determined by using the 

time-series mode

Monitored tributary 

inflow—Mill Creek at 

Opportunity (site 5, fig. 1, 

table 1). Fitted trend 

determined by using 

multiple linear regression

Monitored tributary 

inflow—Willow Creek at 

Opportunity (site 7, fig. 1, 

table 1). Fitted trend 

determined by using 

multiple linear regression

Reach outflow—Silver Bow 

Creek at Warm Springs (site 

8, fig. 1, table 1). Fitted trend 

determined by using the 

time-series model.

Fitted trend value at start or 

end of period

Bold values indicate 

statistical significance 

(p-value less than 0.01) for 

period before value 

presented in bold

A. Unfiltered-recoverable copper

B. Unfiltered-recoverable arsenic

C. Suspended sediment

Flow-adjusted suspended-sediment

concentration, in milligrams per liter

Flow-adjusted unfiltered-recoverable copper

concentration, in micrograms per liter

Flow-adjusted ufniltered-recoverable arsenic

concentration, in micrograms per liter

5.8

4.1

1.9

5.4

20 22 21 23

16 17 18

8.8

8.8

18

8.8 8.0 7.0

5.1

3.4

27

18

38

31

4.0

3.4

6.3 6.9

9.3 7.9

130 120

94

44

1

Period

2 3

 Water year (October–September)

1995 2000 2005 2010

24

18

21 19

Figure 9. Flow-adjusted fitted trends determined by using the time-series model (TSM) for selected constituents for sites in reach 3, 

extending from Silver Bow Creek at Opportunity (site 3, fig. 1, table 1) to Silver Bow Creek at Warm Springs, (site 8, fig. 1, table 1), water 

years 1996–2010.
44 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

Figure 10. Drainage area, streamflow, and estimated normalized constituent loads contributed from reach inflow and within-reach sources for reach 3, extending from Silver Bow Creek at Opportunity (site 3, fig. 1, table 1) to Silver Bow Creek at Warm Springs (site 8, fig. 1, table 1) for selected periods. 

12

1.61.6

1.6

2.1

1.3

2,000

850

740

1,900

2,000

430

2.0

10

1.3

6.5

1.1

363

110

22

40

1.7

2.01.3

1.1

3.1

3.3

3.4

850

740

430

473

62

Drainage area,

in square miles

Drainage area and

streamflow information

Unfiltered-recoverable arsenic Suspended sediment Unfiltered-recoverable copper

Estimated normalized load1, in kilograms per day

Geometric mean streamflow,

in cubic feet per second

(water years 1996–2010)

Water years1996–2000(period 1)Water years2001–05(period 2)Water years2006–10(period 3)

Value

Load transported pastreach outflow[Silver ]Bow Creek at Warm Springs (site 8, fig. 1, table 1)]

Reach outflow(net mobilization from within-reachsources)

Value Drainage area, in square

miles,or geometric meanstreamflow, in cubicfeet per second

Drainage area orgeometric mean streamflowat reach outflow[Silver Bow Creek at Warm Springs (site 8, fig. 1, table 1)]

EXPLANATION, estimated normalized load pie charts

EXPLANATION, drainage area and streamflow pie charts

Value

Value

Reach inflow[Silver Bow Creek at Opportunity (site 3, fig. 1, table 1)]

Reach inflow[Silver Bow Creek  at Opportunity (site 3, fig. 1, table 1)]

All within-reach sources (including groundwaterinflow, the MWB, the outflow from WSP, and the main-stem channel and floodplain downstream from WSP)

Within-reach change in load(net mobilization from within-reach sources, including groundwater inflow, the MWB, the outflow from WSP, and the main-stem channel and floodplain downstream from WSP)

Estimatedflow-normalizedload, in kilograms per day

Squares indicate net accumulation in reach channel when reach inflow is greater than reach outflow. In reach 3, when net accumulation is indicated, all of the reach inflow at site 3 is presumed to be deposited in WSP. In this case, the within-reach change in load is not directly indicated.

Circular pie charts represent loadstransported pastreach outflow [Silver Bow Creek at Warm Springs (site 8, fig. 1, table 1)] Size (area) of circular pie charts and squares are proportional to largest estimated normalized load (for given constituent)

1Estimated flow-normalized load calculated by multiplying the mean annual fitted trend 

concentration (determined by using the time-series model) for the indicated period times the geometric mean streamflow for water years 1996–2010 and a units conversion factor. Calculation of the estimated normalized load is described in detail in the section of this report "Estimation of Normalized Constituent Loads."
Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin 45

16

EXPLANATION

[Water year is defined as the 12-month 

period from October 1 through 

September 30 and is designated by the 

year in which it ends. p-value, statistical 

probability level]

Reach inflow—Silver Bow 

Creek at Warm Springs 

(site 8, fig. 1, table 1)

Monitored tributary 

inflow—Warm Springs 

Creek at Warm Springs 

(site 10, fig. 1, table 1)

Reach outflow—Reach 

outflow Clark Fork near 

Galen (site 11, fig. 1, table 1)

Fitted trend value at start or 

end of period

Bold values indicate 

statistical significance 

(p-value less than 0.01) for 

period before value 

presented in bold

1

10

100

1

10

100

1

10

100

A. Unfiltered-recoverable copper

B. Unfiltered-recoverable arsenic

C. Suspended sediment

 Water year (October–September)

Flow-adjusted suspended-sediment

concentration, in milligrams per liter

Flow-adjusted unfiltered-recoverable copper

concentration, in micrograms per liter

Flow-adjusted ufniltered-recoverable arsenic

concentration, in micrograms per liter

1995 2000 2005 2010

1

Period

2 3

20 22 21 23

5.8

4.1

1.9

5.4

18

8.8

8.8

8.0

7.0

9.9

7.4 7.7

9.8

16 10 11

10

6.2 5.6 5.9 5.2

16

14 16

12

8.7

4.7

3.1

7.7

6.0

5.8 4.8

4.8

Figure 11. Flow-adjusted fitted trends determined by using the time-series model (TSM) for selected constituents for sites in reach 4, 

extending from Silver Bow Creek at Warm Springs (site 8, fig. 1, table 1) to Clark Fork near Galen (site 11, fig. 1, table 1), based on data 

collected during water years 1985–2010.
46 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

Figure 12. Drainage area, streamflow, and estimated normalized constituent loads contributed from reach inflow and within-reach sources for reach 4, extending from Silver Bow Creek at Warm Springs (site 8, fig. 1, table 1) to Clark Fork near Galen (site 11, fig. 1, table 1) for selected periods.

1.0

3.1

0.66

3.4

850780

19

740

430

2.0

0.78

1.3

0.74

1.1

473

163

48

62

3.3

0.67

0.89

1.0

0.29

0.23

0.12

260 460

610

290

0.70

15

3.3

0.68

651

113

3.6

2.9

2.9

4.1

4.2

3.9

Value

Value

1,600

1,500

1,300

Drainage area and

streamflow information

Drainage area,

in square miles

Unfiltered-recoverable arsenic Suspended sediment Unfiltered-recoverable copper

Value

Value

Drainage area, in squaremiles,or geometric meanstreamflow, in cubicfeet per secondDrainage area or geometric mean streamflow at reachoutflow [Clark Fork near Galen(site 11, fig. 1, table 1)]

Within-reach change in load afteraccounting formonitored tributary[net accumulation in reach channel when combined reach inflow (reach inflow plus monitoredtrbutary inflow)is greater than reach outflow]

EXPLANATION, estimated normalized load pie charts

EXPLANATION, drainage area and streamflow pie charts

Other within-reach sources (including groundwater inflow, unmonitored tributaries, and the main-stem channel and floodplain)

Reach inflow[Silver Bow Creek at Warm Springs(site 8, fig. 1, table 1)]

Reach inflow[Silver Bow Creek at Warm Springs(site 8, fig. 1, table 1)]

Circular pie charts represent loadstransported pastreach outflow[Clark Fork near Galen (site 11, fig. 1, table 1)]

Estimatedflow-normalizedload, in kilograms per day

Size (area) of circular pie charts and squares areproportional to largest estimated normalizedload (for given constituent)

Monitored tributary inflow within reach [Warm Springs Creek at Warm Springs (site 10, fig. 1, table 1)]

Monitored tributary inflow within reach [Warm Springs Creek at Warm Springs (site 10, fig. 1, table 1)] Load transported past reach outflow

[Clark Fork near Galen (site 11, fig. 1, table 1)]

Within-reach change in load after accounting for monitored tributary(net mobilization from other within-reach sources, including groundwater inflow, unmonitored tributaries and the main-stem channel and floodplain)

Geometric mean streamflow,

in cubic feet per second

(water years 1996–2010)

Estimated normalized load1, in kilograms per day

Water years1996–2000(period 1)Water years2001–05(period 2)Water years2006–10(period 3)

1Estimated normalized load calculated by multiplying the mean annual fitted trend 

concentration (determined by using the time-series model) for the indicated period times the geometric mean streamflow for water years 1996–2010 and a units conversion factor. Calculation of the estimated normalized load is described in detail in the section of this report "Estimation of Normalized Constituent Loads."
Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin 47

period 3. Overall, minor decreases are indicated for unfilteredrecoverable copper and arsenic, and suspended sediment from 

the start of period 1 to the end of period 3.

The TSM trend results for the reach 4 outflow (site 11) 

indicate minor to small decreases in geometric mean FACs 

of unfiltered-recoverable copper and arsenic, and suspended 

sediment from the start of period 1 to the end of period 3 

(figs. 11 and 4–11; tables 4 and 4–1). The decreases in FACs 

of unfiltered-recoverable copper and suspended sediment 

for the reach 4 outflow (site 11) are of lower magnitude than 

decreases for the reach 4 inflow (site 8). Trend results for 

unfiltered-recoverable copper for site 11 indicate a small 

decrease of 38 percent from 16 μg/L at the start of period 1 to 

10 μg/L at the end of period 3; whereas trend results for site 

8 indicate a large decrease of 61 percent. Trend results for 

suspended sediment for site 11 indicate a minor decrease of 

20 percent from 6.0 mg/L at the start of period 1 to 4.8 mg/L 

at the end of period 3; whereas trend results for site 8 indicate 

a large decrease of 65 percent. Trend results for unfilteredrecoverable arsenic for site 11 indicate a small decrease of 

25 percent from 16 μg/L at the start of period 1 to 12 μg/L at 

the end of period 3; whereas trend results for site 8 indicate a 

minor increase of 15 percent. 

Results of the transport analysis indicate how temporal changes in geometric mean FACs for sites 8, 10, and 11 

translate into temporal changes in relative contributions from 

upstream source areas to the reach 4 outflow loads (fig. 12, 

table 5–4). For unfiltered-recoverable copper, the reach 4 

outflow load (site 11) decreases by about 20 percent from 

period 1 (3.6 kg/d) to period 3 (2.9 kg/d), largely because of a 

decrease in the reach 4 inflow load (site 8). Contributions from 

Warm Springs Creek (site 10) and net mobilization from other 

sources within reach 4 have minor change during periods 1 

through 3. For all periods, contributions of unfilteredrecoverable copper from Warm Springs Creek (site 10) are 

proportionally smaller than streamflow contributed from 

Warm Springs Creek. For all periods, contributions of unfiltered-recoverable copper from reach 4 sources other than 

Warm Springs Creek (site 10) are proportionally larger than 

streamflow contributed from the other within-reach sources. 

For unfiltered-recoverable arsenic, reach 4 outflow decreases 

by about 6 percent from period 1 (4.1 kg/d) to period 3 

(3.9 kg/d). Net accumulation of arsenic in the reach 4 channel is indicated for period 3. Accumulation of arsenic in reach 

4 at near-median conditions at times is plausible. In reach 4, 

water from the Warm Springs Ponds (with high pH and arsenic 

concentration) mixes with water from the Mill-Willow bypass 

(with lower pH and higher arsenic concentrations) and also 

water from Warm Springs Creek (with lower pH), which 

might result in precipitation of arsenic. However, reach 4 

is short and nearly all inflows are accounted for (resulting 

in tight balance accounting) and small errors can result in 

uncertainty in distinguishing between net mobilization and net 

accumulation. Thus, indication of net accumulation of arsenic 

in reach 4 for period 3 is not confirmed, but cannot be discounted based on available data. Whether or not the indication 

of arsenic accumulation for period 3 is accurate, the transport 

analysis indicates that contributions of arsenic from reach 

4 sources other than Warm Springs Creek (site 10) are not 

substantial and the reach 4 outflow largely is sourced from the 

reach 4 inflow (site 8) with much less contribution from Warm 

Springs Creek (site 10). For suspended sediment, reach 4 outflow load (site 11) decreases by about 19 percent from period 

1 (1,600 kg/d) to period 3 (1,300 kg/d). Large variability in 

relative source-area contributions is indicated for suspended 

sediment. Overall, for periods 1 through 3, relative contributions of suspended-sediment from Warm Springs Creek (site 

10) generally are proportionally similar to or larger than the 

relative contribution of streamflow from Warm Springs Creek. 

For periods 2 and 3, contributions of suspended sediment from 

reach 4 sources other than Warm Springs Creek (site 10) are 

proportionally larger than streamflow contributed from the 

other within-reach sources.

Reach 5

Sites in reach 5 include the inflow [Clark Fork near Galen 

(site 11)], monitored tributary sites [Lost Creek near Anaconda 

(site 12) and Lost Creek near Galen (site 13)], and the outflow 

[Clark Fork at Deer Lodge (site 14)]. Trend results for the 

reach inflow and outflow sites (sites 11 and 14) were determined by using the TSM and are presented in tables 4 and 4–1, 

figure 13, and also figures 4–11 and 4–14, for sites 11 and 14, 

respectively. Trend results for the monitored tributary sites on 

Lost Creek [sites 12 and 13, which had short periods of data 

collection] were determined by using MLR. Transport analysis 

results for reach 5 are graphically presented in figure 14.

Detailed trend results for the monitored tributary sites 

on Lost Creek are presented in tables 6 and 4–3, and also 

figures 4–12 and 4–13, for sites 12 and 13, respectively. Fitted 

trends for the downstream site on Lost Creek (site 13) also are 

shown in figure 13, in reference to fitted trends for the mainstem Silver Bow Creek inflow site and Clark Fork outflow 

site that bound the reach (site 11 and site 14, respectively). 

Because MLR and the TSM use different flow-adjustment procedures, caution should be used in directly comparing results 

of the two methods in terms of the estimated fitted trend 

values presented in figure 13. However, magnitudes and directions of trends probably are reasonably comparable between 

the two methods within the short time frame of period 3. Trend 

results for the upstream Lost Creek site (site 12) indicate 

moderate to large decreases in geometric mean FACs of most 

trace-elements for period 3 (fig. 4–12, tables 6 and 4–3). Trend 

results for the downstream Lost Creek site (site 13) indicate 

decreases in some trace elements for period 3 (figs. 13 and 

4–13, tables 6 and 4–3). For example, unfiltered-recoverable 

copper results for period 3 for site 13 indicate a moderate 

decrease of 49 percent (from 5.1 to 2.6 μg/L). Minor increases 

are indicated for arsenic and suspended sediment for site 13.

The TSM trend results for the reach 5 outflow (site 14) 

indicate minor decreases in geometric mean FACs of unfil

tered-recoverable copper and suspended sediment from the 
48 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

A.

B.

C.

 Water year (October–September)

Flow-adjusted suspended-sediment

concentration, in milligrams per liter

Flow-adjusted unfiltered-recoverable copper

concentration, in micrograms per liter

Flow-adjusted ufniltered-recoverable arsenic

concentration, in micrograms per liter

Unfiltered-recoverable copper

Unfiltered-recoverable arsenic

Suspended sediment

EXPLANATION

[Water year is defined as the 12-month 

period from October 1 through 

September 30 and is designated by the 

year in which it ends. p-value, statistical 

probability level]

Reach inflow—Clark Fork 

near Galen (site 11, fig. 1, 

table 1). Fitted trend 

determined by using the 

time-series model

Monitored tributary 

inflow—Lost Creek near 

Galen (site 13, fig. 1, 

table 1). Fitted trend 

determined by using 

multiple linear regression

Reach outflow—Clark Fork 

at Deer Lodge (site 14, fig. 1, 

table 1). Fitted trend 

determined by using the 

time-series model

Fitted trend value at start or 

end of period

Bold values indicate 

statistical significance 

(p-value less than 0.01) for 

period before value 

presented in bold

1

10

100

1

10

100

1

10

100

5.1

2.6

2.6

16 10 11

10

26

23 22 23

14 15

16 14 16

12

14 14 15

14

11

13

6.0 5.8 4.8

4.8

17

14 13 13

1995 2000 2005 2010

1

Period

2 3

26

Figure 13. Flow-adjusted fitted trends determined by using the time-series model (TSM) and multiple linear regression on time, 

streamflow, and season (MLR) for selected constituents for selected sites in reach 5, extending from Clark Fork near Galen (site 11, fig. 1, 

table 1) to Clark Fork at Deer Lodge (site 14, fig. 1, table 1), based on data collected during water years 1985–2010.
Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin 49Figure 14. Drainage area, streamflow, and estimated normalized constituent loads contributed from reach inflow and within-reach sources for reach 5, extending from Clark Fork near Galen (site 11, fig. 1, table 1) to Clark Fork at Deer Lodge (site 14, fig.1, table 1) for selected periods. 

8.4 2.8

4.1

2.8

3.1

3.9

1,600

5,900

1,500

1,300

3.6

2.9

2.9

651

344

87

113

4.2

8.1

8.1

5,1005,000

12

1111

6.9

7.0

7.0

7,500

6,600

6,300

995200

Drainage area and

streamflow information

Drainage area,

in square miles

Unfiltered-recoverable arsenic Suspended sediment Unfiltered-recoverable copper

Estimated normalized load1, in kilograms per day

Geometric mean streamflow,

in cubic feet per second

(water years 1996–2010)

Water years1996–2000(period 1)Water years2001–05(period 2)Water years2006–10(period 3)

Value

Load transported pastreach outflow [Clark Fork at Deer Lodge(site 14, fig. 1, table 1)]

Value

Drainage area, in squaremiles,or geometric meanstreamflow, in cubicfeet per second

Drainage area orgeometric mean streamflowat reach outflow [Clark Fork at Deer Lodge(site 14, fig. 1, table 1)]

EXPLANATION, estimated normalized load pie charts

EXPLANATION, drainage area and streamflow pie charts

Value

Reach inflow[Clark Fork near Galen (site 11, fig. 1, table 1)]

Value

Circular pie charts represent loadstransported pastreach outflow[Clark Fork at Deer Lodge(site 14, fig. 1, table 1)]

Size (area) of circular pie charts are proportional to largest estimatednormalized load (for given constituent)

Estimated lownormalized load,in kilograms per day

Reach inflow[Clark Fork near Galen, (site 11, fig. 1, table 1)] All within-reach sources 

(including groundwater inflow, unmonitored tributaries, and the mainstem channel and floodplain) Total within-reach change in load (net mobilization from within-reach

sources, including groundwater inflow,unmonitored tributaries, and the mainstem channel and floodplain) 1Estimated normalized load calculated by multiplying the mean annual fitted trend concentration (determined by using the time-series model) for the indicated period 

times the geometric mean streamflow for water years 1996–2010 and a units conversion factor. Calculation of the estimated normalized load is described in detail in the section of this report "Estimation of Normalized Constituent Loads."
50 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

start of period 1 to the end of period 3 (figs. 11 and 4–11; 

tables 4 and 4–1) and no change in the FAC of unfilteredrecoverable arsenic from the start of period 1 to the end of 

period 3. Changes in FACs of unfiltered-recoverable copper 

and arsenic for the reach 5 outflow (site 14) are smaller than 

the decreases for the reach 5 inflow (site 11). Trend results 

for unfiltered-recoverable copper for site 14 indicate a minor 

decrease of about 12 percent from 26 μg/L at the start of 

period 1 to 23 μg/L at the end of period 3; whereas trend 

results for site 11 indicate a small decrease of 38 percent. 

Trend results for unfiltered-recoverable arsenic for site 14 

indicate no change in FAC from the start of period 1 to the 

end of period 3, with both values equal to 14 μg/L; whereas 

trend results for site 11 indicate a small decrease of about 

25 percent. The decreases in FACs of suspended sediment for 

the reach 5 outflow (site 14) are of similar magnitude to the 

decreases for the reach 5 inflow (site 11). Trend results for 

suspended sediment for site 14 indicate a small decrease of 

24 percent from 17 mg/L at the start of period 1 to 13 mg/L 

at the end of period 3 and trend results for site 11 indicate a 

similar decrease of about 20 percent.

Results of the transport analysis indicate how temporal 

changes in geometric mean FACs for sites 11 and 14 translate 

into temporal changes in relative contributions from upstream 

source areas to the reach 5 outflow loads (fig. 14, table 5–5). 

Data for the monitored tributary sites on Lost Creek were not 

included in the transport analysis because of factors that complicate directly combining the TSM and MLR results within 

a single analysis. The unquantified contributions from Lost 

Creek are part of the total undifferentiated contributions 

from within-reach sources. For unfiltered-recoverable copper, 

the reach 5 outflow load (site 14) decreases by about 8 percent from period 1 (12 kg/d) to period 3 (11 kg/d), largely 

because of a decrease in the reach 5 inflow load (site 11). Net 

mobilization from sources within reach 5 has minor change 

during periods 1 through 3. For all periods, contributions of 

unfiltered-recoverable copper from sources within reach 5 are 

proportionally larger than streamflow contributed from the 

within-reach sources. For unfiltered-recoverable arsenic, 

reach 5 outflow increases slightly from period 1 (6.9 kg/d) to 

period 3 (7.0 kg/d). For all periods, contributions of unfilteredrecoverable arsenic from sources within reach 5 are proportionally similar to or slightly larger than streamflow contributed from the within-reach sources. For suspended sediment, 

reach 5 outflow load (site 14) decreases by about 16 percent 

from period 1 (7,500 kg/d) to period 3 (6,300 kg/d). For all 

periods, contributions of suspended sediment from sources 

within reach 5 are proportionally larger than streamflow contributed from the within-reach sources.

Reach 6

Sites in reach 6 include the inflow [Clark Fork at Deer 

Lodge (site 14)], the monitored tributary site [Little Blackfoot 

River (site 15)], and the outflow [Clark Fork at Goldcreek 

(site 16)]. Trend results for all sites in reach 6 were determined 

by using the TSM and are presented in tables 4 and 4–1, figure 15, and also figures 4–14, 4–15, and 4–16, for sites 14, 15, 

and 16, respectively. Transport analysis results for reach 6 are 

graphically presented in figure 16.

Little Blackfoot River (site 15) had long-term waterquality data collection that was discontinued in water year 

2005. Available data for site 15 were analyzed by using the 

TSM, but no results are presented for period 3. Trend results 

for periods 1 and 2 for site 15 indicate minor decreases (within 

a small range at low concentrations) in geometric mean FACs 

of unfiltered-recoverable copper and arsenic, and suspended 

sediment.

The TSM trend results for the reach 6 outflow (site 16) 

indicate minor to small decreases in geometric mean FACs of 

unfiltered-recoverable copper and suspended sediment from 

the start of period 1 to the end of period 3 (figs. 15 and 4–16; 

tables 4 and 4–1) and no change in the FAC of unfilteredrecoverable arsenic from the start of period 1 to the end of 

period 3. The decrease in FAC of unfiltered-recoverable 

copper for the reach 6 outflow (site 16) is slightly larger than 

the decrease for the reach 6 inflow (site 14). Trend results 

for unfiltered-recoverable copper for site 16 indicate a small 

decrease of about 27 percent from 22 μg/L at the start of 

period 1 to 16 μg/L at the end of period 3, and trend results for 

site 14 indicate a minor decrease of 12 percent. The absence 

of change in FAC of unfiltered-recoverable arsenic is consistent between the reach 6 outflow (site 16) and the reach 

6 inflow (site 14). Trend results for unfiltered-recoverable 

arsenic for site 16 indicate no change in FAC from the start 

of period 1 to the end of period 3, with both values equal to 

11 μg/L. The decrease in FAC of suspended sediment for the 

reach 6 outflow (site 16) is smaller than the decrease for the 

reach 6 inflow (site 14). Trend results for suspended sediment for site 16 indicate a minor decrease of about 7 percent 

from 15 mg/L at the start of period 1 to 14 mg/L at the end of 

period 3, and trend results for site 14 indicate a small decrease 

of about 24 percent. Fitted trends for suspended sediment for 

site 16 show substantial variability between periods. Specific 

hydraulic and geomorphologic characteristics of reach 6 might 

contribute to complexity in suspended-sediment concentration and flow relations. Factors affecting the relations might 

include bedload and suspended-load relations, and sand-sized 

and fine-sized suspended-sediment transport relations. During 

extended wet or dry periods, variability in the suspendedsediment concentration and flow relations might happen over 

longer time scales than those accounted for in the TSM.

Results of the transport analysis indicate how temporal 

changes in geometric mean FACs for sites 14, 15, and 16 

translate into temporal changes in relative contributions from 

upstream source areas to the reach 6 outflow loads (fig. 16, 

table 5–6). Data for Little Blackfoot River (site 15) were 

included in transport analysis, but no data were available 

for period 3. For period 3, the unquantified contributions 

from Little Blackfoot River (site 15) are part of the total 

undifferentiated contributions from within-reach sources. 

For unfiltered-recoverable copper, the reach 6 outflow load 
Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin 51

EXPLANATION

[Water year is defined as the 12-month 

period from October 1 through 

September 30 and is designated by the 

year in which it ends. p-value, statistical 

probability level]

Reach inflow—Clark Fork at 

Deer Lodge (site 14, fig. 1, 

table 1)

Monitored tributary 

inflow—Little Blackfoot 

River (site 15, fig. 1, table 1)

Reach outflow—Clark Fork 

at Goldcreek (site 16, fig. 1, 

table 1)

Fitted trend value at start or 

end of period

Bold values indicate 

statistical significance 

(p-value less than 0.01) for 

period before value 

presented in bold

1

10

100

1

10

100

1

10

100

A.

B.

C.

 Water year (October–September)

Flow-adjusted suspended-sediment

concentration, in milligrams per liter

Flow-adjusted unfiltered-recoverable copper

concentration, in micrograms per liter

Flow-adjusted ufniltered-recoverable arsenic

concentration, in micrograms per liter

Suspended sediment

Unfiltered-recoverable arsenic

Unfiltered-recoverable copper

1.0

22

19

15 16

11

9.8

9.8

10 11

15

17

8.1

14

1.1 0.94

5.0 4.7

6.5

4.0

3.0

5.4

26

23 22 23

23

14 14 15 14

17

14

13

13

1995 2000 2005 2010

1

Period

2 3

Figure 15. Flow-adjusted fitted trends determined by using the time-series model (TSM) for selected constituents for sites in reach 6, 

extending from Clark Fork at Deer Lodge (site 14, fig. 1, table 1) to Clark Fork at Goldcreek (site 16, fig. 1, table 1), based on data collected 

during water years 1985–2010.
52 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

Figure 16. Drainage area, streamflow, and estimated normalized constituent loads contributed from reach inflow and within-reach for reach 6, extending from Clark Fork at Deer Lodge (site 14, fig. 1, table 1) to Clark Fork at Goldcreek (site 16, fig. 1, table 1) for selected periods. 

7.1 1.9

6.9

1.6

3.3

7.0

7,500

6,900

6,600

4,000

12

11

11

995

302

99

200

7.0

5.1

4.0

800

6,300

407 0.24

0.23

1.3

1.1

1,100

4,500

95

191615

109.7

10

16,00012,000

10,000

1,704

394

Drainage area and

streamflow information

Drainage area,

in square miles

Unfiltered-recoverable arsenic Suspended sediment Unfiltered-recoverable copper

Value

Value

Value

Drainage area, in squaremiles,or geometric meanstreamflow, in cubicfeet per second

EXPLANATION, estimated normalized load pie charts

EXPLANATION, drainage area and streamflow pie charts

Value

Monitored tributary inflow within reach [Little Blackfoot River (site 15, fig. 1, table 1)]

Monitored tributary inflow within reach [Little Blackfoot River (site 15, fig. 1, table 1)]

Load transported pastreach outflow [Clark Fork at Goldcreek (site 16, fig. 1, table 1)]

Drainage area orgeometric mean streamflowat reach outflow [Clark Fork at Goldcreek (site 16, fig. 1, table 1)]

Circular pie chart represent loadstransported pastreach outflow[Clark Fork at Goldcreek (site 16, fig. 1, table 1)]

Estimated flownormalized load, in kilograms per day Other within-reach sources

(including groundwater inflow, unmonitored tributaries, and the mainstem channel and floodplain)

Reach inflow [Clark Fork at Deer Lodge (site 14, fig. 1, table 1)]

Reach inflow [Clark Fork at Deer Lodge (site 14, fig. 1, table 1)]

Size (area) of circular pie charts are proportional to largest estimatednormalized load (for given constituent)

Within-reach change in load after accounting for monitored tributary(net mobilization from otherwithin-reach sources, including groundwater inflow,unmonitored tributaries, and the main-stem channel and floodplain)

Total within-reach change in load for period 3 when no dataavailable for site 15(net mobilization from all within-reach sources, including groundwaterinflow, unmonitored tributaries, and the main-stem channel and floodplain)

Geometric mean streamflow,

in cubic feet per second

(water years 1996–2010)

Estimated normalized load1, in kilograms per day

Water years1996–2000(period 1)Water years2001–05(period 2)Water years2006–10(period 23)

1Estimated normalized load calculated by multiplying the mean annual fitted trend 

concentration (determined by using the time-series model) for the indicated period times the geometric mean streamflow for water years 1996–2010 and a units conversion factor. Calculation of the estimated normalized load is described in detail in the section of this report "Estimation of Normalized Constituent Loads."

2No data available for site 15 for water years 2006–10 (period 3); thus loads from site 15 

are not shown and are included in net mobilization reach sources for period 3.
Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin 53

(site 16) decreases by about 21 percent from period 1 

(19 kg/d) to period 3 (15 kg/d). Contributions of copper from 

Little Blackfoot River (site 15) have minor change during 

periods 1 and 2, and the contributions are proportionally much 

smaller than streamflow contributed from Little Blackfoot 

River (site 15). Net mobilization from all sources within 

reach 6 (including Little Blackfoot River) decreases by about 

45 percent from period 1 (7.3 kg/d) to period 3 (4.0 kg/d). For 

all periods, contributions of copper from all sources within 

reach 6 (including Little Blackfoot River) are proportionally 

smaller than streamflow contributed from all of the withinreach sources. For unfiltered-recoverable arsenic, reach 6 outflow load (site 16) does not change from period 1 to period 3, 

with outflow loads for both periods equal to 10 kg/d. Contributions of arsenic from Little Blackfoot River (site 15) have 

minor change during periods 1 and 2, and the contributions 

are proportionally much smaller than streamflow contributed 

from Little Blackfoot River (site 15). Net mobilization from 

all sources within reach 6 (including Little Blackfoot River) 

slightly increases by about 3 percent from period 1 

(3.2 kg/d) to period 3 (3.3 kg/d). For all periods, contributions 

of unfiltered-recoverable arsenic from all sources within 

reach 6 (including Little Blackfoot River) are proportionally 

smaller than streamflow contributed from all of the withinreach sources. For suspended sediment, reach 6 outflow 

load (site 16) decreases by about 38 percent from period 1 

(16,000 kg/d) to period 3 (10,000 kg/d). Contributions of 

suspended sediment from Little Blackfoot River (site 15) have 

small change during periods 1 and 2, and the contributions 

are proportionally much smaller than streamflow contributed 

from Little Blackfoot River (site 15). Net mobilization from 

all sources within reach 6 (including Little Blackfoot River) 

decreases by about 50 percent from period 1 (8,000 kg/d) 

to period 3 (4,000 kg/d). For all periods, contributions of 

suspended sediment from all sources within reach 6 (including 

Little Blackfoot River) are proportionally smaller than streamflow contributed from all of the within-reach sources.

Reach 7

Sites in reach 7 include the inflow [Clark Fork at Goldcreek (site 16)], the monitored tributary site [Flint Creek 

(site 17)], and the outflow [Clark Fork near Drummond 

(site 18)]. Trend results for all sites in reach 7 were determined 

by using the TSM and are presented in tables 4 and 4–1, 

figure 17, and also figures 4–16, 4–17, and 4–17, for sites 16, 

17, and 18, respectively. Transport analysis results for reach 7 

are graphically presented in figure 18.

Flint Creek (site 17) had long-term water-quality data 

collection that was discontinued in water year 2005. Available 

data for site 17 were analyzed by using the TSM, but 

no results are presented for period 3. Trend results for periods 1 and 2 for site 17 indicate minor decreases (within a 

small range at low concentrations) in geometric mean FAC of 

unfiltered-recoverable copper. Trend results for periods 1 and 

2 for site 17 indicate small to moderate decreases in FACs of 

unfiltered-recoverable arsenic and suspended sediment.

The TSM trend results for the reach 7 outflow (site 18) 

indicate minor to moderate decreases in geometric mean FACs 

of unfiltered-recoverable copper and arsenic, and suspended 

sediment from the start of period 1 to the end of period 3 

(figs. 17 and 4–18; tables 4 and 4–1). The decrease in FAC of 

unfiltered-recoverable copper for the reach 7 outflow (site 18) 

is similar to the decrease for the reach 7 inflow (site 16). Trend 

results for unfiltered-recoverable copper for site 18 indicate a 

small decrease of about 32 percent from 19 μg/L at the start of 

period 1 to 13 μg/L at the end of period 3, and trend results for 

site 16 indicate a small decrease of 27 percent. The decrease in 

FAC of unfiltered-recoverable arsenic for the reach 7 outflow 

(site 18) differs slightly from trend results for the reach 7 

inflow (site 16). Trend results for unfiltered-recoverable arsenic for site 18 indicate a minor decrease of about 17 percent 

from 12 μg/L at the start of period 1 to 10 μg/L at the end of 

period 3, and trend results for site 16 indicate no change. The 

decrease in FAC of suspended sediment for the reach 7 outflow (site 18) is larger than the decrease for the reach 7 inflow 

(site 16). Trend results for suspended sediment for site 18 

indicate a moderate decrease of about 43 percent (mostly during period 1) from 21 mg/L at the start of period 1 to 12 mg/L 

at the end of period 3, and trend results for site 16 indicate a 

small decrease of 7 percent.

Results of the transport analysis indicate how temporal 

changes in geometric mean FACs for sites 16, 17, and 18 

translate into temporal changes in relative contributions from 

upstream source areas to the reach 7 outflow loads (fig. 18, 

table 5–7). Data for Flint Creek (site 17) were included in the 

transport analysis, but no data were available for period 3. For 

period 3, the unquantified contributions from Flint Creek 

(site 17) are part of the total undifferentiated contributions 

from within-reach sources. For unfiltered-recoverable copper, 

the reach 7 outflow load (site 18) decreases by about 

21 percent from period 1 (24 kg/d) to period 3 (18 kg/d). 

Contributions of copper from Flint Creek (site 17) have minor 

change during periods 1 and 2, and the contributions are proportionally much smaller than streamflow contributed 

from Flint Creek (site 17). Net mobilization from sources 

within reach 7 decreases by about 20 percent from period 1 

(4.8 kg/d) to period 3 (3.3 kg/d). For all periods, contributions of copper from sources within reach 7 are proportionally 

smaller than streamflow contributed from the within-reach 

sources. For unfiltered-recoverable arsenic, reach 7 outflow 

load (site 18) decreases by about 6 percent from period 1 

(16 kg/d) to period 3 (15 kg/d). Contributions of arsenic from 

Flint Creek (site 17) have a minor decrease during periods 1 

and 2, and the contributions are proportionally similar 

to streamflow contributed from Flint Creek (site 17). Net 

mobilization from sources within reach 7 decreases by about 

18 percent from period 1 (5.5 kg/d) to period 3 (4.5 kg/d). For 

all periods, contributions of unfiltered-recoverable arsenic 

from sources within reach 7 are proportionally similar to 

streamflow contributed from the within-reach sources. For 
54 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

1

10

100

1

10

100

1

10

100

Flow-adjusted suspended-sediment

concentration, in milligrams per liter

Flow-adjusted unfiltered-recoverable copper

concentration, in micrograms per liter

Flow-adjusted ufniltered-recoverable arsenic

concentration, in micrograms per liter

 Water year (October–September)

1995 2000 2005 2010

2.6 2.4

22

19

15 16

19

15

13 13

13

11

9.8 10

11 12 10 10

10

15

17

8.1

14

21

13

11

12

3.4

11

11

10

13

17

10

18

1

Period

2 3

A.Unfiltered-recoverable copper

B.Unfiltered-recoverable arsenic

C. Suspended sediment

EXPLANATION

[Water year is defined as the 12-month 

period from October 1 through 

September 30 and is designated by the 

year in which it ends. p-value, statistical 

probability level]

Reach inflow—Clark Fork at 

Goldcreek (site 16, fig. 1, 

table 1)

Monitored tributary 

inflow—Flint Creek (site 17, 

fig. 1, table 1)

Reach outflow—Clark Fork 

near Drummond (site 18, 

fig. 1, table 1)

Fitted trend value at start or 

end of period

Bold values indicate 

statistical significance 

(p-value less than 0.01) for 

period before value 

presented in bold

Figure 17. Flow-adjusted fitted trends determined by using the time-series model (TSM) for selected constituents for sites in 

reach 7, extending from Clark Fork at Goldcreek (site 16, fig. 1, table 1) to Clark Fork near Drummond (site 18, fig. 1, table 1), based on 

data collected during water years 1985–2010.
Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin 55 Figure 18. Drainage area, streamflow, and estimated normalized constituent loads contributed from reach inflow and within-reach for reach 7, extending from Clark Fork at Goldcreek (site 16, fig. 1, table 1) to Clark Fork near Drummond (site 18, fig. 1, table 1).

4.1

2.8

10

2.3

4.5

10

16,000

3,900

12,000

6,300

1916

15

1,704

307

87

394

9.7

2.5

3.3

2,100

10,000

490

0.57

2.7

2.4

4,100

3,000

95

0.69 24

1918

16

1515

24,000

17,00017,000

2,501

577

Drainage area and

streamflow information

Drainage area,

in square miles

Unfiltered-recoverable arsenic Suspended sediment Unfiltered-recoverable copper

Value

Value

Drainage area, in squaremiles,or geometric meanstreamflow, in cubicfeet per second

EXPLANATION, estimated normalized load pie charts

EXPLANATION, drainage area and streamflow pie charts

Value

Other within-reach sources(including groundwater inflow,umonitored tributaries, and the main-stem channel and floodplain)

Reach inflow[Clark Fork at Goldcreek(site 16, fig. 1, table 1)]

Reach inflow[Clark Fork at Goldcreek(site 16, fig. 1, table 1)]

Monitored tributary inflowwithin reach [Flint Creek near Drummond (site 17, fig. 1, table 1)]

Monitored tributary inflowwithin reach [Flint Creek near Drummond (site 17, fig. 1, table 1)]

Load transported pastreach outflow[Clark Fork near Drummond (site 18, fig. 1, table 1)]

Drainage area or geometric mean streamflow at reach outflow [Clark Fork near Drummond (site 18, fig. 1, table 1)]

Value

Circular pie charts represent loadstransported pastreach outflow[Clark Fork near Drummond (site 18, fig. 1, table 1)]

Estimatedflow-normalizedload, in kilograms per day

Size (area) of circular pie charts are proportional to largest estimatednormalized load (for given constituent)

Within-reach change in load after accounting for monitored tributary(net mobilization from otherwithin-reach sources, including groundwater inflow,unmonitored tributaries, and the main-stem channel and floodplain)

Total within-reach change in load for period 3 when no dataavailable for site 17(net mobilization from all within-reach sources, including groundwaterinflow, unmonitored tributaries, and the main-stem channel and floodplain)

Geometric mean streamflow,

in cubic feet per second

(water years 1996–2010)

Estimated normalized load1, in kilograms per day

Water years1996–2000

(period 1)Water years2001–05

(period 2)Water years2006–10

(period 23)

1Estimated normalized load calculated by multiplying the mean annual fitted trend 

concentration (determined by using the time-series model) for the indicated period times the geometric mean streamflow for water years 1996–2010 and a units conversion factor. Calculation of the estimated normalized load is described in detail in the section of this report "Estimation of Normalized Constituent Loads."

2No data available for site 17 for water years 2006–10 (period 3); thus loads from site 17 

are not shown and are included in net mobilization from within reach sources for period 3.
56 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

suspended sediment, reach 7 outflow load (site 18) decreases 

by about 29 percent from period 1 (24,000 kg/d) to period 3 

(17,000 kg/d). Contributions of suspended sediment from Flint 

Creek (site 17) have a small decrease during periods 1 and 2, 

and the contributions are proportionally similar to streamflow 

contributed from Flint Creek (site 17). Net mobilization from 

sources within reach 7 decreases by about 21 percent from 

period 1 (8,000 kg/d) to period 3 (6,300 kg/d). For all periods, 

contributions of suspended sediment from sources within 

reach 7 are proportionally similar to streamflow contributed 

from the within-reach sources.

Reach 8

Sites in reach 8 include the inflow [Clark Fork near 

Drummond (site 18)], the monitored tributary site [Rock 

Creek (site 19)], and the outflow [Clark Fork at Turah Bridge 

(site 20)]. Trend results for all sites in reach 8 were determined 

by using the TSM and are presented in tables 4 and 4–1, 

figure 19, and also figures 4–18, 4–19, and 4–20, for sites 18, 

19, and 20, respectively. Transport analysis results for reach 8 

are graphically presented in figure 20.

Rock Creek (site 19) had long-term water-quality data 

collection that was discontinued in water year 2005. Available data for Rock Creek (site 19) were analyzed by using 

the TSM, but no results are presented for period 3. Further, 

because trace-element concentrations for site 19 are low and 

typically less than LRLs, no results for copper or arsenic are 

presented. Trend results for periods 1 and 2 for suspended 

sediment for site 19 indicate an overall small decrease. However, fitted trends for suspended sediment for site 19 show 

substantial variability between periods.

The TSM trend results for the reach 8 outflow (site 

20) indicate minor increases in geometric mean FACs of 

unfiltered-recoverable copper and arsenic, and no change 

in FAC of suspended sediment from the start of period 1 to 

the end of period 3 (figs. 19 and 4–20; tables 4 and 4–1). 

The changes indicated for the reach 8 outflow (site 20) are 

somewhat in contrast to the changes indicated for the reach 

8 inflow (site 18). Trend results for unfiltered-recoverable 

copper for site 20 indicate a minor increase of about 1 percent 

from 9.4 μg/L at the start of period 1 to 9.5 μg/L at the end of 

period 3, and trend results for site 18 indicate a small decrease 

of 32 percent. Trend results for unfiltered-recoverable arsenic for site 20 indicate a minor increase of about 6 percent 

from 6.8 μg/L at the start of period 1 to 7.2 μg/L at the end of 

period 3, and trend results for site 18 indicate a minor decrease 

of 17 percent. Trend results for suspended sediment for site 

20 indicate no change in FAC from the start of period 1 to the 

end of period 3, with both values equal to 13 mg/L; whereas 

trend results for site 18 indicate a moderate decrease of about 

43 percent. Site 20 is the only main-stem Silver Bow Creek or 

Clark Fork site with increases of at least 20 percent indicated 

for unfiltered-recoverable copper and arsenic, and suspended 

sediment for period 3. A possible explanation for this pattern 

might relate to effects of the removal of the former Milltown 

Dam during period 3. Before the removal of the former Milltown Dam, during high-flow conditions, backwater effects of 

the dam might have extended far enough upstream to affect 

the hydraulic gradient at site 20 and also affect the transport 

of particulate materials from reach 8. After the removal of 

the former Milltown Dam, during high flow conditions, the 

hydraulic gradient at site 20 might have steepened and promoted transport of particulate materials from reach 8.

Results of the transport analysis indicate how temporal 

changes in geometric mean FACs for sites 18, 19, and 20 

translate into temporal changes in relative contributions from 

upstream source areas to the reach 8 outflow loads (fig. 20, 

table 5–8). Data for Rock Creek (site 19) were included 

in the transport analysis, but no data were available for 

period 3. Further, because trace-element concentrations for 

site 19 are low and typically less than LRLs, no results for 

copper or arsenic are presented. Thus, for copper and arsenic 

for all periods and for suspended sediment for period 3, the 

unquantified contributions from Rock Creek (site 19) are part 

of the total undifferentiated contributions from within-reach 

sources. For unfiltered-recoverable copper, the reach 8 outflow 

load (site 20) decreases by about 9 percent from period 1 

(23 kg/d) to period 3 (21 kg/d). Net accumulation of copper in 

the reach 8 channel is indicated for period 1, which is difficult 

to interpret. Concentration and streamflow relations within 

reach 8 are complex and affected by the mixing of dilute water 

from Rock Creek with main-stem Clark Fork and potential 

backwater effects of the former Milltown Dam, which was 

alternately present and absent during the period of trend 

analysis. Accurate definition of concentration and streamflow 

relations in reach 8 during period 1 also might have been 

affected by extremely high unfiltered-recoverable copper and 

suspended-sediment concentrations in water year 1996 (at the 

start of period 1) at site 20. Thus, indication of net accumulation of copper in reach 8 for period 1 is not confidently determined. Net mobilization of copper from sources within reach 8 

is indicated for periods 2 and 3, increasing from 1.1 kg/d in 

period 2 to 3.0 kg/d in period 3. The increase in net mobilization of copper from sources within reach 8 during period 3 

might have been affected by an increased hydraulic gradient 

(during high streamflow conditions) following the removal of 

Milltown Dam. For all periods, contributions of copper from 

sources within reach 8 are proportionally much smaller 

than streamflow contributed from the within-reach sources. 

For unfiltered-recoverable arsenic, reach 8 outflow load 

(site 20) decreases by about 6 percent from period 1 

(17 kg/d) to period 3 (16 kg/d). Net mobilization from sources 

within reach 8 increases by about 100 percent from period 1 

(0.89 kg/d) to period 3 (1.8 kg/d). For all periods, contributions of unfiltered-recoverable arsenic from sources within 

reach 8 are proportionally much smaller than streamflow 

contributed from the within-reach sources. For suspended 

sediment, reach 8 outflow load (site 20) decreases by about 

16 percent from period 1 (32,000 kg/d) to period 3 (27,000 

kg/d). Contributions of suspended sediment from Rock Creek 

(site 19) have a minor decrease during periods 1 and 2, and the 
Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin 57

Results for site 19 not reported because 

greater than 6 percent of values were affected by 

recensoring at study reporting level, as discussed 

in the section of this report "Time-Series Model."

Results for site 19 not reported because 

greater than 6 percent of values were affected by 

recensoring at study reporting level, as discussed 

in the section of this report "Time-Series Model."

 Water year (October–September)

1995 2000 2005 2010

EXPLANATION

[Water year is defined as the 12-month 

period from October 1 through 

September 30 and is designated by the 

year in which it ends. p-value, statistical 

probability level]

Reach inflow—Clark Fork near 

Drummond (site 18, fig. 1, 

table 1)

Monitored tributary 

inflow—Rock Creek (site 19, 

fig. 1, table 1)

Reach outflow—Clark Fork at 

Turah Bridge (site 20, fig. 1, 

table 1)

Fitted trend value at start or 

end of period

Bold values indicate 

statistical significance 

(p-value less than 0.01) for 

period before value 

presented in bold

1

10

100

1

10

100

1

10

100

A.

B.Unfiltered-recoverable arsenic

C. Suspended sediment

Flow-adjusted suspended-sediment

concentration, in milligrams per liter

Flow-adjusted unfiltered-recoverable copper

concentration, in micrograms per liter

Flow-adjusted ufniltered-recoverable arsenic

concentration, in micrograms per liter

Unfiltered-recoverable copper

19

15

13 13

9.4 8.9

7.4

9.5

9.5

10 10

6.3 5.9

7.2

21

13

13

11

12 13 12

8.4

13

6.8

12

7.1

4.7

10

3.2

1

Period

2 3

Figure 19. Flow-adjusted fitted trends determined by using the time-series model (TSM) for selected constituents for sites in 

reach 8, extending from Clark Fork near Drummond (site 18, fig. 1, table 1) to Clark Fork at Turah Bridge (site 20, fig. 1, table 1), based 

on data collected during water years 1985–2010.
58 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

Figure 20. Drainage area, streamflow, and estimated normalized constituent loads contributed from reach inflow and within-reach sources for reach 8, extending fromClark Fork near Drummond (site 18, fig.1, table 1) to Clark Fork at Turah Bridge (site 20, fig. 1, table 1) for selected periods.

Total within-reach change in load (net accumulation in reachchannel when reach inflow is greater than reach outflow

1.0

0.89

16

0.92

1.8

15

24,000

4,000

17,000

10,000

241918

2,501

255113

577

15

1.1

3.0

5,100

17,000

885

4,900

342

3,900

23

21

21

17

15

16

32,000

26,000

27,000

3,641

1,030

Drainage area and

streamflow information

Drainage area,

in square miles

Unfiltered-recoverable arsenic2 Suspended sediment Unfiltered-recoverable copper2

Value

Drainage area, in squaremiles,or geometric meanstreamflow, in cubicfeet per second

EXPLANATION, estimated normalized load pie charts

EXPLANATION, drainage area and streamflow pie charts

Value

Other within-reach sources(including groundwater inflow,unmonitored tributaries, and the mainstem channel)

Reach inflow [Clark Fork near Drummond(site 18, fig. 1, table 1)]

Drainage area orgeometric mean streamflowat reach outflow[Clark Fork at Turah Bridge (site 20, fig. 1, table 1)]

Monitored tributary inflow within reach [Rock Creek (site 19,fig. 1, table 1)]

Value

Value

Circular pie chart represent loadstransported pastreach outflow[Clark Fork at Goldcreek (site 16, fig. 1, table 1)]

Estimatedflow-normalizedload, in kilograms per day

Size (area) of circular pie charts are proportional to largest estimatedflow-normalizedload (for given constituent)

Within-reach change in load after accounting for monitored tributary(net mobilization from otherwithin-reach sources, including groundwater inflow,unmonitored tributaries, and the main-stem channel and floodplain)

Load transported pastreach outflow[Clark Fork at Turah Bridge (site 20, fig. 1, table 1)]

Reach inflow [Clark Fork near Drummond (site 18, fig. 1, table 1)]

Monitored tributary inflow within reach [Rock Creek (site 19,fig. 1, table 1)]

Total within-reach change in load when load data unavailable for site 19 (net mobilization from all within-reach sources, including groundwater inflow, unmonitored tributaries, and the main-stem channel and floodplain)

Geometric mean streamflow,

in cubic feet per second

(water years 1996–2010)

Estimated normalized load1, in kilograms per day

Water years1996–2000

(period 1)Water years2001–05

(period 2)Water years2006–10

(period 23)

1Estimated flow-normalized load calculated by multiplying the mean annual fitted trend 

concentration (determined by using the time-series model) for the indicated period times the geometric mean streamflow for water years 1996–2010 and a units conversion factor. Calculation of the estimated normalized load is described in detail in the section of this report "Estimation of Normalized Constituent Loads."

2Load results for Rock Creek (site 19) for copper and arsenic not reported because greater than 

6 percent of data values were affected by recensoring at study reporting level, as discussed in the section of this report "Time-Series Model." No suspended-sediment data available for site 19 for period 3; thus loads from site 19 of copper and arsenic for all periods and suspended sediment for period 3 are are not shown and contribute to within-reach change in load (net mobilization or net accumulation).
Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin 59

contributions are proportionally much smaller than streamflow 

contributed from Rock Creek (site 19). Net mobilization from 

sources within reach 8 increases by about 12 percent from 

period 1 (8,900 kg/d) to period 3 (10,000 kg/d). Contributions 

of suspended sediment from sources within reach 8 are proportionally smaller than or similar to streamflow contributed 

from the within-reach sources. The increases in net mobilization of copper, arsenic, and suspended sediment from sources 

within reach 8 between periods 2 and 3 are notable and might 

have been affected by an increased hydraulic gradient (during 

high streamflow conditions) following the removal of Milltown Dam.

Reach 9

Sites in reach 9 include the inflow [Clark Fork at Turah 

Bridge (site 20)], the monitored tributary site [Blackfoot River 

(site 21)], and the outflow [Clark Fork above Missoula 

(site 22)]. Trend results for all sites in reach 9 were determined 

by using the TSM and are presented in tables 4, 5, and 4–1, 

figure 21, and also figures 4–20, 4–21, and 4–22, for sites 20, 

21, and 22, respectively. Transport analysis results for reach 9 

are graphically presented in figure 22.

Trace-element concentrations for Blackfoot River 

(site 21) are low and typically less than LRLs; thus, no results 

for copper or arsenic are presented. Trend results for site 21 

during periods 1 through 3 indicate a minor decrease (within a 

small range at low concentrations) in geometric mean FAC of 

suspended sediment.

The TSM trend results for the reach 9 outflow (site 22) 

indicate minor to small increases in geometric mean FACs of 

unfiltered-recoverable copper and arsenic, and suspended 

sediment from the start of period 1 to the end of period 3 

(figs. 21 and 4–22; tables 5 and 4–2). Minor decreases in 

FACs of unfiltered-recoverable copper and arsenic, and suspended sediment are indicated for period 1. Minor to moderate increases in FACs of unfiltered-recoverable copper and 

arsenic, and suspended sediment are indicated for period 2. 

The increases in FACs during period 2 might be associated 

with preliminary remediation activities (including temporary 

reservoir drawdowns; Sando and Lambing, 2011) in preparation for breaching and subsequent removal of the former 

Milltown Dam. However, the period 2 increases in FACs 

also might be affected by smoothing procedures of the TSM 

that force connectivity of the fitted trend for period 2 with 

elevated FACs for water 2006 that probably were associated with preliminary drawdown for breaching of the former 

Milltown Dam. Because of the substantial effect of the breach 

of Milltown Dam on March 28, 2008, for site 22, period 3 was 

subdivided into period 3A (October 1, 2005–March 27, 2008) 

and period 3B (March 28, 2008–September 30, 2010). A large 

increase in FACs of unfiltered-recoverable copper is indicated 

for period 3A (associated with the breaching of the former 

Milltown Dam), from 7.0 μg/L at the start of water year 2006 

to 14 μg/L at March 28, 2008. After the breach, a large 

decrease is indicated for period 3B from 14 to 5.9 μg/L at the 

end of water year 2010. For unfiltered-recoverable arsenic, a 

small increase is indicated for period 3A, from 4.1 μg/L at the 

start of water year 2006 to 5.1 μg/L at March 28, 2008. After 

the breach a minor decrease is indicated for period 3B from 

5.1 to 5.0 μg/L at the end of water year 2010. For suspended 

sediment, a large increase is indicated for period 3A, from 9.4 

mg/L at the start of water year 2006 to 23 mg/L at March 28, 

2008, with a large decrease to 9.3 mg/L at the end of water 

year 2010. It is notable that for unfiltered-recoverable copper 

and suspended sediment, FACs at the end of water year 2010 

have only a minor increase relative to FACs at the start of 

water year 1996, which indicates that the sediment and associated metallic contaminants that had accumulated in the former 

Milltown Reservoir were quickly flushed past the reach 9 outflow (site 22) following the breach of Milltown Dam on March 

28, 2008. However, for unfiltered-recoverable arsenic, FAC 

at the end of water year 2010 (5.0 μg/L) has a small increase 

relative to FAC at the start of water year 1996 (3.8 μg/L). The 

small increase in FAC of unfiltered-recoverable arsenic might 

relate to increased contribution from groundwater inflow with 

elevated arsenic concentrations.

Results of the transport analysis indicate how temporal 

changes in geometric mean FACs for sites 20, 21, and 22 

translate into temporal changes in relative contributions from 

upstream source areas to the reach 9 outflow loads (fig. 22, 

table 5–9). Because trace-element concentrations for site 21 

are low and typically less than LRLs, no results for copper 

or arsenic are presented. Thus, for copper and arsenic for all 

periods, the unquantified contributions from Blackfoot River 

(site 21) are part of the total undifferentiated contributions 

from within-reach sources. For unfiltered-recoverable copper, 

the reach 9 outflow load (site 22) increases by about 100 percent from period 1 (25 kg/d) to period 3 (50 kg/d). Net mobilization of copper from sources with reach 9 is indicated for 

all periods, increasing from 2.4 kg/d in period 1 to 28 kg/d in 

period 3. The increase in net mobilization of copper (and also 

arsenic and suspended sediment) from sources within reach 9 

during period 3 indicates effects of the removal of the former 

Milltown Dam. For periods 1 and 2, contributions of copper 

from all sources within reach 9 are proportionally smaller than 

streamflow contributed from the within-reach sources. For 

period 3, contributions of copper from all sources within 

reach 9 are proportionally larger than streamflow contributed 

from the within-reach sources. For unfiltered-recoverable 

arsenic, reach 9 outflow load (site 22) increases by about 

33 percent from period 1 (18 kg/d) to period 3 (24 kg/d). Net 

mobilization of arsenic from sources within reach 9 is indicated for all periods, increasing from 1.9 kg/d in period 1 to 

7.4 kg/d in period 3. For all periods, contributions of unfiltered-recoverable arsenic from all sources within reach 9 are 

proportionally smaller than streamflow contributed from the 

within-reach sources. For suspended sediment, reach 9 outflow 

load (site 22) increases by about 97 percent from period 1 

(39,000 kg/d) to period 3 (77,000 kg/d). Contributions of suspended sediment from Blackfoot River (site 21) have a minor 

decrease during periods 1 through 3, and the contributions are 
60 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

A B

Results for site 21 not reported because 

greater than 6 percent of values were affected by 

recensoring at study reporting level, as discussed 

in the section of this report "Time-Series Model."

 Water year (October–September)

1995 2000 2005 2010

EXPLANATION

[Water year is defined as the 12-month 

period from October 1 through 

September 30 and is designated by the 

year in which it ends. p-value, statistical 

probability level]

Reach inflow—Clark Fork at 

Turah Bridge site 20, fig. 1, 

table 1)

Monitored tributary 

inflow—Blackfoot River 

(site 21, fig. 1, table 1)

Reach outflow—Clark Fork 

above Missoula (site 22, fig. 1, 

table 1)

Fitted trend value at start or 

end of period

Bold values indicate 

statistical significance 

(p-value less than 0.01) for 

period before value 

presented in bold

1

10

100

1

10

100

1

10

100

Results for site 21 not reported because 

greater than 6 percent of values were affected by 

recensoring at study reporting level, as discussed 

in the section of this report "Time-Series Model."

5.6

4.6

7.0

5.9

5.9

3.8 3.6

4.1

5.0

3.9

4.4

8.5

7.2

9.4

9.3

4.0

3.1

23

5.1

14

9.4 8.9

7.4

9.5

9.5

6.3 5.9

7.2

13 12

8.4

13

6.8

A.

B.

C.

Flow-adjusted suspended-sediment

concentration, in milligrams per liter

Flow-adjusted unfiltered-recoverable copper

concentration, in micrograms per liter

Flow-adjusted ufniltered-recoverable arsenic

concentration, in micrograms per liter

1

Period

2 3

Unfiltered-recoverable copper

Unfiltered-recoverable arsenic

Suspended sediment

Figure 21. Fitted trends determined by using the time-series model (TSM) for selected constituents for sites in reach 9, extending from 

Clark Fork at Turah Bridge (site 20, fig. 1, table 1) to Clark Fork above Missoula (site 22, fig. 1, table 1), based on data collected during 

water years 1985–2010.
Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin 61Figure 22. Drainage area, streamflow, and estimated normalized constituent loads contributed from reach inflow and within-reach sources for reach 9, extending from Clark Fork at Turah Bridge (site 20, fig. 1, table 1) to Clark Fork above Missoula (site 22, fig. 1, table 1) for selected periods.

2.4 1.9

17

3.6

7.4

16

32,000

9,900

26,000

42,000

2321

21

3,641 68

26

1,030

15

8.0

28

5,600

27,000

2,290

3,300

972

9,400

8,400

2529

50

1819

24

39,000

41,000

77,000

5,999

2,030

Drainage area and

streamflow information

Drainage area,

in square miles

Suspended sediment Unfiltered-recoverable arsenic2 Unfiltered-recoverable copper2

Geometric mean streamflow,

in cubic feet per second

(water years 1996–2010)

Estimated normalized load1, in kilograms per day

Water years1996–2000

(period 1)Water years2001–05

(period 2)Water years2006–10

(period 3)

1Estimated flow-normalized load calculated by multiplying the mean annual fitted trend concentration 

(determined by using the time-series model) for the indicated period times the geometric mean streamflow for water years 1996–2010 and a units conversion factor. Calculation of the estimated normalized load is described in detail in the section of this report "Estimation of Normalized Constituent Loads."

2Load results for Blackfoot River (site 21) for copper and arsenic not reported because greater than 

6 percent of data values were affected by recensoring at study reporting level, as discussed in the section of this report "Time-Series Model." Thus loads from site 21 of copper and arsenic for all periods and suspended sediment for period 3 are are not shown and are included in net mobilization from within-reach sources.

Value

Drainage area, in squaremiles,or geometric meanstreamflow, in cubicfeet per second

EXPLANATION, drainage area and streamflow pie charts

Value

Reach inflow [Clark Fork at Turah Bridge (site 20, fig. 1,table 1)]

Drainage area or geometric mean streamflow at reach outflow[Clark Fork above Missoula (site 22, fig. 1, table 1)

EXPLANATION, estimated normalized load pie charts

Other within-reach sources(including groundwater inflow,unmonitored tributaries, and the main-stem channel)

Monitored tributary inflow within reach [Blackfoot River (site 21,fig. 1, table 1)]

Value

Value

Load transported pastreach outflow[Clark Fork above Missoula (site 22, fig. 1, table 1)

Circular pie charts represent loadstransported pastreach outflow[Clark Fork above Missoula (site 22, fig. 1, table 1)

Monitored tributary inflow within reach [Blackfoot River (site 21,fig. 1, table 1)]

Reach inflow [Clark Fork at TurahBridge (site 20, fig. 1, table 1)]

Estimatedflow-normalizedload, in kilograms per day

Size (area) of circular pie charts are proportional to largest estimatednormalizedload (for given constituent)

Total within-reach change in load when load data unavailable for site 21 (net mobilization from all within-reach sources, including groundwater inflow, unmonitored tributaries and the main-stem channel and floodplain)

Within-reach change in load after accounting for monitored tributary(net accumulation in reach channel when reach inflow is greater than reach outflow

Within-reach change in load after accounting for monitored tributary (net mobilization from other within-reach sources, including groundwater inflow, unmonitored tributaries, and the main-stem channel and floodplain)
62 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

proportionally much smaller than streamflow contributed from 

Blackfoot River (site 21). Net accumulation of suspended sediment within reach 9 is indicated for period 1, during which the 

former Milltown Dam was in place. Indication of accumulation of suspended sediment within reach 9 for period 1 but net 

mobilization of copper from reach 9 for period 1 is difficult to 

interpret but might be plausible and affected by typical metals 

and suspended-sediment relations. Metals tend to have higher 

solid-phase concentrations in fine-grained suspended sediment 

than in coarse-grained suspended sediment (sand; Horowitz, 

1991). In relation to the entire suspended-sediment load (that 

is, the combined fine-grained and coarse-grained fractions), 

during high streamflow conditions, fine-grained suspended 

sediment (with higher solid-phase metal concentrations) might 

be disproportionately transported through a reservoir, whereas 

coarse-grained suspended sediment (sand) might be disproportionately deposited in the reservoir. Thus, the transport 

analysis results for reach 9 for period 1 might be affected by 

the high-flow conditions during the period and especially by 

the unusually high-flow conditions of water years 1996–97 

early in period 1. For periods 2 and 3, net mobilization of 

suspended sediment from sources within reach 9 increased 

from 5,600 kg/d in period 2 to 42,000 kg/d in period 3. For 

periods 2 and 3, contributions of suspended sediment from 

sources within reach 9 other than the Blackfoot River (site 21) 

are proportionally larger than streamflow contributed from the 

other within-reach sources.

Overview of Water-Quality Trend Results for 

Data-Summary Reaches

This section of the report summarizes trend and transport 

analysis results for the data-summary reaches. In the first part 

of the section, transport analysis results are concisely summarized with respect to evaluation of within-reach contributions of constituents relative to within-reach contributions of 

streamflow. In the second part of the section, an overview of 

temporal changes in FACs and estimated normalized loads is 

provided, with general discussion of processes and transport 

characteristics that might contribute to the observed changes.

Summary of Estimated Normalized Loads and WithinReach Contributions of Constituents and Streamflow

Pie charts illustrating temporal patterns in estimated 

normalized loads for all data-summary reaches are presented 

in figures 23 through 25 for unfiltered-recoverable copper, 

unfiltered-recoverable arsenic, and suspended sediment, 

respectively. The estimated normalized loads do not represent 

actual magnitudes of total mass transport, but rather provide 

information on relative temporal changes in constituent transport characteristics in the upper Clark Fork Basin quantified 

with respect to near-median conditions. In figures 23 through 

25, geometric mean streamflow (water years 1996–2010) is 

shown at the top of each figure, with the size of each pie chart 

being proportional to the geometric mean streamflow for Clark 

Fork above Missoula (site 22; the reach 9 outflow). Pie charts 

that illustrate transport analysis results for each data-summary 

reach for periods 1 through 3 are shown below the pie charts 

showing geometric mean streamflow. Unlike the pie charts 

presented in the previous sections of this report (figures 6, 8, 

10, 12, 14, 16, 18, 20, and 22), the simplified constituent-load 

pie charts in figures 23 through 25 show only the contribution at reach inflow and the total within-reach change in load 

(reflecting either net accumulation in the main-stem channel 

or net mobilization from all within-reach sources). Thus, for 

reaches with monitored tributaries, the contributions from the 

monitored tributaries are considered part of the total undifferentiated contributions from within-reach sources. Sizes of pie 

charts illustrating estimated normalized constituent loads are 

sized proportionally to the period 1 reach 9 outflow. Results 

for reach 9 are not shown for periods 2 and 3 because of 

effects on constituent loads of remediation activities associated with the breach and subsequent removal of the former 

Milltown Dam and difficulties in presenting those results in 

conjunction with results for other reaches. The pie charts in 

figures 23 through 25 provide a side-by-side graphical summary for evaluation of spatial and temporal variability in 

constituent transport relative to streamflow contributions in the 

upper Clark Fork Basin.

For unfiltered-recoverable copper (fig. 23), contributions from sources within reach 1 are proportionally much 

larger than within-reach contributions of streamflow. A large 

decrease in reach 1 outflow is indicated for periods 1 through 

3. Contributions of unfiltered-recoverable copper from sources 

within reach 2 are proportionally much larger than withinreach contributions of streamflow. A moderate decrease in 

reach 2 outflow is indicated for periods 1 through 3, because 

of combined effects of a large decrease in unfiltered-recoverable copper load transported from reach 1 and a small 

decrease in contributions from sources within reach 2. Within 

reach 3, most of the unfiltered-recoverable copper load 

transported from reach 2 is deposited in Warm Springs Ponds. 

A moderate decrease in both the estimated load of unfilteredrecoverable copper deposited in Warm Springs Ponds and the 

reach 3 outflow copper load is indicated for periods 1 through 

3. Contributions of unfiltered-recoverable copper from sources 

within reach 4 are proportionally larger than within-reach contributions of streamflow. A small decrease in reach 4 outflow is 

indicated for periods 1 through 3, because of combined effects 

of a moderate decrease in load transported from reach 3 and a 

minor increase in contributions from sources within reach 4. 

Contributions of unfiltered-recoverable copper from sources 

within reach 5 are proportionally much larger than withinreach contributions of streamflow. Minor changes in 

reach 5 outflow or contributions from sources within reach 5 

are indicated for periods 1 through 3. Contributions of unfiltered-recoverable copper from sources within reach 6 are proportionally smaller than within-reach contributions of streamflow. A small decrease in reach 6 outflow is indicated for 

periods 1 through 3, because of combined effects of a minor 

decrease in load transported from reach 5 and a moderate 
Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin 63Figure 23. Streamflow, and estimated normalized unfiltered-recoverable copper loads contributed from reach inflow and within-reach sources for data-summary reaches for 

selected periods.

Reach 1 Reach 2 Reach 3 Reach 4 Reach 5 Reach 6 Reach 7 Reach 8 Reach 9

25 23 24 19

12

3.6

2.0 12 5.0

21 19 16

11

2.9

1.3 10

1.9

21 18 15 11

2.9 1.1 6.5

1.1

Geometric mean streamflow, in cubic feet per second (water years 1996–2010)

Value

EXPLANATION, estimated normalized load pie charts EXPLANATION, streamflow pie charts

Value

Reach inflow

For reach 3, all of the reach inflow is presumed to be deposited in WSP

For reach 8, all of the reach inflow is presumed to contribute to the reach outflow 

Squares indicate net accumulation in reach channel 

when reach inflow is greater than reach outflow. 

Total within-reach change in load [net accumulation in reach channel ]

Reach outflow (net mobilization from all within-reach sources, including groundwater inflow, the MWB, the outflow from WSP, and the main-stem channel and floodplain downstream from WSP)

1Estimated normalized load calculated by multiplying 

the mean annual fitted trend concentration (determined by using the time-series model) for the indicated period times the geometric mean streamflow for water years 1996–2010 and a units conversion factor. Calculation of the estimated normalized constituent load is described in detail in the section of this report "Estimation of Normalized Constituent Loads."

Estimated normalized unfiltered-recoverable copper load1, in kilograms per day

Water years1996–2000

(period 1)Water years2001–05

(period 2)Water years2006–10

(period 3)

Load transported past reach outflow, in kilograms per day

All within-reachsources

Total within-reach change in load(net mobilizationfrom all withinreach sources)

Reach inflow Reach inflow

Geometric mean streamflowat reach outflow

Circular pie charts represent loadstransported pastreach outflow Size (area) of circular pie charts and squares are proportional to period 1 estimatednormalized load for Clark Fork at Missoula (site 22) 

Circular pie chart represents geometricmean streamflowat reach outflow Size (area) of circular pie charts are proportional to geometric meanstreamflow for Clark Fork at Missoula (site 22)

2,030

1,030

577 394

200 113 62 40 21

Data for reach 9 for periods4 and 5 not shown because of the large effect of activities associated with the breach of the former Milltown Dam on constituent loads and difficulties in presenting those results in conjunction with results for other reaches.
64 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

Figure 24. Streamflow, and estimated normalized unfiltered-recoverable arsenic loads contributed from reach inflow and within-reach sources for data-summary reaches for selected periods.

Reach 1 Reach 2 Reach 3 Reach 4 Reach 5 Reach 6 Reach 7 Reach 8 Reach 9

18 17 16

10

6.9 4.1 3.1

1.6 0.59

15 15 9.7 7.0

4.2 3.3 1.7

0.39

16 15 10

7.0

3.9 3.4

1.3

0.30

Estimated normalized unfiltered-recoverable arsenic load1, in kilograms per day

Data for reach 9 for periods4 and 5 not shown because of the large effect of activities associated with the breach of the former Milltown Dam on constituent loads and difficulties in presenting those results in conjunction with results for other reaches.

EXPLANATION, streamflow pie charts

Value Value

EXPLANATION, estimated normalized load pie charts

Load transported past reach outflow, in kilograms per day

All within-reachsources

Reach inflow

Reach inflow

Geometric mean streamflowat reach outflow

Circular pie charts represent geometricmean streamflowat reach outflow Size (area) of circular pie charts are proportional to geometric meanstreamflow for Clark Fork at Missoula (site 22)

Circular pie charts represent loadstransported pastreach outflow Size (area) of circular pie charts and squares are proportional to period 3 estimatednormalized load for Clark Fork at Missoula(site 22) 

Total within-reach change in load(net mobilizationfrom all withinreach sources)

1Estimated flow-normalized load calculated by multiplying 

the mean annual fitted trend concentration (determined by using the time-series model) for the indicated period times the geometric mean streamflow for water years 1996–2010 and a units conversion factor. Calculation of the estimatednormalized constituent load is described in detail in the section of this report "Estimation of Normalized Constituent Loads."

Geometric mean streamflow, in cubic feet per second (water years 1996–2010)

Water years1996–2000

(period 3)Water years2001–05

(period 4)Water years2006–10

(period 5)

2,030

1,030

577

394 200 113 62 40 21
Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin 65Figure 25. Streamflow, and estimated normalized suspended-sediment loads contributed from reach inflow and within-reach sources for data-summary reaches for selected periods.

Reach 1 Reach 2 Reach 3 Reach 4 Reach 5 Reach 6 Reach 7 Reach 8 Reach 9

39,000 32,000 24,000 16,000

7,500

1,600 850 2,000 740

26,000 17,000 12,000 6,600

1,500 740 1,900 530

27,000

17,000

10,000 6,300

1,300 430 2,000 380

Estimated normalized suspended-sediment load1, in kilograms per day

Squares indicate net accumulation in reach channel when reach inflow is greater than reach outflow. 

For reach 3, all 

of the reach inflow 

is presumed to be 

deposited in WSP

Data for reach 9 for periods4 and 5 not shown because of the large effect of the removal of the former Milltown Dam on constituent loads and difficulties in presenting those results in conjunction with results for other reaches.

EXPLANATION, streamflow pie charts

Value

Geometric mean streamflowat reach outflow

Value

EXPLANATION, estimated normalized load pie charts

Geometric mean streamflow, in cubic feet per second (water years 1996–2010)

Load transported past reach outflow, in kilograms per day

All within-reachsources

Reach inflow Reach inflow

Reach outflow(net mobilization from all within-reach sources, including groundwaterinflow, the MWB, the outflow from WSP, and the main-stem channel and floodplain downstream from WSP)

Circular pie charts represent geometricmean streamflowat reach outflow Size (area) of circular pie charts are proportionalto geometric meanstreamflow for Clark Fork at Missoula(site 22)

Circular pie charts represent loadstransported pastreach outflow Size (area) of circular pie charts and squares are proportionalto period 1 estimatednormalizedload for Clark Fork at Missoula(site 22) 

Total within-reach change in load(net mobilizationfrom all withinreach sources)

Reach inflow

2,030

1,030

577 394

200 113 62 40 21

Water years1996–2000

(period 1)Water years2001–05

(period 2)Water years2006–10

(period 3)

1Estimated normalized load calculated by multiplying the 

mean annual fitted trend concentration (determined by using the time-series model) for the indicated period times the geometric mean streamflow for water years 1996–2010 and a units conversion factor. Calculation of the estimated normalized constituent load is described in detail in the section of this report "Estimation of Normalized Constituent Loads."
66 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

decrease in contributions from sources within reach 6. Contributions of unfiltered-recoverable copper from sources within 

reach 7 are proportionally smaller than within-reach contributions of streamflow. A small decrease in reach 7 outflow is 

indicated for periods 1 through 3, because of combined effects 

of a small decrease in load transported from reach 6 and a 

small decrease in contributions from sources within reach 7. 

Contributions of unfiltered-recoverable copper from sources 

within reach 8 are proportionally smaller than within-reach 

contributions of streamflow. Accumulation of unfiltered-recoverable copper in the reach 8 main-stem channel for period 1 

is difficult to interpret and is not confidently determined. A 

minor decrease in reach 8 outflow is indicated for periods 1 

through 3, because of combined effects of a small decrease in 

load transported from reach 7 and a large increase (primarily 

during period 3) in contributions from sources within reach 8. 

Contributions of unfiltered-recoverable copper from sources 

within reach 9 for period 1 are proportionally smaller 

than within-reach contributions of streamflow. Results for 

periods 2 and 3 for reach 9 are not presented in figure 23 

because of the large effect of the removal of the former Milltown Dam on constituent loads and difficulties in presenting 

those results in conjunction with results for other reaches.

For unfiltered-recoverable arsenic (fig. 24), contributions 

from sources within reach 1 are proportionally larger than 

within-reach contribution of geometric mean streamflow; however, this pattern is much weaker for unfiltered-recoverable 

arsenic than for unfiltered-recoverable copper. A moderate 

decrease in reach 1 outflow is indicated for periods 1 through 

3, because of a large decrease in contributions from sources 

within reach 1. Contributions of unfiltered-recoverable arsenic 

from sources within reach 2 also are proportionally larger than 

within-reach contributions of streamflow and, similar to 

reach 1, the pattern is weaker for unfiltered-recoverable arsenic than for unfiltered-recoverable copper. A minor decrease in 

reach 2 outflow is indicated for periods 1 through 3, because 

of combined effects of a small decrease in load transported 

from reach 1 and no change in contributions from sources 

within reach 2. Contributions of unfiltered-recoverable arsenic 

from sources within reach 3 are proportionally larger than 

within-reach contributions of streamflow. A minor increase in 

reach 3 outflow is indicated for periods 1 through 3, because 

of combined effects of a small decrease in load transported 

from reach 2 and a small increase in contributions from 

sources within reach 3. Contributions of unfiltered-recoverable 

arsenic from sources within reach 4 are proportionally smaller 

than within-reach contributions of streamflow. A minor 

decrease in reach 4 outflow is indicated for periods 1 through 

3, because of combined effects of a minor increase in load 

transported from reach 3 and a moderate decrease in contributions from sources within reach 4. Contributions of unfilteredrecoverable arsenic from sources within reach 5 are proportionally similar to within-reach contributions of streamflow. 

A minor increase in reach 5 outflow and contributions from 

sources within reach 5 is indicated for periods 1 through 3. 

Contributions of unfiltered-recoverable arsenic from sources 

within reach 6 are proportionally smaller than within-reach 

contributions of streamflow. Minor changes in reach 6 outflow 

or contributions from sources within reach 6 are indicated for 

periods 1 through 3. Contributions of unfiltered-recoverable 

arsenic from sources within reach 7 are proportionally similar 

to within-reach contributions of streamflow. Minor changes in 

reach 7 outflow or contributions from sources within reach 7 is 

indicated for periods 1 through 3. Contributions of unfilteredrecoverable arsenic from sources within reach 8 are proportionally smaller than within-reach contributions of streamflow. 

A minor decrease in reach 8 outflow is indicated for periods 1 

through 3, because of combined effects of a minor decrease in 

load transported from reach 7 and a large increase (primarily 

during period 3) in contributions from sources within reach 8. 

Contributions of unfiltered-recoverable arsenic from sources 

within reach 9 for period 1 are proportionally smaller 

than within-reach contributions of streamflow. Results for 

periods 2 and 3 for reach 9 are not presented in figure 24 

because of the large effect of the removal of the former Milltown Dam on constituent loads and difficulties in presenting 

those results in conjunction with results for other reaches.

For suspended sediment (fig. 25), contributions from 

sources within reach 1 are proportionally larger than withinreach contributions of streamflow; however, this pattern is 

much weaker for suspended sediment than for unfilteredrecoverable copper. A moderate decrease in reach 1 outflow is 

indicated for periods 1 through 3. Contributions of suspended 

sediment from sources within reach 2 also are proportionally larger than within-reach contributions of streamflow 

and, similar to reach 1, the pattern is weaker for suspended 

sediment than for unfiltered-recoverable copper. No change in 

reach 2 outflow is indicated for periods 1 through 3, because 

of combined effects of a moderate decrease in load transported from reach 2 and a small increase in contributions 

from sources within reach 2. Similar to unfiltered-recoverable 

copper, within reach 3 most of the suspended-sediment load 

transported from reach 2 is deposited in Warm Springs Ponds. 

No change in the estimated load of suspended sediment deposited in Warm Springs Ponds is indicated for periods 1 through 

3. A moderate decrease in reach 3 outflow is indicated for 

periods 1 through 3. Overall for periods 1 through 3, combined contributions from sources within reach 4 of suspended 

sediment generally are proportionally similar to or larger than 

within-reach contributions of streamflow. Large variability in 

relative source-area contributions is indicated for reach 4 for 

suspended sediment. A small decrease in reach 4 outflow is 

indicated for periods 1 through 3. Contributions of suspended 

sediment from sources within reach 5 are proportionally much 

larger than within-reach contributions of streamflow. Minor 

changes in reach 5 outflow or contributions from sources 

within reach 5 are indicated for periods 1 through 3. Contributions of suspended sediment from sources within reach 6 

are proportionally similar to (period 1) or smaller than 

(periods 2 and 3) within-reach contributions of streamflow. 

A small decrease in reach 6 outflow is indicated for periods 1 

through 3, because of combined effects of a minor decrease 
Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin 67

in load transported from reach 5 and a moderate decrease in 

contributions from sources within reach 6. Contributions of 

suspended sediment from sources within reach 7 are proportionally similar to within-reach contributions of streamflow. 

A small decrease in reach 7 outflow is indicated for periods 1 

through 3, because of combined effects of a small decrease in 

load transported from reach 6 and a small decrease in contributions from sources within reach 7. Overall for periods 1 

through 3, combined contributions from sources within 

reach 8 of suspended sediment are proportionally smaller than 

or similar to within-reach contributions of streamflow. A minor 

decrease in reach 8 outflow is indicated for periods 1 through 

3, because of combined effects of a small decrease in load 

transported from reach 7 and a minor increase (primarily 

during period 3) in contributions from sources within 

reach 8. Combined contributions from sources within reach 9 

of suspended sediment for period 1 are proportionally much 

smaller than within-reach contributions of streamflow. Results 

for periods 2 and 3 for reach 9 are not presented in figure 25 

because of the large effect of the removal of the former Milltown Dam on constituent loads and difficulties in presenting 

those results in conjunction with results for other reaches.

Overview of Temporal Changes in Flow-Adjusted 

Concentrations (FACs) and Estimated Normalized 

Constituent Loads

The following paragraphs discuss results of trend analysis 

of FACs in combination with results of the transport analysis 

to provide an overview of the most substantial findings. With 

respect to copper and suspended-sediment results for water 

years 1996–2010, in general most sites have decreases or 

minor changes in FACs, and increases were uncommon. The 

most substantial changes indicated in the upper Clark Fork 

Basin are moderate to large decreases in FACs and loads of 

copper and suspended sediment in reach 1 outflow (Silver 

Bow Creek at Butte, site 2). Also indicated are moderate 

to large decreases in FACs and loads of copper for reach 2 

outflow (Silver Bow Creek at Opportunity, site 3), moderate 

to large decreases in FACs and loads of copper and suspended 

sediment for reach 3 outflow (Silver Bow Creek at Warm 

Springs, site 8), and a moderate decrease in the estimated load 

of copper deposited in Warm Springs Ponds. Accumulation 

of metallic contaminants and suspended sediment within the 

Warm Springs Ponds substantially reduces the transport of 

those constituents to downstream reaches. Generally minor to 

small decreases in FACs and loads of copper and suspended 

sediment are indicated for reach 4 outflow (Clark Fork near 

Galen, site 11). 

Reach 5 is a large source of metallic contaminants and 

suspended sediment, which strongly affects downstream transport of those constituents. Mobilization of copper and suspended sediment from floodplain tailings and the streambed 

of the Clark Fork and its tributaries within reach 5 results in 

contribution of those constituents from within reach 5 that is 

proportionally much larger than the contribution of streamflow 

from within reach 5. In reach 5, copper loads in the Clark 

Fork increase by a factor of about 4 and suspended-sediment 

loads increase by a factor of about 5, whereas streamflow 

increases by a factor of slightly less than 2. With respect to the 

effect of reach 5 on downstream constituent transport, copper 

and suspended-sediment loads sourced from within reach 5 

account for about 40 and 20 percent, respectively, of the 

reach 8 outflow (Clark Fork at Turah Bridge, site 20); whereas, 

streamflow sourced from within reach 5 accounts for about 

8 percent of the reach 8 outflow. During water years 1996–

2010, minor changes in FACs and loads of copper and suspended sediment are indicated for reach 5 outflow (Clark Fork 

at Deer Lodge, site 14).

For the reaches downstream from reach 5, contributions 

of copper loads sourced from within the reaches are proportionally less than contributions of streamflow sourced from 

within the reaches. Thus, the lower reaches contribute proportionally much less than reach 5 to copper loading in the Clark 

Fork. In general, minor to small changes in loads and FACs 

of copper and suspended sediment are indicated for outflows 

of reaches 6, 7, and 8 during water years 1996–2010. Thus, 

although large decreases in FACs and loads of copper and suspended sediment are indicated upstream from reach 3, those 

large decreases are not translated to the more downstream 

reaches. The effect of reach 5 as a large source of copper and 

suspended sediment, in combination with general temporal 

stationarity in those constituents for reach 5 outflow, contributes to this pattern. However, small to moderate decreases in 

within-reach contributions of copper and suspended-sediment 

loads are indicated for reaches 6 and 7.

In general, minor changes in FACs of copper and suspended sediment are indicated for reach 9 outflow from the 

start of water year 1996 to the end of water year 2010. Large 

magnitude trends are indicated for short periods before and 

after removal of the former Milltown Dam in March 2008. 

For the period of remediation activities leading up to the dam 

removal, copper and suspended-sediment FACs increase by a 

factor of about 2. For the period after the dam removal, FACs 

decrease by a similar magnitude, such that at the end of water 

year 2010 FACs are similar to pre-remediation levels.

With respect to arsenic trend results for water years 

1996–2010, in general most sites have minor changes in FACs. 

The most substantial changes indicated in the upper Clark 

Fork Basin are small to moderate decreases in FACs and loads 

of arsenic in outflows of reaches 1 and 2. However, because 

of consistent loading of arsenic to the main-stem channels of 

Silver Bow Creek and the Clark Fork throughout the upper 

Clark Fork Basin, the decreases in FACs in reaches 1 and 2 

do not substantially affect downstream transport relations. In 

general, for reaches downstream from reach 2, minor changes 

are indicated for FACs and loads of arsenic. Chemical characteristics of the metalloid element arsenic are substantially 

different from copper and other metallic contaminants. Unlike 

metallic contaminants, arsenic in streams in the upper Clark 

Fork Basin typically is mostly in dissolved phase, has less 

variability in concentrations, and has weaker direct relations 
68 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

with suspended-sediment concentrations and streamflow. 

Thus, arsenic transport characteristics in the upper Clark Fork 

Basin downstream from reach 2 differ from those of copper, 

and there is less variability in contributions of arsenic loads 

sourced from within the different reaches. Downstream from 

reach 2, three reaches (reaches 3, 5, and 7) contribute arsenic 

loads sourced from within their reaches that are proportionally 

larger than contributions of streamflow sourced from within 

the reaches. Reach 3 is a relatively large source of arsenic, 

probably affected by geochemical processes in Warm Springs 

Ponds in combination with contributions of arsenic from the 

Mill-Willow bypass. In reach 3, arsenic loads in the Clark 

Fork increase by a factor of about 2, whereas streamflow 

increases by a factor of about 1.5. With respect to the effect 

of reach 3 on downstream constituent transport, arsenic loads 

sourced from within reach 3 account for about 11 percent of 

the reach 8 outflow; whereas, streamflow sourced from within 

reach 3 accounts for about 2 percent of the reach 8 outflow. 

During water years 1996–2010, minor increases in FACs and 

loads of arsenic are indicated for reach 3 outflow. Reaches 5 

and 7 are relatively smaller arsenic source areas than reach 3. 

With respect to the effect of reach 5 on downstream constituent transport, arsenic loads sourced from within reach 5 

account for about 18 percent of the reach 8 outflow; whereas, 

streamflow sourced from within reach 5 accounts for about 

8 percent of the reach 8 outflow. With respect to the effect of 

reach 7 on downstream constituent transport, arsenic loads 

sourced from within reach 7 account for about 30 percent of 

the reach 8 outflow; whereas, streamflow sourced from within 

reach 7 accounts for about 18 percent of the reach 8 outflow. 

During water years 1996–2010, minor changes in FACs and 

loads of arsenic are indicated for outflows of reaches 5 and 7.

Summary and Conclusions

The primary purposes of this report are to (1) characterize temporal trends in flow-adjusted concentrations (filtered 

and unfiltered) of mining-related contaminants and (2) assess 

those trends in the context of source areas and transport of 

those contaminants through the upper Clark Fork Basin. A 

large-scale trend analysis was done on specific conductance, 

selected trace elements (arsenic, cadmium, copper, iron, lead, 

manganese and zinc), and suspended sediment for 22 sites 

for water years 1996–2010. Trend analysis was conducted by 

using two parametric methods: the time-series model (Vecchia, 2005) and multiple linear regression on time, streamflow, 

and season (Helsel and Hirsch, 2002). The report presents 

background information on mining and remediation activities in the upper Clark Fork Basin, trend-analysis methods, 

streamflow conditions, and various data-related factors that 

affect trend results. This information is presented to assist in 

evaluating trend results; however, it is beyond the scope of 

this report to provide detailed explanations for all observed 

temporal changes. 

The main-stem Clark Fork begins at the confluence of 

Silver Bow and Warm Springs Creeks near Warm Springs, 

Montana and flows about 485 miles through Montana and 

Idaho. The study area encompasses the upper Clark Fork 

Basin in west-central Montana upstream from Clark Fork 

above Missoula (site 22) with a drainage area of 5,999 square 

miles (mi2

).

Mining in the upper Clark Fork Basin began in 1864 

when small-scale placer mining operations extracted gold 

from Silver Bow Creek and its tributaries in and near Butte. 

Large amounts of waste materials enriched with trace elements, including the metallic elements cadmium, copper, lead, 

and zinc, as well as the metalloid trace element arsenic, were 

generated from mining operations in the Butte area and the 

milling and smelting operations in the Anaconda area. Extensive deposition of mining wastes in the Silver Bow Creek and 

Clark Fork channels and floodplains had substantial effects 

on water quality. Federal Superfund remediation activities in 

the upper Clark Fork Basin began in 1983 and have included 

substantial remediation in the Butte area and removal of the 

former Milltown Dam near Missoula. 

Water-quality data collection by the U.S. Geological 

Survey began during 1985–88 with the establishment of a 

small long-term monitoring program that has expanded over 

time and continued through present (2013). Some substantial 

changes in field sampling and laboratory analytical methods in the monitoring program occurred before 1993. There 

have been a total of 22 sites in the monitoring program with 

variable periods of record. Many of the study sites were not 

sampled before 1993. To maintain consistency in trend analysis among sites, trend analysis was restricted to the period 

of water years 1996–2010. In general, during water years 

1996–2010 there was reasonable consistency in the frequency 

and timing of sample collection among the sites to conduct 

trend analysis. Review of quality-assurance data collected 

in the monitoring program indicates the data are of suitable 

quality (with respect to precision, accuracy, and contamination 

issues) for analysis of long-term trends.

Two parametric trend-analysis methods were used in 

this study: the time-series model (TSM) and multiple linear 

regression on time, streamflow, and season (MLR). Both of 

the trend-analysis methods analyze trends in flow-adjusted 

concentrations (FACs); that is, the methods compute FACs, 

estimate best-fit trend lines that represent temporal changes in 

FACs, and determine statistical significance of changes. The 

TSM was selected as the preferred trend-analysis method to 

MLR because it incorporates multiple measures of antecedent and concurrent streamflow. The TSM was used to analyze trends for 15 of the 22 study sites. For seven sites, data 

requirements of the TSM were not met. In those cases, MLR 

was used to analyze trends. 

To provide temporal resolution of changes in water quality, trend analysis was conducted on three sequential 5-year 

periods: period 1 (water years 1996–2000); period 2 (water 

years 2001–05); and period 3 (water years 2006–10). For 

recently established sites that did not satisfy data requirements 
Summary and Conclusions 69

for the TSM, MLR trend-analysis was conducted on 

period 3. Because of the substantial effect of the breach of 

Milltown Dam on March 28, 2008, for Clark Fork above 

Missoula (site 22), period 3 was subdivided into period 3A 

(October 1, 2005–March 27, 2008) and period 3B (March 28, 

2008–September 30, 2010). 

The TSM accounts for many hydrological factors that 

contribute to complexity in concentration and streamflow 

relations. In this study, the TSM, was applied as consistently 

as possible among sites, and is considered to be a useful tool 

for simplifying the environmental complexity in the upper 

Clark Fork Basin to provide a large-scale evaluation of general 

temporal changes in constituent transport independent from 

streamflow variability. The TSM best-fit trend lines are considered to provide important information beyond the strict statistical characteristics of the trend results (in terms of p-values 

and levels of significance) because they aid comparing and 

summarizing large-scale patterns among sites. 

To assist in the presentation of results, Silver Bow Creek 

and the Clark Fork were divided into nine reaches based on 

the location of sites along the main-stems of those streams. 

The reaches include the following:

• Reach 1, which extends from Blacktail Creek (site 1) 

to Silver Bow Creek at Butte (site 2), is a 3.5-mile (mi) 

reach through an area affected by large-scale mining 

activities in the Butte Area, including the Berkeley Pit. 

Effluent from the Butte wastewater treatment plant 

discharges within this reach.

• Reach 2, which extends from Silver Bow Creek below 

Blacktail Creek (site 2) to Silver Bow Creek at Opportunity (site 3), is a 15-mi reach that meanders through a 

floodplain with extensive deposits of mining wastes. 

• Reach 3, which extends from Silver Bow Creek at 

Opportunity (site 3) to Silver Bow Creek at Warm 

Springs (site 8), is a 6-mi reach that passes through 

Warm Springs Ponds where there is deposition of particulate materials, treatment (liming) to remove metallic elements, and complex biogeochemical processes. 

Tributary basins within this reach are affected by 

pollution from milling and smelting operations of the 

former Anaconda Mining Company (AMC) Smelter. 

The Mill-Willow bypass diverts the combined flows of 

Mill Creek and Willow Creek into Silver Bow Creek 

between the outlet from Warm Springs Ponds and the 

confluence with Warm Springs Creek.

• Reach 4, which extends from Silver Bow Creek at 

Warm Springs (site 8) to Clark Fork near Galen 

(site 11), is short (2-mi), but environmentally complex. 

In this reach, Warm Springs Creek joins with Silver 

Bow Creek to form the Clark Fork, and there also is 

mixing of waters contributed from the Warm Springs 

Ponds outlet, the Mill-Willow bypass, and Warm 

Springs Creek. The Warm Springs Creek Basin is 

affected by pollution from milling and smelting operations of the former AMC Smelter. 

• Reach 5, which extends from Clark Fork near Galen 

(site 11) to Clark Fork at Deer Lodge (site 14), is a 

21-mi reach that meanders through a broad valley with 

extensive floodplain tailings deposits. Lost Creek, 

which drains a basin affected by pollution from milling 

and smelting operations of the former AMC Smelter, 

discharges into this reach.

• Reach 6, which extends from Clark Fork at Deer 

Lodge (site 14) to Clark Fork at Goldcreek (site 16), 

is a 26-mi reach with a broad meandering valley from 

Deer Lodge to Garrison. The Little Blackfoot River 

discharges into this reach near Garrison and the Clark 

Fork valley narrows. Downstream from Garrison, 

floodplain tailings are less extensive than in the valley 

upstream.

• Reach 7, which extends from Clark Fork at Goldcreek 

(site 16) to Clark Fork near Drummond (site 18), is 

a 31-mi reach with less extensive floodplain tailings. 

Flint Creek discharges into this reach.

• Reach 8, which extends from Clark Fork near Drummond (site 18) to Clark Fork at Turah Bridge (site 20), 

is a 34-mi reach with little or no visible floodplain tailings. Rock Creek discharges into this reach.

• Reach 9, which extends from Clark Fork at Turah 

Bridge (site 20) to Clark Fork above Missoula 

(site 22), is a 9-mi reach that includes the former Milltown Reservoir where substantial amounts of mining 

wastes had been deposited. The former Milltown Dam 

was breached and removed in 2008. The Blackfoot 

River discharges into this reach.

For the reaches and their inclusive sites that were analyzed by 

using the TSM, normalized loads were estimated to evaluate 

temporal changes in relative contributions of selected trace 

elements and suspended sediment from upstream source areas 

to reach outflows. 

Trend results are presented for all constituents investigated; however, in the discussion emphasis is placed on 

copper, arsenic, and suspended sediment. Copper and arsenic 

represent large differences in chemical characteristics and 

are constituents of concern with respect to potential toxicity 

issues. Also, trend patterns for copper generally are similar to 

other metallic contaminants. Suspended-sediment data provide 

information on transport of particulate materials, which is a 

factor that can strongly affect transport of metallic contaminants because of their tendency to adsorb to particulate materials. For most sites with sufficient periods of data collection, 

copper, arsenic, and suspended-sediment data met all requirements for application of the TSM. 

With respect to copper and suspended-sediment results 

for water years 1996–2010, in general most sites have 
70 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

decreases or minor changes in FACs and increases were uncommon. The most substantial changes indicated in the upper Clark 

Fork Basin are moderate to large decreases in FACs and loads 

of copper and suspended sediment in reach 1 outflow (Silver 

Bow Creek at Butte, site 2). Also indicated 

are moderate to large decreases in FACs and loads of copper for 

reach 2 outflow (Silver Bow Creek at Opportunity, 

site 3), moderate to large decreases in FACs and loads of copper and suspended sediment for reach 3 outflow (Silver Bow 

Creek at Warm Springs, site 8), and a moderate decrease in the 

estimated load of copper deposited in Warm Springs Ponds. 

Accumulation of metallic contaminants and suspended sediment within the Warm Springs Ponds substantially reduces the 

transport of those constituents to downstream reaches. Generally minor to small decreases in FACs and loads of copper and 

suspended sediment are indicated for reach 4 outflow (Clark 

Fork near Galen, site 11). 

Reach 5 is a large source of metallic contaminants and 

suspended sediment, which strongly affects downstream 

transport of those constituents. Mobilization of copper and 

suspended sediment from floodplain tailings and the streambed 

of the Clark Fork and its tributaries within reach 5 results in 

contribution of those constituents from within reach 5 that is 

proportionally much larger than the contribution of streamflow 

from within reach 5. In reach 5, copper loads in the Clark Fork 

increase by a factor of about 4 and suspended-sediment loads 

increase by a factor of about 5, whereas streamflow increases by 

a factor of slightly less than 2. With respect to the effect of reach 

5 on downstream constituent transport, copper and suspendedsediment loads sourced from within reach 5 account for about 

40 and 20 percent, respectively, of the 

reach 8 outflow (Clark Fork at Turah Bridge, site 20); whereas, 

streamflow sourced from within reach 5 accounts for about 

8 percent of the reach 8 outflow. During water years 1996–2010, 

minor changes in FACs and loads of copper and suspended 

sediment are indicated for reach 5 outflow (Clark Fork at Deer 

Lodge, site 14).

For the reaches downstream from reach 5, contributions of 

copper loads sourced from within the reaches are proportionally 

less than contributions of streamflow sourced from within the 

reaches. Thus, the lower reaches contribute proportionally much 

less than reach 5 to copper loading in the Clark Fork. In general, 

minor to small changes in loads and FACs of copper and suspended sediment are indicated for outflows of reaches 6, 7, and 

8 during water years 1996–2010. Thus, although large decreases 

in FACs and loads of copper and suspended sediment are 

indicated upstream from reach 3, those large decreases are not 

translated to the more downstream reaches. The effect of reach 

5 as a large source of copper and suspended sediment, in combination with general temporal stationarity in those constituents 

for reach 5 outflow, contributes to this pattern. However, small 

to moderate decreases in within-reach contributions of copper 

and suspended-sediment loads are indicated for reaches 6 and 7.

In general, minor changes in FACs of copper and suspended sediment are indicated for reach 9 outflow from the 

start of water year 1996 to the end of water year 2010. Large 

magnitude trends are indicated for short periods before and after 

removal of the former Milltown Dam in March 2008. For the 

period of remediation activities leading up to the dam removal, 

copper and suspended-sediment FACs increase by a factor of 

about 2. For the period after the dam removal, FACs decrease 

by a similar magnitude, such that at the end of water year 2010 

FACs are similar to pre-remediation levels.

With respect to arsenic trend results for water years 

1996–2010, in general, most sites have minor changes in FACs. 

The most substantial changes indicated in the upper Clark Fork 

Basin are small to moderate decreases in FACs and loads of 

arsenic in outflows of reaches 1 and 2. However, because of 

consistent loading of arsenic to the main-stem channels of Silver 

Bow Creek and the Clark Fork throughout the upper Clark Fork 

Basin, the decreases in FACs in 

reaches 1 and 2 do not substantially affect downstream transport 

relations. In general, for reaches downstream from 

reach 2, minor changes are indicated for FACs and loads of 

arsenic. Chemical characteristics of the metalloid element arsenic are substantially different from copper and other metallic 

contaminants. Unlike metallic contaminants, arsenic in streams 

in the upper Clark Fork Basin typically is mostly in dissolved 

phase, has less variability in concentrations, and has weaker 

direct relations with suspended-sediment concentrations and 

streamflow. Thus, arsenic transport characteristics in the upper 

Clark Fork Basin downstream from reach 2 differ from those of 

copper, and there is less variability in contributions of arsenic 

loads sourced from within the different reaches. Downstream 

from reach 2, three reaches (reaches 3, 5, and 7) contribute arsenic loads sourced from within their reaches that are proportionally larger than or similar to contributions of streamflow sourced 

from within the reaches. 

Reach 3 is a relatively large source of arsenic, probably 

affected by geochemical processes in Warm Springs Ponds in 

combination with contributions of arsenic from the Mill-Willow 

bypass. In reach 3, arsenic loads in the Clark Fork increase by 

a factor of about 2, whereas streamflow increases by a factor of 

about 1.5. With respect to the effect of reach 3 on downstream 

constituent transport, arsenic loads sourced from within reach 3 

account for about 11 percent of the 

reach 8 outflow; whereas, streamflow sourced from within reach 

3 accounts for about 2 percent of the reach 8 outflow. During 

water years 1996–2010, minor increases in FACs and loads of 

arsenic are indicated for reach 3 outflow. Reaches 5 and 7 are 

relatively smaller arsenic source areas than reach 3. With respect 

to the effect of reach 5 on downstream constituent transport, 

arsenic loads sourced from within reach 5 account for about 18 

percent of the reach 8 outflow; whereas, streamflow sourced 

from within reach 5 accounts for about 8 percent of the reach 

8 outflow. With respect to the effect of reach 7 on downstream 

constituent transport, arsenic loads sourced from within reach 

7 account for about 30 percent of the reach 8 outflow; whereas, 

streamflow sourced from within reach 7 accounts for about 18 

percent of the reach 8 outflow. During water years 1996–2010, 

minor changes in FACs and loads of arsenic are indicated for 

outflows of reaches 5 and 7.
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Standards Section, 76 p.

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suspended sediment and selected trace elements transported 

through the Clark Fork Basin, Montana, in selected periods 

before and after the breach of Milltown Dam (water years 

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flood-plain tailings, and metal transport in the upper Clark 

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Appendixes 1–5
78 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

Appendix 1—Summary Information 

Relating to Quality-Control, WaterQuality, and Continuous Streamflow 

Data

Summary information is presented relating to qualitycontrol, water-quality, and continuous streamflow data. Results 

for quality-control equipment blank and replicate samples 

collected during water years 1993–2010 are summarized in 

table 1–1. Spike recoveries for laboratory-spiked deionizedwater blank samples collected during water years 1993–2010 

are presented in table 1–2. Spike recoveries for laboratoryspiked stream-water blank samples collected during water 

years 1993–2010 are presented in table 1–3. Statistical 

summaries of water-quality data collected during water years 

2001–10 are presented in table 1–4. For reference, aquatic life 

standards (based on median hardness for water years 2001–10, 

Montana Department of Environmental Quality, 2012a) are 

presented in table 1–5. Statistical summaries of continuous 

streamflow data collected during water years 2001–10 are 

presented in table 1–6.

Evaluation of long-term spike-recovery data is of particular relevance to the long-term trend analysis. Spike-recoveries 

during water years 1993–2010 for laboratory-spiked 

deionized-water blank samples (table 1–2 and fig. 1–1) and 

laboratory-spiked stream-water samples (table 1–3 and 

fig. 1–2) indicate generally consistent recoveries over time, 

typically varying within plus or minus 10 percent of 100 percent recovery. However, before about water year 2000, spike 

recoveries for unfiltered-recoverable copper in spiked streamwater samples generally were near 100 percent (mean annual 

spike recovery for water years 1993–1999 of 99.1 percent), 

whereas after about water year 2000, spike recoveries were 

consistently less than 100 percent (mean annual spike recovery 

for water years 2000–2010 of 93.9 percent). Changes in spike 

recoveries in about water year 2000 probably were related 

to a change in about water year 2000 by the U.S. Geological 

Survey National Water Quality Laboratory from analysis of 

most metallic elements by graphite furnace atomic absorption 

spectrophotometry (Fishman, 1993) to inductively coupled 

plasma-mass spectrometry (Garbarino and Struzeski, 1998; 

Garbarino and others, 2006). Potential effects on trend results 

of differences in spike recoveries between pre-2000 and post2000 data relate only to sites with long-term data that were 

analyzed by using the TSM. 

The potential effects of temporal differences in spike 

recoveries on trend results were evaluated by using two 

approaches: (1) exploratory TSM analysis with inclusion 

of a step trend in the trend model, and (2) exploratory TSM 

analysis on constituent concentrations adjusted based on 

annual mean spike recoveries. For the exploratory step-trend 

approach, for each site and constituent combination a step 

trend for the period water years 1996–1999 was included in 

the TSM model in addition to including applicable trends for 

periods 1 through 3 (depending on available data for the given 

site and constituent combination). Inclusion of a step trend 

allowed evaluation of whether there was a distinct change in 

data structure between pre-2000 and post-2000 data that might 

have affected trend results. Results of the exploratory steptrend analysis indicated that for all site and constituent combinations, the step trend was nonsignificant, and inclusion of 

the step trend had minor effects on trend results with respect to 

magnitude, direction, and significance on constituent concentration trends. For the exploratory spike-recovery adjustment 

approach, before TSM analysis constituent concentrations for 

each year were adjusted by multiplying the concentrations 

divided by the annual mean spike recovery for laboratoryspiked stream-water samples. Results of the exploratory 

spike-recovery adjustment analysis were more variable than 

results for the exploratory step-trend approach but resulted in 

the same general conclusion that temporal differences in spike 

recoveries had minor effects on trend results.
Table 1–1. Summary information relating to quality-control samples (equipment blank and replicate samples) collected at sites in the upper Clark Fon data collected during water years 1993–2010.[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. LRL, laboratory reporting level; SRL, study reporting deviation; μS/cm, microsiemens per centimeter at 25 degrees Celsius; NA, not applicable; μg/L, micrograms per liter; mg/L, milligrams per liter]

Constituent or property, units of 

measurement

Summary information relating to field blank samples

Number of field 

blank samples

Number of field 

blank samples with 

detected concen-

trations greater 

than the LRL at the 

time of analysis

Percent of field 

blank samples with 

detected concen-

trations greater 

than the LRL at the 

time of analysis

Maximum detected 

concentration for 

field blank samples

Median concentra-

tion in field blank 

samples with de-

tected concentra-

tions greater than 

the LRL at the time 

of analysis

SRL used in ap-

plication of the 

time-series model

Percent of detec-

tions in blank 

samples at con-

centrations greater 

than the SRL used 

in the application 

of the time-series 

model

Specific conductance, μS/cm NA NA NA NA NA 8 NA 

Cadmium, filtered, μg/L 158 3 1.9 0.34 0.07 0.05 3.2 Cadmium, unfiltered-recoverable, 

μg/L

154 0 0 no detections no detections 0.09 0 

Copper, filtered, μg/L 157 8 5.1 3.6 0.52 1 1.3 Copper, unfiltered-recoverable, 

μg/L

154 6 3.9 3.0 1.0 1 1.9 

Iron, filtered,μg/L 154 0 0 no detections no detections 6 0 Iron unfiltered-recoverable, μg/L 150 4 2.7 36 7 20 2.0 Lead, filtered, μg/L 158 4 2.5 0.60 0.23 0.5 0.6 Lead, unfiltered-recoverable, μg/L 154 3 1.9 0.16 0.06 0.5 0 Manganese, filtered, μg/L 153 0 0 no detections no detections 1 0 Manganese, unfiltered-recoverable, 

μg/L

150 2 1.3 0.30 0.15 1 0 

Zinc, filtered, μg/L 156 23 14.7 6.20 0.90 5 0.6 Zinc, unfiltered-recoverable, μg/L 152 10 6.6 3.40 1.40 2 3.3 Arsenic, filtered, μg/L 158 0 0 no detections no detections 1 0 Arsenic, unfiltered-recoverable, 

μg/L

154 0 0 no detections no detections 1 0 

Suspended sediment, mg/L NA NA NA NA NA 1 NA 

1The RSD is calculated according to the following equation (Taylor, 1987):

where

RSD is the relative standard deviation;

S is the standard deviation; and

x is the mean contentration for all replicates.

RSD = x 100, Sx
Table 1–2. Summary information relating to quality-control samples (laboratory-spiked deionized-water blank samples) collected at sites in the uppMontana, based on data collected during water years 1993–2010. [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. F, filtered; UFR, unfiltered-recoverable] Water year Mean spike recovery, percent (values in parentheses indicate 95-percent confidence intervals) Cadmium, F Cadmium, UFR Copper, F Copper, UFR Iron, F Iron, UFR Lead, F Lead, UFR Manganese, F Manganese, UFR Zinc, F Zinc

UFR 

1993 93.4 (85.9, 101) 97 (93.5, 101) 99.5 (95.9, 103) 101.7 (94.4, 109) 94 (90.0, 98.0) 103.3 (92.4, 114) 105.8 (99.5, 112) 100.5 (95.2, 106) 96.9 (96.3, 97.5) 95.6 (82.2, 109) 106.5 (99.7, 113) 96.3 (94

98.

1994 97.5 (89.1, 106) 98.8 (90.6, 107) 101.1 (98.4, 104) 99.7 (94.3, 105) 100 (93.0, 107) 94.6 (84.2, 105) 100.5 (98.5, 102) 99.1 (94.3, 104) 95.7 (90.8, 100) 101.5 (96.2, 107) 106.5 (95.8, 117) 102.6 (91

114

1995 100 (97.3, 103) 101.3 (97.5, 105) 102.7 (101, 105) 97.6 (92.3, 103) 102.2 (97.8, 107) 93.8 (87.9, 99.7) 1023 (97.7, 107) 100.8 (96.6, 105) 96.5 (92.0, 101) 98.5 (93.1, 104) 102.3 (97.1, 108) 101.5 (97

106

1996 95.3 (92.2, 98.4) 82.3 (79.7, 84.9) 99.2 (91.4, 107) 99.6 (93.5, 106) 89.8 (76.0, 104) 90.8 (70.9, 111) 100.5 (93.3, 108) 97.4 (80.2, 115) 89.2 (77.9, 100) 96.5 (91.6, 101) 96.1 (84.3, 108) 87.8 (82

92.

1997 98.5 (92.1, 105) 85.7 (77.7, 93.7) 101.1 (86.2, 116) 106.4 (82.0, 131) 94.7 (78.5, 111) 96.1 (80.2, 112) 101 (93.4, 109) 101.1 (88.9, 113) 90.3 (82.7, 97.9) 99.3 (95.8, 103) 97.9 (78.1, 118) 92.7 (86

99.

1998 104 (93.8, 114) 97.4 (87.0, 108) 100.4 (93.4, 107) 103.4 (98.8, 108) 101.8 (90.7, 113) 95.7 (89.9, 102) 100.2 (91.8, 109) 104.8 (88.8, 121) 102.8 (94.4, 111) 99 (92.1, 106) 95.2 (85.9, 104) 101.3 (86

116

1999 100.9 (92.6, 109) 103.4 (99.9, 107) 107.5 (99.5, 116) 105 (102, 108) 97.7 (94.3, 101) 96.5 (90.0, 103) 97.4 (87.9, 107) 96.2 (85.2, 107) 96 (91.8, 100) 95.9 (86.3, 106) 96.9 (92.9, 101) 93.3 (88

97.

2000 103.8 (97.3, 110) 105 (96.0, 114) 104 (96.0, 112) 100.3 (92.4, 108) 97.4 (92.3, 102) 100.6 (89.2, 112) 98.3 (88.9, 108) 102.6 (97.3, 108) 100.8 (93.3, 108) 103.2 (96.8, 110) 107.8 (95.8, 120) 102.6 (90

115

2001 102.9 (98.9, 107) 107.9 (101, 115) 105.2 (98.6, 112) 96.8 (93.7, 99.9) 101.3 (95.5, 107) 98.3 (86.7, 110) 97.3 (91.9, 103) 96.4 (93.7, 99.1) 101.9 (79.0, 125) 103.7 (89.9, 118) 102 (87.9, 116) 99.1 (82

116

2002 101.1 (98.8, 103) 97.6 (96.3, 98.9) 99.4 (95.0, 104) 98.8 (96.7, 101) 95.1 (89.3, 101) 102.3 (93.0, 112) 98.5 (89.9, 107) 96.9 (90.5, 103) 98.5 (95.4, 102) 96.5 (88.8, 104) 103.9 (94.4, 113) 98.3 (91

105

2003 98.6 (92.6, 105) 97.5 (94.1, 101) 100.4 (93.0, 108) 97.6 (93.2, 102) 101.6 (96.4, 107) 93.1 (87.4, 8.8) 97.2 (92.3, 102) 96 (93.9, 98.1) 95.8 (90.7, 101) 96.6 (79.7, 114) 101.4 (89.8, 113) 99.1 (93

105

2004 97.4 (95.6, 99.2) 100 (98.6, 101) 98.9 (92.7, 105) 99.6 (95.4, 104) 101 (96.3, 106) 96.1 (88.8, 103) 96 (91.9, 100) 98.9 (97.3, 100) 99.1 (92.3, 106) 98.6 (90.6, 107) 102 (91.7, 112) 100 (96

104

2005 102 (97.3, 106) 97.5 (88.1, 107) 102 (97.4, 107) 97.6 (88.4, 107) 97.6 (90.5, 105) 100 (95.2, 105) 101 (95.5, 106) 104 (99.4, 108) 93.8 (82.2, 105) 102 (86.4, 117) 102 (88.3, 116) 96.1 (83

109
2006 100 (92.6, 107) 98.9 (94.1, 104) 102 (97.7, 107) 98.7 (93.8, 104) 106 (101, 112) 103 (95.4, 111) 99 (89.3, 109) 98 (91.2, 105) 97 (90.7, 103) 105 (95.3, 115) 105 (95.4, 115) 94.9 (90

100

2007 107 (103, 112) 103 (94.4, 111) 105 (99.2, 111) 98.4 (86.9, 110) 99.9 (92.1, 108) 104 (98.5, 110) 99.6 (93.9, 105) 103 (100, 106) 107 (99.9, 114) 107 (97.0, 116) 107 (102, 113) 103 (96

110

2008 102 (88.2, 116) 101 (91.9, 110) 105 (88, 121) 97.9 (87.2, 109) 103 (95.9, 110) 101 (96.5, 106) 101 (89, 112) 101 (98, 105) 102 (92.9, 111) 102 (92.5, 112) 99.8 (87.9, 112) 103 (9

111

2009 102 (97.4, 107) 97.2 (93.6, 101) 102 (92.0, 113) 96 (94.0, 97.0) 102 (91.4, 112) 104 (78.8, 130) 102 (96.0, 107) 98.4 (96.1, 101) 105 (103, 106) 99.7 (94.6, 105) 111 (104, 118) 93.3 (88

98.

2010 106 (94.9, 117) 100 (88.4, 112) 97.2 (84.9, 109) 98.6 (84.0, 113) 108 (101, 115) 102 (95.8, 108) 102 (91.5, 113) 102 (91.0, 113) 103 (95.2, 111) 105 (97.2, 112) 113 (94.7, 132) 101 (89

113

Table 1–2. Summary information relating to quality-control samples (laboratory-spiked deionized-water blank samples) collected at sites in the uppMontana, based on data collected during water years 1993–2010.—Continued[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. F, filtered; UFR, unfiltered-recoverable]

Water 

year

Mean spike recovery, percent (values in parentheses indicate 95-percent confidence intervals)

Cadmium, 

F

Cadmium, 

UFR

Copper, 

F

Copper, 

UFR

Iron, 

F

Iron, 

UFR

Lead, 

F

Lead, 

UFR

Manganese, 

F

Manganese, 

UFR

Zinc, 

F

Zinc

UFR
Table 1–3. Summary information relating to quality-control samples (laboratory-spiked stream-water samples) collected at sites in the upper Cla

based on data collected during water years 1993–2010. [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. F, filtered; UFR, unfiltered-recoverable] Water year Mean spike recovery, percent (values in parentheses indicate 95-percent confidence intervals) Cadmium, F Cadmium, UFR Copper, F Copper, UFR Iron, F Iron, UFR Lead, F Lead, UFR Manganese, F Manganese, UFR Zinc, F Zin

UF

1993 97.1 (92.3, 102) 98.1 (95.2, 101) 97.4 (95.8, 99.0) 97.2 (92.3, 102) 94.6 (86.7, 103) 102.2 (94.4, 110) 104.7 (98.5, 111) 96 (93.0, 99.0) 95.7 (92.1, 99.3) 100.2 (96.4, 104) 105.7 (93.4, 118) 95.7 (92

99.

1994 101.3 (97.5, 105) 97.9 (94.4, 101) 96.6 (93.3, 99.8) 98.4 (91.1, 106) 98.2 (94.8, 102) 99.3 (90.6, 108) 103 (101, 105) 99.3 (95.6, 103) 98.1 (95.4, 101) 100.4 (95.4, 105) 97.5 (92.4, 102) 106 (95

117

1995 101.3 (96.7, 106) 102.9 (98.0, 108) 99.8 (96.2, 103) 98 (92.7, 103) 99.5 (96.1, 103) 101.4 (96.2, 107) 102.9 (98.6, 107) 100 (96.7, 103) 97.4 (92.9, 102) 103.8 (99.0, 109) 104.7 (101, 108) 101.1 (99

103

1996 100.2 (91.5, 109) 88.4 (57.8, 119) 101.1 (91.9, 110) 100.3 (92.3, 108) 93.8 (73.3, 114) 101.5 (88.5, 114) 105.1 (90.4, 120) 105.6 (98.4, 113) 90.3 (79.1, 102) 99.5 (92.9, 106) 103.2 (90.2, 116) 99.3 (74

124

1997 98.1 (83.5, 113) 84.3 (75.0, 93.6) 97.3 (88.3, 106) 100.5 (71.9, 129) 99.3 (81.0, 118) 97.5 (78.2, 117) 100.8 (91.6, 110) 102.1 (99.1, 105) 93 (84.0, 102) 99.8 (94.5, 105) 97 (89.9, 104) 92.7 (74

111

1998 104.4 (97.3, 112) 99.5 (92.7, 106) 97.2 (90.6, 104) 99.1 (88.4, 110) 97.5 (82.8, 112) 101.8 (90.2, 113) 102.2 (94.3, 110) 105 (92.9, 117) 99.5 (85.8, 113) 101.5 (98.0, 105) 99.5 (89.1, 110) 98.8 (85

112

1999 102.6 (92.4, 113) 103 (100, 106) 102.7 (89.1, 116) 100.5 (97.5, 104) 97.2 (93.5, 101) 99.9 (90.6, 109) 100.2 (94.0, 106) 101.1 (93.7, 108) 99.8 (92.8, 107) 98.8 (89.3, 108) 98.6 (95.7, 102) 96.2 (91

10

2000 104.2 (100, 108) 98.1 (88.9, 107) 101.6 (97.3, 106) 94.6 (87.7, 102) 96.5 (88.0, 105) 98 (88.3, 108) 101.4 (97.3, 106) 105.3 (103, 108) 97.3 (83.3, 111) 101.7 (91.4, 112) 101.5 (90.9, 112) 97.8 (91

104

2001 103.2 (100, 106) 105.8 (95.9, 116) 106.8 (104, 110) 91.8 (87.7, 95.9) 95.8 (91.4, 100) 101.6 (92.1, 111) 99.7 (95.2, 104) 97.3 (95.3, 99.3) 100 (84.4, 116) 100.9 (90.3, 112) 100.8 (85.7, 116) 96.9 (75

118

2002 106 (97.5, 114) 102 (98.6, 101) 97.3 (91.2, 103) 96.9 (92.9, 101) 92.6 (83.3, 102) 107.1 (103, 111) 101.4 (91.9, 111) 98.9 (92.2, 106) 98.3 (92.5, 104) 94.3 (88.4, 100) 101.3 (92.6, 110) 95.8 (89

102

2003 100.5 (91.4, 110) 99 (94.4, 104) 95.8 (88.9, 103) 91.6 (89.7, 93.5) 106.4 (100, 113) 96.7 (91.6, 102) 96 (90.2, 102) 96.8 (93.7, 99.9) 93.9 (78.8, 109) 99.3 (86.2, 112) 98.4 (93.6, 103) 93 (87

98.

2004 101 (94.2, 108) 101 (100, 103) 95.4 (93.8, 97) 93.8 (89.5, 98.1) 104 (99.5, 108) 111 (91.2, 130) 98.7 (93, 104) 100 (98.6, 102) 103 (89.8, 117) 96 (91.8, 100) 100 (95.3, 105) 94.4 (

97.

2005 97.8 (62.7, 133) 98.2 (88.5, 108) 93.6 (57.9, 129) 93 (84.8, 101) 102 (95.9, 108) 99.3 (95.6, 103) 102 (96.1, 109) 103 (99.7, 106) 88.3 (78.3, 98.3) 97.5 (87.3, 108) 94.3 (60.8, 128) 91.6 (80

102
2006 104 (99.0, 108) 99.6 (94.7, 104) 101 (96.7, 104) 94.8 (91.0, 98.6) 105 (102, 109) 102 (93.6, 110) 102 (94.2, 111) 100 (92.9, 106) 94.9 (88.2, 102) 106 (97.9, 113) 108 (93.3, 123) 91.2 (87

94.

2007 108 (102, 114) 98 (92.2, 104) 100 (89.8, 110) 96.3 (91.8, 101) 107 (103, 111) 103 (94.7, 112) 109 (103, 115) 104 (102, 107) 106 (100, 113) 101 (96.1, 106) 104 (95.7, 113) 98 (89

107

2008 101 (91, 112) 97 (93.6, 100) 98.9 (92, 106) 92.8 (86.4, 99.1) 105 (94.1, 117) 99.4 (92, 107) 100 (91.3, 109) 103 (99.5, 106) 98.9 (90.3, 108) 98.4 (92.5, 104) 106 (88.1, 124) 95.7 (93

98.

2009 106 (101, 112) 94.7 (89.5, 99.8) 96.2 (91.2, 101) 91.4 (87.8, 95.0) 107 (89.7, 124) 102 (86.9, 118) 100 (97.0, 103) 100 (98.8, 101) 97 (88.0, 106) 92.8 (81.7, 104) 114 (104, 124) 89.8 (80

99.

2010 110 (87.6, 132) 98.2 (87.1, 109) 93.8 (83.6, 104) 96.5 (84.4, 108) 105 (91.7, 119) 111 (103, 118) 101 (87.7, 115) 104 (91.5, 116) 104 (93.3, 114) 98.7 (86.4, 111) 109 (101, 118) 94 (81

107

Table 1–3. Summary information relating to quality-control samples (laboratory-spiked deionized-water blank samples) collected at sites in the uppMontana, based on data collected during water years 1993–2010.—Continued[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. F, filtered; UFR, unfiltered-recoverable]

Mean spike recovery, percent (values in parentheses indicate 95-percent confidence intervals) Water 

year Cadmium, F Cadmium, UFR Copper, F Copper, UFR Iron, F Iron, UFR Lead, F Lead, UFR Manganese, F Manganese, UFR Zinc, F Zinc

UFR 
84 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

Figure 1–1. Spike recoveries for laboratory-spiked deionized-water blank samples, based on data collected during water 

years 1993–2010. A, Copper, filtered; B, Copper, unfiltered-recoverable; C, Arsenic, filtered; D, Arsenic, unfiltered-recoverable.

1990 1995 2000 2005 2010

75

150

125

100

50

75

150

125

100

50

75

150

125

100

50

75

150

125

100

50

95-percent confidence interval about the mean spike recovery

Mean spike recovery 

EXPLANATION

Water year (October–September)

B. Copper, unfiltered-recoverable

A. Copper, filtered

C. Arsenic, filtered

D. Arsenic, unfiltered-recoverable

Spike recovery, in percent
Appendix 1—Summary Information Relating to Quality-Control, Water-Quality, and Continuous Streamflow Data 85

Figure 1–2. Spike recoveries for laboratory-spiked stream-water samples, based on data collected during water years 

1993–2010. A, Copper, filtered; B, Copper, unfiltered-recoverable; C, Arsenic, filtered; D, Arsenic, unfiltered-recoverable.

A. Copper, filtered

B. Copper, unfiltered-recoverable

C. Arsenic, filtered

D. Arsenic, unfiltered-recoverable

Spike recovery, in percent

1990 1995 2000 2005 2010

75

150

125

100

50

75

150

125

100

50

75

150

125

100

50

75

150

125

100

50

95-percent confidence interval about the mean spike recovery

Mean spike recovery 

EXPLANATION

Water year (October–September)
Table 1–4. Summary information relating to water-quality constituents and properties in samples collected at sites in the upper Clark Fork Basin, Mcollected during water years 2001–2010. [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. ft3/s, cubic feet per second; NA, not applicable; μS/cm

at 25 degrees Celsius; CaCO3, calcium carbonate; μg/L, micrograms per liter; mg/L, milligrams per liter; ND, distributional parameter not determined because of excessive num

than] Period of water- quality sampling during water years 2001–10 Constituent or property, unadjusted (not flow adjusted) units of measurement Statistical summaries of water-quality data1 

Number of samples (values in parentheses indicate number of censored values) Minimum uncensored value2 25th percentile Median Mean 75th percentile Max

Blacktail Creek (site 1, fig. 1, table 1) 10/2000– 8/2010 Streamflow, instantaneous, ft3/s 72 1.9 4.6 8.8 12 17 5

Specific conductance, μS/cm 72 161 226 265 267 316 36

pH, standard units 72 7.3 7.6 7.7 7.7 7.8 

Hardness, filtered, mg/L as CaCO3 72 65 87 106 105 122 14

Calcium, filtered, mg/L 72 19.2 25.0 30.5 30.1 35.0 4

Magnesium, filtered, mg/L 72 4.09 6.00 7.23 7.24 8.68 1

Cadmium, filtered, μg/L 70 (12) 0.02 0.03 0.04 0.04 0.05 

Cadmium, unfiltered-recoverable, μg/L 72 (18) 0.02 0.02 0.03 0.04 0.05 Copper, filtered, μg/L 71 (1) 0.80 1.6 3.0 3.6 5.2 

Copper, unfiltered-recoverable, μg/L 72 1.7 3.4 5.0 5.8 7.6 1

Iron, filtered, μg/L 72 15 68 163 180 277 64

Iron unfiltered-recoverable, μg/L 72 150 334 544 561 693 1,64

Lead, filtered, μg/L 72 (20) 0.04 0.06 0.11 0.17 0.21 

Lead, unfiltered-recoverable, μg/L 72 (9) 0.10 0.35 0.59 0.79 0.92 Manganese, filtered, μg/L 72 14 29 34 41 44 14

Manganese, unfiltered-recoverable, μg/L 72 24 42 48 55 60 17Zinc, filtered, μg/L 70 (2) 0.82 2.2 2.9 3.2 3.9 

Zinc, unfiltered-recoverable, μg/L 72 2.0 3.0 4.3 6.0 6.3 3

Arsenic, filtered, μg/L 71 1.1 2.2 3.1 3.9 5.1 

Arsenic, unfiltered-recoverable, μg/L 72 (1) 1.0 3.0 4.2 4.9 6.5 1Suspended sediment, mg/L 72 1 4 7 8 10 3

Suspended sediment, percent fines4 72 54 77 82 81 87 9
10/2000– 8/2010 Streamflow, instantaneous, ft3/s 80 13 17 22 27 33 7

Specific conductance, μS/cm 80 238 402 484 468 546 61pH, standard units 80 7.4 7.6 7.6 7.6 7.7 Hardness, filtered, mg/L as CaCO3 80 79 126 154 149 170 21

Calcium, filtered, mg/L 80 23.4 36.7 43.9 42.5 48.1 6Magnesium, filtered, mg/L 80 5.02 8.86 10.6 10.4 12.0 1Cadmium, filtered, μg/L 80 0.05 0.10 0.13 0.33 0.24 

Cadmium, unfiltered-recoverable, μg/L 80 0.09 0.15 0.22 0.46 0.57 Copper, filtered, μg/L 80 3.20 8.3 11 11 13 2

Copper, unfiltered-recoverable, μg/L 80 9.5 16 20 24 25 11Iron, filtered, μg/L 80 10.0 28 51 85 143 26

Iron unfiltered-recoverable, μg/L 80 85 278 414 480 571 2,97Lead, filtered, μg/L 79 (8) 0.12 0.20 0.25 0.27 0.33 

Lead, unfiltered-recoverable, μg/L 80 (1) 0.64 1.4 2.0 2.8 2.8 3Manganese, filtered, μg/L 80 21 71 109 150 177 50

Manganese, unfiltered-recoverable, μg/L 80 26 98 129 180 200 55Zinc, filtered, μg/L 80 16 34 47 100 104 47

Zinc, unfiltered-recoverable, μg/L 80 29 44 59 120 173 47Arsenic, filtered, μg/L 80 2.3 4.3 5.2 5.4 6.5 

Arsenic, unfiltered-recoverable, μg/L 80 3.0 5.0 6.3 7.0 8.9 1Suspended sediment, mg/L 80 2 6 8 11 11 9Suspended sediment, percent fines4 80 70 81 86 85 89 9

Table 1–4. Summary information relating to water-quality constituents and properties in samples collected at sites in the upper Clark Fork Basin, Mcollected during water years 2001–2010.—Continued[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. ft3/s, cubic feet per second; NA, not applicable; μS/cm

at 25 degrees Celsius; CaCO3, calcium carbonate; μg/L, micrograms per liter; mg/L, milligrams per liter; ND, distributional parameter not determined because of excessive num

than]

Period of 

waterquality 

sampling 

during 

water years 

2001–10

Constituent or property, 

unadjusted (not flow adjusted) 

units of measurement

Statistical summaries of water-quality data1 

Number of 

samples (values 

in parentheses 

indicate number 

of censored 

values)

Minimum 

uncensored 

value2

25th 

percentile Median Mean 75th percentile Max

Silver Bow Creek at Butte (site 2, fig. 1, table 1)
Silver Bow Creek at Opportunity (site 3, fig. 1, table 1) 10/2000– 8/2010 Streamflow, instantaneous, ft3/s 80 13 29 44 65 88 22

Specific conductance, μS/cm 80 248 348 441 437 527 63pH, standard units 80 7.8 8.2 8.4 8.4 8.8 Hardness, filtered, mg/L as CaCO3 80 81 129 159 157 184 24

Calcium, filtered, mg/L 80 24.2 38.9 47.3 46.5 54.3 7Magnesium, filtered, mg/L 80 4.96 7.30 9.89 9.87 12.0 1Cadmium, filtered, μg/L 79 0.17 0.26 0.40 0.55 0.70 

Cadmium, unfiltered-recoverable, μg/L 80 0.38 0.67 0.80 1.2 1.3 Copper, filtered, μg/L 78 12 20 25 31 36 14

Copper, unfiltered-recoverable, μg/L 80 31 61 79 120 110 86Iron, filtered, μg/L 80 (1) 7.0 14 23 42 67 24

Iron unfiltered-recoverable, μg/L 79 240 463 621 1,136 889 9,19Lead, filtered, μg/L 80 (8) 0.20 0.29 0.44 0.54 0.60 

Lead, unfiltered-recoverable, μg/L 80 5.0 10 13 25 20 26Manganese, filtered, μg/L 80 30 120 223 254 342 93

Manganese, unfiltered-recoverable, μg/L 80 85 200 290 361 437 1,52Zinc, filtered, μg/L 79 11 48 80 129 175 61

Zinc, unfiltered-recoverable, μg/L 80 70 139 197 273 284 1,23Arsenic, filtered, μg/L 80 6.4 9.5 11 12 13 2

Arsenic, unfiltered-recoverable, μg/L 80 9.1 14 16 20 20 9Suspended sediment, mg/L 80 5 13 18 33 30 28Suspended sediment, percent fines4 80 45 78 84 81 86 9

Table 1–4. Summary information relating to water-quality constituents and properties in samples collected at sites in the upper Clark Fork Basin, Mcollected during water years 2001–2010.—Continued [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. ft3/s, cubic feet per second; NA, not applicable; μS/cm

at 25 degrees Celsius; CaCO3, calcium carbonate; μg/L, micrograms per liter; mg/L, milligrams per liter; ND, distributional parameter not determined because of excessive num

than] Period of water- quality sampling during water years 2001–10 Constituent or property, unadjusted (not flow adjusted) units of measurement Statistical summaries of water-quality data1 

Number of samples (values in parentheses indicate number of censored values) Minimum uncensored value2 25th percentile Median Mean 75th percentile Max
Mill Creek near Anaconda (site 4, fig. 1, table 1) 12/2004– 8/2010 Streamflow, instantaneous, ft3/s 48 7.4 13 26 58 102 21

Specific conductance, μS/cm 48 56 87 133 132 177 21pH, standard units 48 7.6 7.9 8.1 8.1 8.2 Hardness, filtered, mg/L as CaCO3 48 24 36 60 57 79 9

Calcium, filtered, mg/L 48 7.00 10.5 16.9 15.8 21.7 2Magnesium, filtered, mg/L 48 1.45 2.39 4.22 4.28 5.96 Cadmium, filtered, μg/L 47 (2) 0.02 0.03 0.04 0.04 0.05 

Cadmium, unfiltered-recoverable, μg/L 48 0.03 0.05 0.07 0.08 0.09 Copper, filtered, μg/L 48 0.72 1.3 2.1 2.3 3.1 

Copper, unfiltered-recoverable, μg/L 48 (1) 1.3 2.1 3.5 3.9 5.4 1Iron, filtered, μg/L 48 21.0 30 41 47 63 12

Iron unfiltered-recoverable, μg/L 48 78 123 163 192 203 61Lead, filtered, μg/L 48 (3) 0.02 0.08 0.11 0.12 0.16 

Lead, unfiltered-recoverable, μg/L 48 0.15 0.42 0.59 0.76 0.88 Manganese, filtered, μg/L 48 3.2 4.2 5.9 5.9 6.9 1

Manganese, unfiltered-recoverable, μg/L 48 7.4 11 13 14 15 3Zinc, filtered, μg/L 48 (5) 0.73 1.1 1.4 1.5 1.9 

Zinc, unfiltered-recoverable, μg/L 48 1.0 1.8 2.6 3.0 3.9 Arsenic, filtered, μg/L 48 7.3 12 15 16 20 3

Arsenic, unfiltered-recoverable, μg/L 48 8.6 13 17 18 23 3Suspended sediment, mg/L 48 1 2 3 6 6 2Suspended sediment, percent fines4 48 28 57 67 64 75 8

Table 1–4. Summary information relating to water-quality constituents and properties in samples collected at sites in the upper Clark Fork Basin, Mcollected during water years 2001–2010.—Continued[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. ft3/s, cubic feet per second; NA, not applicable; μS/cm

at 25 degrees Celsius; CaCO3, calcium carbonate; μg/L, micrograms per liter; mg/L, milligrams per liter; ND, distributional parameter not determined because of excessive num

than]

Period of 

waterquality 

sampling 

during 

water years 

2001–10

Constituent or property, 

unadjusted (not flow adjusted) 

units of measurement

Statistical summaries of water-quality data1 

Number of 

samples (values 

in parentheses 

indicate number 

of censored 

values)

Minimum 

uncensored 

value2

25th 

percentile Median Mean 75th percentile Max
Mill Creek at Opportunity (site 5, fig. 1, table 1) 3/2003– 8/2010 Streamflow, instantaneous, ft3/s 64 0.43 3.6 9.7 35 48 26

Specific conductance, μS/cm 64 59 95 153 148 196 23pH, standard units 64 7.7 7.9 8.0 8.0 8.1 Hardness, filtered, mg/L as CaCO3 64 24 39 67 63 85 10

Calcium, filtered, mg/L 64 7.01 11.5 18.8 17.6 23.5 2Magnesium, filtered, mg/L 64 1.56 2.64 4.67 4.56 6.30 Cadmium, filtered, μg/L 64 0.02 0.05 0.06 0.06 0.07 

Cadmium, unfiltered-recoverable, μg/L 64 0.04 0.07 0.10 0.15 0.13 Copper, filtered, μg/L 64 1.0 1.9 2.7 3.0 3.9 

Copper, unfiltered-recoverable, μg/L 64 1.5 2.8 4.4 6.8 7.2 3Iron, filtered, μg/L 64 16 36 43 48 60 9

Iron unfiltered-recoverable, μg/L 64 44 113 145 299 264 1,96Lead, filtered, μg/L 64 (6) 0.02 0.09 0.13 0.14 0.17 

Lead, unfiltered-recoverable, μg/L 64 0.07 0.23 0.45 1.5 1.6 1Manganese, filtered, μg/L 64 2.2 4.5 5.6 7.7 9.2 3

Manganese, unfiltered-recoverable, μg/L 64 3.3 10 13 20 18 11

Zinc, filtered, μg/L 63 (1) 1.3 2.1 2.8 3.0 3.8 

Zinc, unfiltered-recoverable, μg/L 64 1.7 3.0 4.9 6.7 7.0 4Arsenic, filtered, μg/L 64 9.0 16 21 22 28 5

Arsenic, unfiltered-recoverable, μg/L 64 10 20 25 26 31 5Suspended sediment, mg/L 64 1 1 2 12 10 10Suspended sediment, percent fines4 64 26 57 74 69 82 9

Table 1–4. Summary information relating to water-quality constituents and properties in samples collected at sites in the upper Clark Fork Basin, Mcollected during water years 2001–2010.—Continued [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. ft3/s, cubic feet per second; NA, not applicable; μS/cm

at 25 degrees Celsius; CaCO3, calcium carbonate; μg/L, micrograms per liter; mg/L, milligrams per liter; ND, distributional parameter not determined because of excessive num

than] Period of water- quality sampling during water years 2001–10 Constituent or property, unadjusted (not flow adjusted) units of measurement Statistical summaries of water-quality data1 

Number of samples (values in parentheses indicate number of censored values) Minimum uncensored value2 25th percentile Median Mean 75th percentile Max
Willow Creek near Anaconda (site 6, fig. 1, table 1) 12/2004– 8/2010 Streamflow, instantaneous, ft3/s 44 1.0 2.5 7.6 14 16 7

Specific conductance, μS/cm 44 66 88 108 102 114 14pH, standard units 44 7.5 7.6 7.7 7.7 7.8 Hardness, filtered, mg/L as CaCO3 44 22 32 38 37 41 5

Calcium, filtered, mg/L 44 7.56 10.8 13.0 12.4 13.8 1Magnesium, filtered, mg/L 44 0.78 1.18 1.43 1.44 1.64 Cadmium, filtered, μg/L 42 (5) 0.02 0.02 0.03 0.03 0.04 

Cadmium, unfiltered-recoverable, μg/L 44 (3) 0.02 0.04 0.05 0.06 0.07 Copper, filtered, μg/L 44 0.90 1.3 1.9 2.0 2.4 

Copper, unfiltered-recoverable, μg/L 44 (1) 1.0 2.1 3.1 3.7 4.1 1Iron, filtered, μg/L 44 28 51 65 80 85 27

Iron unfiltered-recoverable, μg/L 44 86 134 216 342 405 2,38Lead, filtered, μg/L 44 0.03 0.07 0.13 0.14 0.16 

Lead, unfiltered-recoverable, μg/L 44 0.10 0.24 0.49 0.87 0.94 Manganese, filtered, μg/L 44 6.0 11 13 14 18 3

Manganese, unfiltered-recoverable, μg/L 44 14 19 23 26 28 10Zinc, filtered, μg/L 44 (4) 0.65 1.2 1.6 1.7 2.0 

Zinc, unfiltered-recoverable, μg/L 44 (4) 1.0 1.5 2.1 3.1 3.9 1Arsenic, filtered, μg/L 44 9.9 13 14 15 17 2

Arsenic, unfiltered-recoverable, μg/L 44 10 14 15 16 18 2Suspended sediment, mg/L 44 1 3 6 17 13 19Suspended sediment, percent fines4 44 25 62 78 73 88 9

Table 1–4. Summary information relating to water-quality constituents and properties in samples collected at sites in the upper Clark Fork Basin, Mcollected during water years 2001–2010.—Continued[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. ft3/s, cubic feet per second; NA, not applicable; μS/cm

at 25 degrees Celsius; CaCO3, calcium carbonate; μg/L, micrograms per liter; mg/L, milligrams per liter; ND, distributional parameter not determined because of excessive num

than]

Period of 

waterquality 

sampling 

during 

water years 

2001–10

Constituent or property, 

unadjusted (not flow adjusted) 

units of measurement

Statistical summaries of water-quality data1 

Number of 

samples (values 

in parentheses 

indicate number 

of censored 

values)

Minimum 

uncensored 

value2

25th 

percentile Median Mean 75th percentile Max
Willow Creek at Opportunity (site 7, fig. 1, table 1) 3/2003– 8/2010 Streamflow, instantaneous, ft3/s 64 4.5 6.2 9.1 17 20 7

Specific conductance, μS/cm 64 116 227 297 276 315 37pH, standard units 64 7.7 8.0 8.1 8.1 8.3 Hardness, filtered, mg/L as CaCO3 64 65 98 130 120 137 16

Calcium, filtered, mg/L 64 20.10 28.8 36.9 34.8 39.8 4Magnesium, filtered, mg/L 64 3.52 6.27 8.56 7.98 9.40 1Cadmium, filtered, μg/L 64 (5) 0.02 0.03 0.04 0.04 0.06 

Cadmium, unfiltered-recoverable, μg/L 64 (1) 0.02 0.05 0.07 0.10 0.14 Copper, filtered, μg/L 64 1.1 2.4 3.6 5.6 8.6 2

Copper, unfiltered-recoverable, μg/L 64 2.8 4.6 8.1 12 16 4Iron, filtered, μg/L 64 7.0 14 36 44 68 17

Iron unfiltered-recoverable, μg/L 64 27 108 205 272 332 1,42Lead, filtered, μg/L 64 (1) 0.04 0.10 0.19 0.22 0.30 

Lead, unfiltered-recoverable, μg/L 64 0.27 0.85 1.5 2.3 2.6 1Manganese, filtered, μg/L 64 3.3 15 24 32 45 20

Manganese, unfiltered-recoverable, μg/L 64 4.7 19 35 45 58 22Zinc, filtered, μg/L 64 (3) 0.84 2.3 3.9 5.3 7.0 2

Zinc, unfiltered-recoverable, μg/L 64 1.1 4.8 9.5 13 17 6Arsenic, filtered, μg/L 64 11 17 32 42 63 16

Arsenic, unfiltered-recoverable, μg/L 64 12 19 33 45 65 16Suspended sediment, mg/L 64 1 3 5 11 15 8Suspended sediment, percent fines4 64 55 80 88 85 92 9

Table 1–4. Summary information relating to water-quality constituents and properties in samples collected at sites in the upper Clark Fork Basin, Mcollected during water years 2001–2010.—Continued [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. ft3/s, cubic feet per second; NA, not applicable; μS/cm

at 25 degrees Celsius; CaCO3, calcium carbonate; μg/L, micrograms per liter; mg/L, milligrams per liter; ND, distributional parameter not determined because of excessive num

than] Period of water- quality sampling during water years 2001–10 Constituent or property, unadjusted (not flow adjusted) units of measurement Statistical summaries of water-quality data1 

Number of samples (values in parentheses indicate number of censored values) Minimum uncensored value2 25th percentile Median Mean 75th percentile Max
Silver Bow Creek at Warm Springs (site 8, fig. 1, table 1) 10/2000– 8/2010 Streamflow, instantaneous, ft3/s 80 21 41 65 112 146 48

Specific conductance, μS/cm 80 253 386 507 488 582 78pH, standard units 80 8.1 8.5 8.8 8.8 9.0 Hardness, filtered, mg/L as CaCO3 80 104 154 207 203 242 31

Calcium, filtered, mg/L 80 29.1 45.4 60.7 58.8 69.6 9Magnesium, filtered, mg/L 80 6.04 10.5 13.7 13.5 16.5 2Cadmium, filtered, μg/L 80 (14) 0.02 0.03 0.04 0.05 0.06 

Cadmium, unfiltered-recoverable, μg/L 80 (4) 0.03 0.07 0.10 0.12 0.14 Copper, filtered, μg/L 80 1.7 3.3 4.3 5.0 6.0 2

Copper, unfiltered-recoverable, μg/L 80 2.4 5.7 8.0 11 12 9Iron, filtered, μg/L 80 (7) 4.0 8.0 15 18 24 7

Iron unfiltered-recoverable, μg/L 80 36 143 218 258 272 1,42Lead, filtered, μg/L 80 (23) 0.03 0.05 0.08 0.10 0.13 

Lead, unfiltered-recoverable, μg/L 80 (7) 0.15 0.58 1.2 1.6 1.8 4Manganese, filtered, μg/L 80 13 50 78 122 167 87

Manganese, unfiltered-recoverable, μg/L 80 24 95 138 175 221 89Zinc, filtered, μg/L 80 (2) 0.59 2.1 3.5 4.5 5.2 3

Zinc, unfiltered-recoverable, μg/L 80 2.0 6.5 11 15 17 15Arsenic, filtered, μg/L 80 6.8 15 26 24 32 4

Arsenic, unfiltered-recoverable, μg/L 80 10.0 18 29 28 35 5Suspended sediment, mg/L 80 1 3 5 7 7 4Suspended sediment, percent fines4 80 47 79 84 82 88 9

Table 1–4. Summary information relating to water-quality constituents and properties in samples collected at sites in the upper Clark Fork Basin, Mcollected during water years 2001–2010.—Continued[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. ft3/s, cubic feet per second; NA, not applicable; μS/cm

at 25 degrees Celsius; CaCO3, calcium carbonate; μg/L, micrograms per liter; mg/L, milligrams per liter; ND, distributional parameter not determined because of excessive num

than]

Period of 

waterquality 

sampling 

during 

water years 

2001–10

Constituent or property, 

unadjusted (not flow adjusted) 

units of measurement

Statistical summaries of water-quality data1 

Number of 

samples (values 

in parentheses 

indicate number 

of censored 

values)

Minimum 

uncensored 

value2

25th 

percentile Median Mean 75th percentile Max
Warm Springs Creek near Anaconda (site 9, fig. 1, table 1) 10/2005– 8/2010 Streamflow, instantaneous, ft3/s 30 41 61 84 137 170 57

Specific conductance, μS/cm 30 125 184 235 217 253 27pH, standard units 30 8.0 8.4 8.5 8.5 8.6 Hardness, filtered, mg/L as CaCO3 30 58 89 112 105 124 13

Calcium, filtered, mg/L 30 18.5 27.5 33.8 31.6 36.5 3Magnesium, filtered, mg/L 30 2.96 4.90 6.90 6.36 7.85 Cadmium, filtered, μg/L 30 (9) 0.02 0.02 0.02 0.02 0.02 

Cadmium, unfiltered-recoverable, μg/L 30 (3) 0.01 0.02 0.03 0.04 0.04 Copper, filtered, μg/L 30 (2) 0.57 0.68 0.91 1.1 1.3 

Copper, unfiltered-recoverable, μg/L 30 (3) 1.1 1.5 2.1 3.2 3.3 2Iron, filtered, μg/L 30 (3) 2.0 5.0 6.0 7.1 10 1

Iron unfiltered-recoverable, μg/L 30 28 64 75 144 123 1,00Lead, filtered, μg/L 30 (21) 0.02 0.01 0.02 0.03 0.03 

Lead, unfiltered-recoverable, μg/L 30 0.08 0.20 0.26 0.52 0.43 Manganese, filtered, μg/L 30 (1) 0.50 0.70 0.95 1.2 1.5 

Manganese, unfiltered-recoverable, μg/L 30 1.20 2.8 4.3 6.9 6.9 4Zinc, filtered, μg/L 30 (14) 0.30 0.50 0.74 0.89 1.1 

Zinc, unfiltered-recoverable, μg/L 30 (4) 1.0 1.0 2.0 3.2 3.6 2Arsenic, filtered, μg/L 30 1.8 2 2 2 2 

Arsenic, unfiltered-recoverable, μg/L 30 2 2 3 3 3 Suspended sediment, mg/L 30 1 3 4 9 6 6Suspended sediment, percent fines4 30 32 61 66 64 70 8

Table 1–4. Summary information relating to water-quality constituents and properties in samples collected at sites in the upper Clark Fork Basin, Mcollected during water years 2001–2010.—Continued [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. ft3/s, cubic feet per second; NA, not applicable; μS/cm

at 25 degrees Celsius; CaCO3, calcium carbonate; μg/L, micrograms per liter; mg/L, milligrams per liter; ND, distributional parameter not determined because of excessive num

than] Period of water- quality sampling during water years 2001–10 Constituent or property, unadjusted (not flow adjusted) units of measurement Statistical summaries of water-quality data1 

Number of samples (values in parentheses indicate number of censored values) Minimum uncensored value2 25th percentile Median Mean 75th percentile Max
Warm Springs Creek at Warm Springs (site 10, fig. 1, table 1) 10/2000– 8/2010 Streamflow, instantaneous, ft3/s 63 14 37 48 83 102 38

Specific conductance, μS/cm 63 155 219 308 287 339 43pH, standard units 63 7.8 8.1 8.2 8.3 8.4 Hardness, filtered, mg/L as CaCO3 63 40 106 148 138 167 22

Calcium, filtered, mg/L 63 10.5 32.5 45.4 42.2 50.8 6Magnesium, filtered, mg/L 63 3.29 5.84 8.49 7.93 9.48 1Cadmium, filtered, μg/L 63 (14) 0.02 0.02 0.03 0.03 0.04 

Cadmium, unfiltered-recoverable, μg/L 63 (9) 0.03 0.04 0.05 0.08 0.08 Copper, filtered, μg/L 63 (1) 1.1 2.0 2.7 2.9 3.2 1Copper, unfiltered-recoverable, μg/L 63 3.9 6.4 8.1 18 14 14Iron, filtered, μg/L 63 (4) 5.0 8.0 10 11 14 2

Iron unfiltered-recoverable, μg/L 63 39 76 102 272 219 2,11Lead, filtered, μg/L 63 (31) 0.03 0.03 0.04 0.05 0.06 

Lead, unfiltered-recoverable, μg/L 63 (10) 0.21 0.34 0.60 1.3 1.2 1Manganese, filtered, μg/L 63 19 48 78 98 135 39

Manganese, unfiltered-recoverable, μg/L 63 37 95 142 180 203 1,27Zinc, filtered, μg/L 62 (7) 0.60 0.92 1.5 1.6 1.9 

Zinc, unfiltered-recoverable, μg/L 63 1.0 2.7 3.0 7.1 6.0 4Arsenic, filtered, μg/L 63 3.0 3.9 4.5 5.0 5.7 1

Arsenic, unfiltered-recoverable, μg/L 63 3.7 5.0 5.5 6.9 7.1 2Suspended sediment, mg/L 63 1 3 6 15 12 12Suspended sediment, percent fines4 63 43 65 68 68 73 8

Table 1–4. Summary information relating to water-quality constituents and properties in samples collected at sites in the upper Clark Fork Basin, Mcollected during water years 2001–2010.—Continued[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. ft3/s, cubic feet per second; NA, not applicable; μS/cm

at 25 degrees Celsius; CaCO3, calcium carbonate; μg/L, micrograms per liter; mg/L, milligrams per liter; ND, distributional parameter not determined because of excessive num

than]

Period of 

waterquality 

sampling 

during 

water years 

2001–10

Constituent or property, 

unadjusted (not flow adjusted) 

units of measurement

Statistical summaries of water-quality data1 

Number of 

samples (values 

in parentheses 

indicate number 

of censored 

values)

Minimum 

uncensored 

value2

25th 

percentile Median Mean 75th percentile Max
Clark Fork near Galen (site 11, fig. 1, table 1) 10/2000– 8/2010 Streamflow, instantaneous, ft3/s 80 33 79 127 203 258 90

Specific conductance, μS/cm 80 216 320 416 405 480 60pH, standard units 80 8.1 8.4 8.6 8.6 8.8 Hardness, filtered, mg/L as CaCO3 80 91 135 188 176 211 28

Calcium, filtered, mg/L 80 27.5 41.0 55.5 52.2 62.0 8Magnesium, filtered, mg/L 80 5.11 8.30 11.8 11.2 13.8 1Cadmium, filtered, μg/L 80 (13) 0.02 0.03 0.04 0.05 0.05 

Cadmium, unfiltered-recoverable, μg/L 80 (5) 0.04 0.07 0.10 0.12 0.15 Copper, filtered, μg/L 80 1.7 3.4 4.2 4.8 5.6 2

Copper, unfiltered-recoverable, μg/L 80 4.1 8.4 12 19 17 9Iron, filtered, μg/L 80 (5) 4.0 7.0 11 14 19 6

Iron unfiltered-recoverable, μg/L 80 56 135 224 332 324 2,03Lead, filtered, μg/L 80 (26) 0.04 0.04 0.06 0.08 0.10 

Lead, unfiltered-recoverable, μg/L 80 (8) 0.36 0.69 1.3 2.3 2.1 3Manganese, filtered, μg/L 80 24 42 67 94 120 46

Manganese, unfiltered-recoverable, μg/L 80 47 104 149 177 227 78Zinc, filtered, μg/L 80 (3) 0.90 1.7 2.7 3.5 4.7 3

Zinc, unfiltered-recoverable, μg/L 80 2.7 7.5 11 16 18 11

Arsenic, filtered, μg/L 80 5.7 10 16 16 20 3

Arsenic, unfiltered-recoverable, μg/L 80 7.5 12 18 19 23 5Suspended sediment, mg/L 80 1 4 7 13 12 9Suspended sediment, percent fines4 80 40 70 76 75 80 9

Table 1–4. Summary information relating to water-quality constituents and properties in samples collected at sites in the upper Clark Fork Basin, Mcollected during water years 2001–2010.—Continued [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. ft3/s, cubic feet per second; NA, not applicable; μS/cm

at 25 degrees Celsius; CaCO3, calcium carbonate; μg/L, micrograms per liter; mg/L, milligrams per liter; ND, distributional parameter not determined because of excessive num

than] Period of water- quality sampling during water years 2001–10 Constituent or property, unadjusted (not flow adjusted) units of measurement Statistical summaries of water-quality data1 

Number of samples (values in parentheses indicate number of censored values) Minimum uncensored value2 25th percentile Median Mean 75th percentile Max
Lost Creek near Anaconda (site 12, fig. 1, table 1) 12/2004– 8/2010 Streamflow, instantaneous, ft3/s 47 0.37 4.3 8.0 10 13 5

Specific conductance, μS/cm 47 121 173 211 198 221 25pH, standard units 47 7.4 8.2 8.2 8.2 8.3 Hardness, filtered, mg/L as CaCO3 47 50 85 99 94 106 12

Calcium, filtered, mg/L 47 15.7 26.3 30.0 28.5 32.2 3Magnesium, filtered, mg/L 47 2.71 4.54 5.78 5.50 6.45 Cadmium, filtered, μg/L 46 (5) 0.01 0.02 0.03 0.03 0.03 

Cadmium, unfiltered-recoverable, μg/L 47 (5) 0.01 0.03 0.04 0.12 0.07 14Copper, filtered, μg/L 47 0.86 1.3 1.8 3.9 2.9 9

Copper, unfiltered-recoverable, μg/L 47 (1) 1.7 2.8 4.4 16.3 8.3 29,10Iron, filtered, μg/L 47 (4) 4.0 6.0 9.0 10 12 2

Iron unfiltered-recoverable, μg/L 47 22 61 98 2,300 217 99,70Lead, filtered, μg/L 47 (26) 0.02 0.02 0.03 0.04 0.05 

Lead, unfiltered-recoverable, μg/L 47 0.10 0.28 0.42 28 1.0 1,29Manganese, filtered, μg/L 47 (1) 0.40 0.80 1.1 1.6 1.5 4

Manganese, unfiltered-recoverable, μg/L 47 1.2 2.8 4.6 194 7.6 8,83Zinc, filtered, μg/L 47 (14) 0.62 0.71 1.1 1.3 1.5 3

Zinc, unfiltered-recoverable, μg/L 47 (2) 1.0 2.0 2.4 7.7 4.0 7,78Arsenic, filtered, μg/L 47 1.8 2.7 3.4 7.8 5.8 15

Arsenic, unfiltered-recoverable, μg/L 47 2.0 3.2 3.7 87 7.7 3,86Suspended sediment, mg/L 47 1 3 5 1,270 15 58,90Suspended sediment, percent fines4 47 22 49 61 58 67 9

Table 1–4. Summary information relating to water-quality constituents and properties in samples collected at sites in the upper Clark Fork Basin, Mcollected during water years 2001–2010.—Continued[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. ft3/s, cubic feet per second; NA, not applicable; μS/cm

at 25 degrees Celsius; CaCO3, calcium carbonate; μg/L, micrograms per liter; mg/L, milligrams per liter; ND, distributional parameter not determined because of excessive num

than]

Period of 

waterquality 

sampling 

during 

water years 

2001–10

Constituent or property, 

unadjusted (not flow adjusted) 

units of measurement

Statistical summaries of water-quality data1 

Number of 

samples (values 

in parentheses 

indicate number 

of censored 

values)

Minimum 

uncensored 

value2

25th 

percentile Median Mean 75th percentile Max
Lost Creek near Galen (site 13, fig. 1, table 1) 3/2003– 8/2010 Streamflow, instantaneous, ft3/s 64 1.3 3.5 13 21 41 7

Specific conductance, μS/cm 64 540 611 631 648 672 93pH, standard units 64 8.0 8.2 8.3 8.3 8.5 Hardness, filtered, mg/L as CaCO3 64 203 281 298 301 316 45

Calcium, filtered, mg/L 64 48.5 78.5 85.2 84.7 92.2 12Magnesium, filtered, mg/L 64 17.3 19.9 21.0 21.7 23.2 3Cadmium, filtered, μg/L 63 (10) 0.01 0.02 0.02 0.03 0.03 

Cadmium, unfiltered-recoverable, μg/L 64 (5) 0.01 0.03 0.04 0.04 0.05 Copper, filtered, μg/L 64 0.99 1.6 2.4 2.4 2.9 

Copper, unfiltered-recoverable, μg/L 64 1.6 3.7 4.4 5.5 6.3 2Iron, filtered, μg/L 64 (3) 4.0 7.0 10 13 16 6

Iron unfiltered-recoverable, μg/L 64 14 47 77 99 128 29Lead, filtered, μg/L 63 (35) 0.02 0.02 0.03 0.04 0.05 

Lead, unfiltered-recoverable, μg/L 64 0.04 0.14 0.24 0.35 0.44 Manganese, filtered, μg/L 64 1.9 6.9 14 15 20 5

Manganese, unfiltered-recoverable, μg/L 64 2.2 9.6 18 20 29 5Zinc, filtered, μg/L 63 (9) 0.40 0.87 1.3 1.5 1.8 

Zinc, unfiltered-recoverable, μg/L 64 (6) 1.0 1.3 2.0 2.8 3.9 Arsenic, filtered, μg/L 64 6.0 10 13 14 16 4

Arsenic, unfiltered-recoverable, μg/L 64 6.0 11 14 15 16 4Suspended sediment, mg/L 64 2 8 15 16 22 7Suspended sediment, percent fines4 64 18 46 60 57 68 8

Table 1–4. Summary information relating to water-quality constituents and properties in samples collected at sites in the upper Clark Fork Basin, Mcollected during water years 2001–2010.—Continued [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. ft3/s, cubic feet per second; NA, not applicable; μS/cm

at 25 degrees Celsius; CaCO3, calcium carbonate; μg/L, micrograms per liter; mg/L, milligrams per liter; ND, distributional parameter not determined because of excessive num

than] Period of water- quality sampling during water years 2001–10 Constituent or property, unadjusted (not flow adjusted) units of measurement Statistical summaries of water-quality data1 

Number of samples (values in parentheses indicate number of censored values) Minimum uncensored value2 25th percentile Median Mean 75th percentile Max
Clark Fork at Deer Lodge (site 14, fig. 1, table 1) 10/2000– 8/2010 Streamflow, instantaneous, ft3/s 80 38 151 215 282 343 1,13

Specific conductance, μS/cm 80 245 360 489 452 524 60pH, standard units 80 8.0 8.2 8.3 8.3 8.5 Hardness, filtered, mg/L as CaCO3 80 102 152 210 196 230 28

Calcium, filtered, mg/L 80 31.8 45.3 62.3 58.1 67.5 8Magnesium, filtered, mg/L 80 5.53 10.1 13.2 12.5 14.7 1Cadmium, filtered, μg/L 79 (9) 0.02 0.04 0.06 0.06 0.07 

Cadmium, unfiltered-recoverable, μg/L 79 (1) 0.02 0.10 0.15 0.22 0.24 Copper, filtered, μg/L 80 3.2 5.6 7.1 7.9 9.5 1

Copper, unfiltered-recoverable, μg/L 79 8.2 18 27 49 57 46Iron, filtered, μg/L 80 (9) 3.0 6.0 8.0 12 17 4

Iron unfiltered-recoverable, μg/L 80 27 218 393 711 823 6,96

Lead, filtered, μg/L 79 (19) 0.04 0.05 0.08 0.10 0.13 

Lead, unfiltered-recoverable, μg/L 80 (2) 0.33 1.8 3.0 6.7 7.0 6Manganese, filtered, μg/L 80 4.0 24 34 38 49 9

Manganese, unfiltered-recoverable, μg/L 80 12 76 112 143 164 1,01

Zinc, filtered, μg/L 80 0.90 4.5 5.9 6.5 8.1 1

Zinc, unfiltered-recoverable, μg/L 78 4.0 16 26 40 44 35Arsenic, filtered, μg/L 80 6.0 11 15 15 17 2

Arsenic, unfiltered-recoverable, μg/L 79 4.8 13 17 20 24 7Suspended sediment, mg/L 80 1 11 18 37 40 38Suspended sediment, percent fines4 80 31 67 73 71 79 9

Table 1–4. Summary information relating to water-quality constituents and properties in samples collected at sites in the upper Clark Fork Basin, Mcollected during water years 2001–2010.—Continued[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. ft3/s, cubic feet per second; NA, not applicable; μS/cm

at 25 degrees Celsius; CaCO3, calcium carbonate; μg/L, micrograms per liter; mg/L, milligrams per liter; ND, distributional parameter not determined because of excessive num

than]

Period of 

waterquality 

sampling 

during 

water years 

2001–10

Constituent or property, 

unadjusted (not flow adjusted) 

units of measurement

Statistical summaries of water-quality data1 

Number of 

samples (values 

in parentheses 

indicate number 

of censored 

values)

Minimum 

uncensored 

value2

25th 

percentile Median Mean 75th percentile Max
Little Blackfoot River (site 15, fig. 1, table 1) 11/2000– 8/2004 Streamflow, instantaneous, ft3/s 20 19.0 56.3 135 176 297 45

Specific conductance, μS/cm 20 174 211 259 255 287 34pH, standard units 20 8.0 8.1 8.2 8.3 8.4 Hardness, filtered, mg/L as CaCO3 20 79 94 116 117 136 17

Calcium, filtered, mg/L 20 22.9 27.6 34.0 34.1 39.9 4Magnesium, filtered, mg/L 20 5.33 6.03 7.56 7.69 8.83 1Cadmium, filtered, μg/L 20 (18) 0.02 ND ND ND ND 

Cadmium, unfiltered-recoverable, μg/L 20 (14) 0.02 0.01 0.02 0.02 0.03 Copper, filtered, μg/L 20 (3) 0.60 0.75 1.1 1.3 1.5 

Copper, unfiltered-recoverable, μg/L 20 0.80 1.2 1.7 1.9 2.4 Iron, filtered, μg/L 20 (3) 5.0 5.5 16 27 33 11Iron unfiltered-recoverable, μg/L 20 38 63 178 236 305 70Lead, filtered, μg/L 19 (13) 0.05 0.03 0.04 0.05 0.06 

Lead, unfiltered-recoverable, μg/L 20 (8) 0.06 0.11 0.24 0.45 0.51 Manganese, filtered, μg/L 20 2.5 7.9 8.5 11 11 4

Manganese, unfiltered-recoverable, μg/L 20 6.8 18 23 28 32 9Zinc, filtered, μg/L 20 (4) 0.50 0.50 1.2 1.1 1.4 

Zinc, unfiltered-recoverable, μg/L 20 (5) 1.0 0.89 2.0 2.8 3.5 Arsenic, filtered, μg/L 20 3.9 4.6 5.1 5.2 5.8 

Arsenic, unfiltered-recoverable, μg/L 20 4.0 5.0 6.0 5.7 6.0 Suspended sediment, mg/L 20 2 3 8 13 14 4Suspended sediment, percent fines4 20 54 73 83 80 85 9

Table 1–4. Summary information relating to water-quality constituents and properties in samples collected at sites in the upper Clark Fork Basin, Mcollected during water years 2001–2010.—Continued [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. ft3/s, cubic feet per second; NA, not applicable; μS/cm

at 25 degrees Celsius; CaCO3, calcium carbonate; μg/L, micrograms per liter; mg/L, milligrams per liter; ND, distributional parameter not determined because of excessive num

than] Period of water- quality sampling during water years 2001–10 Constituent or property, unadjusted (not flow adjusted) units of measurement Statistical summaries of water-quality data1 

Number of samples (values in parentheses indicate number of censored values) Minimum uncensored value2 25th percentile Median Mean 75th percentile Max
Clark Fork at Goldcreek (site 16, fig. 1, table 1) 10/2000– 8/2010 Streamflow, instantaneous, ft3/s 80 100 310 495 690 876 2,30

Specific conductance, μS/cm 80 206 304 384 370 435 49pH, standard units 80 8.0 8.3 8.4 8.4 8.6 Hardness, filtered, mg/L as CaCO3 80 90 132 169 163 196 22

Calcium, filtered, mg/L 80 27.0 39.4 50.3 48.2 57.9 6Magnesium, filtered, mg/L 80 5.40 8.17 10.6 10.3 12.8 1Cadmium, filtered, μg/L 80 (18) 0.02 0.03 0.04 0.04 0.05 

Cadmium, unfiltered-recoverable, μg/L 80 (3) 0.03 0.08 0.12 0.17 0.21 Copper, filtered, μg/L 80 2.1 4.1 4.9 5.6 6.6 1

Copper, unfiltered-recoverable, μg/L 80 5.2 12 20 31 43 12Iron, filtered, μg/L 80 (10) 3.0 5.1 11 18 25 7

Iron unfiltered-recoverable, μg/L 80 27 198 371 640 870 3,02Lead, filtered, μg/L 79 (30) 0.04 0.04 0.07 0.10 0.12 

Lead, unfiltered-recoverable, μg/L 80 (2) 0.14 1.3 2.4 4.8 5.8 2Manganese, filtered, μg/L 80 4.0 11 15 17 21 5

Manganese, unfiltered-recoverable, μg/L 80 11 56 76 99 117 34Zinc, filtered, μg/L 80 (1) 0.70 2.2 3.1 4.0 5.2 1

Zinc, unfiltered-recoverable, μg/L 80 2.0 12 20 30 42 12Arsenic, filtered, μg/L 80 5.8 7.8 10 10 12 1

Arsenic, unfiltered-recoverable, μg/L 80 7.0 10 12 13 15 2Suspended sediment, mg/L 80 1 10 17 35 44 19Suspended sediment, percent fines4 80 44 66 77 74 82 9

Table 1–4. Summary information relating to water-quality constituents and properties in samples collected at sites in the upper Clark Fork Basin, Mcollected during water years 2001–2010.—Continued[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. ft3/s, cubic feet per second; NA, not applicable; μS/cm

at 25 degrees Celsius; CaCO3, calcium carbonate; μg/L, micrograms per liter; mg/L, milligrams per liter; ND, distributional parameter not determined because of excessive num

than]

Period of 

waterquality 

sampling 

during 

water years 

2001–10

Constituent or property, 

unadjusted (not flow adjusted) 

units of measurement

Statistical summaries of water-quality data1 

Number of 

samples (values 

in parentheses 

indicate number 

of censored 

values)

Minimum 

uncensored 

value2

25th 

percentile Median Mean 75th percentile Max
Flint Creek (site 17, fig. 1, table 1) 11/2000– 8/2004 Streamflow, instantaneous, ft3/s 23 5.4 29 76 84 113 33

Specific conductance, μS/cm 23 276 305 350 373 430 52pH, standard units 23 8.1 8.3 8.4 8.4 8.5 Hardness, filtered, mg/L as CaCO3 23 115 140 161 172 204 25

Calcium, filtered, mg/L 23 30.5 38.5 43.8 46.8 56.6 7Magnesium, filtered, mg/L 23 9.51 10.9 12.5 13.3 15.4 1Cadmium, filtered, μg/L 23 (20) 0.02 ND ND ND ND 

Cadmium, unfiltered-recoverable, μg/L 23 (9) 0.02 0.02 0.04 0.06 0.07 Copper, filtered, μg/L 23 (2) 0.70 1.0 1.2 1.4 1.4 

Copper, unfiltered-recoverable, μg/L 23 1.4 1.8 2.3 3.3 3.3 1Iron, filtered, μg/L 23 (2) 5.0 8.0 15 23 21 11Iron unfiltered-recoverable, μg/L 23 56 127 245 374 432 2,05Lead, filtered, μg/L 21 (6) 0.05 0.08 0.13 0.16 0.19 

Lead, unfiltered-recoverable, μg/L 23 (2) 0.65 1.5 3.1 5.3 5.6 3Manganese, filtered, μg/L 23 22 38 51 56 59 13

Manganese, unfiltered-recoverable, μg/L 23 53 78 101 143 142 59Zinc, filtered, μg/L 23 (1) 1.1 1.2 1.6 2.0 1.9 

Zinc, unfiltered-recoverable, μg/L 23 2.0 5.0 10 15 15 8Arsenic, filtered, μg/L 23 6.1 7.8 10 9.5 11 1

Arsenic, unfiltered-recoverable, μg/L 23 7.0 11 12 13 14 3Suspended sediment, mg/L 23 3 8 15 30 26 19Suspended sediment, percent fines4 23 30 84 88 83 90 9

Table 1–4. Summary information relating to water-quality constituents and properties in samples collected at sites in the upper Clark Fork Basin, Mcollected during water years 2001–2010.—Continued [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. ft3/s, cubic feet per second; NA, not applicable; μS/cm

at 25 degrees Celsius; CaCO3, calcium carbonate; μg/L, micrograms per liter; mg/L, milligrams per liter; ND, distributional parameter not determined because of excessive num

than] Period of water- quality sampling during water years 2001–10 Constituent or property, unadjusted (not flow adjusted) units of measurement Statistical summaries of water-quality data1 

Number of samples (values in parentheses indicate number of censored values) Minimum uncensored value2 25th percentile Median Mean 75th percentile Max
Clark Fork near Drummond (site 18, fig. 1, table 1) 10/2000– 8/2010 Streamflow, instantaneous, ft3/s 80 157 464 709 964 1,158 3,35

Specific conductance, μS/cm 80 236 346 415 414 480 60pH, standard units 80 7.9 8.2 8.3 8.3 8.4 Hardness, filtered, mg/L as CaCO3 80 102 154 184 186 221 28

Calcium, filtered, mg/L 80 30.6 44.5 53.4 53.4 63.4 8Magnesium, filtered, mg/L 80 6.07 10.1 12.6 12.8 15.3 2Cadmium, filtered, μg/L 79 (14) 0.02 0.03 0.04 0.05 0.05 

Cadmium, unfiltered-recoverable, μg/L 80 (2) 0.02 0.08 0.14 0.22 0.27 Copper, filtered, μg/L 79 2.0 3.8 4.7 5.9 7.0 2

Copper, unfiltered-recoverable, μg/L 80 4.6 10 20 37 43 21Iron, filtered, μg/L 80 (18) 3.0 4.0 8.0 16 23 8

Iron unfiltered-recoverable, μg/L 79 20 179 379 822 1,140 5,77Lead, filtered, μg/L 79 (23) 0.04 0.04 0.09 0.14 0.17 

Lead, unfiltered-recoverable, μg/L 80 (2) 0.18 1.3 3.2 7.4 8.3 4Manganese, filtered, μg/L 79 3.3 10 12 16 18 6

Manganese, unfiltered-recoverable, μg/L 80 8.0 53 84 129 157 69Zinc, filtered, μg/L 80 (1) 0.95 2.9 4.1 4.7 5.8 1

Zinc, unfiltered-recoverable, μg/L 80 2.9 12 25 46 57 27Arsenic, filtered, μg/L 80 3.2 8.4 10 10 12 1

Arsenic, unfiltered-recoverable, μg/L 80 8.0 10 13 15 17 4Suspended sediment, mg/L 80 2 11 23 52 63 31Suspended sediment, percent fines4 80 50 66 74 74 81 9

Table 1–4. Summary information relating to water-quality constituents and properties in samples collected at sites in the upper Clark Fork Basin, Mcollected during water years 2001–2010.—Continued[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. ft3/s, cubic feet per second; NA, not applicable; μS/cm

at 25 degrees Celsius; CaCO3, calcium carbonate; μg/L, micrograms per liter; mg/L, milligrams per liter; ND, distributional parameter not determined because of excessive num

than]

Period of 

waterquality 

sampling 

during 

water years 

2001–10

Constituent or property, 

unadjusted (not flow adjusted) 

units of measurement

Statistical summaries of water-quality data1 

Number of 

samples (values 

in parentheses 

indicate number 

of censored 

values)

Minimum 

uncensored 

value2

25th 

percentile Median Mean 75th percentile Max
Rock Creek (site 19, fig. 1, table 1) 11/2000– 8/2004 Streamflow, instantaneous, ft3/s 20 149 209 356 608 915 2,08

Specific conductance, μS/cm 20 68 88 129 118 145 16pH, standard units 20 7.7 8.0 8.2 8.1 8.3 Hardness, filtered, mg/L as CaCO3 20 28 38 57 53 67 7

Calcium, filtered, mg/L 20 7.23 9.90 14.5 13.7 17.2 1Magnesium, filtered, mg/L 20 2.46 3.32 4.78 4.51 5.77 Cadmium, filtered, μg/L 20 (17) 0.02 ND ND ND ND 

Cadmium, unfiltered-recoverable, μg/L 20 (18) 0.02 ND ND ND ND Copper, filtered, μg/L 20 (6) 0.20 0.34 0.46 0.52 0.70 

Copper, unfiltered-recoverable, μg/L 20 (3) 0.30 0.45 0.60 0.80 1.2 Iron, filtered, μg/L 20 (1) 6.0 8.0 15 19 25 5

Iron unfiltered-recoverable, μg/L 20 23 41 72 88 114 26Lead, filtered, μg/L 20 (19) 0.05 ND ND ND ND 

Lead, unfiltered-recoverable, μg/L 20 (15) 0.03 0.02 0.04 0.05 0.06 Manganese, filtered, μg/L 20 0.60 1.0 1.6 1.5 1.8 

Manganese, unfiltered-recoverable, μg/L 20 1.5 3.5 5.2 5.6 7.2 1Zinc, filtered, μg/L 20 (15) 0.40 0.15 0.25 0.32 0.41 

Zinc, unfiltered-recoverable, μg/L 20 (15) 1.0 0.11 0.27 0.66 0.80 Arsenic, filtered, μg/L 20 0.50 0.50 0.60 0.61 0.63 

Arsenic, unfiltered-recoverable, μg/L 18 (15) 1.0 ND ND ND ND Suspended sediment, mg/L 20 1 3 4 5 6 1Suspended sediment, percent fines4 20 57 73 76 75 80 8

Table 1–4. Summary information relating to water-quality constituents and properties in samples collected at sites in the upper Clark Fork Basin, Mcollected during water years 2001–2010.—Continued [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. ft3/s, cubic feet per second; NA, not applicable; μS/cm

at 25 degrees Celsius; CaCO3, calcium carbonate; μg/L, micrograms per liter; mg/L, milligrams per liter; ND, distributional parameter not determined because of excessive num

than] Period of water- quality sampling during water years 2001–10 Constituent or property, unadjusted (not flow adjusted) units of measurement Statistical summaries of water-quality data1 

Number of samples (values in parentheses indicate number of censored values) Minimum uncensored value2 25th percentile Median Mean 75th percentile Max
Clark Fork at Turah Bridge (site 20, fig. 1, table 1) 10/2000– 8/2010 Streamflow, instantaneous, ft3/s 80 331 744 1,200 1,940 2,550 7,56

Specific conductance, μS/cm 80 139 226 303 290 359 41pH, standard units 80 7.9 8.1 8.3 8.3 8.5 Hardness, filtered, mg/L as CaCO3 80 54 98 130 128 159 19

Calcium, filtered, mg/L 80 14.9 28.1 36.9 36.1 44.0 5Magnesium, filtered, mg/L 80 3.97 6.82 9.19 9.29 11.9 1Cadmium, filtered, μg/L 79 (29) 0.02 0.02 0.03 0.03 0.04 

Cadmium, unfiltered-recoverable, μg/L 80 (7) 0.03 0.05 0.10 0.14 0.16 Copper, filtered, μg/L 80 1.1 2.4 3.0 3.8 4.4 1

Copper, unfiltered-recoverable, μg/L 80 2.7 5.8 12 21 25 11Iron, filtered, μg/L 80 (16) 3.0 4.3 11.0 22 30 9

Iron unfiltered-recoverable, μg/L 80 33 117 289 590 711 4,25Lead, filtered, μg/L 80 (30) 0.03 0.04 0.07 0.10 0.13 

Lead, unfiltered-recoverable, μg/L 80 (6) 0.17 0.58 2.0 4.2 4.3 3Manganese, filtered, μg/L 80 2.9 4.8 7.3 8.1 10 3

Manganese, unfiltered-recoverable, μg/L 80 8.9 24 53 80 92 62Zinc, filtered, μg/L 78 0.94 2.1 3.1 3.6 4.6 1

Zinc, unfiltered-recoverable, μg/L 80 2.9 9.0 18 31 35 23Arsenic, filtered, μg/L 80 3.1 4.9 5.6 5.9 6.7 1

Arsenic, unfiltered-recoverable, μg/L 80 3.0 6.0 7.0 8.3 9.3 2Suspended sediment, mg/L 80 2 7 17 39 43 30Suspended sediment, percent fines4 80 54 69 77 76 83 9

Table 1–4. Summary information relating to water-quality constituents and properties in samples collected at sites in the upper Clark Fork Basin, Mcollected during water years 2001–2010.—Continued[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. ft3/s, cubic feet per second; NA, not applicable; μS/cm

at 25 degrees Celsius; CaCO3, calcium carbonate; μg/L, micrograms per liter; mg/L, milligrams per liter; ND, distributional parameter not determined because of excessive num

than]

Period of 

waterquality 

sampling 

during 

water years 

2001–10

Constituent or property, 

unadjusted (not flow adjusted) 

units of measurement

Statistical summaries of water-quality data1 

Number of 

samples (values 

in parentheses 

indicate number 

of censored 

values)

Minimum 

uncensored 

value2

25th 

percentile Median Mean 75th percentile Max
Blackfoot River (site 21, fig. 1, table 1) 10/2000– 8/2010 Streamflow, instantaneous, ft3/s 57 450 634 1,310 2,430 3,980 9,32

Specific conductance, μS/cm 57 140 174 205 215 260 28pH, standard units 57 8.0 8.2 8.4 8.4 8.5 Hardness, filtered, mg/L as CaCO3 56 68 86 108 107 130 14

Calcium, filtered, mg/L 56 18.6 22.5 27.9 27.3 32.0 3Magnesium, filtered, mg/L 56 5.37 7.14 9.46 9.48 11.9 1Cadmium, filtered, μg/L 55 (48) 0.01 ND ND ND ND 

Cadmium, unfiltered-recoverable, μg/L 57 (44) 0.01 <0.01 <0.01 0.01 0.01 Copper, filtered, μg/L 55 (10) 0.23 0.44 0.63 0.72 0.93 

Copper, unfiltered-recoverable, μg/L 57 (8) 0.30 0.72 1.3 1.8 2.2 Iron, filtered, μg/L 56 (13) 3.0 3.6 7.0 13 17 10

Iron unfiltered-recoverable, μg/L 57 14 30 86 232 387 2,20Lead, filtered, μg/L 55 (45) 0.02 ND ND ND ND 

Lead, unfiltered-recoverable, μg/L 57 (18) 0.04 0.05 0.12 0.33 0.31 Manganese, filtered, μg/L 56 0.50 1.2 1.7 2.0 2.5 

Manganese, unfiltered-recoverable, μg/L 57 2.0 5.0 8.6 19 28 15Zinc, filtered, μg/L 55 (24) 0.30 0.36 0.57 0.70 0.88 

Zinc, unfiltered-recoverable, μg/L 57 (26) 1.0 0.49 1.0 2.0 2.7 1Arsenic, filtered, μg/L 56 0.41 0.80 0.97 1.0 1.2 

Arsenic, unfiltered-recoverable, μg/L 54 (10) 0.93 1.0 1.1 1.2 1.3 Suspended sediment, mg/L 57 1 2 5 19 28 22Suspended sediment, percent fines4 57 69 79 82 82 85 9

Table 1–4. Summary information relating to water-quality constituents and properties in samples collected at sites in the upper Clark Fork Basin, Mcollected during water years 2001–2010.—Continued [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. ft3/s, cubic feet per second; NA, not applicable; μS/cm

at 25 degrees Celsius; CaCO3, calcium carbonate; μg/L, micrograms per liter; mg/L, milligrams per liter; ND, distributional parameter not determined because of excessive num

than] Period of water- quality sampling during water years 2001–10 Constituent or property, unadjusted (not flow adjusted) units of measurement Statistical summaries of water-quality data1 

Number of samples (values in parentheses indicate number of censored values) Minimum uncensored value2 25th percentile Median Mean 75th percentile Max
Clark Fork above Missoula (site 22, fig. 1, table 1) 10/2000– 8/2010 Streamflow, instantaneous, ft3/s 80 772 1,320 3,300 4,280 6,510 15,4

Specific conductance, μS/cm 80 148 196 256 252 311 35pH, standard units 80 7.9 8.2 8.3 8.3 8.4 

Hardness, filtered, mg/L as CaCO3 80 70 91 118 117 142 16

Calcium, filtered, mg/L 80 19.3 24.7 31.5 31.5 38.3 4

Magnesium, filtered, mg/L 80 5.34 7.08 9.08 9.22 11.6 1

Cadmium, filtered, μg/L 79 (40) 0.01 0.01 0.02 0.02 0.03 

Cadmium, unfiltered-recoverable, μg/L 80 (8) 0.02 0.05 0.08 0.14 0.15 Copper, filtered, mg/L 80 0.90 1.7 2.3 2.7 3.3 1

Copper, unfiltered-recoverable, μg/L 80 2.5 6.0 10 23 20 38Iron, filtered, μg/L 80 (9) 3.0 7.0 13.5 20 25 10

Iron unfiltered-recoverable, μg/L 80 43 141 246 603 724 5,98Lead, filtered, μg/L 78 (20) 0.04 0.05 0.07 0.10 0.12 

Lead, unfiltered-recoverable, μg/L 80 (7) 0.18 0.77 1.7 3.4 3.8 5Manganese, filtered, μg/L 80 5.9 9.5 13 14 17 3

Manganese, unfiltered-recoverable, μg/L 80 11.7 28 45 64 72 31Zinc, filtered, μg/L 79 (2) 0.80 1.5 2.4 2.7 3.4 

Zinc, unfiltered-recoverable, μg/L 80 3.3 8.5 15 35 33 49Arsenic, filtered, μg/L 80 1.6 3.0 3.6 3.7 4.4 

Arsenic, unfiltered-recoverable, μg/L 80 2.0 3.8 4.3 5.6 6.0 2Suspended sediment, mg/L 80 2 9 15 52 56 95

Suspended sediment, percent fines4 80 14 69 83 77 90 9

1Distributional parameters affected by censored observations (that is, concentrations reported as less than the laboratory reporting level) were estimated by using adjusted ma

(Cohn, 1988). 2Minimum uncensored value refers to the smallest concentration reported as detected above any of the various laboratory reporting levels applicable for a given constituent. 3Ratio of median filtered to unfiltered-recoverable concentration greater than 100 percent affected by low median concentrations near minimum laboratory reporting levels (t

concentrations. 4Percent fines refers to the percentage of suspended sediment smaller than 0.062-millimeter diameter.

Table 1–4. Summary information relating to water-quality constituents and properties in samples collected at sites in the upper Clark Fork Basin, Mcollected during water years 2001–2010.—Continued[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. ft3/s, cubic feet per second; NA, not applicable; μS/cm

at 25 degrees Celsius; CaCO3, calcium carbonate; μg/L, micrograms per liter; mg/L, milligrams per liter; ND, distributional parameter not determined because of excessive num

than]

Period of 

waterquality 

sampling 

during 

water years 

2001–10

Constituent or property, 

unadjusted (not flow adjusted) 

units of measurement

Statistical summaries of water-quality data1 

Number of 

samples (values 

in parentheses 

indicate number 

of censored 

values)

Minimum 

uncensored 

value2

25th 

percentile Median Mean 75th percentile Max
Table 1–5. Aquatic life standards (based on median hardness for water years 2001–10) for selected sites in the upper Clark Fork Basin, Montana. [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. CaCO3, calcium carbonate] Site number (fig. 1, table 1) Site name (fig. 1, table 1) Median hard- ness for water years 2001–10, in milligrams per liter as CaCO3 Aquatic life standards (Montana Department of Environmental Quality, 2012a), in mic

(adjusted for hardness) Cadmium Copper Lead 

Acute Chronic Acute Chronic Acute Chronic 

1 Blacktail Creek 106 2.26 0.283 14.8 9.81 87.9 3.43 

2 Silver Bow Creek at Butte 154 3.31 0.373 21.0 13.5 141 5.51 

3 Silver Bow Creek at Op- portunity 159 3.42 0.382 21.7 13.9 147 5.74 

4 Mill Creek near Anaconda 59.8 1.26 0.185 8.62 6.01 42.4 1.65 

5 Mill Creek at Opportunity 67.0 1.42 0.201 9.60 6.63 49.0 1.91 

6 Willow Creek near Ana- conda 38.4 0.806 0.133 5.68 4.12 24.1 0.941 

7 Willow Creek at Oppor- tunity 130 2.79 0.329 17.9 11.7 114 4.44 

8 Silver Bow Creek at Warm Springs 207 4.47 0.464 27.8 17.4 206 8.03 

9 Warm Springs Creek near Anaconda 112 2.39 0.294 15.6 10.3 94.3 3.68 

10 Warm Springs Creek at Warm Springs 148 3.18 0.362 20.3 13.0 134 5.24 

11 Clark Fork near Galen 188 4.05 0.432 25.4 16.0 182 7.11 

12 Lost Creek near Anaconda 98.8 2.11 0.268 13.8 9.23 80.4 3.13 

13 Lost Creek near Galen 298 6.47 0.608 39.2 23.7 328 12.8 14 Clark Fork at Deer Lodge 210 4.54 0.469 28.2 17.6 210 8.18 

15 Little Blackfoot River 116 2.48 0.302 16.1 10.6 98.6 3.84 16 Clark Fork at Goldcreek 169 3.64 0.399 23.0 14.6 159 6.21 

17 Flint Creek near Drum- mond 161 3.46 0.385 21.9 14.0 150 5.83 

18 Clark Fork near Drum- mond 184 3.96 0.425 24.9 15.7 177 6.91 

19 Rock Creek 56.9 1.20 0.178 8.23 5.76 39.8 1.55 

20 Clark Fork at Turah Bridge 130 2.79 0.329 17.9 11.7 114 4.44 

21 Blackfoot River 108 2.31 0.287 15.1 9.96 90.0 3.51 

22 Clark Fork above Missoula 118 2.52 0.306 16.4 10.7 101 3.93 
Table 1–6. Summary information relating to continuous streamflow data for sites in the upper Clark Fork Basin, Montana , based on data collected d2001–2010.[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends]

Site 

number 

(fig. 1, 

table 1)

Site name 

(fig. 1, table 1)

Drainage 

area, 

square 

miles

Water years 

summarized 

based on available data during 

water years 

2001–10

Daily mean streamflow, cubic feet per secon

Minimum 25th percentile Median Mean 

p

1 Blacktail Creek at Harrison Avenue, at 

Butte, Mont.

85 2001–10 4.4 7.2 8.9 13 

2 Silver Bow Creek below Blacktail 

Creek, at Butte, Mont.

103 2001–10 11 15 17 21 

3 Silver Bow Creek at Opportunity, Mont. 363 2001–10 11 24 30 44 4 Mill Creek near Anaconda, Mont. 34 2005–10 5.5 10 14 33 

5 Mill Creek at Opportunity, Mont. 43 2004–10 0.20 1.7 4.3 15 

6 Willow Creek near Anaconda, Mont. 14 2005–10 0.50 1.5 2.2 7.0 7 Willow Creek at Opportunity, Mont. 31 2004–10 2.5 5.1 6.6 10 

8 Silver Bow Creek at Warm Springs, 

Mont.

473 2001–10 15 35 47 70 

9 Warm Springs Creek near Anaconda, 

Mont.

157 2001–10 13 46 61 84 

10 Warm Springs Creek at Warm Springs, 

Mont.

163 2001–10 5.0 30 40 58 

11 Clark Fork near Galen, Mont. 651 2001–10 26 68 89 130 

12 Lost Creek near Anaconda, Mont. 26 2005–10 0.00 3.1 4.7 6.5 

13 Lost Creek near Galen, Mont. 61 2004–10 1.4 8.5 36 29 14 Clark Fork at Deer Lodge, Mont. 995 2001–10 27 147 187 216 

15 Little Blackfoot River near Garrison, 

Mont.

407 2001–10 16 52 72 145 

16 Clark Fork at Goldcreek, Mont. 1,704 2001–10 73 271 332 460 

17 Flint Creek near Drummond, Mont. 490 2001–10 2.1 66 98 123 18 Clark Fork near Drummond, Mont. 2,501 2001–10 100 401 503 648 

19 Rock Creek near Clinton, Mont. 885 2001–10 40 180 240 471 

20 Clark Fork at Turah Bridge, near Bonner, Mont. 3,641 2001–10 177 654 816 1,200 

21 Blackfoot River near Bonner, Mont. 2,290 2001–10 180 490 624 1,330 22 Clark Fork above Missoula, Mont. 5,999 2001–10 450 1,130 1,420 2,500 
110 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

Appendix 2—Summary of the TimeSeries Model (TSM) as Applied in this 

Study

The theory and parameter estimation for the model are 

described in detail in Vecchia (2005). In the time-series model, 

log-transformed concentration data were partitioned into several components according to equation 2:

log(C) = MC+ANNC+SEASC+TREND+HFVC (2)

where

log is the base-10 logarithm;

C is the concentration, in milligrams per liter;

MC is the long-term mean of the log-transformed 

concentration, as the base-10 logarithm of 

milligrams per liter;

ANNC is the annual concentration anomaly 

(dimensionless);

SEASC is the seasonal concentration anomaly 

(dimensionless);

TREND is the concentration trend (dimensionless); 

and

HFVC is the high-frequency variability of the 

concentration (dimensionless).

In equation 2, the annual concentration anomaly (ANNC), 

seasonal concentration anomaly (SEASC), and high-frequency 

variability (HFVC) terms represent natural variability in concentration for different time scales. ANNC is an estimate of the 

interannual variability in concentration that can be attributed 

to long-term variability in streamflow. ANNC is quantified by 

relating annual means (for the 365-day period immediately 

before a given sample) of log concentration and log streamflow to long-term means (for the entire period of record). 

Extended droughts and wet periods can change the chemical and suspended-material composition of streamflow by 

changing the degree of contact between surface runoff and soil 

particles, availability of particulate material in stream channels 

and near-stream areas, and changing the relative composition 

of runoff among groundwater, overland flow, and subsurface 

flow (Vecchia, 2005).

SEASC is an estimate of the seasonal variability in 

concentration that can be attributed to seasonal variability in 

streamflow or to factors other than variability in streamflow. 

SEASC is quantified by relating seasonal means (for the 90-day 

period immediately before a given sample was collected) of 

log concentration and log streamflow to annual means (for 

the 365-day period immediately before a given sample was 

collected). For example, the seasonal snow-accumulation and 

snowmelt cycle causes seasonal fluctuations in streamflow 

and water quality. Seasonal differences in the relative amount 

of streamflow that comes from natural sources compared to 

anthropogenic contributions (such as wastewater inputs) also 

might cause seasonal fluctuations in concentration that are 

more complicated than a simple relation between concentration and streamflow could produce. 

HFVC is an estimate of the variability in concentration for 

time scales that are smaller than the seasonal time scale (time 

scales of several days to several weeks). Thus, high-frequency 

variability is the variability that remains after the removal of 

seasonal and annual anomalies and trends. HFVC is quantified by relating log concentration and log streamflow for the 

day of sampling to log concentration and log streamflow for 

each of the two 10-day periods immediately before a given 

sample. Short-term changes in meteorological conditions 

might cause high-frequency variability in concentration and 

streamflow. The high-frequency variability depends on a timeseries model, called a periodic autoregressive moving average model, that accounts for the presence of serial correlation 

among concentrations (for example, the tendency for high or 

low values to persist for several days to several weeks before 

returning to normal levels; Vecchia, 2005).

TREND is an estimate of the long-term systematic 

changes in concentration during the study period that are unrelated to long-term variability in streamflow. For this report, a 

significant trend might indicate changes in the extent to which 

mining wastes affect chemical composition of surface water 

or changes in other activities, such as agricultural practices, 

that can change the amount of suspended sediment or trace 

elements that reach the stream. TREND consists of piecewise 

monotonic trends during specified trend-analysis periods. The 

overall significance of TREND (determined by using 

the generalized likelihood ratio principle; Vecchia, 2005, 

appendix 1) specifies whether there were any significant 

changes during any of the specified trend-analysis periods. 

For a given site and constituent combination, if TREND was 

determined to be nonsignificant, the trends for all of the specified trend-analysis periods were considered nonsignificant 

and p-values were not reported. Infrequently, overall significance of TREND could not be determined (and thus TREND

was assumed to be nonsignificant), but the individual trend 

coefficient for a specified trend-analysis period was highly significant and of large magnitude. In those cases, with TREND

included in the model, the numerical procedure for minimizing 

the likelihood function apparently converged to a local, rather 

than global, minimum and produced unrealistic results relative to the model without TREND included. However, trend 

directions and magnitudes for those infrequent cases generally 

were consistent with trends for other constituents that would 

be expected to behave in a similar manner, and with trends 

for upstream or downstream sites. Therefore, the TSM was 

presumed to provide reasonably accurate trend magnitudes for 

the specified trend-analysis period and overall trend patterns 

were not strongly affected. For a given site and constituent 

combination, if TREND was determined to be significant, the 

slope coefficient (γ; Vecchia, 2005, appendix 1) for the trend 

for each specified trend-analysis period was used to determine 

the significance and magnitude of the trend for the specified 

trend-analysis period. The null hypothesis in the test for trend 

significance in a given trend analysis period is that there is 
Appendix 2—Summary of the Time-Series Model (TSM) as Applied in this Study 111

no trend (that is, γ = 0). If the two-tailed p-value for γ was 

less than the selected alpha level (0.01 in this report), the null 

hypothesis was rejected and the trend was determined to be 

significant. Determination of a nonsignificant trend (that is, a 

p-value greater than 0.01) does not imply that the null hypothesis is accepted (that is, that there is no trend). It indicates that 

within the statistical framework of the analysis, a significant 

trend was not detected. The magnitude of the trend for a specified trend-analysis period is expressed as the percent difference between the geometric mean concentration at the end of 

the period and the geometric mean concentration at the start of 

the period and is determined by the equation:

%∆FAC=100(10γ-1), (3)

where

%∆FAC is the percent change in the geometric mean 

of the flow-adjusted concentration, and 

γ is the slope coefficient of the trend for the 

specified trend-analysis period in logtransformed units.

Log-transformed concentrations that have ANNC and 

SEASC removed are referred to in this report as flow-adjusted 

concentrations. Using equation 2, the flow-adjusted concentration is defined as:

 FAC =log(C)–ANNC–SEASC=MC+TREND+HFVC (4)

where

FAC is the flow-adjusted value, as the base10 logarithm of the original units of 

measurement. 

The FACs defined by equation 4 are analogous to FACs 

defined in previous publications as the residuals from a 

regression model that relates concentration to concurrent daily 

streamflow (Helsel and Hirsch, 2002); however, the TSM 

approach generally is more effective than a regression-based 

approach for removing streamflow-related variability (Vecchia, 2005). Time-series plots showing the FACs along with 

the fitted trend (MC+TREND) illustrate long-term changes in 

geometric mean concentration that might indicate changes 

in effects of mining wastes on water-quality in the selected 

watersheds. 

The key to making TSM a powerful trend analysis tool is 

that the entire time series of daily streamflow data are used in 

the model, not just streamflow for the days when concentration samples are available. The model uses a three-per-month, 

or approximately 10-day, sampling frequency. Each month 

is divided into three intervals—days 1 through 10, days 11 

through 20, and day 21 through the end of the month. If a 

water-quality sample is available for a particular interval, it is 

paired with daily streamflow for the same day of the waterquality sample. If no water-quality sample is available, the 

concentration value for the interval is missing and streamflow 

for the middle of the interval (day 5, 15, or 25) is used. If more 

than one concentration sample is available for the interval, 

the value nearest to the midpoint of the interval is used. The 

log-transformed streamflow time series (consisting of three 

values per month) is divided into an annual anomaly, seasonal 

anomaly, and high-frequency variability according to the following equation,

log(Q) = MQ+ANNQ+SEASQ+HFVQ (5)

where

Q is daily mean streamflow, in cubic feet per 

second;

MQ is the mean of the log-transformed streamflow 

for the entire trend analysis period, as the 

base-10 logarithm of cubic feet per second;

ANNQ is the annual streamflow anomaly, computed 

as the 1-year lagged moving average of 

log(Q)-MQ (dimensionless);

SEASQ is the seasonal streamflow anomaly, computed 

as the 3-month lagged moving average of 

log(Q)-MQ-ANNQ (dimensionless); and

HFVQ is log(Q)-MQ-ANNQ-SEASQ is the highfrequency streamflow variability 

(dimensionless). 

The water-quality time-series model (equation 2) is 

directly tied to the streamflow time-series model because the 

streamflow anomalies (ANNQ and SEASQ from equation 5) 

are used as predictor variables for concentration (equation 2). 

For example, ANNC is assumed to equal a constant coefficient 

(estimated from the time-series model) times ANNQ. The different scales of streamflow variability often affect concentration in different ways. The relation between HFVC and HFVQ

can be particularly complicated, changing depending on the 

time-of-year and the degree of serial correlation in the concentration data and cross-correlation between concentration and 

streamflow. 

The TSM residuals for each site and constituent combination were examined graphically to verify the model assumptions that the residuals had constant variance, were serially 

uncorrelated, and were approximately normally distributed. 

Because of the application of the TSM to the large number 

of site and constituent combinations, and practical considerations to keep the trend periods comparable among sites and 

constituents, some minor deviations of the residuals from 

model assumptions were tolerated. Such deviations included 

small changes in residual variance through time and shortterm (about 1 to 2 years) unresolved trending in the residuals. 

In cases where unresolved residual trends were considered to 

be large enough to possibly affect the magnitudes and significance levels of reported fitted trends, more complicated trend 

models were tested and in nearly all cases the more complicated models did not change the general findings and conclusions of this report. Therefore, the reported TSM results were 

judged to provide acceptable fits representative of linearity 

through nearly all of the range in FACs for a given site and 

constituent combination. Standard errors of estimates (SEEs) 

for the TSM analyses are presented in table 2–1. In this report, 
112 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

SEEs are expressed in percent and were converted from log 

units by using procedures described by Tasker (1978). For all 

trace elements, mean SEEs range from 20.9 to 64.5 percent. 

Mean SEEs for unfiltered-recoverable copper and arsenic 

concentrations are 47.0 and 28.8 percent, respectively. Mean 

SEE for suspended-sediment concentration (62.8) is substantially higher than for mean SEEs for trace elements. The 

SEEs indicate reasonably accurate definition of concentration 

and streamflow relations for the purpose of trend analysis. 

However, higher mean SEE for suspended sediment than mean 

SEEs for trace elements indicates lower confidence in results. 

For each site and constituent combination, the fit of the TSM 

can be assessed by examination of the fitted trends in relation 

to the FACs that are shown in figures 4–1 through 4–3, 4–8 

through 4–11, and 4–14 through 4–22. The distribution of the 

FACs about the fitted trend lines shows the extent to which the 

residuals might exhibit nonconstant variance or unresolved 

trends.

Table 2–1. Statistical summaries of standard errors of estimates (SEEs) for the time-series 

model (TSM) analyses1.

[NA, not applicable]

Constituent or property

Number of sites for 

which trend results 

are reported

SEE, percent

Minimum Mean Maximum

Specific conductance 15 6.1 10.5 15.5

Cadmium, filtered 2 54.0 64.5 74.9

Cadmium, unfiltered-recoverable 2 52.1 56.5 61.0

Copper, filtered 11 22.0 32.0 40.2

Copper, unfiltered-recoverable 13 33.9 47.0 63.4

Iron, filtered 7 29.9 44.4 54.0

Iron unfiltered-recoverable 15 30.1 48.5 65.5

Lead, filtered 0 NA NA NA

Lead, unfiltered-recoverable 6 51.9 58.8 65.9

Manganese, filtered 14 29.0 38.9 60.7

Manganese, unfiltered-recoverable 14 33.7 42.8 51.2

Zinc, filtered 2 56.4 62.0 67.6

Zinc, unfiltered-recoverable 11 42.4 52.1 77.1

Arsenic, filtered 13 12.8 20.9 28.2

Arsenic, unfiltered-recoverable 13 18.0 28.8 45.1

Suspended sediment 15 48.6 62.8 73.8

1

The TSM was applied to the following 15 sites (fig. 1, table 1): Blacktail Creek (site 1), Silver Bow Creek 

at Butte (site 2), Silver Bow Creek at Opportunity (site 3), Silver Bow Creek at Warm Springs (site 8), Warm 

Springs Creek at Warm Springs (site 10), Clark Fork near Galen (site 11), Clark Fork at Deer Lodge (site 14), 

Little Blackfoot River (site 15), Clark Fork at Goldcreek (site 16), Flint Creek (site 17), Clark Fork near Drummond (site 18), Rock Creek (site 19), Clark Fork at Turah Bridge (site 20), Blackfoot River (site 21), and Clark 

Fork above Missoula (site 22). For some of the site and constituent combinations, trend results are not reported 

because greater than 6 percent of values were affected by recensoring at study reporting level, as discussed in 

the section of this report “Time-Series Model.” The statistical summaries present the mean and ranges of SEE 

for the TSM analyses for each constituent or property for which trend results are reported.
Appendix 3—Summary of Multiple Linear Regression of Water-Quality Constituents on Time, Streamflow, and Season 113

Appendix 3—Summary of Multiple 

Linear Regression of Water-Quality 

Constituents on Time, Streamflow, and 

Season, as Applied in this Study

Multiple linear regression of water-quality constituents 

on time, streamflow, and season (MLR) was applied in this 

study following guidelines presented in Helsel and Hirsch 

(2002). The basic multiple linear regression model used is 

represented by the equation:

log(Ct

) = b0 + b1 Tt + b2logQt+ b3 sin(2πTt) + 

 b4

 cos(2πTt) + b5 sin(4πTt) + b6 cos(4πTt) + Et (6)

where

log denotes the base-10 logarithm;

Ct is the value of the water-quality constituent 

or property, in indicated units of 

measurement, at time t;

b0 is the intercept;

b1 through b6 are the estimated slope coefficients associated 

with the various explanatory variables;

Tt is decimal time (day of year expressed in 

decimal form; for example, June 30, 2010, 

is expressed as 2010.5) at time t;

Qt is instantaneous streamflow at the time of 

sampling, in cubic feet per second;

sin(2πTt), cos(2πTt), sin(4πTt), and cos(4πTt)

are periodic functions that describe 

seasonal variability; and

Et is an approximately normally distributed 

random error.

The MLR used ordinary least squares if the concentration 

data contained no censored observations. If censored observations were present, the MLR used adjusted maximum-likelihood estimation (Cohn, 1988, 2005) with the same formulation of dependent and explanatory variables.

Use of MLR for trend analysis involves regression of 

constituent concentration [log(Ct

), equation 6] on streamflow 

(Qt

, equation 6), which inherently provides for flow adjustment and quantifies concentration and streamflow relations. 

The residuals from the regression of concentration on streamflow represent flow-adjusted concentrations (FACs; Helsel and 

Hirsch, 2002). Including periodic functions that describe seasonal variability [sin(2πTt

), cos(2πTt), sin(4πTt), and cos(4πTt), 

equation 6] accounts for the effect of repetitive seasonal 

variability on concentration and streamflow relations. The 

residuals from the regression of concentration on streamflow 

and the periodic functions represent changes in concentration 

and streamflow relations through the trend-analysis period. 

Including decimal time (Tt

, equation 6) in the model provides 

quantification of the change in concentration and streamflow 

relations through time and describes the temporal trend in 

FACs for the specified trend-analysis period. The slope coefficient for decimal time (b1

, equation 6) is used to determine the 

significance and magnitude of the trend. The null hypothesis in 

the test for trend significance is that there is no trend (that is, 

b1

 = 0). If the two-tailed p-value for b1 is less than the selected 

alpha level (0.01 in this report), the null hypothesis is rejected 

and the trend is determined to be significant. Determination 

of a nonsignificant trend (that is, a p-value greater than 0.01) 

does not imply that the null hypothesis is accepted (that is, that 

there is no trend). It indicates that within the statistical framework of the analysis, a significant trend was not detected. The 

magnitude of the trend is expressed as the percent difference 

between the geometric mean concentration at the end of the 

period and the geometric mean concentration at the start of the 

period and is determined by the equation:

%∆FAC = 100(10(Nb1) – 1) (7)

where

%∆FAC is the percent change in the geometric mean 

of the flow-adjusted concentration; and

N is the number of years in the trend-analysis 

period.

Application of linear regression for flow-adjusted trend 

analysis assumes that the data are normally distributed and 

that relations between the response variable (a given waterquality constituent) and the combined explanatory variables 

(time, streamflow, and periodic functions that describe seasonal variability) can be represented appropriately by a linear 

fit. Further, the relation between the water-quality constituent 

and streamflow must be statistically significant to accurately 

determine significance level. Data for many water-quality 

constituents typically do not conform to a normal distribution 

because of positive skew (Helsel and Hirsch, 2002). To approximate normality, constituent concentrations and streamflow were transformed to logarithm (base 10) units. 

In accounting for seasonal variability, 2π and 4π sine and 

cosine terms were included in the regression model for all site 

and constituent combinations. During exploratory analysis, 

different multiples of π were added to the model and evaluated 

for significant effect. The 2π and 4π terms frequently, but not 

always, were significant. Inclusion of the periodic functions 

when they were not significant in the regression model for 

some site and constituent combinations probably had small 

effect on the trend analysis results.

Effects of serial correlation on MLR results were evaluated for each site and constituent combination. Significant 

serial correlation was determined if Spearman’s correlation coefficient on the lag-one residuals produced a p-value less than 

0.05 (Helsel and Hirsch, 2002). Significant serial correlation 

was infrequent, but for site and constituent combinations with 

significant serial correlation the significance level of the trend 

results was not reported. It is notable that the simple measure 

of serial correlation used in this report can be affected by 

many factors and also might not completely represent all serial 

correlation effects in a given MLR model. Serial correlation 

does not affect the unbiased estimate of the trend line, but can 
114 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

result in unrepresentatively deflated SEEs and p-values.

The regression model results for each site and constituent combination were evaluated by examining the SEEs, the 

significance of the concentration and streamflow relation, 

influence and leverage statistics, and homoscedasticity and 

normality of residuals. Statistical summaries of SEE for the 

regression models are presented in table 3–1. In this report, 

SEEs are expressed in percent and were converted from log 

units by using procedures described by Tasker (1978). For 

all trace elements, mean SEEs range from 29.7–58.6 percent. 

Mean SEEs for unfiltered-recoverable copper and arsenic concentrations are 41.2 and 29.7 percent, respectively. SEEs for 

trace-element MLR models are similar to SEEs for regressions 

of constituents on streamflow previously used to estimate 

trace-element loads in the Clark Fork Basin (Lambing, 1991; 

Hornberger and others, 1997; Lambing, 1998; Lambing and 

Sando, 2008, 2009; Sando and Lambing, 2011). Mean SEE 

for suspended-sediment concentration (70.9) is substantially 

higher than for mean SEEs for trace elements, but also generally is less than or similar to SEEs for regressions previously 

used to estimate suspended-sediment loads in the Clark Fork 

Basin (Lambing, 1991; Hornberger and others, 1997; Lambing, 1998; Lambing and Sando, 2008, 2009; Sando and Lambing, 2011). The SEEs indicate reasonably accurate definition 

of concentration and streamflow relations for the purpose of 

trend analysis. However, higher mean SEE for suspended sediment than mean SEEs for trace elements indicates lower confidence in results. For a given site and constituent combination, 

the significance level of the trend results was not reported if 

the concentration and streamflow relation was nonsignificant 

(p-value greater than 0.05). No data values were determined 

to have significant high influence for any of the MLR models. 

Because of the application of a consistent regression model 

to the large number of site and constituent combinations, and 

practical considerations to keep the trend periods comparable 

among sites and constituents, some minor deviations of the 

residuals from model assumptions was tolerated. However, 

the reported regression model results were judged to provide 

acceptable fits representative of linearity through nearly all of 

the range in FACs for a given site and constituent combination. For each site and constituent combination, the fit of the 

regression model can be assessed by examination of the fitted 

trends in relation to the FACs that are shown in figures 4–4 

through 4–7, 4–9, and 4–12 through 4–13. For plotting purposes, the FACs were determined by adding the residuals from 

the regression of concentration on streamflow to the geometric 

mean concentration based on data collected during water years 

2001–2010. The distribution of the FACs about the fitted trend 

lines shows the extent to which the regression model results 

were affected by factors such as residual heteroscedasticity 

and curvature.

Table 3–1. Statistical summaries of standard errors of 

estimates (SEEs) for multiple linear regression models of waterquality constituents on time, streamflow, and season (MLR)1

.

Constituent or property SEE, percent

Minimum Mean Maximum

Specific conductance 4.7 9.4 17.5

Cadmium, filtered 21.4 34.9 43.9

Cadmium, unfiltered-recoverable34.0 42.4 50.1

Copper, filtered 23.9 30.5 35.6

Copper, unfiltered-recoverable 27.1 41.2 57.3

Iron, filtered 36.3 50.2 63.9

Iron unfiltered-recoverable 34.0 43.1 64.9

Lead, filtered 43.8 48.2 57.6

Lead, unfiltered-recoverable 42.3 58.6 68.8

Manganese, filtered 19.5 43.1 63.6

Manganese, unfiltered-recoverable35.4 51.8 69.1

Zinc, filtered 24.2 35.9 51.9

Zinc, unfiltered-recoverable 35.9 46.4 61.5

Arsenic, filtered 17.4 30.1 38.4

Arsenic, unfiltered-recoverable 20.3 29.7 36.3

Suspended sediment 48.1 70.9 81.6

1

MLR was applied to the following seven sites (fig. 1, table 1): Mill 

Creek near Anaconda (site 4); Mill Creek at Opportunity (site 5); Willow 

Creek near Anaconda (site 6); Willow Creek at Opportunity (site 7); Warm 

Springs Creek near Anaconda (site 9); Lost Creek near Anaconda (site 12); 

and Lost Creek near Galen (site 13). The statistical summaries present the 

mean and ranges of SEE for the seven regression models for each constituent or property.
Table 4–1. Flow-adjusted trend results determined by using the time-series model (TSM) for selected water-quality constituents and properties for the upper Clark Fork Basin, Montana, water years 1996–2010. [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. Values in parentheses indicate p-values for associated Gray shading indicates statistical significance at p-value less than 0.01. p-value, statistical probability level; SEASC, seasonal concentration anomaly coefficient; ANNC, annual 

ficient; SEE, standard error of estimate in percent; NR, not reported; <, less than] Constituent or property Number of samples Total percent change for water years 1996–2000 (period 1) Total percent change for water years 2001–05 (period 2) Total percent change for water years 2006–2010 (period 3) p-value for overall trend analysis1 SEASC ANNC SEE 

Blacktail Creek (site 1, fig. 1, table 1) Specific conductance 131 7 (NR3) -1 (NR3) -2 (NR3) 1.000 -0.32 (<0.001) -0.17 (<0.001) 8.7 

Cadmium, filtered 129 NR4 NR4 NR4 NR4 NR4 NR4 NR4 

Cadmium, unfiltered-recoverable 131 NR4 NR4 NR4 NR4 NR4 NR4 NR4 Copper, filtered 130 -11 (NR3) -2 (NR3) -5 (NR3) 0.038 1.43 (<0.001) -0.13 (0.136) 40.2 

Copper, unfiltered-recoverable 131 -17 (0.060) 0 (0.974) -12 (0.327) 0.005 1.31 (<0.001) -0.11 (0.416) 33.9 

Iron, filtered 131 -38 (0.001) 56 (0.004) 27 (0.215) <0.001 1.89 (<0.001) 1.61 (<0.001) 43.4 

Iron unfiltered-recoverable 131 -22 (NR3) -14 (NR3) 48 (NR3) 0.030 1.17 (<0.001) 0.38 (0.007) 37.4 

Lead, filtered 129 NR4 NR4 NR4 NR4 NR4 NR4 NR4 

Lead, unfiltered-recoverable 131 NR4 NR4 NR4 NR4 NR4 NR4 NR4 Manganese, filtered 131 1 (NR3) -19 (NR3) 31 (NR3) 0.579 -0.40 (0.006) -0.03 (0.705) 33.3 

Manganese, unfiltered-recoverable 131 -16 (0.007) -17 (0.010) 45 (<0.001) 0.001 -0.42 (0.004) -0.20 (0.010) 34.1 

Zinc, filtered 129 NR4 NR4 NR4 NR4 NR4 NR4 NR4 

Zinc, unfiltered-recoverable 131 NR4 NR4 NR4 NR4 NR4 NR4 NR4 Arsenic, filtered 130 -27 (<0.001) 18 (0.003) 7 (0.371) <0.001 0.88 (<0.001) 0.31 (<0.001) 22.0 Arsenic, unfiltered-recoverable 130 -26 (<0.001) 9 (0.246) 7 (0.499) 0.001 0.98 (<0.001) 0.51 (<0.001) 22.3 Suspended sediment 131 7 (0.648) -21 (0.150) -10 (0.599) 0.089 1.09 (<0.001) 0.20 (0.335) 61.8 

Silver Bow Creek at Butte (site 2, fig. 1, table 1)

Specific conductance 139 -5 (NR3) 0 (NR3) -3 (NR3) 0.010 -0.36 (<0.001) -0.19 (<0.001) 9.7 

Cadmium, filtered 138 -69 (<0.001) -66 (<0.001) -68 (<0.001) <0.001 1.07 (0.043) 0.97 (0.009) 74.9 

Cadmium, unfiltered-recoverable 137 -58 (<0.001) -64 (<0.001) -73 (<0.001) <0.001 1.65 (<0.001) 0.47 (0.103) 61.0 

Copper, filtered 139 -79 (<0.001) 6 (0.717) -56 (<0.001) <0.001 1.96 (<0.001) 0.96 (<0.001) 36.5 

Copper, unfiltered-recoverable 139 -76 (<0.001) -29 (<0.001) -55 (<0.001) <0.001 1.71 (<0.001) 0.89 (<0.001) 34.1 

Iron, filtered 139 -68 (<0.001) 67 (<0.001) 73 (0.003) <0.001 1.54 (<0.001) 0.74 (0.005) 46.2 

Iron unfiltered-recoverable 139 -55 (<0.001) -17 (0.080) 28 (0.072) <0.001 2.07 (<0.001) 0.73 (<0.001) 47.6 

Lead, filtered 134 NR4 NR4 NR4 NR4 NR4 NR4 NR4 

Lead, unfiltered-recoverable 138 -66 (<0.001) -50 (<0.001) -9 (0.535) <0.001 2.20 (<0.001) 0.73 (0.004) 65.5 
Table 4–1. Flow-adjusted trend results determined by using the time-series model (TSM) for selected water-quality constituents and properties for the upper Clark Fork Basin, Montana, water years 1996–2010.—Continued [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. Values in parentheses indicate p-values for associated Gray shading indicates statistical significance at p-value less than 0.01. p-value, statistical probability level; SEAS , seasonal concentration anomaly coefficient; ANN , annual 

C C ficient; SEE, standard error of estimate in percent; NR, not reported; <, less than] Constituent or property Number of samples Total percent change for water years 1996–2000 (period 1) Total percent change for water years 2001–05 (period 2) Total percent change for water years 2006–2010 (period 3) p-value for overall trend analysis1 SEAS C ANN C SEE 

Silver Bow Creek at Butte (site 2, fig. 1, table 1)—Continued Manganese, filtered Manganese, unfiltered-recoverable Zinc, filtered Zinc, unfiltered-recoverable Arsenic, filtered Arsenic, unfiltered-recoverable Suspended sediment 139 139 139 139 139 139 138 -56 (<0.001) -49 (<0.001) -69 (<0.001) -66 (<0.001) 4 (0.525) -26 (<0.001) -9 (0.530) -7 (0.234) -35 (<0.001) -73 (<0.001) -72 (<0.001) -41 (<0.001) -44 (<0.001) -46 (<0.001) -75 (<0.001) -54 (<0.001) -59 (<0.001) -57 (<0.001) 7 (0.420) 8 (0.381) -2 (0.939) <0.001 0.003 <0.001 <0.001 <0.001 <0.001 <0.001 1.32 (<0.001) 1.24 (<0.001) 1.16 (0.004) 1.40 (<0.001) 0.21 (0.150) 0.73 (<0.001) 1.19 (0.001) 0.00 (0.988) 0.07 (0.576) 0.42 (0.134) 0.13 (0.580) -0.28 (0.021) -0.19 (0.136) 0.00 (0.990) 39.1 42.3 56.4 49.7 19.9 25.9 68.5 

Silver Bow Creek at Opportunity (site 3, fig. 1, table 1) Specific conductance Cadmium, filtered Cadmium, unfiltered-recoverable Copper, filtered Copper, unfiltered-recoverable Iron, filtered Iron unfiltered-recoverable Lead, filtered Lead, unfiltered-recoverable Manganese, filtered Manganese, unfiltered-recoverable Zinc, filtered Zinc, unfiltered-recoverable Arsenic, filtered Arsenic, unfiltered-recoverable Suspended sediment 141 139 138 139 141 139 140 137 141 141 141 140 141 141 141 122 9 (<0.001) -25 (0.018) -6 (0.581) -11 (0.197) -8 (0.197) -4 (NR3) -4 (NR3) NR4 35 (0.080) -20 (<0.001) -10 (0.276) -25 (0.038) -13 (0.250) 19 (0.005) 7 (0.121) -26 (NR3) 1 (0.187) -36 (<0.001) -28 (<0.001) -33 (<0.001) -23 (<0.001) -8 (NR3) 6 (NR3) NR4 2 (0.877) -36 (<0.001) -36 (<0.001) -39 (<0.001) -42 (<0.001) 14 (0.005) 6 (0.036) 20 (NR3) -7 (0.008) -45 (<0.001) -60 (<0.001) -40 (<0.001) -53 (<0.001) 19 (NR3) -22 (NR3) NR4 -65 (<0.001) -49 (<0.001) -45 (<0.001) -60 (<0.001) -52 (<0.001) -40 (<0.001) -51 (<0.001) -10 (NR3) <0.001 <0.001 <0.001 <0.001 <0.001 0.267 0.885 NR4 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 0.671 -0.31 (<0.001) 0.44 (0.017) 0.51 (<0.001) 0.47 (<0.001) 0.35 (<0.001) 0.88 (<0.001) 0.71 (<0.001) NR4 0.86 (<0.001) 0.01 (0.916) 0.14 (0.303) 0.32 (0.139) 0.46 (0.006) -0.17 (0.016) 0.02 (0.730) 0.62 (0.004) -0.25 (<0.001) 0.36 (0.016) 0.38 (0.001) 0.22 (0.034) 0.40 (<0.001) 0.86 (<0.001) 0.73 (<0.001) NR4 0.81 (<0.001) -0.19 (0.051) 0.00 (0.977) 0.17 (0.357) 0.23 (0.110) -0.16 (0.014) 0.15 (0.008) 0.76 (0.001) 7.8 54.0 52.1 34.6 63.4 54.0 58.7 NR4 65.9 55.2 48.4 67.6 54.3 28.2 45.1 54.3 
Table 4–1. Flow-adjusted trend results determined by using the time-series model (TSM) for selected water-quality constituents and properties for the upper Clark Fork Basin, Montana, water years 1996–2010.—Continued[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. Values in parentheses indicate p-values for associated Gray shading indicates statistical significance at p-value less than 0.01. p-value, statistical probability level; SEAS , seasonal concentration anomaly coefficient; ANN , annual 

C C

ficient; SEE, standard error of estimate in percent; NR, not reported; <, less than]

Constituent or property

Number 

of 

samples

Total percent 

change for 

water years 

1996–2000 

(period 1)

Total percent 

change for 

water years 

2001–05 

(period 2)

Total percent 

change for 

water years 

2006–2010 

(period 3)

p-value 

for overall 

trend 

analysis1

SEAS C ANN C SEE

Silver Bow Creek at Warm Springs (site 8, fig. 1, table 1)

Specific conductanceCadmium, filteredCadmium, unfiltered-recoverableCopper, filtered

Copper, unfiltered-recoverable

Iron, filtered

Iron unfiltered-recoverable

Lead, filtered

Lead, unfiltered-recoverableManganese, filteredManganese, unfiltered-recoverable

Zinc, filtered

Zinc, unfiltered-recoverableArsenic, filtered

Arsenic, unfiltered-recoverableSuspended sediment

146146146146

146

146

146

145

146146146

146

139146

146148

0 (0.918)

NR4

NR4

-54 (<0.001)

-51 (<0.001)

NR4

-27 (0.009)

NR4

NR4

-8 (NR3)

-16 (NR3)

NR4

-64 (<0.001)

11 (NR3)

12 (NR3)

9 (0.679)

-4 (0.063)

NR4

NR4

-12 (0.201)

-9 (0.328)

NR4

-10 (0.336)

NR4

NR4

10 (NR3)

2 (NR3)

NR4

-47 (<0.001)

0 (NR3)

-3 (NR3)

-30 (0.026)

5 (0.114)

NR4

NR4

-5 (0.683)

-13 (0.234)

NR4

-18 (0.147)

NR4NR4

-38 (NR3)

-35 (NR3)

NR4

1 (0.963)

4 (NR3)

7 (NR3)

-55 (<0.001)

0.002

NR4

NR4

<0.001

<0.001

NR4

<0.001

NR4

NR4

1.0000.039

NR4

<0.001

0.5620.630

<0.001

-0.24 (<0.001)

NR4

NR4

0.31 (0.020)

0.49 (<0.001)

NR4

0.46 (0.002)

NR4NR4

0.38 (0.099)

-0.01 (0.955)

NR4

0.42 (0.008)

0.26 (0.001)

0.28 (<0.001)

0.29 (0.147)

-0.28 (<0.001)

NR4

NR4

0.24 (0.034)

0.27 (0.014)

NR4

0.03 (0.780)

NR4

NR4

0.10 (0.647)

-0.17 (0.322)

NR4

0.17 (0.180)

0.00 (0.996)0.01 (0.930)

0.16 (0.403)

10.4

NR4

NR4

31.9

38.1

NR4

39.7

NR4

NR4

60.7

43.0

NR4

43.5

25.926.3

64.7

Warm Springs Creek at Warm Springs (site 10, fig. 1, table 1)

Specific conductance

Cadmium, filtered

Cadmium, unfiltered-recoverableCopper, filteredCopper, unfiltered-recoverableIron, filteredIron unfiltered-recoverable

Lead, filtered

Lead, unfiltered-recoverable

108

108

108107108108108

108

108

-12 (<0.001)

NR4

NR4

-2 (0.837)

-26 (<0.001)

NR4

-32 (NR3)

NR4

NR4

0 (0.853)

NR4

NR4

-6 (0.451)

5 (0.589)

NR4

11 (NR3)

NR4

NR4

-7 (0.040)

NR4

NR4

-34 (<0.001)

27 (0.070)

NR4

58 (NR3)

NR4

NR4

<0.001

NR4

NR4

<0.001

<0.001

NR4

0.044

NR4

NR4

-0.26 (<0.001)

NR4

NR4

-0.15 (0.029)

0.08 (0.336)

NR4

0.34 (<0.001)

NR4

NR4

-0.17 (<0.001)

NR4

NR4

-0.03 (0.576)

0.15 (<0.001)

NR4

0.30 (0.010)

NR4

NR4

15.5

NR4

NR4

32.360.9

NR4

60.8

NR4

NR4
Manganese, filtered 108 -40 (<0.001) 21 (0.150) -52 (<0.001) <0.001 -0.30 (0.047) -0.23 (0.010) 44.8 

Manganese, unfiltered-recoverable 108 -37 (NR3) 24 (NR3) -34 (NR3) 0.081 -0.33 (0.010) -0.29 (0.001) 46.9 

Zinc, filtered 108 NR4 NR4 NR4 NR4 NR4 NR4 NR4 

Zinc, unfiltered-recoverable 108 NR4 NR4 NR4 NR4 NR4 NR4 NR4 Arsenic, filtered 108 13 (<0.001) -1 (0.685) -24 (<0.001) <0.001 -0.13 (<0.001) -0.24 (<0.001) 24.4 Arsenic, unfiltered-recoverable 108 -10 (NR3) 5 (NR3) -12 (NR3) 0.353 -0.09 (0.171) -0.17 (<0.001) 35.8 

Suspended sediment 109 -46 (0.004) -33 (0.051) 144 (<0.001) <0.001 -0.19 (0.226) 0.26 (0.072) 68.3 

Clark Fork near Galen (site 11, fig. 1, table 1) Specific conductance 147 -2 (NR3) -2 (NR3) -2 (NR3) 0.016 -0.23 (<0.001) -0.27 (<0.001) 11.8 

Cadmium, filtered 145 NR4 NR4 NR4 NR4 NR4 NR4 NR4 

Cadmium, unfiltered-recoverable 145 NR4 NR4 NR4 NR4 NR4 NR4 NR4 Copper, filtered 147 -49 (<0.001) -6 (0.409) -12 (0.194) <0.001 -0.17 (0.015) -0.19 (0.026) 27.1 

Copper, unfiltered-recoverable 147 -38 (<0.001) 9 (0.409) -10 (0.437) <0.001 0.12 (0.211) 0.13 (0.226) 46.5 

Iron, filtered 145 NR4 NR4 NR4 NR4 NR4 NR4 NR4 

Iron unfiltered-recoverable 147 -36 (<0.001) 17 (0.194) 0 (0.986) 0.023 0.45 (<0.001) 0.32 (0.003) 44.9 Lead, filtered 145 NR4 NR4 NR4 NR4 NR4 NR4 NR4 Lead, unfiltered-recoverable 145 NR4 NR4 NR4 NR4 NR4 NR4 NR4 Manganese, filtered 147 -16 (0.114) 20 (0.114) -42 (<0.001) <0.001 -0.07 (0.575) -0.09 (0.391) 34.3 

Manganese, unfiltered-recoverable 147 -23 (0.007) 7 (0.491) -29 (0.011) <0.001 0.09 (0.468) -0.24 (0.008) 33.7 

Zinc, filtered 145 NR4 NR4 NR4 NR4 NR4 NR4 NR4 

Zinc, unfiltered-recoverable 140 -59 (<0.001) -32 (<0.001) 9 (0.576) <0.001 0.32 (0.029) 0.23 (0.043) 42.4 

Arsenic, filtered 147 -4 (0.667) 9 (0.276) -17 (0.078) 1.000 -0.13 (0.091) 0.17 (0.032) 25.6 

Arsenic, unfiltered-recoverable 147 -10 (0.187) 14 (0.009) -25 (<0.001) <0.001 -0.07 (0.180) 0.10 (0.174) 26.4 

Suspended sediment 148 -3 (0.859) -18 (0.208) 1 (0.966) 1.000 0.53 (0.002) 0.55 (<0.001) 57.5 

Table 4–1. Flow-adjusted trend results determined by using the time-series model (TSM) for selected water-quality constituents and properties for the upper Clark Fork Basin, Montana, water years 1996–2010.—Continued [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. Values in parentheses indicate p-values for associated Gray shading indicates statistical significance at p-value less than 0.01. p-value, statistical probability level; SEASC, seasonal concentration anomaly coefficient; ANNC, annual 

ficient; SEE, standard error of estimate in percent; NR, not reported; <, less than] Constituent or property Number of samples Total percent change for water years 1996–2000 (period 1) Total percent change for water years 2001–05 (period 2) Total percent change for water years 2006–2010 (period 3) p-value for overall trend analysis1 SEASC ANNC SEE 

Warm Springs Creek at Warm Springs (site 10, fig. 1, table 1)—Continued
Clark Fork at Deer Lodge (site 14, fig. 1, table 1)

Specific conductance 146 -2 (0.355) -1 (0.394) -7 (0.002) <0.001 -0.04 (0.001) -0.20 (<0.001) 11.2 

Cadmium, filtered 146 NR4 NR4 NR4 NR4 NR4 NR4 NR4 

Cadmium, unfiltered-recoverable 145 NR4 NR4 NR4 NR4 NR4 NR4 NR4 Copper, filtered 146 -18 (0.002) 3 (0.691) -17 (0.030) 0.002 0.18 (<0.001) 0.16 (0.023) 22.0 Copper, unfiltered-recoverable 145 -14 (NR3) -2 (NR3) 2 (NR3) 0.753 0.23 (0.034) 0.28 (0.030) 45.0 

Iron, filtered 146 NR4 NR4 NR4 NR4 NR4 NR4 NR4 

Iron unfiltered-recoverable 146 -25 (NR3) 9 (NR3) 18 (NR3) 0.059 0.30 (0.042) 0.43 (0.002) 51.8 

Lead, filtered 146 NR4 NR4 NR4 NR4 NR4 NR4 NR4 

Lead, unfiltered-recoverable 143 -3 (NR3) -11 (NR3) 33 (NR3) 0.666 0.26 (0.101) 0.35 (0.039) 59.5 

Manganese, filtered 146 -15 (NR3) 45 (NR3) -31 (NR3) 0.013 -0.08 (0.464) -0.08 (0.443) 42.8 

Manganese, unfiltered-recoverable 146 -29 (NR3) 13 (NR3) -7 (NR3) 0.090 -0.16 (0.258) -0.09 (0.465) 46.9 Zinc, filtered 146 NR4 NR4 NR4 NR4 NR4 NR4 NR4 

Zinc, unfiltered-recoverable 142 -36 (<0.001) -8 (0.490) -5 (0.775) <0.001 0.27 (0.054) 0.25 (0.049) 46.6 

Arsenic, filtered 146 -2 (0.575) 19 (<0.001) -14 (0.001) 0.004 0.17 (<0.001) 0.21 (<0.001) 14.7 

Arsenic, unfiltered-recoverable 146 -3 (NR3) 5 (NR3) -7 (NR3) 0.554 0.24 (<0.001) 0.24 (<0.001) 26.5 

Suspended sediment 147 -15 (0.144) -11 (0.380) 3 (0.870) <0.001 -0.11 (0.565) 0.31 (0.033) 64.4 

Little Blackfoot River (site 15, fig. 1, table 1)

Specific conductance 65 3 (NR3) 1 (NR3) NR5 0.357 -0.10 (<0.001) -0.06 (<0.001) 10.7 

Cadmium, filtered 64 NR4 NR4 NR5 NR4 NR4 NR4 NR4 

Cadmium, unfiltered-recoverable 64 NR4 NR4 NR5 NR4 NR4 NR4 NR4 Copper, filtered 63 NR4 NR4 NR5 NR4 NR4 NR4 NR4 Copper, unfiltered-recoverable 65 -15 (NR3) 11 (NR3) NR5 0.705 -0.29 (0.144) -0.09 (0.643) 47.4 

Iron, filtered 60 18 (NR3) -49 (NR3) NR5 0.019 0.30 (<0.001) 0.50 (<0.001) 51.9 

Iron unfiltered-recoverable 65 -48 (<0.001) 33 (0.027) NR5 <0.001 0.36 (<0.001) -0.33 (0.019) 51.4 

Lead, filtered 65 NR4 NR4 NR5 NR4 NR4 NR4 NR4 

Lead, unfiltered-recoverable 65 NR4 NR4 NR5 NR4 NR4 NR4 NR4 

Table 4–1. Flow-adjusted trend results determined by using the time-series model (TSM) for selected water-quality constituents and properties for the upper Clark Fork Basin, Montana, water years 1996–2010.—Continued[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. Values in parentheses indicate p-values for associated Gray shading indicates statistical significance at p-value less than 0.01. p-value, statistical probability level; SEASC, seasonal concentration anomaly coefficient; ANNC, annual 

ficient; SEE, standard error of estimate in percent; NR, not reported; <, less than]

Constituent or property

Number 

of 

samples

Total percent 

change for 

water years 

1996–2000 

(period 1)

Total percent 

change for 

water years 

2001–05 

(period 2)

Total percent 

change for 

water years 

2006–2010 

(period 3)

p-value 

for overall 

trend 

analysis1

SEASC ANNC SEE
Manganese, filtered 65 -12 (NR3) -13 (NR3) NR5 1.000 -0.55 (<0.001) -0.10 (0.406) 35.4 

Manganese, unfiltered-recoverable 62 -39 (0.003) 32 (0.201) NR5 0.002 0.03 (0.860) -0.30 (0.136) 44.5 

Zinc, filtered 65 NR4 NR4 NR5 NR4 NR4 NR4 NR4 

Zinc, unfiltered-recoverable 65 NR4 NR4 NR5 NR4 NR4 NR4 NR4 Arsenic, filtered 65 -4 (NR3) 12 (NR3) NR5 0.077 0.00 (0.976) 0.03 (0.501) 12.8 

Arsenic, unfiltered-recoverable 65 -23 (<0.001) -5 (0.051) NR5 <0.001 0.15 (<0.001) -0.21 (<0.001) 18.0 Suspended sediment 66 -26 (NR3) -25 (NR3) NR5 0.146 0.03 (0.865) 0.24 (0.267) 67.9 

Clark Fork at Goldcreek (site 16, fig. 1, table 1) Specific conductance 146 0 (NR3) -2 (NR3) -5 (NR3) 0.197 -0.09 (<0.001) -0.14 (<0.001) 10.0 

Cadmium, filtered 146 NR4 NR4 NR4 NR4 NR4 NR4 NR4 

Cadmium, unfiltered-recoverable 146 NR4 NR4 NR4 NR4 NR4 NR4 NR4 Copper, filtered 145 -21 (NR3) 12 (NR3) -6 (NR3) 0.019 0.01 (0.856) 0.35 (<0.001) 23.8 

Copper, unfiltered-recoverable 145 -14 (0.168) -18 (0.047) 3 (0.796) 0.005 0.37 (<0.001) 0.62 (<0.001) 43.1 Iron, filtered 145 NR4 NR4 NR4 NR4 NR4 NR4 NR4 

Iron unfiltered-recoverable 146 -16 (0.121) -28 (0.005) 45 (0.028) <0.001 0.49 (0.001) 0.68 (<0.001) 49.3 Lead, filtered 146 NR4 NR4 NR4 NR4 NR4 NR4 NR4 Lead, unfiltered-recoverable 146 NR4 NR4 NR4 NR4 NR4 NR4 NR4 

Manganese, filtered 146 -6 (NR3) -8 (NR3) 3 (NR3) 0.341 0.05 (0.648) -0.02 (0.833) 32.3 

Manganese, unfiltered-recoverable 146 -18 (0.017) -20 (0.016) 22 (0.139) <0.001 0.06 (0.631) 0.13 (0.208) 38.8 

Zinc, filtered 146 NR4 NR4 NR4 NR4 NR4 NR4 NR4 

Zinc, unfiltered-recoverable 144 -33 (<0.001) -34 (<0.001) 36 (0.046) <0.001 0.34 (0.010) 0.46 (<0.001) 42.7 Arsenic, filtered 146 -9 (NR3) 10 (NR3) -2 (NR3) 0.811 -0.06 (0.089) 0.15 (0.001) 15.7 

Arsenic, unfiltered-recoverable 146 -13 (0.003) 6 (0.056) 5 (0.162) 0.003 0.06 (0.066) 0.23 (<0.001) 23.1 

Suspended sediment 147 18 (0.263) -53 (<0.001) 69 (0.011) <0.001 0.48 (0.006) 0.50 (0.003) 58.9 

Table 4–1. Flow-adjusted trend results determined by using the time-series model (TSM) for selected water-quality constituents and properties for the upper Clark Fork Basin, Montana, water years 1996–2010.—Continued [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. Values in parentheses indicate p-values for associated Gray shading indicates statistical significance at p-value less than 0.01. p-value, statistical probability level; SEASC, seasonal concentration anomaly coefficient; ANNC, annual 

ficient; SEE, standard error of estimate in percent; NR, not reported; <, less than] Constituent or property Number of samples Total percent change for water years 1996–2000 (period 1) Total percent change for water years 2001–05 (period 2) Total percent change for water years 2006–2010 (period 3) p-value for overall trend analysis1 SEASC ANNC SEE 

Little Blackfoot River (site 15, fig. 1, table 1)—Continued
Flint Creek (site 17, fig. 1, table 1)

Specific conductance 89 7 (NR3) 10 (NR3) NR5 0.024 -0.12 (<0.001) -0.12 (<0.001) 15.2 

Cadmium, filtered 85 NR4 NR4 NR5 NR4 NR4 NR4 NR4 

Cadmium, unfiltered-recoverable 85 NR4 NR4 NR5 NR4 NR4 NR4 NR4 Copper, filtered 88 NR4 NR4 NR5 NR4 NR4 NR4 NR4 Copper, unfiltered-recoverable 89 -21 (NR3) -16 (NR3) NR5 0.055 0.32 (<0.001) 0.23 (0.011) 37.9 

Iron, filtered 84 -2 (NR3) 0 (NR3) NR5 1.000 0.37 (<0.001) 0.35 (<0.001) 47.8 

Iron unfiltered-recoverable 89 -41 (NR3) 38 (NR3) NR5 1.000 0.65 (<0.001) 0.36 (<0.001) 37.0 Lead, filtered 85 NR4 NR4 NR5 NR4 NR4 NR4 NR4 

Lead, unfiltered-recoverable 84 -31 (0.002) -9 (0.481) NR5 <0.001 0.88 (<0.001) 0.52 (<0.001) 52.6 

Manganese, filtered 89 -4 (NR3) 32 (NR3) NR5 0.072 0.00 (0.974) -0.45 (<0.001) 33.0 

Manganese, unfiltered-recoverable 89 -23 (0.003) 0 (0.977) NR5 0.003 0.09 (0.095) -0.14 (0.002) 39.6 

Zinc, filtered 85 NR4 NR4 NR5 NR4 NR4 NR4 NR4 

Zinc, unfiltered-recoverable 81 -33 (0.017) -38 (0.047) NR5 <0.001 0.60 (<0.001) 0.30 (0.016) 44.7 

Arsenic, filtered 89 -7 (NR3) -4 (NR3) NR5 0.106 -0.08 (0.009) -0.15 (<0.001) 20.1 

Arsenic, unfiltered-recoverable 89 -19 (0.005) -3 (0.687) NR5 0.040 0.12 (0.017) -0.05 (0.327) 29.6 

Suspended sediment 90 -9 (NR3) -43 (NR3) NR5 1.000 0.62 (<0.001) 0.43 (0.002) 59.7 

Clark Fork near Drummond (site 18, fig. 1, table 1)

Specific conductance 146 -3 (0.103) -3 (0.026) -3 (0.084) 0.009 -0.19 (<0.001) -0.22 (<0.001) 9.5 Cadmium, filtered 146 NR4 NR4 NR4 NR4 NR4 NR4 NR4 

Cadmium, unfiltered-recoverable 146 NR4 NR4 NR4 NR4 NR4 NR4 NR4 Copper, filtered 143 2 (0.547) 15 (<0.001) -23 (<0.001) 0.002 0.15 (0.029) 0.68 (<0.001) 36.2 

Copper, unfiltered-recoverable 144 -23 (0.014) -15 (0.107) 5 (0.755) <0.001 0.68 (<0.001) 0.47 (<0.001) 52.9 

Iron, filtered 143 NR4 NR4 NR4 NR4 NR4 NR4 NR4 

Iron unfiltered-recoverable 145 -61 (<0.001) 7 (0.194) 44 (0.013) <0.001 0.95 (<0.001) 0.33 (<0.001) 65.5 Lead, filtered 146 NR4 NR4 NR4 NR4 NR4 NR4 NR4 Lead, unfiltered-recoverable 145 NR4 NR4 NR4 NR4 NR4 NR4 NR4 

Table 4–1. Flow-adjusted trend results determined by using the time-series model (TSM) for selected water-quality constituents and properties for the upper Clark Fork Basin, Montana, water years 1996–2010.—Continued[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. Values in parentheses indicate p-values for associated Gray shading indicates statistical significance at p-value less than 0.01. p-value, statistical probability level; SEASC, seasonal concentration anomaly coefficient; ANNC, annual 

ficient; SEE, standard error of estimate in percent; NR, not reported; <, less than]

Constituent or property

Number 

of 

samples

Total percent 

change for 

water years 

1996–2000 

(period 1)

Total percent 

change for 

water years 

2001–05 

(period 2)

Total percent 

change for 

water years 

2006–2010 

(period 3)

p-value 

for overall 

trend 

analysis1

SEASC ANNC SEE
Manganese, filtered 145 -8 (NR3) 1 (NR3) -3 (NR3) 0.736 0.38 (<0.001) 0.15 (0.162) 34.6 

Manganese, unfiltered-recoverable 146 -35 (<0.001) 1 (0.901) 17 (0.353) 0.005 0.54 (<0.001) 0.32 (0.038) 46.6 

Zinc, filtered 145 NR4 NR4 NR4 NR4 NR4 NR4 NR4 

Zinc, unfiltered-recoverable 143 -55 (<0.001) -16 (<0.001) 10 (0.448) <0.001 0.78 (<0.001) 0.30 (<0.001) 56.2 Arsenic, filtered 146 -8 (NR3) 8 (NR3) -10 (NR3) 0.030 0.11 (<0.001) 0.06 (0.153) 15.8 

Arsenic, unfiltered-recoverable 146 -15 (NR3) 1 (NR3) 1 (NR3) 0.163 0.21 (<0.001) 0.10 (0.121) 24.7 

Suspended sediment 147 -39 (<0.001) -11 (0.084) 8 (0.686) <0.001 0.40 (0.009) 0.52 (<0.001) 65.0 

Rock Creek (site 19, fig. 1, table 1) Specific conductance 65 2 (NR3) -8 (NR3) NR5 1.000 -0.27 (<0.001) -0.23 (<0.001) 9.7 

Cadmium, filtered 64 NR4 NR4 NR5 NR4 NR4 NR4 NR4 

Cadmium, unfiltered-recoverable 63 NR4 NR4 NR5 NR4 NR4 NR4 NR4 Copper, filtered 64 NR4 NR4 NR5 NR4 NR4 NR4 NR4 Copper, unfiltered-recoverable 64 NR4 NR4 NR5 NR4 NR4 NR4 NR4 Iron, filtered 64 -15 (NR3) -29 (NR3) NR5 0.013 0.77 (0.001) 0.69 (<0.001) 29.9 

Iron unfiltered-recoverable 64 -26 (<0.001) -27 (0.044) NR5 <0.001 1.20 (<0.001) 0.60 (<0.001) 30.1 

Lead, filtered 64 NR4 NR4 NR5 NR4 NR4 NR4 NR4 

Lead, unfiltered-recoverable 64 NR4 NR4 NR5 NR4 NR4 NR4 NR4 Manganese, filtered 63 NR4 NR4 NR5 NR4 NR4 NR4 NR4 Manganese, unfiltered-recoverable 63 NR4 NR4 NR5 NR4 NR4 NR4 NR4 Zinc, filtered 64 NR4 NR4 NR5 NR4 NR4 NR4 NR4 Zinc, unfiltered-recoverable 64 NR4 NR4 NR5 NR4 NR4 NR4 NR4 Arsenic, filtered 64 NR4 NR4 NR5 NR4 NR4 NR4 NR4 Arsenic, unfiltered-recoverable 64 NR4 NR4 NR5 NR4 NR4 NR4 NR4 Suspended sediment 66 52 (<0.001) -56 (<0.001) NR5 <0.001 3.04 (<0.001) 0.86 (<0.001) 54.5 

Table 4–1. Flow-adjusted trend results determined by using the time-series model (TSM) for selected water-quality constituents and properties for the upper Clark Fork Basin, Montana, water years 1996–2010.—Continued [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. Values in parentheses indicate p-values for associated Gray shading indicates statistical significance at p-value less than 0.01. p-value, statistical probability level; SEASC, seasonal concentration anomaly coefficient; ANNC, annual 

ficient; SEE, standard error of estimate in percent; NR, not reported; <, less than] Constituent or property Number of samples Total percent change for water years 1996–2000 (period 1) Total percent change for water years 2001–05 (period 2) Total percent change for water years 2006–2010 (period 3) p-value for overall trend analysis1 SEASC ANNC SEE 

Clark Fork near Drummond (site 18, fig. 1, table 1)—Continued
Clark Fork at Turah Bridge (site 20, fig. 1, table 1)

Specific conductance 193 -5 (NR3) -1 (NR3) 0 (NR3) 0.016 -0.12 (<0.001) -0.20 (<0.001) 12.8 

Cadmium, filtered 170 NR4 NR4 NR4 NR4 NR4 NR4 NR4 

Cadmium, unfiltered-recoverable 170 NR4 NR4 NR4 NR4 NR4 NR4 NR4 Copper, filtered 169 -20 (NR3) 8 (NR3) -12 (NR3) 0.012 0.57 (<0.001) 0.51 (<0.001) 30.0 Copper, unfiltered-recoverable 168 -6 (NR3) -16 (NR3) 27 (NR3) 0.110 0.65 (<0.001) 0.58 (<0.001) 47.2 

Iron, filtered 150 NR4 NR4 NR4 NR4 NR4 NR4 NR4 

Iron unfiltered-recoverable 170 -28 (<0.001) -15 (0.222) 58 (0.082) <0.001 0.84 (<0.001) 0.44 (<0.001) 57.1 Lead, filtered 170 NR4 NR4 NR4 NR4 NR4 NR4 NR4 Lead, unfiltered-recoverable 170 NR4 NR4 NR4 NR4 NR4 NR4 NR4 Manganese, filtered 170 1 (NR3) -10 (NR3) 18 (NR3) 1.000 0.41 (<0.001) 0.27 (<0.001) 29.0 

Manganese, unfiltered-recoverable 170 -20 (0.131) -16 (0.147) 45 (0.025) <0.001 0.67 (<0.001) 0.19 (0.035) 51.2 Zinc, filtered 170 NR4 NR4 NR4 NR4 NR4 NR4 NR4 

Zinc, unfiltered-recoverable 161 -33 (0.317) -39 (0.001) 82 (0.007) <0.001 0.49 (0.001) -0.10 (0.483) 56.3 

Arsenic, filtered 170 -4 (NR3) 7 (NR3) 2 (NR3) 0.022 0.23 (<0.001) 0.28 (<0.001) 19.7 

Arsenic, unfiltered-recoverable 170 -7 (NR3) -6 (NR3) 21(NR3) 0.173 0.35 (<0.001) 0.21 (<0.001) 31.2 

Suspended sediment 209 -7 (0.895) -32 (0.197) 60 (0.531) 0.002 0.65 (<0.001) 0.18 (<0.001) 58.1 

Blackfoot River (site 21, fig. 1, table 1)

Specific conductance 102 0 (0.796) -4 (<0.001) 6 (<0.001) <0.001 -0.14 (<0.001) -0.05 (0.009) 6.1 

Cadmium, filtered 101 NR4 NR4 NR4 NR4 NR4 NR4 NR4 

Cadmium, unfiltered-recoverable 100 NR4 NR4 NR4 NR4 NR4 NR4 NR4 Copper, filtered 102 NR4 NR4 NR4 NR4 NR4 NR4 NR4 Copper, unfiltered-recoverable 102 NR4 NR4 NR4 NR4 NR4 NR4 NR4 Iron, filtered 102 NR4 NR4 NR4 NR4 NR4 NR4 NR4 Iron unfiltered-recoverable 102 -37 (<0.001) 29 (0.032) -28 (0.021) <0.001 1.13 (<0.001) 0.95 (<0.001) 43.6 Lead, filtered 101 NR4 NR4 NR4 NR4 NR4 NR4 NR4 Lead, unfiltered-recoverable 102 NR4 NR4 NR4 NR4 NR4 NR4 NR4 

Table 4–1. Flow-adjusted trend results determined by using the time-series model (TSM) for selected water-quality constituents and properties for the upper Clark Fork Basin, Montana, water years 1996–2010.—Continued[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. Values in parentheses indicate p-values for associated Gray shading indicates statistical significance at p-value less than 0.01. p-value, statistical probability level; SEASC, seasonal concentration anomaly coefficient; ANNC, annual 

ficient; SEE, standard error of estimate in percent; NR, not reported; <, less than]

Constituent or property

Number 

of 

samples

Total percent 

change for 

water years 

1996–2000 

(period 1)

Total percent 

change for 

water years 

2001–05 

(period 2)

Total percent 

change for 

water years 

2006–2010 

(period 3)

p-value 

for overall 

trend 

analysis1

SEASC ANNC SEE
Manganese, filtered 99 NR4 NR4 NR4 NR4 NR4 NR4 NR4 

Manganese, unfiltered-recoverable 100 NR4 NR4 NR4 NR4 NR4 NR4 NR4 Zinc, filtered 101 NR4 NR4 NR4 NR4 NR4 NR4 NR4 Zinc, unfiltered-recoverable 102 NR4 NR4 NR4 NR4 NR4 NR4 NR4 Arsenic, filtered 101 NR4 NR4 NR4 NR4 NR4 NR4 NR4 Arsenic, unfiltered-recoverable 101 NR4 NR4 NR4 NR4 NR4 NR4 NR4 Suspended sediment 103 -12 (NR3) 4 (NR3) -24 (NR3) 0.403 1.21 (<0.001) 1.31 (<0.001) 48.6 

1Determination of and distinction between p-value for individual trend period and p-value for overall trend analysis are discussed in “Supplement 2: Summary of the Time-S

in this Study.” 2Procedures for determining and applying the study reporting level used in the application of the time-series model are discussed in the section of this report “Time-Series M

3Results not reported because of nonsignificant overall trend analysis (p-value greater than 0.01). 4Results not reported because greater than 6 percent of values were affected by recensoring at study reporting level, as discussed in the section of this report “Time-Series Mo5Results not reported because of no or insufficient data for application of the TSM during indicated trend-analysis period. Table 4–1. Flow-adjusted trend results determined by using the time-series model (TSM) for selected water-quality constituents and properties for 

the upper Clark Fork Basin, Montana, water years 1996–2010.—Continued [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. Values in parentheses indicate p-values for associated Gray shading indicates statistical significance at p-value less than 0.01. p-value, statistical probability level; SEASC, seasonal concentration anomaly coefficient; ANNC, annual 

ficient; SEE, standard error of estimate in percent; NR, not reported; <, less than] Constituent or property Number of samples Total percent change for water years 1996–2000 (period 1) Total percent change for water years 2001–05 (period 2) Total percent change for water years 2006–2010 (period 3) p-value for overall trend analysis1 SEASC ANNC SEE 

Blackfoot River (site 21, fig. 1, table 1)—Continued
Table 4–2. Flow-adjusted trend results determined by using the time-series model (TSM) for selected water-quality constituents and properties for (site 22, fig. 1, table 1), water years 1996–2010.[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. Values in parentheses indicate p-values for associated Gray shading indicates statistical significance at p-value less than 0.01. p-value, statistical probability level; SEASC, seasonal concentration anomaly coefficient; ANNC, annual 

ficient; SEE, standard error of estimate in percent; NR, not reported; <, less than]

Constituent or property

Number 

of 

samples

Total percent 

change for 

water years 

1996–2000 

(period 1)

Total percent 

change for 

water years 

2001–05 

(period 2)

Total percent 

change for 

October 1, 

2005–March 27, 

2008 (period 3A)

Total percent 

change for 

March 28, 2008–

September 30, 

2010 (period 3B)

p-value 

for overall 

trend 

analysis1

SEASC ANNC S

Clark Fork above Missoula (site 22, fig. 1, table 1)

Specific conductance 186 0 (NR3) -2 (NR3) 0 (NR3) 5 (NR3) 0.031 -0.18 (<0.001) -0.19 (<0.001) 

Cadmium, filtered 166 NR4 NR4 NR4 NR4 NR4 NR4 NR4 

Cadmium, unfilteredrecoverable 167 NR4 NR4 NR4 NR4 NR4 NR4 NR4 Copper, filtered 166 -21 (NR3) 21 (NR3) 7 (NR3) 11 (NR3) 0.027 0.53 (<0.001) 0.37 (0.007) 3

Copper, unfilteredrecoverable 165 -18 (0.026) 52 (<0.001) 104 (<0.001) -59 (0.002) <0.001 0.55 (<0.001) 0.62 (<0.001) 6

Iron, filtered 167 NR4 NR4 NR4 NR4 NR4 NR4 NR4 

Iron unfiltered-recoverable 167 -30 (<0.001) 61 (<0.001) 79 (<0.001) -58 (0.001) <0.001 0.77 (<0.001) 0.76 (<0.001) 

Lead, filtered 160 NR4 NR4 NR4 NR4 NR4 NR4 NR4 N

Lead, unfiltered-recoverable 162 NR4 NR4 NR4 NR4 NR4 NR4 NR4 NManganese, filtered 167 -8 (0.312) 1 (0.938) 2 (0.854) -44 (0.001) <0.001 -0.17 (0.303) 0.01 (0.930) 3

Manganese, unfilteredrecoverable 167 -20 (0.023) 17 (0.242) 71 (<0.001) -45 (0.005) <0.001 0.13 (0.461) 0.17 (0.267) 4

Zinc, filtered 166 NR4 NR4 NR4 NR4 NR4 NR4 NR4 N

Zinc, unfiltered-recoverable 150 NR4 NR4 NR4 NR4 NR4 NR4 NR4 NArsenic, filtered 167 -2 (0.636) 12 (0.078) 7 (0.365) 12 (0.276) 0.005 0.19 (0.015) -0.02 (0.765) 2

Arsenic, unfilteredrecoverable 167 -6 (0.410) 14 (0.184) 24 (0.070) -2 (0.921) 0.008 0.49 (<0.001) 0.08 (0.540) 3

Suspended sediment 209 -15 (0.211) 32 (0.150) 147 (<0.001) -60 (0.004) <0.001 1.28 (<0.001) 0.72 (<0.001) 7

1Determination of and distinction between p-value for individual trend period and p-value for overall trend analysis are discussed in “Supplement 2: Summary of the Time-S

in this Study.”

2Procedures for determining and applying the study reporting level used in the application of the time-series model are discussed in the section of this report “Time-Series M3Results not reported because of nonsignificant overall trend analysis (p-value greater than 0.01).4Results not reported because greater than 6 percent of values were affected by recensoring at study reporting level, as discussed in the section of this report “Time-Series M
126 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

Table 4–3. Flow-adjusted trend results determined by using multiple linear regression on time, streamflow, and season (MLR) 

for selected water-quality constituents and properties for selected sampling sites in the upper Clark Fork Basin, Montana, water 

years 2006–2010.

[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. Values in parentheses indicate 

95-percent confidence intervals. Gray shading indicates statistical significance at p-value less than 0.01. p-value, statistical probability level; SEE, standard 

error of estimate; NR, not reported; <, less than]

Constituent or property

Number 

of 

samples

Total percent change 

for water years 

2006–2010 (period 5)

Percent 

censored 

values

p-value for 

streamflow 

coefficient

SEE p-value for 

regression

Mill Creek near Anaconda (site 4, fig. 1, table 1)

Specific conductance 48 7 (0.031) 0 <0.001 7.0 <0.001

Cadmium, filtered 47 0 (NR1

) 4 0.989 41.7 0.081

Cadmium, unfiltered-recoverable48 -11 (0.433) 0 0.024 35.6 <0.001

Copper, filtered 48 -15 (0.253) 0 <0.001 33.8 <0.001

Copper, unfiltered-recoverable 48 -21 (0.091) 2 0.003 32.6 <0.001

Iron, filtered 48 -33 (NR1

) 0 0.507 34.9 <0.001

Iron unfiltered-recoverable 48 -25 (0.055) 0 0.022 36.3 <0.001

Lead, filtered 48 -36 (NR1

) 6 0.280 57.6 <0.001

Lead, unfiltered-recoverable 48 -29 (0.046) 0 0.010 19.5 <0.001

Manganese, filtered 48 -4 (NR1

) 0 0.210 19.5 <0.001

Manganese, unfiltered-recoverable48 -15 (NR1

) 0 0.260 35.4 <0.001

Zinc, filtered 48 16 (0.258) 10 <0.001 29.6 <0.001

Zinc, unfiltered-recoverable 48 0 (0.982) 0 <0.001 35.9 <0.001

Arsenic, filtered 48 -9 (NR1

) 0 0.758 38.4 0.002

Arsenic, unfiltered-recoverable 48 -13 (NR1

) 0 0.960 34.7 <0.001

Suspended sediment 48 -20 (0.252) 0 <0.001 48.1 <0.001

Mill Creek at Opportunity (site 5, fig. 1, table 1)

Specific conductance 64 11 (0.010) 0 <0.001 10.4 <0.001

Cadmium, filtered 64 -21 (NR1

) 0 0.343 27.5 <0.001

Cadmium, unfiltered-recoverable64 -33 (0.007) 0 <0.001 41.1 <0.001

Copper, filtered 64 -36 (<0.001) 0 <0.001 26.2 <0.001

Copper, unfiltered-recoverable 64 -35 (0.003) 0 <0.001 38.0 <0.001

Iron, filtered 64 -27 (NR1

) 0 0.733 34.0 <0.001

Iron unfiltered-recoverable 64 -26 (0.116) 0 <0.001 54.0 <0.001

Lead, filtered 64 -40 (0.003) 9 <0.001 47.0 <0.001

Lead, unfiltered-recoverable 64 -23 (0.214) 0 <0.001 47.9 <0.001

Manganese, filtered 64 -32 (0.027) 0 <0.001 47.9 <0.001

Manganese, unfiltered-recoverable64 -23 (NR1

) 0 0.222 69.1 <0.001

Zinc, filtered 63 -42 (NR1

) 2 0.534 24.2 <0.001

Zinc, unfiltered-recoverable 64 -33 (0.013) 0 <0.001 45.4 <0.001

Arsenic, filtered 64 -33 (NR1

) 0 0.071 33.0 <0.001

Arsenic, unfiltered-recoverable 64 -34 (<0.001) 0 0.009 30.7 <0.001

Suspended sediment 64 -15 (0.504) 0 <0.001 72.4 <0.001
Appendix 4—Trend Analysis Results 127

Table 4–3. Flow-adjusted trend results determined by using multiple linear regression on time, streamflow, and season (MLR) 

for selected water-quality constituents and properties for selected sampling sites in the upper Clark Fork Basin, Montana, water 

years 2006–2010.—Continued

[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. Values in parentheses indicate 

95-percent confidence intervals. Gray shading indicates statistical significance at p-value less than 0.01. p-value, statistical probability level; SEE, standard 

error of estimate; NR, not reported; <, less than]

Constituent or property

Number 

of 

samples

Total percent change 

for water years 

2006–2010 (period 5)

Percent 

censored 

values

p-value for 

streamflow 

coefficient

SEE p-value for 

regression

Willow Creek near Anaconda (site 6, fig. 1, table 1)

Specific conductance 44 24 (<0.001) 0 <0.001 8.5 <0.001

Cadmium, filtered 42 -14 (0.314) 12 0.040 33.1 0.005

Cadmium, unfiltered-recoverable44 -33 (0.054) 7 0.001 47.9 <0.001

Copper, filtered 44 -23 (0.016) 0 <0.001 23.9 <0.001

Copper, unfiltered-recoverable 44 -32 (0.064) 2 0.009 48.3 <0.001

Iron, filtered 44 -16 (0.388) 0 0.017 47.0 <0.001

Iron unfiltered-recoverable 44 -29 (0.096) 0 <0.001 49.0 <0.001

Lead, filtered 44 -38 (0.014) 0 <0.001 44.2 <0.001

Lead, unfiltered-recoverable 44 -44 (0.022) 0 <0.001 30.2 <0.001

Manganese, filtered 44 -24 (NR1

) 0 0.086 30.2 <0.001

Manganese, unfiltered-recoverable44 -26 (NR1

) 0 0.114 39.8 0.006

Zinc, filtered 44 -26 (0.055) 9 0.003 34.6 <0.001

Zinc, unfiltered-recoverable 44 -18 (0.317) 9 <0.001 44.7 <0.001

Arsenic, filtered 44 -16 (0.038) 0 0.042 18.3 <0.001

Arsenic, unfiltered-recoverable 44 -18 (0.026) 0 0.020 20.3 <0.001

Suspended sediment 44 -54 (0.015) 0 <0.001 80.8 <0.001

Willow Creek at Opportunity (site 7, fig. 1, table 1)

Specific conductance 64 -13 (0.027) 0 0.020 17.5 <0.001

Cadmium, filtered 64 -25 (0.064) 8 <0.001 43.9 <0.001

Cadmium, unfiltered-recoverable64 7 (0.611) 2 <0.001 39.7 <0.001

Copper, filtered 64 -26 (0.008) 0 <0.001 31.8 <0.001

Copper, unfiltered-recoverable 64 -15 (0.199) 0 <0.001 35.4 <0.001

Iron, filtered 64 54 (0.008) 0 <0.001 46.6 <0.001

Iron unfiltered-recoverable 64 61 (0.004) 0 <0.001 47.3 <0.001

Lead, filtered 64 32 (0.066) 2 <0.001 43.8 <0.001

Lead, unfiltered-recoverable 64 19 (0.358) 0 <0.001 57.4 <0.001

Manganese, filtered 64 41 (NR1

) 0 0.886 57.4 <0.001

Manganese, unfiltered-recoverable

64 70 (NR1

) 0 0.239 54.1 <0.001

Zinc, filtered 64 -21 (0.142) 5 <0.001 47.3 <0.001

Zinc, unfiltered-recoverable 64 6 (0.734) 0 <0.001 50.0 <0.001

Arsenic, filtered 64 -23 (0.041) 0 <0.001 37.1 <0.001

Arsenic, unfiltered-recoverable 64 -19 (0.090) 0 <0.001 36.3 <0.001

Suspended sediment 64 10 (0.657) 0 <0.001 66.4 <0.001
128 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

Table 4–3. Flow-adjusted trend results determined by using multiple linear regression on time, streamflow, and season (MLR) 

for selected water-quality constituents and properties for selected sampling sites in the upper Clark Fork Basin, Montana, water 

years 2006–2010.—Continued

[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. Values in parentheses indicate 

95-percent confidence intervals. Gray shading indicates statistical significance at p-value less than 0.01. p-value, statistical probability level; SEE, standard 

error of estimate; NR, not reported; <, less than]

Constituent or property

Number 

of 

samples

Total percent change 

for water years 

2006–2010 (period 5)

Percent 

censored 

values

p-value for 

streamflow 

coefficient

SEE p-value for 

regression

Warm Springs Creek near Anaconda (site 9, fig. 1, table 1)

Specific conductance 30 16 (<0.001) 0 <0.001 4.7 <0.001

Cadmium, filtered 30 24 (NR1

) 30 0.224 21.4 <0.001

Cadmium, unfiltered-recoverable30 1 (NR1

) 10 0.246 50.1 <0.001

Copper, filtered 30 -15 (NR1

) 7 0.060 32.1 <0.001

Copper, unfiltered-recoverable 30 24 (NR1

) 10 0.193 57.3 <0.001

Iron, filtered 30 -38 (NR1

) 10 0.768 39.1 <0.001

Iron unfiltered-recoverable 30 -1 (0.969) 0 0.038 57.2 <0.001

Lead, filtered 30 NR2 70 NR2 NR2 NR2

Lead, unfiltered-recoverable 30 4 (0.923) 0 0.016 67.6 <0.001

Manganese, filtered 30 91 (NR1

) 3 0.326 38.8 <0.001

Manganese, unfiltered-recoverable30 18 (0.592) 0 0.029 53.2 <0.001

Zinc, filtered 30 NR2 47 NR2 NR2 NR2

Zinc, unfiltered-recoverable 30 22 (0.564) 13 0.005 61.5 <0.001

Arsenic, filtered 30 -9 (NR1

) 0 0.130 17.4 0.010

Arsenic, unfiltered-recoverable 30 18 (NR1

) 0 0.058 20.3 <0.001

Suspended sediment 30 -24 (0.498) 0 <0.001 72.3 <0.001

Lost Creek near Anaconda (site 12, fig. 1, table 1)

Specific conductance 47 -4 (0.315) 0 <0.001 8.7 <0.001

Cadmium, filtered 46 -44 (NR1

) 11 0.504 41.9 <0.001

Cadmium, unfiltered-recoverable47 -50 (NR1

) 11 0.526 48.4 <0.001

Copper, filtered 47 -46 (NR1

) 0 0.879 30.2 <0.001

Copper, unfiltered-recoverable 47 -42 (NR1

) 2 0.160 49.9 <0.001

Iron, filtered 47 25 (0.193) 9 0.034 35.3 <0.001

Iron unfiltered-recoverable 47 -20 (0.426) 0 0.002 63.9 <0.001

Lead, filtered 47 NR2 57 NR2 NR2 NR2

Lead, unfiltered-recoverable 47 -28 (0.286) 0 0.022 44.3 <0.001

Manganese, filtered 47 -16 (NR1

) 2 0.429 44.3 <0.001

Manganese, unfiltered-recoverable

47 -3 (NR1

) 0 0.067 52.6 <0.001

Zinc, filtered 47 -32 (NR1

) 30 0.245 28.1 <0.001

Zinc, unfiltered-recoverable 47 -43 (NR1

) 4 0.081 47.4 <0.001

Arsenic, filtered 47 -51 (NR1

) 0 0.161 32.3 <0.001

Arsenic, unfiltered-recoverable 47 -44 (NR1

) 0 0.375 31.3 <0.001

Suspended sediment 47 -56 (0.013) 0 <0.001 74.8 <0.001
Appendix 4—Trend Analysis Results 129

Table 4–3. Flow-adjusted trend results determined by using multiple linear regression on time, streamflow, and season (MLR) 

for selected water-quality constituents and properties for selected sampling sites in the upper Clark Fork Basin, Montana, water 

years 2006–2010.—Continued

[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. Values in parentheses indicate 

95-percent confidence intervals. Gray shading indicates statistical significance at p-value less than 0.01. p-value, statistical probability level; SEE, standard 

error of estimate; NR, not reported; <, less than]

Constituent or property

Number 

of 

samples

Total percent change 

for water years 

2006–2010 (period 5)

Percent 

censored 

values

p-value for 

streamflow 

coefficient

SEE p-value for 

regression

Lost Creek near Galen (site 13, fig. 1, table 1)

Specific conductance 62 4 (NR1

) 0 0.230 7.4 <0.001

Cadmium, filtered 61 8 (NR1

) 16 0.375 35.3 0.026

Cadmium, unfiltered-recoverable62 -51 (<0.001) 8 0.026 34.3 <0.001

Copper, filtered 61 -52 (NR1

) 0 0.360 27.6 <0.001

Copper, unfiltered-recoverable 62 -49 (<0.001) 0 0.003 23.8 <0.001

Iron, filtered 62 129 (NR1

) 5 0.110 58.1 <0.001

Iron unfiltered-recoverable 62 31 (NR1

) 0 0.378 44.1 <0.001

Lead, filtered 61 4 (NR1) 57 0.760 81.2 0.011

Lead, unfiltered-recoverable 62 -10 (0.601) 0 0.002 61.7 <0.001

Manganese, filtered 62 141 (NR1

) 0 0.075 61.7 <0.001

Manganese, unfiltered-recoverable62 140 (NR1

) 0 0.317 57.1 <0.001

Zinc, filtered 61 13 (NR1

) 15 0.911 50.5 <0.001

Zinc, unfiltered-recoverable 62 -24 (0.055) 10 0.004 37.6 <0.001

Arsenic, filtered 62 -3 (0.774) 0 0.030 32.6 <0.001

Arsenic, unfiltered-recoverable 62 6 (0.616) 0 0.038 32.8 <0.001

Suspended sediment 62 16 (0.588) 0 0.526 81.3 <0.001

1

Results not reported because of nonsignificant relation between streamflow and consituent concentration as indicated by p-value greater than 0.05.

2

Results not reported because greater than 45 percent of values were censored (that is, concentrations reported as less than the laboratory reporting level).
130 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

Results not reported because greater than 

6 percent of values were affected by recensoring 

at study reporting level, as discussed in the 

section of this report "Time-Series Model."

[Graph included as a place holder to assist in

comparison with other stations.]

Results not reported because greater than 

6 percent of values were affected by recensoring 

at study reporting level, as discussed in the 

section of this report "Time-Series Model."

[Graph included as a place holder to assist in

comparison with other stations.]

100

1,000 1,000

0.1

1

10

100

0.1

1

10

100

0.1

1

10

100

0.1

1

10

100

100

10

1

279 297 295 289

2.3 2.0 2.0 1.9

2.9 2.1 2.5 2.6

6.7 7.2 5.6 5.1

5.2 4.3 4.3 3.8

3.8

3.8 2.8 3.1 3.3

Flow-adjusted values, in indicated units of measrement

Filtered zinc (micrograms per liter) Unfiltered-recoverable

zinc (micrograms per liter)

Water year (October–September)

1 2 3 1 2 3

Period Period

1990199520002005201019901995200020052010

Filtered arsenic (micrograms per liter) Unfiltered-recoverable

arsenic (micrograms per liter)

Unfiltered-recoverable

copper (micrograms per liter) Filtered copper (micrograms per liter)

Specific conductance (μS/cm) Suspended sediment (milligrams per liter)

EXPLANATION

[Water year is defined as the 12-month 

period from October 1 through September 

30 and is designated by the year in which 

it ends. μS/cm, microsiemens per 

centimeter at 25 degrees Celsius; p-value, 

statistical probability level]

Flow-adjusted concentration (FAC) determined by 

using the time-series 

model

Flow-adjusted fitted trend 

determined by using the 

time-series model

Fitted trend value at start or 

end of period

Bold values indicate 

statistical significance 

(p-value less than 0.01) for 

period before value 

presented in bold

2.8

Figure 4–1. Flow-adjusted fitted trends determined by using the time-series model (TSM) for selected water-quality constituents and 

properties for Blacktail Creek (site 1, fig. 1, table 1), water years 1996–2010.
Appendix 4—Trend Analysis Results 131

522 493 493 476

60 13 13 6.0

6.5 6.8 4.0 4.3

15 14 7.5 7.3

7.3

180 44 31 14

14

13 10 5.6 6.0

890 280 77 32 1,100 350 98 43

Filtered copper (micrograms per liter)

Filtered zinc (micrograms per liter)

Filtered arsenic (micrograms per liter)

Unfiltered-recoverable

copper (micrograms per liter)

Unfiltered-recoverable

zinc (micrograms per liter)

Unfiltered-recoverable

arsenic (micrograms per liter)

1 2 3 1 2 3

Period Period

Specific conductance (μS/cm) Suspended sediment (milligrams per liter)

Flow-adjusted values, in indicated units of measrement

Water year (October–September)

1990199520002005201019901995200020052010

EXPLANATION

[Water year is defined as the 12-month 

period from October 1 through September 

30 and is designated by the year in which 

it ends. μS/cm, microsiemens per 

centimeter at 25 degrees Celsius; p-value, 

statistical probability level]

Flow-adjusted concentration (FAC) determined by 

using the time-series 

model

Flow-adjusted fitted trend 

determined by using the 

time-series model

Fitted trend value at start or 

end of period

Bold values indicate 

statistical significance 

(p-value less than 0.01) for 

period before value 

presented in bold

10

100

1,000

10,000

10

100

1

1,000

10

100

1,000

1,000

10,000

10

100

1,000

1

10

100

1

1,000

10

1

100

1,000

10

1

100

100

1,000

Figure 4–2. Flow-adjusted fitted trends determined by using the time-series model (TSM) for selected water-quality constituents and 

properties for Silver Bow Creek at Butte (site 2, fig. 1, table 1), water years 1996–2010.
132 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

Figure 4.3. Flow-adjusted fitted trends determined by using the time-series model (TSM) for selected constituents and properties for Silver Bow Creek at

Opportunity (site 3, fig. 1, table 1), water years 1996–2010.

424 463 470 440

48 42 29 17

9.0 11 12 7.4

24 18 21 19

19

130 120 94 44

44

16 17 18 8.8

290 220 130 53 540 470 270 130

21

1 2 3 1 2 3

Period Period

Flow-adjusted values, in indicated units of measrement

Water year (October–September)

1990199520002005201019901995200020052010

EXPLANATION

[Water year is defined as the 12-month 

period from October 1 through September 

30 and is designated by the year in which 

it ends. μS/cm, microsiemens per 

centimeter at 25 degrees Celsius; p-value, 

statistical probability level]

Flow-adjusted concentration (FAC) determined by 

using the time-series 

model

Flow-adjusted fitted trend 

determined by using the 

time-series model

Fitted trend value at start or 

end of period

Bold values indicate 

statistical significance 

(p-value less than 0.01) for 

period before value 

presented in bold

10

100

1,000

10,000

10

100

1

1,000

10

100

1,000

1,000

10,000

10

100

1,000

1

10

100

1

1,000

10

1

100

1,000

10

1

100

100

1,000

Specific conductance (μS/cm) Suspended sediment (milligrams per liter)

Unfiltered-recoverable

copper (micrograms per liter)

Unfiltered-recoverable

zinc (micrograms per liter)

Filtered zinc (micrograms per liter)

Filtered copper (micrograms per liter)

Filtered arsenic (micrograms per liter) Unfiltered-recoverable

arsenic (micrograms per liter)

Figure 4–3. Flow-adjusted fitted trends determined by using the time-series model (TSM) for selected water-quality constituents and 

properties for Silver Bow Creek at Opportunity (site 3, fig. 1, table 1), water years 1996–2010.
Appendix 4—Trend Analysis Results 133

120 128

4.1 3.3

3.3

3.6 2.8

2.5 2.5

18 16

2.1 1.8

1.3 1.6

17 15

1 2 3 1 2 3

Period Period

Flow-adjusted values, in indicated units of measrement

Water year (October–September)

1990199520002005201019901995200020052010

EXPLANATION

[Water year is defined as the 12-month 

period from October 1 through September 

30 and is designated by the year in which 

it ends. μS/cm, microsiemens per 

centimeter at 25 degrees Celsius; p-value, 

statistical probability level]

10

100

1

1,000

1

10

100

0.1

1

10

100

0.1

1

10

100

0.1

1

10

100

0.1

1

10

100

0.1

1

10

100

0.1

50

100

500

Specific conductance (μS/cm) Suspended sediment (milligrams per liter)

Unfiltered-recoverable

copper (micrograms per liter)

Unfiltered-recoverable

zinc (micrograms per liter) Filtered zinc (micrograms per liter)

Filtered copper (micrograms per liter)

Filtered arsenic (micrograms per liter) Unfiltered-recoverable

arsenic (micrograms per liter)

Flow-adjusted concentration (FAC) determined by 

using multiple linear 

regression

Flow-adjusted fitted trend 

determined by using 

multiple linear regression

Fitted trend value at start or 

end of period

Figure 4–4. Flow-adjusted fitted trends determined by using multiple linear regression on time, discharge, and season (MLR) for 

selected water-quality constituents and properties for Mill Creek near Anaconda (site 4, fig. 1, table 1), water years 2006–2010.
134 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

134 148

4.0

4.0

3.4

5.1 3.4

3.4

5.0 3.3

27 18

2.9 1.9

2.9 1.7

24 16

1 2 3 1 2 3

Period Period

Flow-adjusted values, in indicated units of measrement

Water year (October–September)

1990199520002005201019901995200020052010

EXPLANATION

[Water year is defined as the 12-month 

period from October 1 through September 

30 and is designated by the year in which 

it ends. μS/cm, microsiemens per 

centimeter at 25 degrees Celsius; p-value, 

statistical probability level]

Flow-adjusted concentration (FAC) determined by 

using multiple linear 

regression

Flow-adjusted fitted trend 

determined by using 

multiple linear regression

Fitted trend value at start or 

end of period

Bold values indicate 

statistical significance 

(p-value less than 0.01) for 

period before value 

presented in bold

10

100

1

1,000

1

10

100

0.1

1

10

100

0.1

1

10

100

0.1

1

10

100

0.1

1

10

100

0.1

1

10

100

0.1

50

100

500

Specific conductance (μS/cm) Suspended sediment (milligrams per liter)

Unfiltered-recoverable

copper (micrograms per liter)

Unfiltered-recoverable

zinc (micrograms per liter)

Filtered zinc (micrograms per liter)

Filtered copper (micrograms per liter)

Filtered arsenic (micrograms per liter) Unfiltered-recoverable

arsenic (micrograms per liter)

Figure 4–5. Flow-adjusted fitted trends determined by using multiple linear regression on time, discharge, and season (MLR) for 

selected water-quality constituents and properties for Mill Creek at Opportunity (site 5, fig. 1, table 1), water years 2006–10.
Appendix 4—Trend Analysis Results 135

Figure 4.6. Flow-adjusted fitted trends determined by using multiple linear regression on time, streamflow, and season (MLR) for selected

constituents and properties for Willow Creek near Anaconda (site 6, fig. 1, table 1), water years 2006–10.

90 111 8.0

8.0

3.6

3.6

3.5 2.4

2.7 2.2

17 14

2.0 1.6

1.7 1.3

16 14

Flow-adjusted values, in indicated units of measrement

Water year (October–September)

1990199520002005201019901995200020052010

EXPLANATION

[Water year is defined as the 12-month 

period from October 1 through September 

30 and is designated by the year in which 

it ends. μS/cm, microsiemens per 

centimeter at 25 degrees Celsius; p-value, 

statistical probability level]

Flow-adjusted concentration (FAC) determined by 

using multiple linear 

regression

Flow-adjusted fitted trend 

determined by using 

multiple linear regression

Fitted trend value at start or 

end of period

Bold values indicate 

statistical significance 

(p-value less than 0.01) for 

period before value 

presented in bold

10

100

1

1,000

1

10

100

0.1

1

10

100

0.1

1

10

100

0.1

1

10

100

0.1

1

10

100

0.1

1

10

100

0.1

50

100

500

Specific conductance (μS/cm) Suspended sediment (milligrams per liter)

Unfiltered-recoverable

copper (micrograms per liter)

Unfiltered-recoverable

zinc (micrograms per liter)

Filtered zinc (micrograms per liter)

Filtered copper (micrograms per liter)

Filtered arsenic (micrograms per liter) Unfiltered-recoverable

arsenic (micrograms per liter)

1 2 3 1 2 3

Period Period

Figure 4–6. Flow-adjusted fitted trends determined by using multiple linear regression on time, discharge, and season (MLR) for 

selected water-quality constituents and properties for Willow Creek near Anaconda (site 6, fig. 1, table 1), water years 2006–10.
136 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

274 238

6.3 6.9

6.9

9.3 7.9

8.4 8.9

38 31

4.7 3.5 3.5

3.9 3.1

36 27

Flow-adjusted values, in indicated units of measrement

Water year (October–September)

1990199520002005201019901995200020052010

EXPLANATION

[Water year is defined as the 12-month 

period from October 1 through September 

30 and is designated by the year in which 

it ends. μS/cm, microsiemens per 

centimeter at 25 degrees Celsius; p-value, 

statistical probability level]

Flow-adjusted concentration (FAC) determined by 

using multiple linear 

regression

Flow-adjusted fitted trend 

determined by using 

multiple linear regression

Fitted trend value at start or 

end of period

Bold values indicate 

statistical significance 

(p-value less than 0.01) for 

period before value 

presented in bold

10

100

1

1,000

1

10

100

0.1

1

10

100

0.1

1

10

100

0.1

1

10

100

0.1

1

10

100

0.1

1

10

100

0.1

50

100

500

Specific conductance (μS/cm) Suspended sediment (milligrams per liter)

Unfiltered-recoverable

copper (micrograms per liter)

Unfiltered-recoverable

zinc (micrograms per liter)

Filtered zinc (micrograms per liter)

Filtered copper (micrograms per liter)

Filtered arsenic 

(micrograms per liter)

Unfiltered-recoverable

arsenic (micrograms per liter)

1 2 3 1 2 3

Period Period

Figure 4–7. Flow-adjusted fitted trends determined by using multiple linear regression on time, discharge, and season (MLR) for 

selected water-quality constituents and properties for Mill Creek at Opportunity (site 7, fig. 1, table 1), water years 2006–10.
Appendix 4—Trend Analysis Results 137

527 526 506 530

11 4.8 4.2 4.0

17 19 19 20

5.4 5.8 4.1 1.9

18 8.8

8.8

8.0 7.0

7.0

20 22 21 23

48 17 9.1 9.2

Flow-adjusted values, in indicated units of measrement

Water year (October–September)

1990199520002005201019901995200020052010

EXPLANATION

[Water year is defined as the 12-month 

period from October 1 through September 

30 and is designated by the year in which 

it ends. μS/cm, microsiemens per 

centimeter at 25 degrees Celsius; p-value, 

statistical probability level]

Flow-adjusted concentration (FAC) determined by 

using the time-series 

model

Flow-adjusted fitted trend 

determined by using the 

time-series model

Fitted trend value at start or 

end of period

Bold values indicate 

statistical significance 

(p-value less than 0.01) for 

period before value 

presented in bold

10

100

1

1,000

10

100

1

1,000

10

100

1

1,000

10

100

1

1,000

10

100

1

1,000

10

100

1

1,000

100

1,000

Specific conductance (μS/cm) Suspended sediment (milligrams per liter)

Unfiltered-recoverable

copper (micrograms per liter)

Unfiltered-recoverable

zinc (micrograms per liter)

Filtered zinc (micrograms per liter)

Filtered copper (micrograms per liter)

Filtered arsenic 

(micrograms per liter)

Unfiltered-recoverable

arsenic (micrograms per liter)

1 2 3 1 2 3

Period Period

Results not reported because greater than 

6 percent of values were affected by recensoring 

at study reporting level, as discussed in the 

section of this report "Time-Series Model."

[Graph included as a place holder to assist in

comparison with other stations.]

Figure 4–8. Flow-adjusted fitted trends determined by using the time-series model (TSM) for selected water-quality constituents and 

properties for Silver Bow Creek at Warm Springs (site 8, fig. 1, table 1), water years 1996–2010.
138 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

194 225

225

5.6 4.3

4.3

2.4 3.0

2.1 2.6

2.4 2.8

1.0 0.88

2.3 2.1

Flow-adjusted values, in indicated units of measrement

Water year (October–September)

1990199520002005201019901995200020052010

EXPLANATION

[Water year is defined as the 12-month 

period from October 1 through September 

30 and is designated by the year in which 

it ends. μS/cm, microsiemens per 

centimeter at 25 degrees Celsius; p-value, 

statistical probability level]

Flow-adjusted concentration (FAC) determined by 

using multiple linear 

regression

Flow-adjusted fitted trend 

determined by using 

multiple linear regression

Fitted trend value at start or 

end of period

Bold values indicate 

statistical significance 

(p-value less than 0.01) for 

period before value 

presented in bold

10

100

1

1,000

1

10

0.1

100

1

10

0.1

100

1

10

0.1

100

1

10

0.1

100

1

10

0.1

100

100

1,000

Specific conductance (μS/cm) Suspended sediment (milligrams per liter)

Unfiltered-recoverable

copper (micrograms per liter)

Unfiltered-recoverable

zinc (micrograms per liter)

Filtered zinc (micrograms per liter)

Filtered copper (micrograms per liter)

Filtered arsenic 

(micrograms per liter)

Unfiltered-recoverable

arsenic (micrograms per liter)

1 2 3 1 2 3

Period Period

Results not presented because greater than

45 percent of values were censored.

[Graph included as a place holder to assist in

comparison with other stations.]

Figure 4–9. Flow-adjusted fitted trends determined by using multiple linear regression on time, discharge, and season (MLR) for 

selected water-quality constituents and properties for Warm Springs Creek near Anaconda (site 9, fig. 1, table 1), water years 2006–10.
Appendix 4—Trend Analysis Results 139

Results not reported because greater than 

6 percent of values were affected by recensoring 

at study reporting level, as discussed in the 

section of this report "Time-Series Model."

[Graph included as a place holder to assist in

comparison with other stations.]

1

426 376 377 351

2.4 2.4 2.2 1.5

4.7 5.3 5.2 4.0

8.7 4.7 3.1

3.1

7.7

7.7

9.9 7.4 7.7 9.8

6.2 5.6 5.9 5.2

Flow-adjusted values, in indicated units of measrement

Water year (October–September)

1990199520002005201019901995200020052010

EXPLANATION

[Water year is defined as the 12-month 

period from October 1 through September 

30 and is designated by the year in which 

it ends. μS/cm, microsiemens per 

centimeter at 25 degrees Celsius; p-value, 

statistical probability level]

Flow-adjusted concentration (FAC) determined by 

using the time-series 

model

Flow-adjusted fitted trend 

determined by using the 

time-series model

Fitted trend value at start or 

end of period

Bold values indicate 

statistical significance 

(p-value less than 0.01) for 

period before value 

presented in bold

10

100

1

1,000

1

10

0.1

100

1

10

0.1

100

1

10

0.1

100

1

10

0.1

100

100

1,000

Specific conductance (μS/cm) Suspended sediment (milligrams per liter)

Unfiltered-recoverable

copper (micrograms per liter)

Unfiltered-recoverable

zinc (micrograms per liter)

Filtered zinc (micrograms per liter)

Filtered copper (micrograms per liter)

Filtered arsenic 

(micrograms per liter)

Unfiltered-recoverable

arsenic (micrograms per liter)

1 2 3 1 2 3

Period Period

Results not reported because greater than

6 percent of values were affected by recensoring

at study reporting level, as discussed in the

section of this report "Time-Series Model."

[Graph included as a place holder to assist in

comparison with other stations.]

Figure 4–10. Flow-adjusted fitted trends determined by using the time-series model (TSM) for selected water-quality constituents and 

properties for Warm Springs Creek at Warm Springs (site 10, fig. 1, table 1), water years 1996–10.
140 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

Results not reported because greater than 

6 percent of values were affected by recensoring 

at study reporting level, as discussed in the 

section of this report "Time-Series Model."

[Graph included as a place holder to assist in

comparison with other stations.]

459 450 440 431

8.0 4.1 3.8 3.3

12 12 13 10

6.0 5.8 4.8 4.8

4.8

16 10

10

11 10

16 14 16 12

32 13 8.8 10

Flow-adjusted values, in indicated units of measrement

Water year (October–September)

1990199520002005201019901995200020052010

EXPLANATION

[Water year is defined as the 12-month 

period from October 1 through September 

30 and is designated by the year in which 

it ends. μS/cm, microsiemens per 

centimeter at 25 degrees Celsius; p-value, 

statistical probability level]

Flow-adjusted concentration (FAC) determined by 

using the time-series 

model

Flow-adjusted fitted trend 

determined by using the 

time-series model

Fitted trend value at start or 

end of period

Bold values indicate 

statistical significance 

(p-value less than 0.01) for 

period before value 

presented in bold

10

100

1

1,000 1,000

10

100

1

1,000

10

100

1

1,000

10

100

1

1,000

10

100

1

1,000

10

100

1

1,000

100

Specific conductance (μS/cm) Suspended sediment (milligrams per liter)

Unfiltered-recoverable

copper (micrograms per liter)

Unfiltered-recoverable

zinc (micrograms per liter)

Filtered zinc (micrograms per liter)

Filtered copper (micrograms per liter)

Filtered arsenic 

(micrograms per liter)

Unfiltered-recoverable

arsenic (micrograms per liter)

1 2 3 1 2 3

Period Period

Figure 4–11. Flow-adjusted fitted trends determined by using the time-series model (TSM) for selected water-quality constituents and 

properties for Clark Fork near Galen (site 11, fig. 1, table 1), water years 1996–2010.
Appendix 4—Trend Analysis Results 141

200 192

7.4 3.3

3.3

6.0 3.5

3.3 1.9

5.4 3.1

2.2 1.2

1.5 1.0

4.9 2.4

Flow-adjusted values, in indicated units of measrement

Water year (October–September)

1990199520002005201019901995200020052010

EXPLANATION

[Water year is defined as the 12-month 

period from October 1 through September 

30 and is designated by the year in which 

it ends. μS/cm, microsiemens per 

centimeter at 25 degrees Celsius; p-value, 

statistical probability level]

Flow-adjusted concentration (FAC) determined by 

using multiple linear 

regression

Flow-adjusted fitted trend 

determined by using 

multiple linear regression

Fitted trend value at start or 

end of period

10

100

1

1,000

1

10

0.1

100

1

10

0.1

100

1

10

0.1

100

1

10

0.1

100

1

10

0.1

100

100

1,000

Specific conductance (μS/cm) Suspended sediment (milligrams per liter)

Unfiltered-recoverable

copper (micrograms per liter)

Unfiltered-recoverable

zinc (micrograms per liter)

Filtered copper (micrograms per liter)

Filtered arsenic 

(micrograms per liter)

Unfiltered-recoverable

arsenic (micrograms per liter)

1 2 3 1 2 3

Period Period

1

10

0.1

100

Filtered zinc (micrograms per liter)

Figure 4–12. Flow-adjusted fitted trends determined by using multiple linear regression on time, discharge, and season (MLR) for 

selected water-quality constituents and properties for Lost Creek near Anaconda (site 12, fig. 1, table 1), water years 2006–10.
142 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

632 659 11 13

13

5.1 2.6 2.6

2.4 1.8

14 15

2.3 1.1

1.3 1.5

13 13

Flow-adjusted values, in indicated units of measrement

Water year (October–September)

1990199520002005201019901995200020052010

EXPLANATION

[Water year is defined as the 12-month 

period from October 1 through September 

30 and is designated by the year in which 

it ends. μS/cm, microsiemens per 

centimeter at 25 degrees Celsius; p-value, 

statistical probability level]

Flow-adjusted concentration (FAC) determined by 

using multiple linear 

regression

Flow-adjusted fitted trend 

determined by using the 

multiple linear regression

Fitted trend value at start or 

end of period

Bold values indicate 

statistical significance 

(p-value less than 0.01) for 

period before value 

presented in bold

10

100

1

1,000

10

100

0.1

1

10

100

0.1

1

10

100

0.1

1

10

100

0.1

1

10

100

0.1

1

10

100

0.1

1

200

1,000

2,000

Specific conductance (μS/cm) Suspended sediment (milligrams per liter)

Unfiltered-recoverable

copper (micrograms per liter)

Unfiltered-recoverable

zinc (micrograms per liter)

Filtered copper (micrograms per liter)

Filtered arsenic 

(micrograms per liter)

Unfiltered-recoverable

arsenic (micrograms per liter)

1

1 2 3 1 2 3

Period Period

Filtered zinc (micrograms per liter)

Figure 4–13. Flow-adjusted fitted trends determined by using multiple linear regression on time, discharge, and season (MLR) for 

selected water-quality constituents and properties for Lost Creek near Galen (site 13, fig. 1, table 1), water years 2006–10.
Appendix 4—Trend Analysis Results 143

Results not reported because greater than 

6 percent of values were affected by recensoring 

at study reporting level, as discussed in the 

section of this report "Time-Series Model."

[Graph included as a place holder to assist in

comparison with other stations.]

495 488 481 449

7.1 5.8 6.0 5.0

11 10 12 11

17 14 13 13

13

26 23 22 23

14 14 15 14

36 23

23

21 20

Flow-adjusted values, in indicated units of measrement

Water year (October–September)

1990199520002005201019901995200020052010

EXPLANATION

[Water year is defined as the 12-month 

period from October 1 through September 

30 and is designated by the year in which 

it ends. μS/cm, microsiemens per 

centimeter at 25 degrees Celsius; p-value, 

statistical probability level]

Flow-adjusted concentration (FAC) determined by 

using the time-series 

model

Flow-adjusted fitted trend 

determined by using the 

time-series model

Fitted trend value at start or 

end of period

Bold values indicate 

statistical significance 

(p-value less than 0.01) for 

period before value 

presented in bold

10

100

1

1,000

10

100

1

1,000

10

100

1

1,000

10

100

1

1,000

10

100

1

1,000

10

100

1

1,000

100

1,000

Specific conductance (μS/cm) Suspended sediment (milligrams per liter)

Unfiltered-recoverable

copper (micrograms per liter)

Unfiltered-recoverable

zinc (micrograms per liter)

Filtered zinc (micrograms per liter)

Filtered copper (micrograms per liter)

Filtered arsenic 

(micrograms per liter)

Unfiltered-recoverable

arsenic (micrograms per liter)

1 2 3 1 2 3

Period Period

Figure 4–14. Flow-adjusted fitted trends determined by using the time-series model (TSM) for selected water-quality constituents and 

properties for Clark Fork at Deer Lodge (site 14, fig. 1, table 1), water years 1996–2010.
144 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

Results not reported because greater than 

6 percent of values were affected by recensoring 

at study reporting level, as discussed in the 

section of this report "Time-Series Model."

[Graph included as a place holder to assist in

comparison with other stations.]

Results not reported because greater than 

6 percent of values were affected by recensoring 

at study reporting level, as discussed in the 

section of this report "Time-Series Model."

[Graph included as a place holder to assist in

comparison with other stations.]

Results not reported because greater than 

6 percent of values were affected by recensoring 

at study reporting level, as discussed in the 

section of this report "Time-Series Model."

[Graph included as a place holder to assist in

comparison with other stations.]

Figure 4.15. Flow-adjusted fitted trends determined by using the time-series model (TSM) for selected constituents and properties for Little Blackfoot

River (site 15, fig. 1, table 1), water years 1996–2005.

253 261 264

4.9 4.7 5.3

5.4 4.0 3.0

6.5 5.0

5.0

4.7

1.1 0.94 1.0

1.0

Flow-adjusted values, in indicated units of measrement

Water year (October–September)

1990199520002005201019901995200020052010

EXPLANATION

[Water year is defined as the 12-month 

period from October 1 through September 

30 and is designated by the year in which 

it ends. μS/cm, microsiemens per 

centimeter at 25 degrees Celsius; p-value, 

statistical probability level]

Flow-adjusted concentration (FAC) determined by 

using the time-series 

model

Flow-adjusted fitted trend 

determined by using the 

time-series model

Fitted trend value at start or 

end of period

Bold values indicate 

statistical significance 

(p-value less than 0.01) for 

period before value 

presented in bold

10

100

1

1,000

1

10

0.1

100

100 100

1

10

0.1

1

10

0.1

100

1,000

Specific conductance (μS/cm) Suspended sediment (milligrams per liter)

Unfiltered-recoverable

copper (micrograms per liter)

Unfiltered-recoverable

zinc (micrograms per liter)

Filtered copper (micrograms per liter)

Filtered arsenic 

(micrograms per liter)

Unfiltered-recoverable

arsenic (micrograms per liter)

1 2 3 1 2 3

Period Period

Filtered zinc (micrograms per liter)

Figure 4–15. Flow-adjusted fitted trends determined by using the time-series model (TSM) for selected water-quality constituents and 

properties for Little Blackfoot River (site 15, fig. 1, table 1), water years 1996–2005.
Appendix 4—Trend Analysis Results 145

Results not reported because greater than 

6 percent of values were affected by recensoring 

at study reporting level, as discussed in the 

section of this report "Time-Series Model."

[Graph included as a place holder to assist in

comparison with other stations.]

422 422 411 391

4.9 3.9 4.3 4.0

9.0 8.2 9.1 8.9

15 17 8.1

8.1

14

22 19 15 16

16

11 9.8 10 11

28 19 12 17

Flow-adjusted values, in indicated units of measrement

Water year (October–September)

1990199520002005201019901995200020052010

EXPLANATION

[Water year is defined as the 12-month 

period from October 1 through September 

30 and is designated by the year in which 

it ends. μS/cm, microsiemens per 

centimeter at 25 degrees Celsius; p-value, 

statistical probability level]

Flow-adjusted concentration (FAC) determined by 

using the time-series 

model

Flow-adjusted fitted trend 

determined by using the 

time-series model

Fitted trend value at start or 

end of period

Bold values indicate 

statistical significance 

(p-value less than 0.01) for 

period before value 

presented in bold

10

100

1

1,000

10

100

1

1,000

10

100

1

1,000

10

100

1

1,000

10

100

1

1,000

10

100

1

1,000

100

1,000

Specific conductance (μS/cm) Suspended sediment (milligrams per liter)

Unfiltered-recoverable

copper (micrograms per liter)

Unfiltered-recoverable

zinc (micrograms per liter)

Filtered zinc (micrograms per liter)

Filtered copper (micrograms per liter)

Filtered arsenic 

(micrograms per liter)

Unfiltered-recoverable

arsenic (micrograms per liter)

1 2 3 1 2 3

Period Period

Figure 4–16. Flow-adjusted fitted trends determined by using the time-series model (TSM) for selected water-quality constituents and 

properties for Clark Fork at Goldcreek (site 16, fig. 1, table 1), water years 1996–2010.
146 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

Results not reported because greater than 

6 percent of values were affected by recensoring 

at study reporting level, as discussed in the 

section of this report "Time-Series Model."

[Graph included as a place holder to assist in

comparison with other stations.]

Results not reported because greater than 

6 percent of values were affected by recensoring 

at study reporting level, as discussed in the 

section of this report "Time-Series Model."

[Graph included as a place holder to assist in

comparison with other stations.]

324 346 376

8.7 8.1 7.8 13 11

11

10

18 17 10

3.4 2.6 2.4

2.4

18 13 8.0

Flow-adjusted values, in indicated units of measrement

Water year (October–September)

1990199520002005201019901995200020052010

EXPLANATION

[Water year is defined as the 12-month 

period from October 1 through September 

30 and is designated by the year in which 

it ends. μS/cm, microsiemens per 

centimeter at 25 degrees Celsius; p-value, 

statistical probability level]

Flow-adjusted concentration (FAC) determined by 

using the time-series 

model

Flow-adjusted fitted trend 

determined by using the 

time-series model

Fitted trend value at start or 

end of period

Bold values indicate 

statistical significance 

(p-value less than 0.01) for 

period before value 

presented in bold

10

100

1

1,000

10

100

1

1,000

10

100

1

1,000

10

100

1

1,000

10

100

1

1,000

100

1,000

Specific conductance (μS/cm) Suspended sediment (milligrams per liter)

Unfiltered-recoverable

copper (micrograms per liter)

Unfiltered-recoverable

zinc (micrograms per liter)

Filtered zinc (micrograms per liter)

Filtered copper (micrograms per liter)

Filtered arsenic 

(micrograms per liter)

Unfiltered-recoverable

arsenic (micrograms per liter)

1 2 3 1 2 3

Period Period

Figure 4–17. Flow-adjusted fitted trends determined by using the time-series model (TSM) for selected water-quality constituents and 

properties for Flint Creek (site 17, fig. 1, table 1), water years 1996–2005.
Appendix 4—Trend Analysis Results 147

Results not reported because greater than 

6 percent of values were affected by recensoring 

at study reporting level, as discussed in the 

section of this report "Time-Series Model."

[Graph included as a place holder to assist in

comparison with other stations.]

486 472 458 445

3.7 3.7 4.3 3.3

9.3 8.6 9.3 8.3

21 13 11 12

19 15 13 13

13

12 10 10 10

40 18 15

15

17

Flow-adjusted values, in indicated units of measrement

Water year (October–September)

1990199520002005201019901995200020052010

EXPLANATION

[Water year is defined as the 12-month 

period from October 1 through September 

30 and is designated by the year in which 

it ends. μS/cm, microsiemens per 

centimeter at 25 degrees Celsius; p-value, 

statistical probability level]

Flow-adjusted concentration (FAC) determined by 

using the time-series 

model

Flow-adjusted fitted trend 

determined by using the 

time-series model

Fitted trend value at start or 

end of period

Bold values indicate 

statistical significance 

(p-value less than 0.01) for 

period before value 

presented in bold

10

100

1

1,000

10

100

1

1,000

10

100

1

1,000

10

100

1

1,000

10

100

1

1,000

10

100

1

1,000

100

1,000

Specific conductance (μS/cm) Suspended sediment (milligrams per liter)

Unfiltered-recoverable

copper (micrograms per liter)

Unfiltered-recoverable

zinc (micrograms per liter)

Filtered zinc (micrograms per liter)

Filtered copper (micrograms per liter)

Filtered arsenic 

(micrograms per liter)

Unfiltered-recoverable

arsenic (micrograms per liter)

1 2 3 1 2 3

Period Period

Figure 4–18. Flow-adjusted fitted trends determined by using the time-series model (TSM) for selected water-quality constituents and 

properties for Clark Fork near Drummond (site 18, fig. 1, table 1), water years 1996–2010.
148 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

Results not reported because greater than 

6 percent of values were affected by recensoring 

at study reporting level, as discussed in the 

section of this report "Time-Series Model."

[Graph included as a place holder to assist in

comparison with other stations.]

Results not reported because greater than 

6 percent of values were affected by recensoring 

at study reporting level, as discussed in the 

section of this report "Time-Series Model."

[Graph included as a place holder to assist in

comparison with other stations.]

Results not reported because greater than 

6 percent of values were affected by recensoring 

at study reporting level, as discussed in the 

section of this report "Time-Series Model."

[Graph included as a place holder to assist in

comparison with other stations.]

Results not reported because greater than 

6 percent of values were affected by recensoring 

at study reporting level, as discussed in the 

section of this report "Time-Series Model."

[Graph included as a place holder to assist in

comparison with other stations.]

Results not reported because greater than 

6 percent of values were affected by recensoring 

at study reporting level, as discussed in the 

section of this report "Time-Series Model."

[Graph included as a place holder to assist in

comparison with other stations.]

Results not reported because greater than 

6 percent of values were affected by recensoring 

at study reporting level, as discussed in the 

section of this report "Time-Series Model."

[Graph included as a place holder to assist in

comparison with other stations.]

Unfiltered-recoverable

zinc (micrograms per liter)

121 123 113 4.7

4.7

7.1

7.1

3.2

Flow-adjusted values, in indicated units of measrement

Water year (October–September)

1990199520002005201019901995200020052010

EXPLANATION

[Water year is defined as the 12-month 

period from October 1 through September 

30 and is designated by the year in which 

it ends. μS/cm, microsiemens per 

centimeter at 25 degrees Celsius; p-value, 

statistical probability level]

Flow-adjusted concentration (FAC) determined by 

using the time-series 

model

Flow-adjusted fitted trend 

determined by using the 

time-series model

Fitted trend value at start or 

end of period

Bold values indicate 

statistical significance 

(p-value less than 0.01) for 

period before value 

presented in bold

10

1

100

1,000

100

1,000

Specific conductance (μS/cm) Suspended sediment (milligrams per liter)

Unfiltered-recoverable

copper (micrograms per liter)

Unfiltered-recoverable

zinc (micrograms per liter)

Filtered zinc (micrograms per liter)

Filtered copper (micrograms per liter)

Filtered arsenic 

(micrograms per liter)

1 2 3 1 2 3

Period Period

Figure 4–19. Flow-adjusted fitted trends determined by using the time-series model (TSM) for selected water-quality constituents and 

properties for Rock Creek (site19, fig. 1, table 1), water years 1996–2005.
Appendix 4—Trend Analysis Results 149

Results not reported because greater than 

6 percent of values were affected by recensoring 

at study reporting level, as discussed in the 

section of this report "Time-Series Model."

[Graph included as a place holder to assist in

comparison with other stations.]

344 327 324 325

3.3 2.7 2.9 2.6

5.3 5.1 5.4 5.5

13 12 8.4 13

9.4 8.9 7.4 9.5

9.5

6.8 6.3 5.9 7.2

22 15 9.0 16

16

Flow-adjusted values, in indicated units of measrement

Water year (October–September)

1990199520002005201019901995200020052010

EXPLANATION

[Water year is defined as the 12-month 

period from October 1 through September 

30 and is designated by the year in which 

it ends. μS/cm, microsiemens per 

centimeter at 25 degrees Celsius; p-value, 

statistical probability level]

Flow-adjusted concentration (FAC) determined by 

using the time-series 

model

Flow-adjusted fitted trend 

determined by using the 

time-series model

Fitted trend value at start or 

end of period

Bold values indicate 

statistical significance 

(p-value less than 0.01) for 

period before value 

presented in bold

10

100

1

1,000

10

100

1

1,000

10

100

1

1,000

10

100

1

1,000

10

100

1

1,000

10

100

1

1,000

100

1,000

Specific conductance (μS/cm) Suspended sediment (milligrams per liter)

Unfiltered-recoverable

copper (micrograms per liter)

Unfiltered-recoverable

zinc (micrograms per liter)

Filtered zinc (micrograms per liter)

Filtered copper (micrograms per liter)

Filtered arsenic 

(micrograms per liter)

Unfiltered-recoverable

arsenic (micrograms per liter)

1 2 3 1 2 3

Period Period

Figure 4–20. Flow-adjusted fitted trends determined by using the time-series model (TSM) for selected water-quality constituents and 

properties for Clark For at Turah Bridge (site 20, fig. 1, table 1), water years 1996–2010.
150 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

Results not reported because greater than 

6 percent of values were affected by recensoring 

at study reporting level, as discussed in the 

section of this report "Time-Series Model."

[Graph included as a place holder to assist in

comparison with other stations.]

Results not reported because greater than 

6 percent of values were affected by recensoring 

at study reporting level, as discussed in the 

section of this report "Time-Series Model."

[Graph included as a place holder to assist in

comparison with other stations.]

Results not reported because greater than 

6 percent of values were affected by recensoring 

at study reporting level, as discussed in the 

section of this report "Time-Series Model."

[Graph included as a place holder to assist in

comparison with other stations.]

Results not reported because greater than 

6 percent of values were affected by recensoring 

at study reporting level, as discussed in the 

section of this report "Time-Series Model."

[Graph included as a place holder to assist in

comparison with other stations.]

Results not reported because greater than 

6 percent of values were affected by recensoring 

at study reporting level, as discussed in the 

section of this report "Time-Series Model."

[Graph included as a place holder to assist in

comparison with other stations.]

Results not reported because greater than 

6 percent of values were affected by recensoring 

at study reporting level, as discussed in the 

section of this report "Time-Series Model."

[Graph included as a place holder to assist in

comparison with other stations.]

Unfiltered-recoverable

arsenic (micrograms per liter)

237 238 229 243

243

4.4 3.9 4.0 3.1

3.1

Flow-adjusted values, in indicated units of measrement

Water year (October–September)

1990199520002005201019901995200020052010

EXPLANATION

[Water year is defined as the 12-month 

period from October 1 through September 

30 and is designated by the year in which 

it ends. μS/cm, microsiemens per 

centimeter at 25 degrees Celsius; p-value, 

statistical probability level]

Flow-adjusted concentration (FAC) determined by 

using the time-series 

model

Flow-adjusted fitted trend 

determined by using the 

time-series model

Fitted trend value at start or 

end of period

Bold values indicate 

statistical significance 

(p-value less than 0.01) for 

period before value 

presented in bold

10

1

100

1,000

100

1,000

Specific conductance (μS/cm) Suspended sediment (milligrams per liter)

Unfiltered-recoverable

copper (micrograms per liter)

Unfiltered-recoverable

zinc (micrograms per liter) Filtered zinc (micrograms per liter)

Filtered copper (micrograms per liter)

Filtered arsenic 

(micrograms per liter)

1 2 3 1 2 3

Period Period

Figure 4–21. Flow-adjusted fitted trends determined by using the time-series model (TSM) for selected water-quality constituents and 

properties for Blackfoot River (site 21, fig. 1, table 1) water years 1996–2010.
Appendix 4—Trend Analysis Results 151

Results not reported because greater than 

6 percent of values were affected by recensoring 

at study reporting level, as discussed in the 

section of this report "Time-Series Model."

[Graph included as a place holder to assist in

comparison with other stations.]

BA BA

274 274 270 286

2.2 1.7 2.1 2.5

2.9 2.9 3.2 3.9

8.5 7.2 9.4 9.3

5.6 4.6

4.6

7.0 5.9

5.9

3.8 3.6 4.1 5.0

272

2.2

3.4

23

14

15 8.6 12 31 13

5.1

Flow-adjusted values, in indicated units of measrement

Water year (October–September)

1990199520002005201019901995200020052010

EXPLANATION

[Water year is defined as the 12-month 

period from October 1 through September 

30 and is designated by the year in which 

it ends. μS/cm, microsiemens per 

centimeter at 25 degrees Celsius; p-value, 

statistical probability level]

Flow-adjusted concentration (FAC) determined by 

using the time-series 

model

Flow-adjusted fitted trend 

determined by using the 

time-series model

Fitted trend value at start or 

end of period

Bold values indicate 

statistical significance 

(p-value less than 0.01) for 

period before value 

presented in bold

10

100

1

1,000

10

100

1

1,000

10

100

1

1,000

10

100

1

1,000

10

100

1

1,000

10

100

1

1,000

100

1,000

Specific conductance (μS/cm) Suspended sediment (milligrams per liter)

Unfiltered-recoverable

copper (micrograms per liter)

Unfiltered-recoverable

zinc (micrograms per liter)

Filtered zinc (micrograms per liter)

Filtered copper (micrograms per liter)

Filtered arsenic 

(micrograms per liter)

Unfiltered-recoverable

arsenic (micrograms per liter)

1 2 3 1 2 3

Period Period

Figure 4–22. Flow-adjusted fitted trends determined by using the time-series model (TSM) for selected water-quality constituents and 

properties for Clark Fork above Missoula (site 22, fig. 1, table 1), water years 1996–2010.
152 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

Table 5–1. Transport-analysis balance calculations for sites analyzed by using the time series model (TSM) in reach 1, extending 

from Blacktail Creek (site 1, fig. 1, table 1) to Silver Bow Creek at Butte (site 2, fig. 1, table 1) for selected periods, water years 

1996–2010.

[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends]

Site name and number or summation category

Estimated normalized load1

(kilograms per day)

Unfiltered-recoverable copperUnfiltered-recoverable arsenic Suspended sediment

Water years 1996–2000 (period 1)

Inflow Blacktail Creek (site 1) 0.13 0.093 200

Outflow Silver Bow Creek at Butte (site 2) 5.0 0.59 740

Total within-reach change in load—outflow (site 2) minus 

inflow (site 1) (positive values indicate net mobilization from 

within-reach sources including groundwater inflow, unmonitored tributaries, and the main-stem channel and floodplain)

4.9 0.50 540

Water years 2001–05 (period 2)

Inflow Blacktail Creek (site 1) 0.12 0.083 180

Outflow Silver Bow Creek at Butte (site 2) 1.9 0.39 530

Total within-reach change in load—outflow (site 2) minus 

inflow (site 1) (positive values indicate net mobilization from 

within-reach sources including groundwater inflow, unmonitored tributaries, and the main-stem channel and floodplain)

1.7 0.30 350

Water years 2006–10 (period 3)

Inflow Blacktail Creek (site 1) 0.11 0.090 150

Outflow Silver Bow Creek at Butte (site 2) 1.1 0.30 380

Total within-reach change in load—outflow (site 2) minus 

inflow (site 1) (positive values indicate net mobilization from 

within-reach sources including groundwater inflow, unmonitored tributaries, and the main-stem channel and floodplain)

1.0 0.21 220

1

The estimated normalized load was computed by multiplying the mean annual fitted trend concentration (determined by using the time-series model) for 

the indicated period times the geometric mean streamflow for water years 1996–2010 and a units conversion factor according to equation 1 in the section of 

this report “Estimation of Normalized Constituent Loads.” Loads are reported to two significant figures; however, before final rounding, calculations used 

three significant figures when necessary. As a result, some of the load values have minor rounding artifacts.
Appendix 5—Transport-Analysis Balance Calculations for Data-Summary Reaches 153

Table 5–2. Transport-analysis balance calculations for sites analyzed by using the time series model (TSM) in reach 2, extending 

from Silver Bow Creek at Butte (site 2, fig. 1, table 1) to Silver Bow Creek at Opportunity (site 3, fig. 1, table 1) for selected periods, 

water years 1996–2010. 

[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends]

Site name and number or summation category

Estimated normalized load1

(kilograms per day)

Unfiltered-recoverable copperUnfiltered-recoverable arsenic Suspended sediment

Water years 1996–2000 (period 1)

Inflow Silver Bow Creek at Butte (site 2) 5.0 0.59 740

Outflow Silver Bow Creek at Opportunity (site 3) 12 1.6 2,000

Total within-reach change in load—outflow (site 3) minus 

inflow (site 2) (positive values indicate net mobilization from 

within-reach sources including groundwater inflow, unmonitored tributaries, and the main-stem channel and floodplain)

7.3 1.0 1,300

Water years 2001–05 (period 2)

Inflow Silver Bow Creek at Butte (site 2) 1.9 0.39 530

Outflow Silver Bow Creek at Opportunity (site 3) 10 1.7 1,900

Total within-reach change in load—outflow (site 3) minus 

inflow (site 2) (positive values indicate net mobilization from 

within-reach sources including groundwater inflow, unmonitored tributaries, and the main-stem channel and floodplain)

8.6 1.3 1,400

Water years 2006–10 (period 3)

Inflow Silver Bow Creek at Butte (site 2) 1.1 0.29 380

Outflow Silver Bow Creek at Opportunity (site 3) 6.5 1.3 2,000

Total within-reach change in load—outflow (site 3) minus 

inflow (site 2) (positive values indicate net mobilization from 

within-reach sources including groundwater inflow, unmonitored tributaries, and the main-stem channel and floodplain)

5.4 1.0 1,600

1

The estimated normalized load was computed by multiplying the mean annual fitted trend concentration (determined by using the time-series model) for 

the indicated period times the geometric mean streamflow for water years 1996–2010 and a units conversion factor according to equation 1 in the section of 

this report “Estimation of Normalized Constituent Loads.” Loads are reported to two significant figures; however, before final rounding, calculations used 

three significant figures when necessary. As a result, some of the load values have minor rounding artifacts.
154 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

Table 5–3. Transport-analysis balance calculations for sites analyzed by using the time series model (TSM) in reach 31, extending 

from Silver Bow Creek at Opportunity (site 3, fig. 1, table 1) to Silver Bow Creek at Warm Springs (site 8, fig. 1, table 1) for selected 

periods, water years 1996–2010. 

[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends]

Site name and number or summation category

Estimated normalized load2

(kilograms per day)

Unfiltered-recoverable copperUnfiltered-recoverable arsenic Suspended sediment

Water years 1996–2000 (period 1)

Inflow Silver Bow Creek at Opportunity (site 3) 12 1.6 2,000

Outflow Silver Bow Creek at Warm Springs (site 8) 2.0 3.1 850

Total within-reach change in load—outflow (site 8) minus 

inflow (site 3) (negative values indicate net accumulation in 

Warm Springs Ponds; positive values indicate net mobilization 

from within-reach sources including groundwater inflow, the 

Mill-Willow bypass, the outflow from Warm Springs Ponds, 

and the main-stem channel and floodplain downstream from 

Warm Springs Ponds)

-10 1.6 -1,200

Water years 2001–05 (period 2)

Inflow Silver Bow Creek at Opportunity (site 3) 10 1.7 1,900

Outflow Silver Bow Creek at Warm Springs (site 8) 1.3 3.3 740

Total within-reach change in load—outflow (site 8) minus 

inflow (site 3) (negative values indicate net accumulation in 

Warm Springs Ponds; positive values indicate net mobilization 

from within-reach sources including groundwater inflow, the 

Mill-Willow bypass, the outflow from Warm Springs Ponds, 

and the main-stem channel and floodplain downstream from 

Warm Springs Ponds)

-9.2 1.6 -1,200

Water years 2006–10 (period 3)

Inflow Silver Bow Creek at Opportunity (site 3) 6.5 1.3 2,000

Outflow Silver Bow Creek at Warm Springs (site 8) 1.1 3.4 430

Total within-reach change in load—outflow (site 8) minus 

inflow (site 3) (negative values indicate net accumulation in 

Warm Springs Ponds; positive values indicate net mobilization 

from within-reach sources including groundwater inflow, the 

Mill-Willow bypass, the outflow from Warm Springs Ponds, 

and the main-stem channel and floodplain downstream from 

Warm Springs Ponds)

-5.4 2.1 -1,600

1

Data for Mill Creek at Opportunity (site 5) and Willow Creek at Opportunity (site 7) were not included as monitored tributary inflows because of factors 

that complicate directly combining the TSM results and multiple linear regression on time, streamflow, and season (MLR) results within a single analysis. The 

inability to distinguish the relative magnitudes of the within-reach contributions from the Mill-Willow bypass and Warm Springs Ponds to the reach outflow 

required simplifying assumptions. As a result, when net accumulation in the reach 3 channel is indicated for unfiltered-recoverable copper and suspended 

sediment, it is presumed that all of the reach inflow at site 3 is stored in Warm Springs Ponds.

2

The estimated normalized load was computed by multiplying the mean annual fitted trend concentration (determined by using the time-series model) for 

the indicated period times the geometric mean streamflow for water years 1996–2010 and a units conversion factor according to equation 1 in the section of 

this report "Estimation of Normalized Constituent Loads." Loads are reported to two significant figures; however, before final rounding, calculations used 

three significant figures when necessary. As a result, some of the load values have minor rounding artifacts.
Appendix 5—Transport-Analysis Balance Calculations for Data-Summary Reaches 155

Table 5–4. Transport-analysis balance calculations for sites analyzed by using the time series model (TSM) in reach 4, extending 

from Silver Bow Creek at Warm Springs (site 8, fig. 1, table 1) to Clark Fork near Galen (site 11, fig. 1, table 1) for selected periods, 

water years 1996–2010. 

[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends]

Site name and number or summation category

Estimated normalized load1

(kilograms per day)

Unfiltered-recoverable copperUnfiltered-recoverable arsenic Suspended sediment

Water years 1996–2000 (period 1)

Inflow Silver Bow Creek at Warm Springs (site 8) 2.0 3.1 850

 Monitored tributary inflow within reach Warm Springs 

Creek at Warm Springs (site 10)

1.0 0.70 780

Combined inflow (sum of sites 8 and 10) 3.0 3.9 1,600

Outflow Clark Fork near Galen (site 11) 3.6 4.1 1,600

Total within-reach change in load—outflow (site 11) minus 

inflow (site 8) (positive values indicate net mobilization from 

all within-reach sources including groundwater inflow, the 

monitored tributary, unmonitored tributaries, and the main-stem 

channel and floodplain)

1.7 1.0 790

Within-reach change in load after accounting for the monitored tributary—outflow (site 11) minus combined inflow 

(sum of sites 8 and 10) (positive values indicate net mobilization from other within-reach sources including groundwater 

inflow, unmonitored tributaries and the main-stem channel and 

floodplain; negative values indicate net accumulation in reach 

channel)

0.67 0.29 19

Water years 2001–05 (period 2)

Inflow Silver Bow Creek at Warm Springs (site 8) 1.3 3.3 740

 Monitored tributary inflow within reach

Warm Springs Creek at Warm Springs (site 10)

0.89 0.68 460

Combined inflow (sum of sites 8 and 10) 2.2 4.0 1,200

Outflow Clark Fork near Galen (site 11) 2.9 4.2 1,500

Total within-reach change in load—outflow (site 11) minus 

inflow (site 8) (positive values indicate net mobilization from 

all within-reach sources including groundwater inflow, the 

monitored tributary, unmonitored tributaries, and the main-stem 

channel and floodplain)

1.7 0.91 720

Within-reach change in load after accounting for the monitored tributary—outflow (site 11) minus combined inflow 

(sum of sites 8 and 10) (positive values indicate net mobilization from other within-reach sources including groundwater 

inflow, unmonitored tributaries and the main-stem channel and 

floodplain)

0.78 0.23 260

Water years 2006–10 (period 3)

Inflow Silver Bow Creek at Warm Springs (site 8) 1.1 3.4 430

 Monitored tributary inflow within reach

Warm Springs Creek at Warm Springs (site 10)

1.0 0.66 610

Combined inflow (sum of sites 8 and 10) 2.1 4.0 1,040

Outflow Clark Fork near Galen (site 11) 2.9 3.9 1,300
156 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

Table 5–4. Transport-analysis balance calculations for sites analyzed by using the time series model (TSM) in reach 4, extending 

from Silver Bow Creek at Warm Springs (site 8, fig. 1, table 1) to Clark Fork near Galen (site 11, fig. 1, table 1) for selected periods, 

water years 1996–2010.—Continued 

[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends]

Site name and number or summation category

Estimated normalized load1

(kilograms per day)

Unfiltered-recoverable copperUnfiltered-recoverable arsenic Suspended sediment

Water years 2006–10 (period 3)—Continued

Total within-reach change in load—outflow (site 11) minus 

inflow (site 8) (positive values indicate net mobilization from 

all within-reach sources including groundwater inflow, the 

monitored tributary, unmonitored tributaries, and the main-stem 

channel and floodplain)

1.8 0.53 900

Within-reach change in load after accounting for the monitored tributary—outflow (site 11) minus combined inflow 

(sum of sites 8 and 10) (positive values indicate net mobilization from other within-reach sources including groundwater 

inflow, unmonitored tributaries and the main-stem channel and 

floodplain; negative values indicate net accumulation in reach 

channel)

0.74 -0.12 290

1

The estimated normalized load was computed by multiplying the mean annual fitted trend concentration (determined by using the time-series model) for 

the indicated period times the geometric mean streamflow for water years 1996–2010 and a units conversion factor according to equation 1 in the section of 

this report “Estimation of Normalized Constituent Loads.” Loads are reported to two significant figures; however, before final rounding, calculations used 

three significant figures when necessary. As a result, some of the load values have minor rounding artifacts.
Appendix 5—Transport-Analysis Balance Calculations for Data-Summary Reaches 157

Table 5–5. Transport-analysis balance calculations for sites analyzed by using the time series model (TSM) in reach 51, extending 

from Clark Fork near Galen (site 11, fig. 1, table 1) to Clark Fork at Deer Lodge (site 14, fig. 1, table 1) for selected periods, water years 

1996–2010. 

[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends]

Site name and number or summation category

Estimated normalized load2

(kilograms per day)

Unfiltered-recoverable copperUnfiltered-recoverable arsenic Suspended sediment

Water years 1996–2000 (period 1)

Inflow Clark Fork near Galen (site 11) 3.6 4.1 1,600

Outflow Clark Fork at Deer Lodge (site 14) 12 6.9 7,500

Total within-reach change in load—outflow (site 14) minus 

inflow (site 11) (positive values indicate net mobilization from 

within-reach sources including groundwater inflow, unmonitored tributaries, and the main-stem channel and floodplain)

8.4 2.8 5,900

Water years 2001–05 (period 2)

Inflow Clark Fork near Galen (site 11) 2.9 4.2 1,500

Outflow Clark Fork at Deer Lodge (site 14) 11 7.0 6,600

Total within-reach change in load—outflow (site 14) minus 

inflow (site 11) (positive values indicate net mobilization from 

within-reach sources including groundwater inflow, unmonitored tributaries, and the main-stem channel and floodplain)

8.1 2.8 5,100

Water years 2006–10 (period 3)

Inflow Clark Fork near Galen (site 11) 2.9 3.9 1,300

Outflow Clark Fork at Deer Lodge (site 14) 11 7.0 6,300

Total within-reach change in load—outflow (site 14) minus 

inflow (site 11) (positive values indicate net mobilization from 

within-reach sources including groundwater inflow, unmonitored tributaries, and the main-stem channel and floodplain)

8.1 3.1 5,000

1

Data for Lost Creek near Galen (site 13) were not included as monitored tributary inflows because of factors that complicate directly combining the TSM 

results and multiple linear regression on time, streamflow, and season (MLR) results within a single analysis. 

2

The estimated normalized load was computed by multiplying the mean annual fitted trend concentration (determined by using the time-series model) for 

the indicated period times the geometric mean streamflow for water years 1996–2010 and a units conversion factor according to equation 1 in the section of 

this report "Estimation of Normalized Constituent Loads." Loads are reported to two significant figures; however, before final rounding, calculations used 

three significant figures when necessary. As a result, some of the load values have minor rounding artifacts.
158 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

Table 5–6. Transport-analysis balance calculations for sites analyzed by using the time series model (TSM) in reach 6, extending 

from Clark Fork at Deer Lodge (site 14, fig. 1, table 1) to Clark Fork at Goldcreek (site 16, fig. 1, table 1) for selected periods, water years 

1996–2010. 

[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends]

Site name and number or summation category

Estimated normalized load1

(kilograms per day)

Unfiltered-recoverable copperUnfiltered-recoverable arsenic Suspended sediment

Water years 1996–2000 (period 1)

Inflow Clark Fork at Deer Lodge (site 14) 12 6.9 7,500

 Monitored tributary inflow within reach

Little Blackfoot River (site 15)

0.24 1.3 1,100

Combined inflow (sum of sites 14 and 15) 12 8.3 8,600

Outflow Clark Fork at Goldcreek (site 16) 19 10 16,000

Total within-reach change in load—outflow (site 16) minus 

inflow (site 14) (positive values indicate net mobilization from 

all within-reach sources including groundwater inflow, the 

monitored tributary, unmonitored tributaries, and the main-stem 

channel and floodplain)

7.3 3.2 8,000

Within-reach change in load after accounting for the monitored tributary—outflow (site 16) minus combined inflow 

(sum of sites 14 and 15) (positive values indicate net mobilization from other within-reach sources including groundwater 

inflow, unmonitored tributaries, and the mainstem channel and 

floodplain)

7.1 1.9 6,900

Water years 2001–05 (period 2)

Inflow Clark Fork at Deer Lodge (site 14) 11 7.0 6,600

 Monitored tributary inflow within reach

Little Blackfoot River (site 15)

0.23 1.1 800

Combined inflow (sum of sites 14 and 15) 11 8.1 7,400

Outflow Clark Fork at Goldcreek (site 16) 16 9.7 12,000

Total within-reach change in load—outflow (site 16) minus 

inflow (site 14) (positive values indicate net mobilization from 

all within-reach sources including groundwater inflow, the 

monitored tributary, unmonitored tributaries, and the main-stem 

channel and floodplain)

5.3 2.7 5,300

Within-reach change in load after accounting for the monitored tributary—outflow (site 16) minus combined inflow 

(sum of sites 14 and 15) (positive values indicate net mobilization from other within-reach sources including groundwater 

inflow, unmonitored tributaries, and the mainstem channel and 

floodplain)

5.1 1.6 4,500

Water years 2006–10 (period 32

)

Inflow Clark Fork at Deer Lodge (site 14) 11 7.0 6,300

Outflow Clark Fork at Goldcreek (site 16) 15 10 10,000

Total within-reach change in load—outflow (site 16) minus 

inflow (site 14) (positive values indicate net mobilization from 

all within-reach sources including groundwater inflow, unmonitored tributaries, and the main-stem channel and floodplain)

4.0 3.3 4,000

1

The estimated normalized load was computed by multiplying the mean annual fitted trend concentration (determined by using the time-series model) for 

the indicated period times the geometric mean streamflow for water years 1996–2010 and a units conversion factor according to equation 1 in the section of 

this report "Estimation of Normalized Constituent Loads." Loads are reported to two significant figures; however, before final rounding, calculations used 

three significant figures when necessary. As a result, some of the load values have minor rounding artifacts.

2

No data available for site 15 for water years 2006–10 (period 3); thus loads from site 15 are not accounted for and contribute to the within-reach change in 

load for period 3. Further, for period 3, loads from site 15 are included in net mobilization from within-reach sources.
Appendix 5—Transport-Analysis Balance Calculations for Data-Summary Reaches 159

Table 5–7. Transport-analysis balance calculations for sites analyzed by using the time series model (TSM) in reach 7, extending 

from Clark Fork at Goldcreek (site 16, fig. 1, table 1) to Clark Fork near Drummond (site 18, fig. 1, table 1) for selected periods, water 

years 1996–2010. 

[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends]

Site name and number or summation category

Estimated normalized load1

(kilograms per day)

Unfiltered-recoverable copperUnfiltered-recoverable arsenic Suspended sediment

Water years 1996–2000 (period 1)

Inflow Clark Fork at Goldcreek (site 16) 19 10 16,000

 Monitored tributary inflow within reach 

 Flint Creek (site 17)

0.69 2.7 4,100

Combined inflow (sum of sites 16 and 17) 20 13 20,000

Outflow Clark Fork near Drummond (site 18) 24 16 24,000

Total within-reach change in load—outflow (site 18) minus 

inflow (site 16) (positive values indicate net mobilization from 

all within-reach sources including groundwater inflow, the 

monitored tributary, unmonitored tributaries, and the main-stem 

channel and floodplain)

4.8 5.5 8,000

Within-reach change in load after accounting for the monitored tributary—outflow (site 18) minus combined inflow 

(sum of sites 16 and 17) (positive values indicate net mobilization from other within-reach sources including groundwater 

inflow, unmonitored tributaries and the main-stem channel and 

floodplain)

4.1 2.8 3,900

Water years 2001–05 (period 2)

Inflow Clark Fork at Goldcreek (site 16) 16 9.7 12,000

 Monitored tributary inflow within reach

Flint Creek (site 17)

0.57 2.4 3,000

Combined inflow (sum of sites 16 and 17) 17 12 15,000

Outflow Clark Fork near Drummond (site 18) 19 15 17,000

Total within-reach change in load—outflow (site 18) minus 

inflow (site 16) (positive values indicate net mobilization from 

all within-reach sources including groundwater inflow, the 

monitored tributary, unmonitored tributaries, and the main-stem 

channel and floodplain)

3.1 4.8 5,100

Within-reach change in load after accounting for the monitored tributary—outflow (site 18) minus combined inflow 

(sum of sites 16 and 17) (positive values indicate net mobilization from other within-reach sources including groundwater 

inflow, unmonitored tributaries and the main-stem channel and 

floodplain)

2.5 2.3 2,100

Water years 2006–10 (period 32

)

Inflow Clark Fork at Goldcreek (site 16) 15 10 10,000

Outflow Clark Fork near Drummond (site 18) 18 15 17,000

Total within-reach change in load—outflow (site 18) minus 

inflow (site 16) (positive values indicate net mobilization from 

all within-reach sources including groundwater inflow, unmonitored tributaries, and the main-stem channel and floodplain)

3.3 4.5 6,300

1

The estimated normalized load was computed by multiplying the mean annual fitted trend concentration (determined by using the time-series model) for 

the indicated period times the geometric mean streamflow for water years 1996–2010 and a units conversion factor according to equation 1 in the section of 

this report "Estimation of Normalized Constituent Loads." Loads are reported to two significant figures; however, before final rounding, calculations used 

three significant figures when necessary. As a result, some of the load values have minor rounding artifacts.

2

No data available for site 17 for water years 2006–10 (period 3); thus loads from site 17 are not accounted for and contribute to the total within-reach 

change in load for period 3. Further, for period 3, loads from site 17 are included in net mobilization from within-reach sources.
160 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

Table 5–8. Transport-analysis balance calculations for sites analyzed by using the time series model (TSM) in reach 8, extending 

from Clark Fork near Drummond (site 18, fig. 1, table 1) to Clark Fork at Turah Bridge (site 20, fig. 1, table 1) for selected periods, water 

years 1996–2010. 

[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends]

Site name and number or summation category

Estimated normalized load1

(kilograms per day)

Unfiltered-recoverable copperUnfiltered-recoverable arsenic Suspended sediment

Water years 1996–2000 (period 1)

InflowClark Fork near Drummond (site 18) 24 16 24,000

 Monitored tributary inflow within reach2

Rock Creek (site 19)

ND ND 4,900

Combined inflow (sum of sites 18 and 19) ND ND 28,000

Outflow Clark Fork at Turah Bridge (site 20) 23 17 32,000

Total within-reach change in load—outflow (site 20) minus 

inflow (site 18) (negative values indicate net accumulation in 

reach channel; positive values indicate net mobilization from 

all within-reach sources including groundwater inflow, the 

monitored tributary, unmonitored tributaries, and the main-stem 

channel and floodplain)

-1.0 0.89 8,900

Within-reach change in load after accounting for the monitored tributary—outflow (site 20) minus combined inflow 

(sum of sites 18 and 19) (positive values indicate net mobilization from other within-reach sources including groundwater 

inflow, unmonitored tributaries, and the main-stem channel and 

floodplain)

ND ND 4,000

Water years 2001–05 (period 2)

Inflow Clark Fork near Drummond (site 18) 19 15 17,000

 Monitored tributary inflow within reach2

Rock Creek (site 19)

ND ND 3,900

Combined inflow (sum of sites 18 and 19) ND ND 21,000

Outflow Clark Fork at Turah Bridge (site 20) 21 15 26,000

Total within-reach change in load—outflow (site 20) minus 

inflow (site 18) (negative values indicate net accumulation in 

reach channel; positive values indicate net mobilization from 

all within-reach sources including groundwater inflow, the 

monitored tributary, unmonitored tributaries, and the main-stem 

channel and floodplain)

1.1 0.92 9,000

Within-reach change in load after accounting for the monitored tributary—outflow (site 20) minus combined inflow 

(sum of sites 18 and 19) (positive values indicate net mobilization from other within-reach sources including groundwater 

inflow, unmonitored tributaries, and the main-stem channel and 

floodplain)

ND ND 5,100

Water years 2006–10 (period 32

)

Inflow Clark Fork near Drummond (site 18) 18 15 17,000

Outflow Clark Fork at Turah Bridge (site 20) 21 16 27,000

Total within-reach change in load—outflow (site 20) minus 

inflow (site 18) (positive values indicate net mobilization from 

all within-reach sources including groundwater inflow, unmonitored tributaries, and the main-stem channel and floodplain)

3.0 1.8 10,000

1

The estimated normalized load was computed by multiplying the mean annual fitted trend concentration (determined by using the time-series model) for 

the indicated period times the geometric mean streamflow for water years 1996–2010 and a units conversion factor according to equation 1 in the section of 

this report "Estimation of Normalized Constituent Loads." Loads are reported to two significant figures; however, before final rounding, calculations used 

three significant figures when necessary. As a result, some of the load values have minor rounding artifacts.

2

Load results for Rock Creek near Clinton (site 19) for copper and arsenic not reported because greater than 6 percent of data values were affected by 

recensoring at study reporting level, as discussed in the section of this report "Time-Series Model." No suspended-sediment data available for site 19 for 

period 3; thus loads from site 19 of copper and arsenic for all periods and suspended sediment for period 3 are not accounted for and contribute to the withinreach change in load.
Appendix 5—Transport-Analysis Balance Calculations for Data-Summary Reaches 161

Table 5–9. Transport-analysis balance calculations for sites analyzed by using the time series model (TSM) in reach 9, extending 

from Clark Fork at Turah Bridge (site 20, fig. 1, table 1) to Clark Fork above Missoula (site 22, fig. 1, table 1) for selected periods, water 

years 1996–2010. 

[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. ND, not determined because of 

greater than 6 percent of values affected by recensoring at study reporting level, as discussed in the section of this report “Time-Series Model.”]

Site name and number or summation category

Estimated normalized load1

(kilograms per day)

Unfiltered-recoverable copperUnfiltered-recoverable arsenic Suspended sediment

Water years 1996–2000 (period 1)

Inflow Clark Fork at Turah Bridge (site 20) 23 17 32,000

 Monitored tributary inflow within reach2

Blackfoot River (site 21)

ND ND 9,900

Combined inflow (sum of sites 20 and 21) ND ND 42,000

Outflow Clark Fork above Missoula (site 22) 25 18 39,000

Total within-reach change in load—outflow (site 22) minus 

inflow (site 20) (positive values indicate net mobilization from 

all within-reach sources including groundwater inflow, the 

monitored tributary, unmonitored tributaries, and the main-stem 

channel and floodplain)

2.4 1.9 6,500

Within-reach change in load after accounting for the monitored tributary—outflow (site 22) minus combined inflow 

(sum of sites 20 and 21) (negative values indicate net accumulation in reach channel)

ND ND -3,300

Water years 2001–05 (period 2)

Inflow Clark Fork at Turah Bridge (site 20) 21 15 26,000

 Monitored tributary inflow within reach2

Blackfoot River (site 21)

ND ND 9,400

Combined inflow (sum of sites 20 and 21) 21 15 35,000

OutflowClark Fork above Missoula (site 22)" 29 19 41,000

Total within-reach change in load—outflow (site 22) minus 

inflow (site 20) (positive values indicate net mobilization from 

all within-reach sources including groundwater inflow, the 

monitored tributary, unmonitored tributaries, and the main-stem 

channel and floodplain)

8.0 3.6 15,000

Within-reach change in load after accounting for the monitored tributary—outflow (site 22) minus combined inflow 

(sum of sites 20 and 21) (positive values indicate net mobilization from other within-reach sources including groundwater 

inflow, unmonitored tributaries, and the main-stem channel and 

floodplain)

ND ND 5,600

Water years 2006–10 (period 3)

Inflow Clark Fork at Turah Bridge (site 20) 21 16 27,000

 Monitored tributary inflow within reach2

Blackfoot River (site 21)

ND ND 8,400

Combined inflow (sum of sites 20 and 21 21 16 36,000

Outflow Clark Fork above Missoula (site 22) 50 24 77,000
162 Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010

Table 5–9. Transport-analysis balance calculations for sites analyzed by using the time series model (TSM) in reach 9, extending 

from Clark Fork at Turah Bridge (site 20, fig. 1, table 1) to Clark Fork above Missoula (site 22, fig. 1, table 1) for selected periods, water 

years 1996–2010.—Continued 

[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends]

Site name and number or summation category

Estimated normalized load1

(kilograms per day)

Unfiltered-recoverable copperUnfiltered-recoverable arsenic Suspended sediment

Water years 2006–10 (period 3)—Continued

Total within-reach change in load—outflow (site 22) minus 

inflow (site 20) (positive values indicate net mobilization from 

all within-reach sources including groundwater inflow, the 

monitored tributary, unmonitored tributaries, and the main-stem 

channel and floodplain)

28 7.4 50,000

Within-reach change in load after accounting for the monitored tributary—outflow (site 22) minus combined inflow 

(sum of sites 20 and 21) (positive values indicate net mobilization from other within-reach sources including groundwater 

inflow, unmonitored tributaries, and the main-stem channel and 

floodplain)

ND ND 42,000

1

The estimated normalized load was computed by multiplying the mean annual fitted trend concentration (determined by using the time-series model) for 

the indicated period times the geometric mean streamflow for water years 1996–2010 and a units conversion factor according to equation 1 in the section of 

this report “Estimation of Normalized Constituent Loads.” Loads are reported to two significant figures; however, before final rounding, calculations used 

three significant figures when necessary. As a result, some of the load values have minor rounding artifacts.

2

Loads results for Blackfoot River near Bonner (site 21) for copper and arsenic not reported because greater than 6 percent of data values were affected 

by recensoring at study reporting level, as discussed in the section of this report “Time-Series Model.” Thus loads from site 21 of copper and arsenic for all 

periods are not accounted for and contribute to the within-reach change in load.
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3162 Bozeman Avenue

Helena, MT 59601

(406) 457–5900

Or visit the Montana Water Science Center Web site at:

http://mt.water.usgs.gov/
Sando and others—Water-Quality Trends for Selected Sampling Sites in the Upper Clark Fork Basin, Montana, Water Years 1996–2010—Scientific Investigations Report 2013–5217

ISSN 2328-031X (print)

ISSN 2328-0328 (online)

http://dx.doi.org/10.3133/sir20135217