Prepared in cooperation with the U.S. Environmental Protection Agency

Water-Quality Trends and Constituent-Transport Analysis 

for Selected Sampling Sites in the Milltown Reservoir/Clark 

Fork River Superfund Site in the Upper Clark Fork Basin, 

Montana, Water Years 1996–2015

Scientific Investigations Report 2016–5100

U.S. Department of the Interior

U.S. Geological Survey

Water-Quality Trends and ConstituentTransport Analysis for Selected Sampling 

Sites in the Milltown Reservoir/Clark Fork 

River Superfund Site in the Upper Clark 

Fork Basin, Montana, Water Years 

1996–2015

By Steven K. Sando and Aldo V. Vecchia

Prepared in cooperation with the U.S. Environmental Protection Agency

Scientific Investigations Report 2016–5100

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, Director

U.S. Geological Survey, Reston, Virginia: 2016

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Suggested citation:

Sando, S.K., and Vecchia, A.V., 2016, Water-quality trends and constituent-transport analysis for selected sampling 

sites in the Milltown Reservoir/Clark Fork River Superfund Site in the upper Clark Fork Basin, Montana, water years 

1996–2015: U.S. Geological Survey Scientific Investigations Report 2016–5100, 82 p., http://dx.doi.org/10.3133/

sir20165100.

ISSN 2328-0328 (online)
iii

Contents

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

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

Purpose and Scope ..............................................................................................................................4

Description of Study Area ...................................................................................................................4

Data-Collection and Analytical Methods...................................................................................................6

Quality Assurance..........................................................................................................................................6

Overview of Streamflow and Water-Quality Characteristics for Water Years 2011–15.....................8

General Streamflow Characteristics for Water Years 2011–15.....................................................8

Water-Quality Characteristics for Water Years 2011–15................................................................8

Reach 4........................................................................................................................................18

Reach 5........................................................................................................................................20

Reach 6........................................................................................................................................20

Reach 7........................................................................................................................................21

Reach 8........................................................................................................................................21

Reach 9........................................................................................................................................21

Water-Quality Trend- and Constituent-Transport Analysis Methods..................................................22

General Description of the Time-Series Model .............................................................................22

Selection of Trend-Analysis Time Periods......................................................................................23

Estimation of Normalized Constituent Loads .................................................................................23

Factors that Affect Trend Analysis and Interpretation..........................................................................24

Streamflow Conditions.......................................................................................................................24

Other Factors.......................................................................................................................................24

Water-Quality Trends and Constituent-Transport Analysis Results....................................................28

Water-Quality Trends in Flow-Adjusted Concentrations..............................................................28

Copper .........................................................................................................................................38

Arsenic.........................................................................................................................................38

Suspended Sediment ................................................................................................................38

Overview of Water-Quality Trend Results..............................................................................38

Constituent-Transport Analysis Results..........................................................................................39

Copper .........................................................................................................................................45

Arsenic.........................................................................................................................................45

Suspended Sediment ................................................................................................................45

Overview of Constituent-Transport Analysis Results ..........................................................46

Summary and Conclusions.........................................................................................................................47

References....................................................................................................................................................48

Appendix 1—Summary Information Relating to Quality-Control Data ......................................52

Appendix 2—Summary of the Time-Series Model as Applied in this Study.............................61

Theoretical and Computational Information .........................................................................61

Specific Aspects of the Application of the Time-Series Model in this Study..................62

Appendix 3—Trend-Analysis Results..............................................................................................65

Appendix 4—Transport-Analysis Balance Calculations for Data-Summary Reaches...........76
iv

Figures

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

reaches in the upper Clark Fork Basin, Montana; the Milltown Reservoir/Clark Fork

River Superfund Site includes the reaches from sampling site 8 to sampling site 22 ......3

2. Graphs showing statistical distributions of selected constituents for selected

sampling sites in the Milltown Reservoir/Clark Fork River Superfund Site in the

upper Clark Fork Basin, Montana, water years 2011–15......................................................19

3. Graphs showing daily mean streamflow for selected sampling sites in the Milltown

Reservoir/Clark Fork River Superfund Site in the upper Clark Fork Basin, Montana,

water years 1993–2015...............................................................................................................25

4. Graphs showing selected streamflow and constituent concentration information

for Clark Fork near Galen, Montana (sampling site 11), water years 1993–2015 .............27

5. Graphs showing flow-adjusted fitted trends for selected constituents for sampling

sites in reach 4, extending from Silver Bow Creek at Warm Springs, Montana

(sampling site 8), to Clark Fork near Galen, Montana (sampling site 11), 

water years 1996–2015...............................................................................................................32

6. Graphs showing flow-adjusted fitted trends for selected constituents for 

sampling sites in reach 5, extending from Clark Fork near Galen, Montana 

(sampling site 11), to Clark Fork at Deer Lodge, Montana (sampling site 14), 

water years 1996–2015...............................................................................................................33

7. Graphs showing flow-adjusted fitted trends for selected constituents for 

sampling sites in reach 6, extending from Clark Fork at Deer Lodge, Montana 

(sampling site 14), to Clark Fork at Goldcreek, Montana (sampling site 16), 

water years 1996–2015...............................................................................................................34

8. Graphs showing flow-adjusted fitted trends for selected constituents for 

sampling sites in reach 7, extending from Clark Fork at Goldcreek, Montana 

(sampling site 16), to Clark Fork near Drummond, Montana (sampling site 18), 

water years 1996–2015...............................................................................................................35

9. Graphs showing flow-adjusted fitted trends for selected constituents for 

sampling sites in reach 8, extending from Clark Fork near Drummond, Montana 

(sampling site 18), to Clark Fork at Turah Bridge near Bonner, Montana 

(sampling site 20), water years 1996–2015 .............................................................................36

10. Graphs showing flow-adjusted fitted trends for selected constituents for 

sampling sites in reach 9, extending from Clark Fork at Turah Bridge near Bonner, 

Montana (sampling site 20), to Clark Fork above Missoula, Montana 

(sampling site 22), water years 1996–2015 .............................................................................37

11. Pie charts representing geometric mean streamflow and estimated normalized

unfiltered-recoverable copper loads contributed from reach inflow and

within-reach sources for data-summary reaches for selected periods ...........................42

12. Pie charts representing geometric mean streamflow and estimated normalized

unfiltered-recoverable arsenic loads contributed from reach inflow and

within-reach sources for data-summary reaches for selected periods ...........................43

13. Pie charts representing geometric mean streamflow and estimated normalized

suspended-sediment loads contributed from reach inflow and within-reach

sources for data-summary reaches for selected periods ...................................................44
v

Appendix Figures

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

samples, water years 1993–2015..............................................................................................59

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

water years 1993–2015...............................................................................................................60

3–1. Graphs showing flow-adjusted fitted trends for selected water-quality constituents and

properties for Silver Bow Creek at Warm Springs, Montana (sampling site 8), water

years 1996–2015 ..........................................................................................................................69

3–2. Graphs showing flow-adjusted fitted trends for selected water-quality constituents

and properties for Clark Fork near Galen, Montana (sampling site 11), water years

1996–2015 .....................................................................................................................................70

3–3. Graphs showing flow-adjusted fitted trends for selected water-quality constituents

and properties for Clark Fork at Deer Lodge, Montana (sampling site 14), water

years 1996–2015 ..........................................................................................................................71

3–4. Graphs showing flow-adjusted fitted trends for selected water-quality constituents

and properties for Clark Fork at Goldcreek, Montana (sampling site 16), water years

1996–2015 .....................................................................................................................................72

3–5. Graphs showing flow-adjusted fitted trends for selected water-quality constituents

and properties for Clark Fork near Drummond, Montana (sampling site 18), water

years 1996–2015 ..........................................................................................................................73

3–6. Graphs showing flow-adjusted fitted trends for selected water-quality constituents

and properties for Clark Fork at Turah Bridge near Bonner, Montana (sampling site

20), water years 1996–2015 .......................................................................................................74

3–7. Graphs showing flow-adjusted fitted trends for selected water-quality constituents

and properties for Clark Fork above Missoula, Montana (sampling site 22), water

years 1996–2015 ..........................................................................................................................75

Tables

1. Information for selected sampling sites and data-summary reaches in the Milltown

Reservoir/Clark Fork River Superfund Site in the upper Clark Fork Basin, Montana........5

2. Properties, constituents, and associated information relating to laboratory and

study reporting levels...................................................................................................................7

3. Statistical summaries of continuous streamflow data for selected sampling sites in

the Milltown Reservoir/Clark Fork River Superfund Site in the upper Clark Fork

Basin, Montana.............................................................................................................................9

4. Statistical summaries of water-quality data collected at selected sampling sites in

the Milltown Reservoir/Clark Fork River Superfund Site in the upper Clark Fork

Basin, Montana, water years 2011–15 ....................................................................................10

5. Percentages of samples with unadjusted unfiltered-recoverable concentrations

exceeding water-quality standards for selected sampling sites in the Milltown

Reservoir/Clark Fork River Superfund Site in the upper Clark Fork Basin, water

years 2011–15 ..............................................................................................................................18

6. Summary of flow-adjusted trend results for selected sampling sites and

constituents, water years 1996–2015 ......................................................................................29

7. Summary of flow-adjusted trend results for Clark Fork above Missoula, Montana

(sampling site 22), for selected constituents, water years 1996–2015...............................31
vi

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

data-summary reaches in the Milltown Reservoir/Clark Fork River Superfund Site in

the upper Clark Fork Basin, Montana, water years 1996–2015...........................................40

Appendix Tables

1–1. Summary information relating to quality-control samples (field equipment blank

and replicate samples) collected at sampling sites in the upper Clark Fork Basin,

Montana, water years 1993–2015 ............................................................................................53

1–2. Summary information relating to quality-control samples (laboratory-spiked

deionized-water blank samples) collected at sampling sites in the upper Clark Fork

Basin, Montana, water years 1993–2015 ................................................................................54

1–3. Summary information relating to quality-control samples (laboratory-spiked

stream-water samples) collected at sampling sites in the upper Clark Fork Basin,

Montana, water years 1993–2015 ............................................................................................56

1–4. Aquatic-life standards (based on median hardness for water years 2011–15) for

selected sampling sites in the Milltown Reservoir/Clark Fork River Superfund Site

in the upper Clark Fork Basin, Montana .................................................................................58

2–1. Statistical summaries of standard errors of estimates for the trend models...................64

3–1. Flow-adjusted trend results for selected water-quality constituents and properties

for selected sampling sites in the Milltown Reservoir/Clark Fork River Superfund

Site in the upper Clark Fork Basin, Montana, water years 1996–2015...............................66

3–2. Flow-adjusted trend results for selected water-quality constituents and properties

for Clark Fork above Missoula, Montana (sampling site 22), water years 1996–2015.....68

4–1. Constituent-transport analysis balance calculations for sampling sites in reach 4,

extending from Silver Bow Creek at Warm Springs, Montana (sampling site 8), to

Clark Fork near Galen, Montana (sampling site 11), for selected periods, water

years 1996–2015 ..........................................................................................................................77

4–2. Constituent-transport analysis balance calculations for sampling sites in reach 5,

extending from Clark Fork near Galen, Montana (sampling site 11), to Clark Fork at

Deer Lodge, Montana (sampling site 14), for selected periods, water years

1996–2015 .....................................................................................................................................78

4–3. Constituent-transport analysis balance calculations for sampling sites in reach 6,

extending from Clark Fork at Deer Lodge, Montana (sampling site 14), to Clark Fork

at Goldcreek, Montana (sampling site 16), for selected periods, water years

1996–2015 .....................................................................................................................................79

4–4. Constituent-transport analysis balance calculations for sampling sites in reach 7,

extending from Clark Fork at Goldcreek, Montana (sampling site 16), to Clark Fork

near Drummond, Montana (sampling site 18), for selected periods, water years

1996–2015 .....................................................................................................................................80

4–5. Constituent-transport analysis balance calculations for sampling sites in reach 8,

extending from Clark Fork near Drummond, Montana (sampling site 18), to Clark

Fork at Turah Bridge near Bonner, Montana (sampling site 20), for selected periods,

water years 1996–2015...............................................................................................................81

4–6. Constituent-transport analysis balance calculations for sampling sites in reach 9,

extending from Clark Fork at Turah Bridge near Bonner, Montana (sampling site 20),

to Clark Fork above Missoula, Montana (sampling site 22), for selected periods,

water years 1996–2015...............................................................................................................82
vii

Conversion Factors

U.S. customary units to International System of Units

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm) 

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2) 259.0 hectare (ha)

square mile (mi2) 2.590 square kilometer (km2)

Volume

gallon (gal) 3.785 liter (L)

Flow rate

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

Mass

pound, avoirdupois (lb) 0.4536 kilogram (kg)

Supplemental Information

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).

Load estimates are given in kilograms per day (kg/d).

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.
viii

Abbreviations

AMC Anaconda Mining Company

FAC flow-adjusted concentration

LRL laboratory reporting level

LOWESS locally weighted scatter plot smooth 

NWQL National Water Quality Laboratory

NWIS National Water Information System

SEE standard error of estimate

SRL study reporting level

TSM time-series model

USGS U.S. Geological Survey
Water-Quality Trends and Constituent-Transport Analysis 

for Selected Sampling Sites in the Milltown Reservoir/

Clark Fork River Superfund Site in the Upper Clark Fork 

Basin, Montana, Water Years 1996–2015

By Steven K. Sando and Aldo V. Vecchia

Abstract

During the extended history of mining in the upper Clark 

Fork Basin in Montana, 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 near Butte and milling and smelting operations near Anaconda. Extensive deposition of mining 

wastes in the Silver Bow Creek and Clark Fork channels and 

flood plains had substantial effects on water quality. Federal 

Superfund remediation activities in the upper Clark Fork Basin 

began in 1983 and have included substantial remediation near 

Butte and removal of the former Milltown Dam near Missoula. To aid in evaluating the effects of remediation activities 

on water quality, the U.S. Geological Survey began collecting 

streamflow and water-quality data in the upper Clark Fork 

Basin in the 1980s.

Trend analysis was done on specific conductance, 

selected trace elements (arsenic, copper, and zinc), and 

suspended sediment for seven sampling sites in the Milltown 

Reservoir/Clark Fork River Superfund Site for water years 

1996–2015. The most upstream site included in trend analysis 

is Silver Bow Creek at Warm Springs, Montana (sampling 

site 8), and the most downstream site is Clark Fork above Missoula, Montana (sampling site 22), which is just downstream 

from the former Milltown Dam. Water year is the 12-month 

period from October 1 through September 30 and is designated 

by the year in which it ends. Trend analysis was done by using 

a joint time-series model for concentration and streamflow. To 

provide temporal resolution of changes in water quality, trend 

analysis was conducted for four sequential 5-year periods: 

period 1 (water years 1996–2000), period 2 (water years 

2001–5), period 3 (water years 2006–10), and period 4 (water 

years 2011–15). Because of the substantial effect of the intentional breach of Milltown Dam on March 28, 2008, period 3 

was subdivided into period 3A (October 1, 2005–March 27, 

2008) and period 3B (March 28, 2008–September 30, 2010) 

for the Clark Fork above Missoula (sampling site 22). Trend 

results were considered statistically significant when the statistical probability level was less than 0.01.

In conjunction with the trend analysis, estimated normalized constituent loads (hereinafter referred to as “loads”) were 

calculated and presented within the framework of a constituent-transport analysis to assess the temporal trends in flowadjusted concentrations (FACs) in the context of sources and 

transport. The transport analysis allows assessment of temporal changes in relative contributions from upstream source 

areas to loads transported past each reach outflow. 

Trend results indicate that FACs of unfiltered-recoverable 

copper decreased at the sampling sites from the start of 

period 1 through the end of period 4; the decreases ranged 

from large for one sampling site (Silver Bow Creek at Warm 

Springs [sampling site 8]) to moderate for two sampling sites 

(Clark Fork near Galen, Montana [sampling site 11] and Clark 

Fork above Missoula [sampling site 22]) to small for four 

sampling sites (Clark Fork at Deer Lodge, Montana [sampling 

site 14], Clark Fork at Goldcreek, Montana [sampling site 16], 

Clark Fork near Drummond, Montana [sampling site 18], and 

Clark Fork at Turah Bridge near Bonner, Montana [sampling 

site 20]). For period 4 (water years 2011–15), the most notable 

changes indicated for the Milltown Reservoir/Clark Fork 

River Superfund Site were statistically significant decreases in 

FACs and loads of unfiltered-recoverable copper for sampling 

sites 8 and 22. The period 4 changes in FACs of unfilteredrecoverable copper for all other sampling sites were not statistically significant.

Trend results indicate that FACs of unfiltered-recoverable 

arsenic decreased at the sampling sites from period 1 through 

period 4 (water years 1996–2015); the decreases ranged from 

minor (sampling sites 8–20) to small (sampling site 22). For 

period 4 (water years 2011–15), the most notable changes indicated for the Milltown Reservoir/Clark Fork River Superfund 

Site were statistically significant decreases in FACs and loads 

of unfiltered-recoverable arsenic for sampling site 8 and near 

statistically significant decreases for sampling site 22. The 

period 4 changes in FACs of unfiltered-recoverable arsenic for 

all other sampling sites were not statistically significant.
2 Water-Quality Trends and Constituent-Transport Analysis for Selected Sampling Sites

Trend results indicate that FACs of suspended sediment 

decreased at the sampling sites from period 1 through period 4 

(water years 1996–2015); the decreases ranged from moderate 

(sampling site 8) to small (sampling sites 11–22). For period 4 

(water years 2011–15), the changes in FACs of suspended sediment were not statistically significant for any sampling sites.

The reach of the Clark Fork from Galen to Deer Lodge 

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 flood-plain tailings and the streambed of the Clark Fork 

and its tributaries within the reach results in a contribution 

of those constituents that is proportionally much larger than 

the contribution of streamflow from within the reach. Within 

the reach from Galen to Deer Lodge, unfiltered-recoverable 

copper loads increased by a factor of about 4 and suspendedsediment loads increased by a factor of about 5, whereas 

streamflow increased by a factor of slightly less than 2. For 

period 4 (water years 2011–15), unfiltered-recoverable copper and suspended-sediment loads sourced from within the 

reach accounted for about 41 and 14 percent, respectively, of 

the loads at Clark Fork above Missoula (sampling site 22), 

whereas streamflow sourced from within the reach accounted 

for about 4 percent of the streamflow at sampling site 22. 

During water years 1996–2015, decreases in FACs and loads 

of unfiltered-recoverable copper and suspended sediment for 

the reach generally were proportionally smaller than for most 

other reaches.

Unfiltered-recoverable copper loads sourced within the 

reaches of the Clark Fork between Deer Lodge and Turah 

Bridge near Bonner (just upstream from the former Milltown Dam) were proportionally smaller than contributions 

of streamflow sourced from within the reaches; these reaches 

contributed proportionally much less to copper loading 

in the Clark Fork than the reach between Galen and Deer 

Lodge. Although substantial decreases in FACs and loads of 

unfiltered-recoverable copper and suspended sediment were 

indicated for Silver Bow Creek at Warm Springs (sampling 

site 8), those substantial decreases were not translated to 

downstream reaches between Deer Lodge and Turah Bridge 

near Bonner. The effect of the reach of the Clark Fork from 

Galen to Deer Lodge as a large source of copper and suspended sediment, in combination with little temporal change 

in those constituents for the reach, contributes to this pattern.

With the removal of the former Milltown Dam in 

2008, substantial amounts of contaminated sediments that 

remained in the Clark Fork channel and flood plain in reach 9 

(downstream from Turah Bridge near Bonner) became more 

available for mobilization and transport than before the dam 

removal. After the removal of the former Milltown Dam, the 

Clark Fork above Missoula (sampling site 22) had statistically 

significant decreases in FACs of unfiltered-recoverable copper 

in period 3B (March 28, 2008, through water year 2010) that 

continued in period 4 (water years 2011–15). Also, decreases 

in FACs of unfiltered-recoverable arsenic and suspended sediment were indicated for period 4 at this site. The decrease in 

FACs of unfiltered-recoverable copper for sampling site 22 

during period 4 was proportionally much larger than the 

decrease for the Clark Fork at Turah Bridge near Bonner 

(sampling site 20). Net mobilization of unfiltered-recoverable 

copper and arsenic from sources within reach 9 are smaller for 

period 4 than for period 1 when the former Milltown Dam was 

in place, providing evidence that contaminant source materials 

have been substantially reduced in reach 9.

Introduction

Mining in the upper Clark Fork Basin in 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; fig. 1). By the early 1900s, the small gold mining operations had transitioned to larger scale underground silver and 

copper mining owned by the former Anaconda Mining Company (AMC), with most of the ore being processed at AMC 

milling and smelting facilities near Anaconda (U.S. Environmental Protection Agency, 2005, 2010; 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. The Berkeley Pit mining 

operations and AMC milling and smelting operations continued until closure in the early 1980s. 

During the extended history of mining in the upper Clark 

Fork Basin, 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 near Butte and the milling and smelting 

operations near Anaconda (Andrews, 1987; Gammons and 

others, 2006). Extensive deposition of mining wastes in the 

Silver Bow Creek and Clark Fork channels and flood plains 

had substantial effects on water quality. Federal Superfund 

remediation activities in the upper Clark Fork Basin began in 

1983 and have included substantial remediation near Butte and 

removal of the former Milltown Dam near Missoula in 2008 

(U.S. Environmental Protection Agency, 2004, 2010; CDM, 

2005; Sando and Lambing, 2011). The various Superfund 

activities are distributed among three National Priorities List 

sites: the Silver Bow Creek/Butte Area Site, the Anaconda 

Smelter Site, and the Milltown Reservoir/Clark Fork River 

Superfund Site, which are described in the “Description of 

Study Area” section of this report. 

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

Survey (USGS) in 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 (2016). Sando and others (2014) analyzed 

the monitoring data and characterized flow-adjusted trends in 

mining-related contaminants for 22 sampling sites in the Silver 

Bow Creek/Butte Area Site, the Anaconda Smelter Site, and 

the Milltown Reservoir/Clark Fork River Superfund Site in the 
Introduction 3

Figure 1. Location of study area, selected sampling sites, and data-summary reaches in the upper Clark Fork Basin, Montana; the Milltown Reservoir/Clark Fork River Superfund Site 

includes the reaches from sampling site 8 to sampling site 22.

Missoula

Bonner

Clinton

Drummond

Goldcreek Garrison

Deer Lodge

Butte

22

20

18

16

15

14

8

11

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 Cree

k

North Fork Blackfoot

Nevada Creek

Upp

er Willo

w

Creek

Dunham Creek

Blacktail Creek

Ross Fork 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

Silver Bow Creek

Clark Fork

Clark

Fork

Cr

Reach 4

Reach 5

Reach 6

Reach 7 Reach 8

Reach 9

113°30'

113°

114°

47°

46°30'

46°

112°30'

Warm Springs Cr eek

Mill Creek

Lost

Creek

Racetrack Creek

Willow Creek

Warm

Springs

Ponds

MillWillow

Bypass

Warm Springs

Anaconda

Opportunity

Galen

Reach 3

Reach 4

Reach 5

8

11

MONTANA

Study

area

IDAHO

Butte

Missoula

0 5 10 15 20 MILES

0 5 10 15 20 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)

16

15

Former

Milltown

Dam

Study area

Data-summary reach basin boundary

Main-stem Silver Bow Creek or Clark Fork channel

Sampling site and sampling site number (table 1) 

Sampling site included in trend analysis

Sampling site not included in trend analysis

EXPLANATION
4 Water-Quality Trends and Constituent-Transport Analysis for Selected Sampling Sites

upper Clark Fork Basin for 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). An update of 

flow-adjusted water-quality trends for the monitoring data was 

needed for seven sampling sites to provide timely information 

for the 2016 5-year review for the Milltown Reservoir/Clark 

Fork River Superfund Site. The USGS, in cooperation with 

the U.S. Environmental Protection Agency, conducted this 

study to test for flow-adjusted trends (water years 1996–2015) 

in water quality at seven sampling sites (fig. 1, table 1) in the 

Milltown Reservoir/Clark Fork River Superfund Site by using 

a joint time-series model (TSM; Vecchia, 2005) for concentration and streamflow; an eighth site (Clark Fork above Little 

Blackfoot River near Garrison, Montana [sampling site 15; 

fig. 1, table 1]) was included in the study for the purpose of 

statistically summarizing water-quality data collected during 

water years 2011–15, but the period of water-quality data collection was insufficient for trend analysis. 

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 Milltown Reservoir/Clark 

Fork River Superfund Site in the upper Clark Fork Basin. 

Trend analysis was done on specific conductance, selected 

trace elements (arsenic, copper, and zinc), and suspended sediment for seven sampling sites for water years 1996–2015. This 

report provides an update of and supersedes the trend results 

reported by Sando and others (2014) for seven sampling sites 

in the Milltown Reservoir/Clark Fork River Superfund Site. 

This report presents the trend results and information on 

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 (not 

shown on fig. 1). 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 (mi) 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, Montana (sampling site 22, 

table 1), with a drainage area of 5,999 square miles (mi2). 

Sando and others (2014) presented somewhat detailed information describing the hydrographic, physiographic, climatic, 

and geologic characteristics of the upper Clark Fork Basin and 

an overview of mining and remediation activities. 

Early Federal Superfund activities in the upper Clark 

Fork Basin involved designation of three areas as National 

Priorities List sites in 1983: the Silver Bow Creek Site, the 

Anaconda Smelter Site, and the Milltown Reservoir Site. The 

Silver Bow Creek Site was redesignated as the Silver Bow 

Creek/Butte Area Site in 1987 and includes remnants from 

mining operations near Butte and about 26 river miles of 

Silver Bow Creek extending from near Butte to the outlet of 

Warm Springs Ponds (U.S. Environmental Protection Agency, 

2000; CDM, 2005). The 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). Many remediation activities 

within the Anaconda Smelter Site are administered within the 

Regional Water, Waste, and Soils Operable Unit (Henry Elsen, 

U.S. Environmental Protection Agency, written commun., 

January 2016). The Milltown Reservoir Site was redesignated 

as the Milltown Reservoir/Clark Fork River Superfund Site 

in 1992. The Milltown Reservoir/Clark Fork River Superfund 

Site includes two primary operable units: the Milltown Reservoir Operable Unit and the Clark Fork Operable Unit. The 

Milltown Reservoir Operable Unit includes about 0.84 mi2

defined by the area inundated by maximum pool elevation of 

the former Milltown Reservoir (U.S. Environmental Protection Agency, 2004). The Clark Fork Operable Unit includes 

streamside areas of the 115-mi reach of the Clark Fork 

extending from the Warm Springs Ponds outlet to the start of 

Milltown Reservoir Operable Unit (Montana Department of 

Environmental Quality, 2016). 

The specific focus of this study is the Milltown Reservoir/Clark Fork River Superfund Site, which includes the 

Clark Fork Operable Unit and the Milltown Reservoir Operable Unit, and extends about 123 river miles from the outlet 

of Warm Springs Ponds on Silver Bow Creek (represented by 

sampling site 8) to the outlet of the former Milltown Reservoir 

(represented by sampling site 22, which is about 3 river miles 

downstream from the former Milltown Dam). Sampling sites 

included in this study are located on the main-stem channels 

of Silver Bow Creek and the Clark Fork. Sando and others 

(2014) included trend analyses for several sampling sites on 

tributaries to Silver Bow Creek or the Clark Fork in the Milltown Reservoir/Clark Fork River Superfund Site; however, 

data collection for most of the tributary sampling sites was discontinued in water year 2004. No tributary sampling sites were 

included in this study. The sampling site numbers and reach 

designations assigned by Sando and others (2014) generally 

have been retained to facilitate comparisons. An exception is 

Clark Fork above Little Blackfoot River near Garrison (USGS 

streamgage 12324400), for which data collection began in 

water year 2009. Streamgage 12324400 was not included in 

Sando and others (2014). A discontinued tributary sampling 

site (Little Blackfoot River near Garrison, Montana; USGS 

streamgage 12324590) was designated as sampling site 15 in 

Sando and others (2014), but in this study Clark Fork above 

Little Blackfoot River near Garrison (USGS streamgage 

12324400) is designated as sampling site 15. The period of 
Introduction 

5

Table 1. Information for selected sampling sites and data-summary reaches in the Milltown Reservoir/Clark Fork River Superfund Site 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; NA, not applicable]

Sampling site 

number1

(fig. 1)

USGS site 

identification 

number

USGS site name Abbreviated sampling site name

Datasummary 

reach1,2

Drainage 

area, 

in square 

miles

Period of 

water-quality 

data collection

Median annual 

sampling frequency, 

in samples per year 

(range)

Trend 

analysis 

periods3

8 12323750 Silver Bow Creek at Warm Springs, Montana Silver Bow Creek at Warm Springs 3 and 4 473 3/1993–8/2015 8 (6–11) 1, 2, 3, 4

11 12323800 Clark Fork near Galen, Montana Clark Fork near Galen 4 and 5 651 7/1988–8/2015 8 (1–13) 1, 2, 3, 4

14 12324200 Clark Fork at Deer Lodge, Montana Clark Fork at Deer Lodge 5 and 6 995 3/1985–8/2015 8 (4–20) 1, 2, 3, 415 12324400 Clark Fork above Little Blackfoot River near Garrison, Montana Clark Fork near Garrison 6 1,139 3/2009–8/2015 8 (7-8) NA416 12324680 Clark Fork at Goldcreek, Montana Clark Fork at Goldcreek 6 and 7 1,704 3/1993–8/2015 8 (6–10) 1, 2, 3, 418 12331800 Clark Fork near Drummond, Montana Clark Fork near Drummond 7 and 8 2,501 3/1993–8/2015 8 (6–10) 1, 2, 3, 420 12334550 Clark Fork at Turah Bridge near Bonner, Montana Clark Fork at Turah Bridge 8 and 9 3,641 3/1985–8/2015 8 (6–23) 1, 2, 3, 422 12340500 Clark Fork above Missoula, Montana Clark Fork above Missoula 9 5,999 7/1986–8/2015 8 (2–18) 1, 2, 3A, 3B, 4

1For this study, the sampling site numbers and reach designations assigned by Sando and others (2014) generally have been retained to facilitate comparisons. 2Where two reach numbers are shown, the site is both an outflow from the upstream reach and an inflow to the downstream reach.3The numerical designations of the trend analysis periods are defined as

 1: water years 1996–2000; 2: water years 2001–5; 3: water years 2006–10; 4: water years 2011–15. Because of the substantial effect of the breach and removal of Milltown Dam in 2008, for Clark Fork above Missoula (station 12340500), period 3 was subdivided into period 3A (October 1, 2005–March 27,  2008) and period 3B (March 28, 2008–September 30, 2010).

4Period of water-quality data collection is insufficient for trend analysis. Site was included in the study for the purpose of statistically summarizing water-quality data collected during water years 2011–15.
6 Water-Quality Trends and Constituent-Transport Analysis for Selected Sampling Sites

water-quality data collection is insufficient for trend analysis 

for sampling site 15, but this site was included in the study 

for the purpose of statistically summarizing water-quality data 

collected during water years 2011–15.

Data-Collection and Analytical 

Methods

Sando and others (2014) present information concerning 

historical aspects of data-collection and analytical methods 

used in the monitoring program. Data collected in the monitoring program are published (typically on an annual basis) in 

data reports that present the methods of data collection, waterquality data, quality-assurance data, and statistical summaries 

of the data (for example, Dodge and others, 2015). A brief 

overview of field and laboratory data-collection and analytical 

methods is presented in the following paragraphs.

The sampling design of the monitoring program provides 

information relevant to several objectives, including evaluating constituent transport, regulatory compliance, and longterm trends. Since 1993, the sampling frequency of the mainstem sampling sites in the monitoring program generally has 

been consistent, with the sites sampled eight times per year in 

most years. In the monitoring program, the seasonal timing of 

sample collection placed greater emphasis on the snowmelt 

runoff period (typically April–July), when streamflow conditions are high and variable and constituent transport is large. 

About 75 percent of samples were collected during April–July. 

In general, the frequency and timing of sample collection 

throughout the period of data collection among the sites are 

reasonably consistent to provide reasonable consistency in 

trend-analysis results.

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, 2015). Specific 

conductance was measured onsite in subsamples from the 

composite water samples. Subsamples of the composite water 

samples 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 (table 2) by 

using methods described by Garbarino and Struzeski (1998) 

and Garbarino and others (2006). Water samples also were 

analyzed for suspended-sediment concentrations by the USGS 

sediment laboratory in Helena, Montana. All water-quality 

data are available in the USGS National Water Information 

System (NWIS; U.S. Geological Survey, 2015).

Quality Assurance

Sando and others (2014) present information concerning 

historical aspects of quality-assurance procedures used in the 

monitoring program. Quality-assurance data collected in the 

monitoring program are reported and statistically summarized 

in annual data reports (for example, Dodge and others, 2015). 

Selected quality-assurance information relevant to this study is 

presented in the following paragraphs.

Analytical results for field quality-assurance samples 

(including field blank and replicate samples) that were 

collected in the monitoring program during water years 

1993–2015 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. 

Summary of analytical results for field blank samples 

(table 1–1 in appendix 1 at the back of the report) provides 

information on potential effects of contamination during the 

sampling process on trend-analysis results. For the traceelement constituents included in the trend analysis (table 2), 

the frequency of detection in field blank samples at concentrations greater than the laboratory reporting level (LRL) at the 

time of analysis ranged from 0.5 percent (filtered arsenic) to 

10.7 percent (unfiltered-recoverable zinc). 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 datasets because contamination indicated by field blank 

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

persistent contamination issues were identified during periodic 

reviews of the data and excluded from data analysis. 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 trend-analysis method 

with respect to handling of low-concentration and censored 

data (that is, analytical results reported as less than the LRL; 

Helsel, 2005) are described in the section of this report “General Description of the Time-Series Model.” 

Summary of analytical results for field replicate samples 

(table 1–1 in appendix 1 at the back of the report) provides 

information on data precision. For the entire study period, the 

relative standard deviations (a measure of overall precision) 

for field replicate sample pairs were within 20 percent for all 

constituents, indicating reasonable precision (Taylor, 1987; 

Dodge and others, 2015). 
Quality Assurance 7

Analytical results for laboratory-spiked deionized-water 

blank samples and stream-water samples that were collected 

in the monitoring program during water years 1993–2015 

are presented in tables 1–2 and 1–3, respectively, in appendix 1 at the back of the report. Annual mean recoveries for 

laboratory-spiked deionized-water blank samples for all 

constituents combined have ranged from 82.3 to 118 percent 

(mean of 104 percent). Annual mean recoveries for laboratoryspiked stream-water samples for all constituents combined 

have ranged from 84.3 to 114 percent (mean of 105 percent). 

Potential effects of temporal variability in spike recoveries on 

trend results are described in appendix 1 and also the section 

“Specific Aspects of the Application of the Time-Series Model 

in this Study” in appendix 2. Based on analysis of all qualityassurance data, the quality of the study datasets were determined to be suitable for trend analysis.

Table 2. Properties, constituents, and associated 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, microsiemen per centimeter at 25 degrees Celsius; NA, not applicable; mg/L, milligram per liter; μg/L, microgram 

per liter]

Property or constituent Units of 

measurement

Number of NWQL 

laboratory reporting 

levels during water years 

1993–2015

Range in NWQL 

laboratory reporting 

levels

Study reporting level 

used in application of the 

time-series model1

Specific conductance2 μS/cm NA NA NA

pH, standard units standard units NA NA NA

Calcium, filtered mg/L 5 0.005–0.022 NA

Magnesium, filtered mg/L 7 0.002–0.011 NA

Cadmium, filtered μg/L 7 0.01–1.0 NA

Cadmium, unfiltered-recoverable μg/L 10 0.007–1.0 NA

Copper, filtered2 μg/L 4 0.2–1 1.0

Copper, unfiltered-recoverable2 μg/L 6 0.3–2 1.0

Lead, filtered μg/L 10 0.015–5 NA

Lead, unfiltered-recoverable μg/L 6 0.03–5 NA

Zinc, filtered μg/L 7 0.9–20 NA

Zinc, unfiltered-recoverable2 μg/L 4 2–31 2.0

Arsenic, filtered2 μg/L 7 0.022–1 1.0

Arsenic, unfiltered-recoverable2 μg/L 7 0.06–1 1.0

Suspended sediment2 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 

“General Description of the Time-Series Model.”

2

Property or constituent was analyzed for temporal trends.
8 Water-Quality Trends and Constituent-Transport Analysis for Selected Sampling Sites

Overview of Streamflow and WaterQuality Characteristics for Water 

Years 2011–15

Statistically summarizing recent streamflow and waterquality characteristics of the study sampling sites (fig. 1, 

table 1) is useful for generally describing water quality and in 

providing comparative information relevant for interpreting 

trend results. Data are summarized for water years 2011–15, 

a summary period that represents recent water-quality conditions and the increment of data collected after the study period 

1996–2010 reported by Sando and others (2014). 

General Streamflow Characteristics for Water 

Years 2011–15

To aid in interpreting water-quality characteristics of the 

sampling sites, statistical summaries of continuous streamflow 

data are presented in table 3. The continuous streamflow data 

are available in NWIS (U.S. Geological Survey, 2015). In 

general, streamflow conditions during water years 2011–15 

were somewhat high. Mean annual streamflows for water 

years 2011–15 generally were about 10–20 percent higher than 

period-of-record mean annual streamflows.

Water-Quality Characteristics for Water 

Years 2011–15

Statistical summaries of water-quality data (water years 

2011–15) for sampling sites in the Milltown Reservoir/

Clark Fork River Superfund Site in the upper Clark Fork 

Basin are presented in table 4. The statistical summaries in 

table 4 are based on unadjusted trace-element concentrations 

(the observed concentrations before flow adjustment). Flow 

adjustment, described in the sections of this report “General 

Description of the Time-Series Model” and “Factors that 

Affect Trend Results and Interpretation,” is relevant when 

interpreting trends in concentrations of water-quality constituents that are strongly dependent on streamflow conditions. 

However, flow adjustment is not relevant for statistically summarizing the observed water-quality data during water years 

2011–15.

In addition to statistical summaries of unadjusted concentrations, ratios of median filtered to unfiltered-recoverable 

trace-element concentrations are reported in table 4 to provide general information on the predominant phase (that is, 

dissolved or particulate) of transport. Values of aquatic-life 

standards (Montana Department of Environmental Quality, 

2012; based on median hardness for each site for water years 

2011–15) for cadmium, copper, lead, and zinc are presented 

in table 1–4 in appendix 1 at the back of the report; those 

values were used for plotting the standards in relation to 

statistical distributions of selected trace elements. The arsenic 

human-health standard is 10 micrograms per liter (μg/L; 

Montana Department of Environmental Quality, 2012). Percentages of samples (water years 2011–15) with unadjusted 

unfiltered-recoverable concentrations exceeding water-quality 

standards for each site are presented in table 5. The exceedance percentages for the hardness-based aquatic-life standards 

for cadmium, copper, lead, and zinc in table 5 were based on 

comparison of trace-element concentrations of each individual 

sample with the aquatic-life standards that were calculated by 

using the hardness for each individual sample.

Statistical distributions of water-quality characteristics of 

the sampling sites are illustrated in figure 2 by using boxplots 

of selected example constituents (unadjusted specific conductance and unadjusted concentrations of copper, arsenic, and 

suspended sediment); the boxplots provide an overview of 

important water-quality characteristics in the upper Clark Fork 

Basin. 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, is strongly correlated 

with hardness (which is used in calculations of aquatic-life 

standards), and provides information on the extent of water 

contact with geologic materials, types of geologic materials 

present in the sampling-site basins, and potential effects of 

remediation activities on ionic strength. Copper and arsenic 

are presented as examples of trace elements because they 

are constituents of concern with respect to potential toxicity 

issues, but they have much different geochemical characteristics. Spatial and temporal variability in copper concentrations 

in the upper Clark Fork Basin generally is similar to variability in other metallic contaminants that tend to adsorb to 

particulates in water (Sando and others, 2014) and is considered generally representative of those constituents. In contrast, 

arsenic in the upper Clark Fork Basin tends to largely exist in 

the dissolved phase and does not exhibit the same variability 

as metallic contaminants (Sando and others, 2014). 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, Sando and others 

(2014) divided Silver Bow Creek and the Clark Fork into 

nine data-summary reaches based on the location of sampling 

sites along the main-stems of those streams. The sampling site 

numbers and reach designations assigned by Sando and others 

(2014) generally have been retained to facilitate comparisons, 

and water-quality characteristics for sampling sites in six 

reaches (reaches 4–9) are presented. Water-quality characteristics within the six reaches are affected by environmental 

characteristics within the delineated reach basin boundaries 

(fig. 1). Water-quality characteristics of the sampling sites are 

described for each of the data-summary reaches. Emphasis 

is placed on describing spatial differences in observed water 

quality in the Milltown Reservoir/Clark Fork River Superfund Site in the upper Clark Fork Basin during water years 

2011–15.
Overview of Streamflow and Water-Quality Characteristics for Water Years 2011–15 9

Table 3. Statistical summaries of continuous streamflow data for selected sampling sites in the Milltown Reservoir/Clark Fork River Superfund Site 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. ft3/s, cubic foot per second; POR, period of record]

Sampling site number 

(fig. 1, table 1)

Abbreviated sampling site name 

(table 1)

Drainage 

area,

 in square 

miles

Analysis period, 

in water years 

(number of years)

Statistical summaries of daily mean streamflow, 

in ft3/s

Minimum 25th percentile Median

Mean 

(also referred to 

as “mean annual 

streamflow”)

75th 

percentile Maximum

8 Silver Bow Creek at Warm Springs 473 2011–15 (5) 22 51 65 96 97 1,060 POR: 1994–2015 (22) 15 41 59 88 88 1,060

11 Clark Fork near Galen 651 2011–15 (5) 35 92 130 172 174 1,390

POR: 1989–2015 (27) 13 70 100 143 152 1,390

14 Clark Fork at Deer Lodge 995 2011–15 (5) 55 187 237 283 302 1,960 POR: 1979–2015 (37) 22 159 219 257 298 2,39015 Clark Fork near Garrison 1,139 2011–15 (5) 61 198 263 315 331 2,560 POR: 2010–15 (6) 61 209 267 323 334 2,56016 Clark Fork at Goldcreek 1,704 2011–15 (5) 112 320 409 570 583 6,100 POR: 1978–2015 (38) 55 280 380 519 556 9,10018 Clark Fork near Drummond 2,501 2011–15 (5) 185 461 595 771 813 7,740 POR: 1994–2015 (22) 77 419 563 718 758 8,43020 Clark Fork at Turah Bridge 3,641 2011–15 (5) 250 790 990 1,490 1,560 12,700 POR: 1985–2015 (31) 177 678 870 1,260 1,260 12,70022 Clark Fork above Missoula 5,999 2011–15 (5) 500 1,400 1,730 3,330 3,760 28,100 POR: 1930–2015 (86) 340 1,270 1,650 2,930 2,960 30,800
10 Water-Quality Trends and Constituent-Transport Analysis for Selected Sampling Sites Table 4. Statistical summaries of water-quality data collected at selected sampling sites in the Milltown Reservoir/Clark Fork River Superfund Site in the upper Clark Fork Basin, Montana, water years 2011–15. [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 foot per second; NA, not applicable; μS/cm, microsiemen per centimeter at 25 degrees Celsius; CaCO3, calcium carbonate; μg/L, microgram per liter; mg/L, milligram per liter] Constituent or property, unadjusted (not flow adjusted) units of measurement Statistical summaries of water-quality data1 Ratios of median filtered to median unfiltered-recoverable concentrations for trace elements, in percent3 Number of samples (values in parentheses indicate number of censored values) Minimum uncensored value2 25th percentile Median Mean 75th percentile Maximum Silver Bow Creek at Warm Springs, Montana (sampling site 8, fig. 1, table 1) Streamflow, instantaneous, ft3/s 40 20 66 89 146 161 1,030 NA Specific conductance, μS/cm 40 182 342 394 407 489 577 NA pH, standard units 40 8.1 8.5 8.8 NA 9.1 9.4 NA Hardness, filtered, mg/L as CaCO3 40 74.9 136 170 169 203 253 NA Calcium, filtered, mg/L 40 22.5 39.7 48.4 48.7 58.6 73.3 NA Magnesium, filtered, mg/L 40 4.52 9.10 11.8 11.5 14.4 16.9 NA Cadmium, filtered, μg/L 40 (4) 0.023 0.031 0.038 0.044 0.054 0.096 45 Cadmium, unfiltered-recoverable, μg/L 40 0.027 0.065 0.085 0.119 0.125 0.567 Copper, filtered, μg/L 40 1.6 2.6 3.5 4.3 4.7 21.4 51 Copper, unfiltered-recoverable, μg/L 40 2.8 5.0 6.8 9.5 11.2 35.2 Iron, filtered, μg/L 40 7.0 16.2 30.0 30.0 38.7 63.0 13 Iron, unfiltered-recoverable, μg/L 40 61.1 159 225 256 313 839 Lead, filtered, μg/L 40 0.044 0.103 0.158 0.162 0.186 0.566 14

Lead, unfiltered-recoverable, μg/L 40 0.37 0.81 1.16 1.80 2.07 6.39Manganese, filtered, μg/L 40 27.1 42.7 61.2 72.6 84.7 208 64Manganese, unfiltered-recoverable, μg/L 40 60.1 77.5 95.2 116 130 332Zinc, filtered, μg/L 40 (11) 1.5 1.7 2.8 2.8 3.3 6.1 33Zinc, unfiltered-recoverable, μg/L 40 (2) 2.3 5.5 8.6 13.3 14.1 69.8Arsenic, filtered, μg/L 40 8.4 13.4 19.2 20.9 28.0 38.1 86Arsenic, unfiltered-recoverable, μg/L 40 10.4 16.9 22.4 22.8 28.8 37.9Suspended sediment, mg/L 40 1 3 6 6 7 21 NASuspended sediment, percent fines4 40 60 84 88 87 92 98 NA
Overview of Streamflow and Water-Quality Characteristics for Water Years 2011–15 11

Constituent or property, 

unadjusted 

(not flow adjusted) 

units of measurement

Statistical summaries of water-quality data1 Ratios of median 

filtered to median 

unfiltered-recoverable 

concentrations for 

trace elements, 

in percent3

Number of samples 

(values in parentheses 

indicate number of 

censored values)

Minimum 

uncensored 

value2

25th 

percentile Median Mean 75th percentile Maximum

Clark Fork near Galen, Montana (sampling site 11, fig. 1, table 1)

Streamflow, instantaneous, ft3/s 40 38 110 175 249 284 1,380 NASpecific conductance, μS/cm 40 182 292 367 360 434 498 NApH, standard units 40 8.2 8.4 8.6 NA 8.7 9.1 NAHardness, filtered, mg/L as CaCO3 40 76.4 125 164 158 191 225 NACalcium, filtered, mg/L 40 23.2 37.1 47.9 46.4 55.4 65.1 NAMagnesium, filtered, mg/L 40 4.44 7.75 10.6 10.2 12.7 15.1 NACadmium, filtered, μg/L 40 (2) 0.020 0.037 0.041 0.044 0.049 0.111 42Cadmium, unfiltered-recoverable, μg/L 40 0.034 0.076 0.098 0.115 0.160 0.287Copper, filtered, μg/L 40 1.4 3.1 3.7 4.3 4.7 19.8 31Copper, unfiltered-recoverable, μg/L 39 4.8 9.2 11.9 15.4 17.5 51.6Iron, filtered, μg/L 40 7.5 11.7 20.0 20.2 27.1 43.0 8Iron, unfiltered-recoverable, μg/L 40 67.5 167 248 297 370 860Lead, filtered, μg/L 40 0.037 0.074 0.112 0.116 0.132 0.387 7Lead, unfiltered-recoverable, μg/L 40 0.40 1.10 1.51 2.06 2.82 6.33Manganese, filtered, μg/L 40 13.1 37.8 41.8 54.7 63.8 130 48Manganese, unfiltered-recoverable, μg/L 40 40.9 73.0 87.5 102 122 220Zinc, filtered, μg/L 40 (7) 1.4 1.8 2.6 2.8 3.3 9.4 24Zinc, unfiltered-recoverable, μg/L 40 2.8 7.1 10.7 13.5 18.0 45.1Arsenic, filtered, μg/L 40 7.0 10.4 12.7 13.8 18.0 27.5 82Arsenic, unfiltered-recoverable, μg/L 40 8.9 12.4 15.4 16.0 19.0 31.5Suspended sediment, mg/L 40 2 5 8 12 12 59 NASuspended sediment, percent fines4 40 32 68 76 75 87 96 NA

Table 4. Statistical summaries of water-quality data collected at selected sampling sites in the Milltown Reservoir/Clark Fork River Superfund Site in the upper Clark Fork Basin, Montana, water years 2011–15.—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 foot per second; NA, not applicable; μS/cm, microsiemen per centimeter at 25 degrees Celsius; CaCO3, calcium carbonate; μg/L, microgram per liter; mg/L, milligram per liter]
12 Water-Quality Trends and Constituent-Transport Analysis for Selected Sampling Sites Constituent or property, unadjusted (not flow adjusted) units of measurement Statistical summaries of water-quality data1 Ratios of median filtered to median unfiltered-recoverable concentrations for trace elements, in percent3 Number of samples (values in parentheses indicate number of censored values) Minimum uncensored value2 25th percentile Median Mean 75th percentile Maximum Clark Fork at Deer Lodge, Montana (sampling site 14, fig. 1, table 1) Streamflow, instantaneous, ft3/s 40 44 197 265 353 357 2,000 NA Specific conductance, μS/cm 40 228 346 436 412 481 525 NA pH, standard units 40 7.9 8.2 8.3 NA 8.4 8.9 NA Hardness, filtered, mg/L as CaCO3 40 97.1 154 200 183 214 231 NA Calcium, filtered, mg/L 40 29.1 46.0 58.8 54.0 62.8 68.8 NA Magnesium, filtered, mg/L 40 5.92 9.56 13.1 11.8 13.7 15.5 NA Cadmium, filtered, μg/L 40 0.035 0.049 0.065 0.069 0.072 0.280 43 Cadmium, unfiltered-recoverable, μg/L 40 0.046 0.094 0.152 0.203 0.221 0.784 Copper, filtered, μg/L 40 3.4 5.6 7.0 8.3 7.7 45.9 25 Copper, unfiltered-recoverable, μg/L 40 9.4 15.2 27.6 46.3 49.3 220 Iron, filtered, μg/L 40 5.5 11.7 18.5 18.7 24.9 45.8 4 Iron, unfiltered-recoverable, μg/L 40 63.0 224 436 708 788 4,290 Lead, filtered, μg/L 40 (1) 0.041 0.082 0.142 0.152 0.189 0.372 4

Lead, unfiltered-recoverable, μg/L 40 0.55 1.61 3.28 5.70 6.63 32.8Manganese, filtered, μg/L 40 11.7 22.6 30.0 32.6 38.7 70.8 36Manganese, unfiltered-recoverable, μg/L 40 22.9 57.4 82.9 97.5 115 364Zinc, filtered, μg/L 40 1.6 3.6 5.5 6.5 6.6 50.6 23Zinc, unfiltered-recoverable, μg/L 40 5.0 15.3 23.2 34.9 37.6 164Arsenic, filtered, μg/L 40 7.7 10.3 13.3 14.0 16.2 36.6 81Arsenic, unfiltered-recoverable, μg/L 40 9.7 13.8 16.4 19.2 20.3 46.6Suspended sediment, mg/L 40 2 8 17 33 31 218 NASuspended sediment, percent fines4 40 39 72 81 77 86 96 NA

Table 4. Statistical summaries of water-quality data collected at selected sampling sites in the Milltown Reservoir/Clark Fork River Superfund Site in the upper Clark Fork Basin, Montana, water years 2011–15.—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 foot per second; NA, not applicable; μS/cm, microsiemen per centimeter at 25 degrees Celsius; CaCO3, calcium carbonate; μg/L, microgram per liter; mg/L, milligram per liter]
Overview of Streamflow and Water-Quality Characteristics for Water Years 2011–15 13

Constituent or property, 

unadjusted 

(not flow adjusted) 

units of measurement

Statistical summaries of water-quality data1 Ratios of median 

filtered to median 

unfiltered-recoverable 

concentrations for 

trace elements, 

in percent3

Number of samples 

(values in parentheses 

indicate number of 

censored values)

Minimum 

uncensored 

value2

25th 

percentile Median Mean 75th percentile Maximum

Clark Fork above Little Blackfoot River near Garrison, Montana (sampling site 15, fig. 1, table 1)

Streamflow, instantaneous, ft3/s 39 71 227 289 410 418 2,310 NASpecific conductance, μS/cm 39 249 363 449 421 479 527 NApH, standard units 39 7.9 8.2 8.4 NA 8.6 8.9 NAHardness, filtered, mg/L as CaCO3 39 107 162 202 186 213 228 NACalcium, filtered, mg/L 39 31.9 47.4 58.8 54.1 61.7 66.5 NAMagnesium, filtered, mg/L 39 6.65 10.4 13.4 12.3 14.4 15.5 NACadmium, filtered, μg/L 39 (1) 0.024 0.050 0.065 0.067 0.072 0.227 42Cadmium, unfiltered-recoverable, μg/L 39 0.027 0.117 0.155 0.227 0.272 0.835Copper, filtered, μg/L 39 2.8 6.2 7.9 9.2 9.7 40.6 25Copper, unfiltered-recoverable, μg/L 39 10.0 19.1 31.9 51.3 54.0 222Iron, filtered, μg/L 38 5.2 9.2 15.7 19.0 25.2 64.4 3Iron, unfiltered-recoverable, μg/L 38 40.7 256 505 806 823 3,860Lead, filtered, μg/L 39 (1) 0.048 0.086 0.135 0.181 0.247 0.715 4Lead, unfiltered-recoverable, μg/L 39 0.33 2.08 3.74 6.40 6.63 32.3Manganese, filtered, μg/L 39 8.6 20.7 27.2 29.4 35.7 65.1 32Manganese, unfiltered-recoverable, μg/L 39 13.4 63.4 84.5 105 129 344Zinc, filtered, μg/L 39 (2) 1.9 3.1 4.9 5.6 6.9 37.1 18Zinc, unfiltered-recoverable, μg/L 39 (1) 3.2 15.9 26.7 43.9 44.5 181Arsenic, filtered, μg/L 39 7.8 10.9 15.2 15.0 17.3 36.7 87Arsenic, unfiltered-recoverable, μg/L 39 10.5 15.2 17.4 20.3 21.2 46.0Suspended sediment, mg/L 39 1 11 21 37 37 205 NASuspended sediment, percent fines4 39 46 72 79 77 83 92 NA

Table 4. Statistical summaries of water-quality data collected at selected sampling sites in the Milltown Reservoir/Clark Fork River Superfund Site in the upper Clark Fork Basin, Montana, water years 2011–15.—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 foot per second; NA, not applicable; μS/cm, microsiemen per centimeter at 25 degrees Celsius; CaCO3, calcium carbonate; μg/L, microgram per liter; mg/L, milligram per liter]
14 Water-Quality Trends and Constituent-Transport Analysis for Selected Sampling Sites Constituent or property, unadjusted (not flow adjusted) units of measurement Statistical summaries of water-quality data1 Ratios of median filtered to median unfiltered-recoverable concentrations for trace elements, in percent3 Number of samples (values in parentheses indicate number of censored values) Minimum uncensored value2 25th percentile Median Mean 75th percentile Maximum Clark Fork at Goldcreek, Montana (sampling site 16, fig. 1, table 1) Streamflow, instantaneous, ft3/s 40 137 393 522 820 902 4,450 NA Specific conductance, μS/cm 40 216 297 364 353 411 456 NA pH, standard units 40 7.9 8.1 8.3 NA 8.6 9.1 NA Hardness, filtered, mg/L as CaCO3 40 98.5 131 165 158 186 211 NA Calcium, filtered, mg/L 40 29.6 38.7 48.2 46.5 55.0 62.1 NA Magnesium, filtered, mg/L 40 5.96 8.21 10.6 10.2 12.2 13.6 NA Cadmium, filtered, μg/L 40 (3) 0.020 0.031 0.041 0.044 0.050 0.124 40 Cadmium, unfiltered-recoverable, μg/L 40 0.021 0.072 0.102 0.158 0.209 0.530 Copper, filtered, μg/L 40 2.1 4.3 5.1 6.1 6.4 23.3 27 Copper, unfiltered-recoverable, μg/L 40 5.6 11.4 18.6 32.1 41.3 133 Iron, filtered, μg/L 40 (1) 3.8 8.8 18.6 25.9 36.0 93.7 5 Iron, unfiltered-recoverable, μg/L 40 31.8 182 360 699 922 2,940 Lead, filtered, μg/L 40 (2) 0.035 0.056 0.111 0.141 0.170 0.677 5

Lead, unfiltered-recoverable, μg/L 40 0.14 1.31 2.24 4.33 5.99 19.9Manganese, filtered, μg/L 40 5.5 12.8 16.1 18.3 20.0 45.1 24Manganese, unfiltered-recoverable, μg/L 40 9.3 46.9 67.4 84.4 107 253Zinc, filtered, μg/L 40 (5) 1.8 2.3 3.5 4.1 5.7 17.7 20Zinc, unfiltered-recoverable, μg/L 40 (1) 2.9 11.0 17.3 29.9 41.7 113Arsenic, filtered, μg/L 40 5.6 7.9 9.0 9.9 11.5 22.5 79Arsenic, unfiltered-recoverable, μg/L 40 7.5 9.7 11.4 13.3 14.4 28.4Suspended sediment, mg/L 40 2 8 16 35 40 176 NASuspended sediment, percent fines4 40 56 71 82 78 87 94 NA

Table 4. Statistical summaries of water-quality data collected at selected sampling sites in the Milltown Reservoir/Clark Fork River Superfund Site in the upper Clark Fork Basin, Montana, water years 2011–15.—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 foot per second; NA, not applicable; μS/cm, microsiemen per centimeter at 25 degrees Celsius; CaCO3, calcium carbonate; μg/L, microgram per liter; mg/L, milligram per liter]
Overview of Streamflow and Water-Quality Characteristics for Water Years 2011–15 15

Constituent or property, 

unadjusted 

(not flow adjusted) 

units of measurement

Statistical summaries of water-quality data1 Ratios of median 

filtered to median 

unfiltered-recoverable 

concentrations for 

trace elements, 

in percent3

Number of samples 

(values in parentheses 

indicate number of 

censored values)

Minimum 

uncensored 

value2

25th 

percentile Median Mean 75th percentile Maximum

Clark Fork near Drummond, Montana (sampling site 18, fig. 1, table 1)

Streamflow, instantaneous, ft3/s 40 248 563 781 1,040 1,090 5,540 NASpecific conductance, μS/cm 40 243 346 417 403 458 560 NApH, standard units 40 7.9 8.1 8.1 NA 8.2 8.5 NAHardness, filtered, mg/L as CaCO3 40 109 158 190 184 211 265 NACalcium, filtered, mg/L 40 32.6 45.1 54.3 52.7 59.7 74.9 NAMagnesium, filtered, mg/L 40 6.75 10.7 13.2 12.9 15.0 19.0 NACadmium, filtered, μg/L 40 (2) 0.021 0.032 0.043 0.045 0.053 0.101 35Cadmium, unfiltered-recoverable, μg/L 40 (1) 0.026 0.072 0.124 0.168 0.241 0.536Copper, filtered, μg/L 40 1.9 3.9 4.8 5.6 6.2 19.8 24Copper, unfiltered-recoverable, μg/L 40 5.4 9.8 19.4 29.9 36.7 107Iron, filtered, μg/L 40 (2) 3.6 9.0 15.0 20.7 26.9 88.7 3Iron, unfiltered-recoverable, μg/L 40 24.8 180 440 710 979 3,170Lead, filtered, μg/L 40 (2) 0.039 0.059 0.115 0.142 0.152 0.592 4Lead, unfiltered-recoverable, μg/L 40 0.17 1.27 3.02 4.61 6.29 19.8Manganese, filtered, μg/L 40 4.2 12.4 15.5 17.4 22.0 37.7 20Manganese, unfiltered-recoverable, μg/L 40 9.9 49.7 76.6 96.0 121 294Zinc, filtered, μg/L 40 (1) 2.2 3.5 4.3 4.7 5.3 13.2 19Zinc, unfiltered-recoverable, μg/L 40 (1) 4.6 12.0 22.7 35.0 48.3 134Arsenic, filtered, μg/L 40 6.3 8.0 9.7 10.1 11.3 23.9 86Arsenic, unfiltered-recoverable, μg/L 40 7.7 10.6 11.3 13.3 13.9 30.7Suspended sediment, mg/L 40 2 9 22 40 57 216 NASuspended sediment, percent fines4 40 42 68 79 76 86 93 NA

Table 4. Statistical summaries of water-quality data collected at selected sampling sites in the Milltown Reservoir/Clark Fork River Superfund Site in the upper Clark Fork Basin, Montana, water years 2011–15.—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 foot per second; NA, not applicable; μS/cm, microsiemen per centimeter at 25 degrees Celsius; CaCO3, calcium carbonate; μg/L, microgram per liter; mg/L, milligram per liter]
16 Water-Quality Trends and Constituent-Transport Analysis for Selected Sampling Sites Constituent or property, unadjusted (not flow adjusted) units of measurement Statistical summaries of water-quality data1 Ratios of median filtered to median unfiltered-recoverable concentrations for trace elements, in percent3 Number of samples (values in parentheses indicate number of censored values) Minimum uncensored value2 25th percentile Median Mean 75th percentile Maximum Clark Fork at Turah Bridge near Bonner, Montana (sampling site 20, fig. 1, table 1) Streamflow, instantaneous, ft3/s 40 462 1,050 1,500 2,230 2,640 10,600 NA Specific conductance, μS/cm 40 140 214 285 277 340 385 NA pH, standard units 40 7.8 8.0 8.1 NA 8.2 8.4 NA Hardness, filtered, mg/L as CaCO3 40 60.1 97.6 132 127 156 186 NA Calcium, filtered, mg/L 40 17.3 27.8 35.9 35.6 43.5 52.8 NA Magnesium, filtered, mg/L 40 4.11 6.76 9.57 9.27 11.6 13.1 NA Cadmium, filtered, μg/L 40 (12) 0.017 0.019 0.027 0.031 0.037 0.083 37 Cadmium, unfiltered-recoverable, μg/L 40 (3) 0.025 0.048 0.073 0.104 0.132 0.404 Copper, filtered, μg/L 40 1.3 2.2 2.9 3.8 3.9 17.9 27 Copper, unfiltered-recoverable, μg/L 40 3.8 5.9 10.5 16.8 20.1 61.9 Iron, filtered, μg/L 40 (3) 3.3 7.1 20.4 29.7 34.0 359 6 Iron, unfiltered-recoverable, μg/L 40 47.7 132 316 507 527 2,450 Lead, filtered, μg/L 40 (6) 0.030 0.039 0.069 0.134 0.137 2.79 4

Lead, unfiltered-recoverable, μg/L 40 0.20 0.58 1.67 2.67 3.33 11.9Manganese, filtered, μg/L 40 3.0 5.4 6.9 9.6 9.8 48.6 16Manganese, unfiltered-recoverable, μg/L 40 9.5 26.9 43.5 57.7 66.7 212Zinc, filtered, μg/L 40 (4) 1.5 2.3 3.3 3.9 4.7 17.4 23Zinc, unfiltered-recoverable, μg/L 40 3.8 8.4 14.2 22.9 27.3 109Arsenic, filtered, μg/L 40 2.7 4.5 5.6 5.6 6.0 14.2 90Arsenic, unfiltered-recoverable, μg/L 40 3.0 5.6 6.2 7.3 8.2 21.0Suspended sediment, mg/L 40 3 7 16 32 30 186 NASuspended sediment, percent fines4 40 44 66 78 74 85 91 NA

Table 4. Statistical summaries of water-quality data collected at selected sampling sites in the Milltown Reservoir/Clark Fork River Superfund Site in the upper Clark Fork Basin, Montana, water years 2011–15.—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 foot per second; NA, not applicable; μS/cm, microsiemen per centimeter at 25 degrees Celsius; CaCO3, calcium carbonate; μg/L, microgram per liter; mg/L, milligram per liter]
Overview of Streamflow and Water-Quality Characteristics for Water Years 2011–15 17

Constituent or property, 

unadjusted 

(not flow adjusted) 

units of measurement

Statistical summaries of water-quality data1 Ratios of median 

filtered to median 

unfiltered-recoverable 

concentrations for 

trace elements, 

in percent3

Number of samples 

(values in parentheses 

indicate number of 

censored values)

Minimum 

uncensored 

value2

25th 

percentile Median Mean 75th percentile Maximum

Clark Fork above Missoula, Montana (sampling site 22, fig. 1, table 1)

Streamflow, instantaneous, ft3/s 40 910 1,710 4,100 5,530 7,240 22,900 NASpecific conductance, μS/cm 40 148 189 230 239 288 341 NApH, standard units 40 8.0 8.2 8.3 NA 8.4 8.7 NAHardness, filtered, mg/L as CaCO3 40 70.7 88.5 109 113 141 163 NACalcium, filtered, mg/L 40 19.3 23.8 29.5 30.3 36.9 44.9 NAMagnesium, filtered, mg/L 40 5.30 6.98 8.48 9.06 11.3 12.9 NACadmium, filtered, μg/L 40 (25) 0.017 0.014 0.018 0.019 0.023 0.046 47Cadmium, unfiltered-recoverable, μg/L 40 (12) 0.020 0.021 0.038 0.056 0.067 0.345Copper, filtered, μg/L 40 1.0 1.5 1.7 2.1 2.1 7.0 35Copper, unfiltered-recoverable, μg/L 40 1.9 3.2 4.8 9.0 9.4 53.1Iron, filtered, μg/L 40 (1) 3.7 6.6 17.3 22.6 34.2 60.5 7Iron, unfiltered-recoverable, μg/L 40 40.9 96.3 255 370 344 2,030Lead, filtered, μg/L 40 (10) 0.026 0.031 0.054 0.068 0.089 0.212 7Lead, unfiltered-recoverable, μg/L 40 0.13 0.41 0.80 1.40 1.57 8.04Manganese, filtered, μg/L 40 3.5 5.5 6.5 7.7 8.5 20.0 24Manganese, unfiltered-recoverable, μg/L 40 8.8 19.7 27.5 36.5 41.4 155Zinc, filtered, μg/L 40 (16) 1.4 1.4 1.9 2.0 2.4 5.5 26Zinc, unfiltered-recoverable, μg/L 40 (4) 2.3 4.9 7.2 12.2 12.2 84.4Arsenic, filtered, μg/L 40 1.2 2.5 3.1 3.1 3.6 7.1 85Arsenic, unfiltered-recoverable, μg/L 40 1.4 2.9 3.7 4.1 4.8 13.2Suspended sediment, mg/L 40 2 5 13 26 20 176 NASuspended sediment, percent fines4 40 61 73 82 79 85 95 NA 1Distributional parameters affected by censored observations (that is, concentrations reported as less than the laboratory reporting level) were estimated by using adjusted maximum likelihood estimation (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 (table 2) and small bias in filtered concentrations.4Percent fines refers to the percentage of suspended sediment smaller than 0.062-millimeter diameter.

Table 4. Statistical summaries of water-quality data collected at selected sampling sites in the Milltown Reservoir/Clark Fork River Superfund Site in the upper Clark Fork Basin, Montana, water years 2011–15.—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 foot per second; NA, not applicable; μS/cm, microsiemen per centimeter at 25 degrees Celsius; CaCO3, calcium carbonate; μg/L, microgram per liter; mg/L, milligram per liter]
18 Water-Quality Trends and Constituent-Transport Analysis for Selected Sampling Sites

Table 5. Percentages of samples with unadjusted unfiltered-recoverable concentrations exceeding water-quality standards for 

selected sampling sites in the Milltown Reservoir/Clark Fork River Superfund Site in the upper Clark Fork Basin, water years 2011–15.

[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]

Sampling 

site 

number 

(fig. 1, 

table 1)

Abbreviated sampling site name 

(table 1)

Percentage of samples exceeding indicated standard

Arsenic 

humanhealth 

standard

Aquatic-life standards

Cadmium Copper Lead Zinc

Acute Chronic Acute Chronic Acute Chronic Acute Chronic

8 Silver Bow Creek at Warm Springs 100 0 3 8 18 0 3 0 0

11 Clark Fork near Galen 98 0 0 26 41 0 8 0 0

14 Clark Fork at Deer Lodge 95 0 15 58 75 0 23 3 3

15 Clark Fork near Garrison 100 0 18 59 79 0 23 3 3

16 Clark Fork at Goldcreek 68 0 18 48 60 0 28 0 0

18 Clark Fork near Drummond 80 0 15 38 58 0 25 3 3

20 Clark Fork at Turah Bridge 13 0 13 28 48 0 25 0 0

22 Clark Fork above Missoula 3 0 5 15 23 0 13 0 0

Reach 4

Reach 4 extends about 2 river miles from Silver Bow 

Creek at Warm Springs, Montana (sampling site 8), to Clark 

Fork near Galen, Montana (sampling site 11). Within the 

reach, water from Warm Springs Ponds mixes and geochemically reacts with water contributed from the Mill-Willow 

Bypass and Warm Springs Creek; thus, complex water-quality 

processes are possible in the short reach.

The Warm Springs Ponds system was originally constructed during 1908–17 (and expanded during the 1950s) 

to trap sediment enriched in trace elements (CDM, 2005). In 

about 1967, the AMC started introducing a lime and water 

suspension into Silver Bow Creek upstream from Warm 

Springs Ponds to raise pH and promote precipitation and 

deposition of metals in Warm Springs Ponds (U.S. Environmental Protection Agency, 2000). The Mill-Willow Bypass 

was constructed in about 1969 to capture streamflows of Mill 

and Willow Creeks near their mouths and divert the combined 

streamflows (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 sampling site 8 (CDM, 2005). Warm 

Springs Creek originates in the mountains west of the AMC 

Smelter, flows generally east through areas adjacent to the 

AMC Smelter and various tailings piles and ponds, and joins 

Silver Bow Creek to form the Clark Fork near Warm Springs. 

The Warm Springs Creek Basin is affected by pollution from 

milling and smelting operations of the AMC Smelter. Thick 

tailings deposits are extensive in the Silver Bow Creek and 

Clark Fork flood plain near Warm Springs (Smith and others, 

1998) and provide a source of sediment enriched with metallic 

contaminants within reach 4. 

In reach 4, the mean annual streamflow for water years 

2011–15 increased by about 79 percent from 96 cubic feet 

per second (ft3/s) at sampling site 8 to 172 ft3/s at sampling 

site 11 (table 3) primarily because of contributions from Warm 

Springs Creek and also ephemeral gulches and groundwater 

inflow. Near the end of reach 4, Warm Springs Creek and 

Silver Bow Creek join to form the Clark Fork. 

Silver Bow Creek at Warm Springs (sampling site 8) is 

about 0.2 river mile downstream from Warm Springs Ponds, 

which were designed to trap suspended sediment and metallic 

contaminants by physical deposition and treatment (liming; 

U.S. Environmental Protection Agency, 2000). Median concentrations of unfiltered-recoverable copper and zinc (6.8 and 

8.6 μg/L, respectively) and suspended sediment (6 milligrams 

per liter [mg/L]) are lower than median concentrations of 

most downstream main-stem Clark Fork sampling sites (fig. 2, 

table 4). The median concentration of unfiltered-recoverable 

arsenic (22.4 μg/L) at sampling site 8 is higher than median 

concentrations at the downstream main-stem Clark Fork sampling sites. The high median arsenic concentration at sampling 

site 8 is affected by contributions of water with high arsenic 

concentrations from the Mill-Willow Bypass and by complex 

hydrologic and limnologic factors that affect arsenic biogeochemical processing in Warm Springs Ponds (Chatham, 2012). 

The median pH for sampling site 8 is 8.8 standard units, 

which is higher than the median pH of the downstream mainstem Clark Fork sampling sites (table 4). 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). Exceedances of most waterquality standards were infrequent (that is, less than or equal 

to 20 percent of samples) for sampling site 8; however, the 
Overview of Streamflow and Water-Quality Characteristics for Water Years 2011–15 19 Figure 2. Statistical distributions of selected constituents for selected sampling sites in the Milltown Reservoir/Clark Fork River Superfund Site in the upper Clark Fork Basin, Montana, water years 2011–15. A, specific conductance; 

B, copper; 

C, arsenic; and 

D, suspended sediment.

A.

C.

D.

reach

4

reach

5

reach

6

reach

7

reach

8

reach

9

B.

Specific conductance

Copper

ArsenicSuspended sediment

Silver Bow Creek 

at Warm Springs

(sampling site 8)

Clark Fork near Galen

(sampling site 11)

Clark Fork 

near Garrison

(sampling site 15)

Clark Fork at 

Deer Lodge

(sampling site 14)

Clark Fork 

at Goldcreek

(sampling site 16)

Clark Fork 

above Missoula

(sampling site 22)

Clark Fork 

near Drummond

(sampling site 18)

Clark Fork at 

Turah Bridge

(sampling site 20)

Sampling site

100

1,000

1

10

200100

1

10

200100

1

10

200100

Unadjusted (not flow adjusted) 

specific conductance, in 

microsiemens per centimeter 

at 25 degrees Celsius

Unadjusted (not flow adusted) 

filtered and unfiltered- 

- recoverable copper concentra

tion, 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

75th percentilemedian25th percentile

90th percentile10th percentileAquatic-life standards for 

copper based on median hardness for the given site for water years 2011–15 (Montana Department ofEnvironmental Quality, 2012)

Human-health standard for 

arsenic (Montana Department of Environmental Quality, 2012)

22

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

Sampling site number (fig. 1, 

table 1)— Site numbers that lie on top of reach dividing lines indicate that the site is both an outflow from the upstream reach and an inflow to the downstream reach 

Interquartile

range

Acute

Chronic

8 11 14 15 16 18 20 22

8 11 14 15 16 18 20 22

8 11 14 15 16 18 20 22

8 11 14 15 16 18 20 22

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]
20 Water-Quality Trends and Constituent-Transport Analysis for Selected Sampling Sites

arsenic human-health standard was exceeded in 100 percent of 

samples (table 5).

Clark Fork near Galen (sampling site 11) is about 2 river 

miles downstream from sampling site 8 and about 1 river mile 

downstream from the start of the Clark Fork at the confluence of Silver Bow Creek and Warm Springs Creek. Spatial 

changes in water quality between sampling sites 8 and 11 in 

water years 2011–15 include increases in median concentrations of unfiltered-recoverable metallic trace elements and 

suspended sediment, as well as decreases in median concentrations of unfiltered-recoverable arsenic (fig. 2, table 4). Factors 

that might contribute to the patterns include mobilization of 

materials from flood-plain tailings deposits near Warm Springs 

and complex processes as water from Warm Springs Ponds 

mixes and geochemically reacts with water contributed from 

the Mill-Willow Bypass and Warm Springs Creek. Exceedances of most water-quality standards were somewhat infrequent for sampling site 11, but the acute aquatic-life standard 

for copper was exceeded in 26 percent of samples, the chronic 

aquatic-life standard for copper was exceeded in 41 percent of 

samples, and the arsenic human-health standard was exceeded 

in 98 percent of samples (table 5). 

Reach 5

Reach 5 extends about 21 river miles from Clark Fork 

near Galen (sampling site 11) to Clark Fork at Deer Lodge, 

Montana (sampling site 14), and meanders through a broad 

valley with extensive flood-plain tailings deposits. Lost Creek 

(a tributary to the Clark Fork in reach 5) originates in the 

mountains northwest of the 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 AMC Smelter (U.S. Environmental Protection Agency, 2010). In reach 5, the mean annual streamflow 

for water years 2011–15 increased by about 65 percent from 

172 ft3/s at sampling site 11 to 283 ft3/s at sampling site 14 

(table 3) partly because of contributions from Lost Creek 

and also numerous other tributaries, ephemeral gulches, and 

groundwater inflow. 

Spatial changes in water quality between sampling 

sites 11 and 14 in water years 2011–15 include substantial 

increases in median concentrations of unfiltered-recoverable 

metallic trace elements and suspended sediment (fig. 2, 

table 4). Mobilization of mining wastes from extensive floodplain tailings deposits and stream banks contribute to the 

pattern. Exceedances of water-quality standards were frequent 

for sampling site 14: the acute aquatic-life standard for copper 

was exceeded in 58 percent of samples, the chronic aquaticlife standard for copper was exceeded in 75 percent of samples, the chronic aquatic-life standard for lead was exceeded in 

23 percent of samples, and the arsenic human-health standard 

was exceeded in 95 percent of samples (table 5).

Reach 6

Reach 6 extends about 26 river miles from Clark Fork 

at Deer Lodge (sampling site 14) to Clark Fork at Goldcreek, 

Montana (sampling site 16). Clark Fork above Little Blackfoot River near Garrison (sampling site 15), is in reach 6 and 

is located about 14 river miles downstream from sampling 

site 14 and about 12 river miles upstream from sampling 

site 16. Water-quality data collection for sampling site 15 

began in water year 2009 (table 1); thus, water-quality data 

for sampling site 15 are suitable for summarizing water years 

2011–15 water-quality characteristics but are not adequate for 

trend analysis.

The Clark Fork meanders through a broad valley from 

Deer Lodge to Garrison, in which flood-plain 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 tributary to the Clark Fork in reach 

6) drains a basin with moderate density of agricultural and 

historical mining activity (in comparison with other tributaries downstream from Deer Lodge) and discharges into reach 

6 near Garrison (about 1 river mile downstream from sampling site 15) where the Clark Fork Valley begins to narrow. 

Downstream from Garrison, flood-plain tailings are less 

extensive than in the valley upstream. In reach 6, the mean 

annual streamflow for water years 2011–15 increased by about 

11 percent from 283 ft3/s at sampling site 14 to 315 ft3/s at 

sampling site 15 and then by about 81 percent to 570 ft3/s at 

sampling site 16 (table 3). The overall increase in streamflow 

from sampling site 14 to sampling site 16 was about 101 percent, mostly because of contributions from the Little Blackfoot 

River and also numerous other tributaries, ephemeral gulches, 

and groundwater inflow. 

Spatial changes in water quality between sampling 

sites 14 and 16 in water years 2011–15 include decreases in 

median concentrations of unfiltered-recoverable metallic trace 

elements, unfiltered-recoverable arsenic, and suspended sediment, despite small increases in most of these values between 

sampling sites 14 and 15. Water-quality changes in reach 6 

primarily were 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 result in decreasing water-quality effects with distance 

downstream from primary source areas. Exceedances of waterquality standards were frequent for sampling site 15: the acute 

aquatic-life standard for copper was exceeded in 59 percent 

of samples, the chronic aquatic-life standard for copper was 

exceeded in 79 percent of samples, the chronic aquatic-life 

standard for lead was exceeded in 23 percent of samples, and 

the arsenic human-health standard was exceeded in 100 percent of samples (table 5). Exceedances of water-quality standards were somewhat frequent for sampling site 16: the acute 

aquatic-life standard for copper was exceeded in 48 percent 
Overview of Streamflow and Water-Quality Characteristics for Water Years 2011–15 21

of samples, the chronic aquatic-life standard for copper was 

exceeded in 60 percent of samples, the chronic aquatic-life 

standard for lead was exceeded in 28 percent of samples, and 

the arsenic human-health standard was exceeded in 68 percent 

of samples (table 5).

Reach 7

Reach 7 extends about 31 river miles from Clark Fork at 

Goldcreek (sampling site 16) to Clark Fork near Drummond, 

Montana (sampling site 18). In reach 7, channel meandering 

and exposed flood-plain tailings are less extensive than in 

upstream reaches (Lambing, 1998; Smith and others, 1998). 

Flint Creek (a tributary that discharges to the Clark Fork in 

reach 7 near Drummond) drains a basin with high density of 

agricultural and historical mining activity (in comparison with 

other tributaries downstream from Deer Lodge). 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 (Lambing, 1998; Smith and others, 1998). In 

reach 7, the mean annual streamflow for water years 2011–15 

increased by about 35 percent from 570 ft3/s at sampling 

site 16 to 771 ft3/s at sampling site 18 (table 3) mostly because 

of contributions from Flint Creek and also numerous other 

tributaries, ephemeral gulches, and groundwater inflow. 

Spatial changes in water quality between sampling 

sites 16 and 18 in water years 2011–15 include generally small 

increases in median concentrations of unfiltered-recoverable 

metallic trace elements and suspended sediment. Although 

the increases were 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 were affected by streamflow contributions from the Flint Creek Basin, which has high density of 

agricultural and historical mining activity (in comparison with 

other tributaries downstream from Deer Lodge). The Clark 

Fork flood plain and stream banks downstream from Flint 

Creek probably also contain mining-waste deposits sourced 

from the Flint Creek Basin. Exceedances of water-quality standards were somewhat frequent for sampling site 18: the acute 

aquatic-life standard for copper was exceeded in 38 percent 

of samples, the chronic aquatic-life standard for copper was 

exceeded in 58 percent of samples, the chronic aquatic-life 

standard for lead was exceeded in 25 percent of samples, and 

the arsenic human-health standard was exceeded in 80 percent 

of samples (table 5). 

Reach 8

Reach 8 extends about 34 river miles from Clark Fork 

near Drummond (sampling site 18) to Clark Fork at Turah 

Bridge near Bonner, Montana (sampling site 20). In reach 8, 

the Clark Fork flows through a narrow flood plain (generally 

less than 1 mi wide) with little or no visible mining tailings. 

Rock Creek (a tributary to the Clark Fork in reach 8) drains 

a heavily forested basin with low density of agricultural and 

historical mining activity (in comparison with other tributaries downstream from Deer Lodge) and discharges into reach 8 

near Clinton, Montana. In reach 8, the mean annual streamflow for water years 2011–15 increased by about 93 percent 

from 771 ft3/s at sampling site 18 to 1,490 ft3/s at sampling 

site 20 (table 3) primarily because of contributions from 

Rock Creek, as well as numerous other tributaries, ephemeral 

gulches, and groundwater inflow.

Spatial changes in water quality between sampling 

sites 18 and 20 in water years 2011–15 include generally 

substantial decreases in median concentrations of unfilteredrecoverable metallic trace elements, unfiltered-recoverable 

arsenic, and suspended sediment. Water-quality changes in 

reach 8 were affected by dilution from Rock Creek. Exceedances of most water-quality standards were somewhat infrequent for sampling site 20, but the acute aquatic-life standard 

for copper was exceeded in 28 percent of samples, the chronic 

aquatic-life standard for copper was exceeded in 48 percent 

of samples, and the chronic aquatic-life standard for lead was 

exceeded in 25 percent of samples (table 5). 

Reach 9

Reach 9 extends about 9 river miles from Clark Fork at 

Turah Bridge (sampling site 20) to Clark Fork above Missoula, Montana (sampling site 22). Reach 9 includes the 

former Milltown Reservoir where large amounts of mining wastes had been deposited. The former Milltown Dam 

was removed in 2008. The Blackfoot River (a tributary that 

discharges to the Clark Fork in reach 9 near Bonner) drains 

a largely forested basin with low density of agricultural and 

historical mining activity (in comparison with other tributaries downstream from Deer Lodge). In reach 9, mean annual 

streamflow increased by about 123 percent from 1,490 ft3/s 

at sampling site 20 to 3,330 ft3/s at sampling site 22 (table 3) 

primarily because of contributions from the Blackfoot River. 

Spatial changes in water quality between sampling 

sites 20 and 22 in water years 2011–15 include generally 

substantial decreases in median concentrations of unfilteredrecoverable metallic trace elements, unfiltered-recoverable 

arsenic, and suspended sediment. Water-quality changes in 

reach 9 were affected by dilution from the Blackfoot River. 

Exceedances of most water-quality standards were infrequent 

for sampling site 22, but the chronic aquatic-life standard for 

copper was exceeded in 23 percent of samples (table 5). 
22 Water-Quality Trends and Constituent-Transport Analysis for Selected Sampling Sites

Water-Quality Trend- and ConstituentTransport Analysis Methods

This section of the report describes methods used to 

analyze trends in flow-adjusted concentrations of water-quality 

constituents. 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.

General Description of the Time-Series Model

The TSM for streamflow and constituent concentration (Vecchia, 2005) was used 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 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 are presented in appendix 2.

The TSM analyzes trends in flow-adjusted concentrations 

(FACs); that is, the TSM computes FACs, estimates unbiased 

best-fit trend lines that represent temporal changes in FACs, 

and determines statistical significance of changes. Flow adjustment is necessary because concentrations of many waterquality constituents are strongly dependent on streamflow 

conditions, which are primarily affected by climatic variability 

in the study area. The intent of flow adjustment is to identify 

and remove streamflow-related variability in concentrations 

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. 

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. The TSM FACs 

provide detailed accounting by incorporating interannual, seasonal, and short-term streamflow variability (Vecchia, 2005), 

which compensates for interannual, seasonal, and short-term 

hysteresis processes that affect concentration and streamflow 

relations (Colby, 1956; Chanat and others, 2002; Vecchia, 

2005). Detailed analysis of continuous streamflow data provides definition of the context of streamflow conditions associated with a given water sample, 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. 

The TSM incorporates 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. 

All of the study datasets (except for Clark Fork near Garrison [sampling site 15], which was not analyzed for trends) 

met the data criteria for applying the TSM, which include 

at least 15 years of continuous streamflow data and at least 

15 years of water-quality data with at least 60 total waterquality samples and at least 10 samples total in each 3-month 

season (Vecchia, 2005). 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 

artificially imposed 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 (table 2) in the datasets of 

the Clark Fork monitoring program complicate the task of 

setting consistent substitution values. In applying the TSM to 

the study datasets, study reporting levels (SRLs; table 2) were 

established to set consistent substitution values for each traceelement 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 that provided information on potential contamination bias of low concentrations. The SRLs were applied to 

the study datasets 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. The study datasets 

largely were unaffected by recensoring for the trace-element 

constituents included in the trend analysis (table 2); unfilteredrecoverable zinc was the only affected constituent, and no 

sampling site had more than 8.5 percent of values affected 

by the recensoring procedures. Further, for individual constituents, the maximum frequency of detection in field blank 

samples at concentrations greater than the SRL was 2.7 percent (for unfiltered-recoverable zinc; table 1–1).

The TSM accounts for many hydrologic factors that 

contribute to complexity in concentration and streamflow 

relations. In this study, the TSM was applied as consistently 

as possible among sampling-site and constituent combinations 

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. As such, the TSM provides a consistent relational 

framework for evaluating temporal water-quality changes 
Water-Quality Trend- and Constituent-Transport Analysis Methods 23

among the sampling sites. The TSM best-fit trend lines were 

considered to provide important information beyond the strict 

statistical characteristics of the trend results (in terms of statistical probability levels [p-values] and levels of significance) 

because they aid in comparing and summarizing large-scale 

patterns among sampling sites. 

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 sampling 

sites with different periods of data collection, (2) distinguish 

somewhat short-term timing of changes in concentration 

and streamflow relations during 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, trend-analysis periods were defined as sequential 

5-year periods that extended from near the start of long-term 

data-collection activities for most sampling sites in the upper 

Clark Fork Basin to the end of water year 2015. Thus, four 

trend-analysis time periods were defined: period 1 (water years 

1996–2000), period 2 (water years 2001–5), period 3 (water 

years 2006–10), and period 4 (water years 2011–15). 

The TSM-fitted trends for a given 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 of the 

trend-analysis period. The flatter slopes near the ends provide 

gradual transition between adjacent trend-analysis periods. In 

some cases, the fitted trends in a given trend-analysis period 

do not precisely follow the patterns in FACs, and there are 

short-term (about 1–2 years) trend patterns in FACs that are 

unresolved in the fitted trends. In those cases, better temporal 

resolution might have been attained by defining two or more 

trend-analysis periods in a given 5-year trend-analysis period. 

This approach generally was avoided because it would have 

required detailed trend analysis for potentially inconsistent 

time periods among the various sampling-site and constituent 

combinations. An important consideration in the design of the 

trend-analysis structure of this study was making general comparisons among the sampling-site and constituent combinations to evaluate large-scale effects of mining and remediation 

activities for 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 

in the affected trend-analysis periods were consistent with 

overall patterns in FACs in the period. However, because of 

the substantial effect of the intentional breach of the former 

Milltown Dam on March 28, 2008, an exception to consistent trend-analysis periods was made. For Clark Fork above 

Missoula (sampling 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 intentional breach 

of the former Milltown Dam was part of an extensive remediation effort from about 2006–8 that resulted in the removal of 

the former Milltown Dam (Sando and Lambing, 2011). 

Estimation of Normalized Constituent Loads

Normalized constituent loads were estimated to assess the 

temporal trends in FACs of mining-related contaminants in the 

context of sources and transport. The fitted trends are unbiased best-fit lines through the FACs, which are independent of 

streamflow variability. The FAC trends at individual sampling 

sites are important descriptors of water-quality changes in the 

upper Clark Fork Basin, but without consideration of differences in streamflow magnitudes among different sampling 

sites, the trends do not provide direct 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 among sampling sites but normalizes streamflow for a given sampling 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 four 

5-year trend-analysis periods. The stationary streamflow index 

used in estimating normalized loads was the geometric mean 

streamflow for each sampling site for water years 1996–2015. 

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 sampling-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 

5-year analysis period times the geometric mean streamflow 

for water years 1996–2015 and a units conversion factor, 

according to the following equation:

LOAD = MAC G* * MQ 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 1996–2015, in 

cubic feet per second; and
24 Water-Quality Trends and Constituent-Transport Analysis for Selected Sampling Sites

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 (in each 

of the four 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”; this terminology indicates temporal averaging of geometric mean concentrations. The temporal averaging of geometric mean concentrations in each 5-year period 

effectively results in the MAC representing the center of the 

5-year period, which introduces a conservative approach to the 

transport analysis. The geometric mean generally is closely 

associated with the median of the original untransformed 

units for data that are approximately log-normally distributed. 

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, but rather provide information on 

relative temporal changes in constituent transport characteristics of the study sampling sites quantified with respect to 

near-median conditions.

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 trendanalysis procedures and interpreting trend results include 

relations between unadjusted concentrations and FACs, and 

data transformation. 

Streamflow Conditions

Daily mean streamflows for water years 1993–2015 

for selected sampling sites in the Milltown Reservoir/Clark 

Fork River Superfund Site in the upper Clark Fork Basin are 

presented in figure 3. Locally weighted scatter plot smooth 

(LOWESS; Cleveland and McGill, 1984; Cleveland, 1985) 

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 1996–2015 are presented to represent overall 

central tendency of streamflow during the period of trend 

analysis. Silver Bow Creek at Warm Springs (sampling site 8), 

Clark Fork at Deer Lodge (sampling site 14), and Clark Fork 

at Turah Bridge (sampling site 20) were selected as examples 

for showing hydrologic patterns (fig. 3) that generally apply to 

the other sampling sites. 

Temporal variability in streamflow conditions during 

the study period generally is similar among sampling 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, streamflows 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 streamflows 

were prevalent during most of period 1 and 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 (fig. 3). 

Streamflow during water year 1997 was particularly unusual 

in that the receding limb of snowmelt runoff was less abrupt 

and less variable than in 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 sampling site 14 (fig. 3B) sometimes exceeded 

annual maximum streamflows during the low streamflow years 

2000–2002. During period 2 (water years 2001–5), streamflows generally were below the geometric mean streamflows. 

During period 3 (water years 2006–10), streamflows gradually increased from below the geometric mean streamflows 

in water year 2006 to above the geometric mean streamflows 

in water year 2010. During period 4 (water years 2011–15), 

streamflows generally were above the geometric mean streamflows in water years 2011–12 and then decreased to near the 

geometric mean streamflows in water year 2013. Streamflows 

in water year 2011 were especially high and generally similar 

to streamflows in water year 1997.

Other Factors

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. Unadjusted concentrations 

are the observed concentrations before flow adjustment.

The 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 

sampling-site and constituent combinations, and also can be 

variable through time for a given sampling-site and constituent combination. Time-series streamflow, unfilteredrecoverable copper, unfiltered-recoverable arsenic, and 
Factors that Affect Trend Analysis and Interpretation 25

Figure 3. Daily mean streamflow for selected sampling sites in the Milltown Reservoir/Clark Fork River Superfund Site in the upper 

Clark Fork Basin, Montana, water years 1993–2015. A, Silver Bow Creek at Warm Springs, Montana; B, Clark Fork at Deer Lodge, 

Montana; and C, Clark Fork at Turah Bridge near Bonner, Montana.

10

100

1,000

10,000

10

100

1,000

10,000

10

100

1,000

10,000

EXPLANATION

Daily mean streamflow

Geometric mean streamflow 

for water years 1996–2015

Locally weighted scatter plot 

smooth (LOWESS; Cleveland, 

1985; Cleveland and McGill, 

1984) line for daily mean 

streamflow

[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]

 Water year (October–September)

1990

1995

2000

2005

2010

2015

1

Period

2 3 4

C. Clark Fork at Turah Bridge (sampling site 20, fig. 1, table 1)

B. Clark Fork at Deer Lodge (sampling site 14, fig. 1, table 1)

A. Silver Bow Creek at Warm Springs (sampling site 8, fig. 1, table 1)

Streamflow, in cubic feet per second
26 Water-Quality Trends and Constituent-Transport Analysis for Selected Sampling Sites

suspended-sediment data for Clark Fork near Galen (sampling 

site 11) are presented in figure 4 to provide examples for 

discussion of relations between unadjusted and flow-adjusted 

concentrations. 

Similarities among the LOWESS lines for streamflow 

(fig. 4A) and unadjusted suspended-sediment concentrations 

(fig. 4D) illustrate the direct relations between streamflow and 

unadjusted suspended-sediment concentrations. Unadjusted 

suspended-sediment 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. Flow-adjustment procedures account for the response of 

suspended-sediment concentrations to variations in streamflow 

and produce FACs that represent temporal variability in consistent streamflow conditions. In the Clark Fork, suspendedsediment FACs in high streamflow conditions are less variable and lower than unadjusted concentrations (for example, 

fig. 4D, water years 1996–99). Suspended-sediment FACs in 

low streamflow conditions are less variable and generally centered within unadjusted concentrations (for example, fig. 4D, 

water years 2000–2001).

Unfiltered-recoverable copper has concentration and 

streamflow relations that are similar to suspended sediment 

because of adsorption on inorganic and organic particulate 

materials; these same relations generally apply to other metallic elements. As a result, patterns in unadjusted concentrations and FACs for unfiltered-recoverable 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 a 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 in 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 unfilteredrecoverable 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. 

Similarities among the LOWESS lines for streamflow 

(fig. 4A), unfiltered-recoverable copper (fig. 4B), and suspended sediment (fig. 4D) indicate that temporal variability 

in streamflow might confound interpretation of temporal 

variability in unadjusted constituent concentrations. Examination of temporal variability during water years 1993–2015 

indicates that, in all cases, the LOWESS lines for streamflow (fig 4A), unfiltered-recoverable copper (fig. 4B), and 

suspended sediment (fig. 4D) are highest about 1996–97 

and lowest about 2000–2001, then variably increase during 

2002–11 and generally decrease during 2012–15. Because 

of the strong association between constituent concentrations 

and streamflow, interpreting temporal changes in unadjusted 

constituent concentrations during specific time periods is difficult. For example, in water years 2000–2002, mean annual 

streamflow was low (about 60 percent of the long-term mean 

annual streamflow). Annual mean streamflow in water year 

2003 somewhat increased to near-normal conditions (about 

90 percent of the long-term mean annual streamflow). Associated with the increase in streamflow in 2003 were somewhat 

abrupt increases in unadjusted concentrations of unfilteredrecoverable copper and suspended sediment that are reflected 

by somewhat abrupt increases in the LOWESS lines for those 

constituents. The somewhat abrupt increases in unadjusted 

concentrations of unfiltered-recoverable copper and suspended 

sediment in water year 2003 probably were affected by the 

near-normal streamflow conditions of water year 2003 immediately following the low streamflow conditions of water years 

2000–2002. During water years 2000–2002, low streamflow 

conditions might have promoted storage of particulate materials in the basin; the stored particulate materials might have 

been readily mobilized during water year 2003. Beginning in 

water year 2005, streamflow conditions gradually transitioned 

from generally low streamflow conditions to high streamflow 

conditions in water year 2011. The gradual transition might 

have affected the response in unadjusted concentrations of 

unfiltered-recoverable copper and suspended sediment to the 

high streamflow conditions of water year 2011, particularly 

in comparison with the more abrupt increase in streamflow in 

water year 2003. Thus, various complexities in concentration 

and streamflow relations contribute to difficulties in interpreting temporal patterns in unadjusted constituent concentrations. 

Temporal variability in streamflow strongly confounds the 

ability to interpret temporal variability in unadjusted constituent concentrations.

The TSM flow-adjustment procedure analyzes concentration and streamflow relations on multiple timescales (interannual, seasonal, and short-term) and accounts for streamflow 

variability. In contrast to the LOWESS lines through the 

unadjusted constituent concentrations, the TSM-fitted trends 

in figure 4 indicate consistent decreases in FACs of unfilteredrecoverable copper and suspended sediment. 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. Log transformation 

results in datasets that are approximately normally distributed 

and allows analysis using rigorous parametric procedures; 

however, log transformation decreases variability in the data 
Factors that Affect Trend Analysis and Interpretation 27

Figure 4. Selected streamflow and constituent concentration information for Clark Fork near Galen, Montana (sampling site 11), water 

years 1993–2015. A, streamflow; B, unfiltered-recoverable copper; C, unfiltered-recoverable arsenic; and D, suspended sediment.

Water year (October–September)

199019952000200520102015

A. Streamflow

B. 

C.Unfiltered-recoverable arsenic

D.Suspended sediment

Unfiltered-recoverable copper

1

Period

2 3 4

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

Daily mean streamflow

Geometric mean streamflow 

for water years 1996–2015

Locally weighted scatter plot 

smooth (LOWESS; 

Cleveland, 1985; 

Cleveland and McGill, 

1984) line for daily mean 

streamflow

Flow-adjusted concentration 

Flow-adjusted fitted trend 

Unadjusted (not flow 

adjusted) concentration

LOWESS line for unadjusted 

concentrations

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]
28 Water-Quality Trends and Constituent-Transport Analysis for Selected Sampling Sites

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 sampling 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 a relatively larger effect than 

expansion of the lower end of the distribution. This factor is 

important in interpreting trend results with respect to various 

regulatory issues, including compliance with human-health or 

aquatic-life standards. 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. Effects 

of data transformation, however, 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 sampling sites. In the trend analyses, all data 

(high as well as low values) affect changes in FAC geometric 

means; thus, the fitted trends appropriately represent unbiased 

estimates of overall changes in central tendency. 

Water-Quality Trends and ConstituentTransport Analysis Results

This section of the report presents water-quality trend 

and transport-analysis results for selected sampling sites in the 

data-summary reaches in the Milltown Reservoir/Clark Fork 

River Superfund Site for water years 1996–2015. Results are 

presented for all constituents investigated, but emphasis is 

placed on copper, arsenic, and suspended sediment in the following subsections. 

Water-Quality Trends in Flow-Adjusted 

Concentrations

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 appendix 3 in tables 3–1 (for 

most sampling sites) and 3–2 (for Clark Fork above Missoula 

[sampling site 22]). Detailed trend results are graphically presented in figures 3–1 through 3–7 in appendix 3. The detailed 

graphical presentations in appendix 3 present fitted trends for 

all constituents and allow evaluation of the fitted trends for a 

given sampling site in conjunction with FACs.

Fitted trend values (that quantify the temporal changes 

in FAC geometric means in terms of concentration units) are 

summarized in tables 6 (for most sampling sites) and 7 (for 

Clark Fork above Missoula [sampling site 22]) and graphically summarized in figures 5–10. The summary graphical 

presentations in figures 5–10 show side-by-side fitted trends 

for the adjacent sampling sites in a given reach and allow 

comparisons in temporal patterns between the reach inflow 

and outflow; these comparisons facilitate interpretation of the 

constituent-transport analysis results.

In this report, qualitative observations are described for 

the overall trend magnitude (percent change) from the start of 

period 1 to the end of period 4. Overall trend magnitude was 

considered to be (1) large, if the absolute value was greater 

than about 60 percent; (2) moderate, if the absolute value was 

in the range of about 40–60 percent; (3) small, if the absolute 

value was in the range of about 20–40 percent; and (4) minor, 

if the absolute value was less than about 20 percent. 

Trend-magnitude and fitted trend values are considered 

semiquantitative estimates determined by complex statistical 

analysis. Throughout this report, trend-magnitude and fitted 

trend values frequently are mentioned in figures, tables, and 

discussion of temporal and spatial changes in water quality 

(reported to two significant figures for all constituents except 

specific conductance, which is reported to three significant 

figures). Reference to specific trend-magnitude and fitted trend 

values is intended to facilitate presentation and discussion of 

relative spatial and temporal differences between values but is 

not intended to represent absolute accuracy at two significant 

figures. The p-values and levels of significance (a p-value less 

than 0.01 is considered statistically significant in this report) 

associated with the trend results are indicated in the tables and 

figures that present trend results. Significance levels were not 

the only factor in evaluating the substance of the trends, but 

rather were considered in conjunction with trend directions 

and relative magnitudes, and patterns among sites and constituents. 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 the 

sampling sites. Factors affecting temporal variability in water 

quality in the upper Clark Fork Basin are complex. Much 

information on changes in water quality is presented herein, 

but it is beyond the scope of this report to provide detailed 

explanations for all of the changes or to link specific trends 

with specific remediation activities. 
Water-Quality Trends and Constituent-Transport Analysis Results 29

Table 6. Summary of flow-adjusted trend results for selected sampling sites and constituents, water years 1996–2015.

[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. Gray shading indicates a statistically 

significant (p-value less than 0.01) trend for the trend period before the shaded value. p-value, statistical probability level; μS/cm, microsiemen per centimeter at 

25 degrees Celsius; μg/L, microgram per liter; mg/L, milligram per liter]

Constituent or property, flow-adjusted 

units of measurement

Fitted trend values

Percent change from 

start of 

period 1 

through end of 

period 41

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)

Start of 

water year 

2011 

(start of 

period 4)

End of 

water year 

2015 

(end of 

period 4)

Silver Bow Creek at Warm Springs, Montana (sampling site 8, fig. 1, table 1)

Specific conductance, μS/cm 521 514 501 513 446 -14

Copper, filtered, μg/L 8.9 4.6 4.1 3.8 2.9 -67

Copper, unfiltered-recoverable, μg/L 15 9.3 7.9 7.0 5.0 -67

Zinc, unfiltered-recoverable, μg/L 35 16 8.4 9.8 6.1 -83

Arsenic, filtered, μg/L 19 19 20 21 17 -11

Arsenic, unfiltered-recoverable, μg/L 22 22 23 23 19 -14

Suspended sediment, mg/L 5.3 6.3 4.6 2.7 3.1 -42

Clark Fork near Galen, Montana (sampling site 11, fig. 1, table 1)

Specific conductance, μS/cm 447 454 415 443 388 -13

Copper, filtered, μg/L 7.6 4.2 4.0 3.3 3.4 -55

Copper, unfiltered-recoverable, μg/L 15 11 11 11 8.1 -46

Zinc, unfiltered-recoverable, μg/L 30 13 9.0 12 7.1 -76

Arsenic, filtered, μg/L 12 11 13 10 11 -8

Arsenic, unfiltered-recoverable, μg/L 15 14 15 12 14 -7

Suspended sediment, mg/L 5.2 5.8 4.7 5.1 3.8 -27

Clark Fork at Deer Lodge, Montana (sampling site 14, fig. 1, table 1)

Specific conductance, μS/cm 479 482 463 454 456 -5

Copper, filtered, μg/L 6.9 5.8 6.1 5.4 5.8 -16

Copper, unfiltered-recoverable, μg/L 30 23 24 25 23 -23

Zinc, unfiltered-recoverable, μg/L 39 24 24 22 19 -51

Arsenic, filtered, μg/L 11 11 13 11 11 0

Arsenic, unfiltered-recoverable, μg/L 16 14 15 14 14 -13

Suspended sediment, mg/L 18 15 14 15 12 -33

Clark Fork at Goldcreek, Montana (sampling site 16, fig. 1, table 1)

Specific conductance, μS/cm 425 418 406 398 398 -6

Copper, filtered, μg/L 4.8 3.8 4.3 3.8 3.9 -19

Copper, unfiltered-recoverable, μg/L 19 19 15 14 15 -21

Zinc, unfiltered-recoverable, μg/L 27 20 13 15 13 -52

Arsenic, filtered, μg/L 9.4 8.2 8.8 8.6 8.2 -13

Arsenic, unfiltered-recoverable, μg/L 12 10 10 10 9.7 -19

Suspended sediment, mg/L 15 17 8.3 13 11 -27
30 Water-Quality Trends and Constituent-Transport Analysis for Selected Sampling Sites

Constituent or property, flow-adjusted 

units of measurement

Fitted trend values

Percent change from 

start of 

period 1 

through end of 

period 41

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)

Start of 

water year 

2011 

(start of 

period 4)

End of 

water year 

2015 

(end of 

period 4)

Clark Fork near Drummond, Montana (sampling site 18, fig. 1, table 1)

Specific conductance, μS/cm 461 459 449 434 461 0

Copper, filtered, μg/L 3.9 3.9 4.3 3.3 3.7 -5

Copper, unfiltered-recoverable, μg/L 17 15 14 13 12 -29

Zinc, unfiltered-recoverable, μg/L 36 19 15 17 13 -64

Arsenic, filtered, μg/L 9.6 9.0 9.4 8.4 8.6 -10

Arsenic, unfiltered-recoverable, μg/L 12 10 11 10 10 -17

Suspended sediment, mg/L 21 16 13 16 13 -38

Clark Fork at Turah Bridge near Bonner, Montana (sampling site 20, fig. 1, table 1)

Specific conductance, μS/cm 347 330 324 334 327 -6

Copper, filtered, μg/L 3.3 2.5 2.8 2.6 2.1 -36

Copper, unfiltered-recoverable, μg/L 10 9.0 8.3 8.2 7.9 -21

Zinc, unfiltered-recoverable, μg/L 21 13 9.2 14 9.7 -54

Arsenic, filtered, μg/L 5.4 5.1 5.4 5.5 4.7 -13

Arsenic, unfiltered-recoverable, μg/L 6.8 6.1 6.1 6.6 5.6 -18

Suspended sediment, mg/L 13 12 8.8 12 9.5 -27

1

Shading represents qualitative observations on overall trend magnitudes (percent change from start of water year 1996 to end of water year 2015) as follows: 

no shading—minor (the absolute value was less than about 20 percent); green shading—small (the absolute value was in the range of about 20–40 percent; tan 

shading—moderate (the absolute value was in the range of about 40–60 percent; and purple shading—large (the absolute value was greater than about 60 percent).

Table 6. Summary of flow-adjusted trend results for selected sampling sites and constituents, water years 1996–2015.—Continued

[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. Gray shading indicates a statistically 

significant (p-value less than 0.01) trend for the trend period before the shaded value. p-value, statistical probability level; μS/cm, microsiemen per centimeter at 

25 degrees Celsius; μg/L, microgram per liter; mg/L, milligram per liter]
Water-Quality Trends and Constituent-Transport Analysis Results 31

Table 7. Summary of flow-adjusted trend results for Clark Fork above Missoula, Montana (sampling site 22), for selected constituents, 

water years 1996–2015.

[Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. Gray shading indicates a statistically 

significant (p-value less than 0.01) trend for the trend period before the shaded value. p-value, statistical probability level; μS/cm, microsiemen per centimeter at 

25 degrees Celsius; μg/L, microgram per liter; mg/L, milligram per liter]

Constituent or property, 

flow-adjusted units 

of measurement

Fitted trend values

Percent change 

from start of 

period 1 through 

end of period 41

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)

Start of 

water year 

2011 

(start of 

period 4)

End of 

water year 

2015 

(end of 

period 4)

Clark Fork above Missoula, Montana (sampling site 22, fig. 1, table 1)

Specific conductance, μS/cm 277 275 270 273 283 265 -4

Copper, filtered, μg/L 2.3 1.7 2.1 2.4 1.9 1.4 -39

Copper, unfiltered-recoverable, μg/L 6.4 4.9 6.9 15 6.3 3.0 -53

Zinc, unfiltered-recoverable, μg/L 14 7.2 10 30 10 5.0 -64

Arsenic, filtered, μg/L 3.3 2.8 3.2 3.6 3.4 2.6 -21

Arsenic, unfiltered-recoverable, μg/L 4.2 3.3 3.9 4.8 4.0 3.0 -29

Suspended sediment, mg/L 7.7 7.4 9.2 25 9.9 6.0 -22

1

Shading represents qualitative observations on overall trend magnitudes (percent change from start of water year 1996 to end of water year 2015) as follows: 

no shading—minor (the absolute value was less than about 20 percent); green shading—small (the absolute value was in the range of about 20–40 percent; tan 

shading—moderate (the absolute value was in the range of about 40–60 percent; and purple shading—large (the absolute value was greater than about 60 percent).
32 Water-Quality Trends and Constituent-Transport Analysis for Selected Sampling Sites

Figure 5. Flow-adjusted fitted trends for selected constituents for sampling sites in reach 4, extending from Silver Bow Creek at Warm 

Springs, Montana (sampling site 8), to Clark Fork near Galen, Montana (sampling site 11), water years 1996–2015.

15

Reach inflow—Silver Bow 

Creek at Warm Springs 

(sampling site 8, fig. 1, 

table 1)

Reach outflow—Clark Fork 

near Galen (sampling 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 unfiltered-recoverable arsenic

concentration, in micrograms per liter

1995 2000 2005 2010 2015

1

Period

2 3 4

22 22 23 19

6.3

4.6

2.7

3.1

5.3

15

7.9

9.3

7.0

5.0

15 11

9.3

11 11

8.1

15 14 15 14

23

12

5.2

5.8 4.7

3.8

5.1

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]
Water-Quality Trends and Constituent-Transport Analysis Results 33

Figure 6. Flow-adjusted fitted trends for selected constituents for sampling sites in reach 5, extending from Clark Fork near Galen, 

Montana (sampling site 11), to Clark Fork at Deer Lodge, Montana (sampling site 14), water years 1996–2015.

15

11 11 11

8.1

15 14 15 14

12

5.2

5.8 4.7

3.8

5.1

30

11

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 unfiltered-recoverable arsenic

concentration, in micrograms per liter

1995 2000 2005 2010 2015

1 2 3 4

Period

15

16

14 15

14

15 14 12

18

24 25 23

30

23

14

Reach inflow—Clark Fork 

near Galen (sampling site 

11, fig. 1, table 1)

Reach outflow—Clark Fork at 

Deer Lodge (sampling site 

14, 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

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]
34 Water-Quality Trends and Constituent-Transport Analysis for Selected Sampling Sites

Figure 7. Flow-adjusted fitted trends for selected constituents for sampling sites in reach 6, extending from Clark Fork at Deer Lodge, 

Montana (sampling site 14), to Clark Fork at Goldcreek, Montana (sampling site 16), water years 1996–2015.

19 19

15 14 15

12

10 10 10 9.7

15

17

8.3

11

13

19

EXPLANATION

Reach inflow—Clark Fork at 

Deer Lodge (sampling site 

14, fig. 1, table 1)

Reach outflow—Clark Fork at 

Goldcreek (sampling 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

8.3

[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]

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 unfiltered-recoverable arsenic

concentration, in micrograms per liter

1995 2000 2005 2010 2015

1 2 3 4

Period

16

14 15

14

15

14 15

12

18

24 25

23

30

23

14
Water-Quality Trends and Constituent-Transport Analysis Results 35

Figure 8. Flow-adjusted fitted trends for selected constituents for sampling sites in reach 7, extending from Clark Fork at Goldcreek, 

Montana (sampling site 16), to Clark Fork near Drummond, Montana (sampling site 18), water years 1996–2015.

19 19

15

14

15

12

10 10 10 9.7

15

17

8.3

11

13

17

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 (sampling site 

16, fig. 1, table 1)

Reach outflow—Clark Fork 

near Drummond (sampling 

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

8.3

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 unfiltered-recoverable arsenic

concentration, in micrograms per liter

1995 2000 2005 2010 2015

1 2 3 4

Period

12 10 11

10

16

13

16

13

21

14

13 12

17

15

10
36 Water-Quality Trends and Constituent-Transport Analysis for Selected Sampling Sites

Figure 9. Flow-adjusted fitted trends for selected constituents for sampling sites in reach 8, extending from Clark Fork near Drummond, 

Montana (sampling site 18), to Clark Fork at Turah Bridge near Bonner, Montana (sampling site 20), water years 1996–2015.

10

9.0 8.3 8.2 7.9

6.8

6.1 6.1 5.6 6.6

13

12

8.8 9.5

12

10

[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]

EXPLANATION

Reach inflow—Clark Fork near 

Drummond (sampling site 

18, fig. 1, table 1)

Reach outflow—Clark Fork at 

Turah Bridge (sampling 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

10

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 unfiltered-recoverable arsenic

concentration, in micrograms per liter

1995 2000 2005 2010 2015

1 2 3 4

Period

12

10 11

10

16

13

16

13

21

14

13 12

17

15

10
Water-Quality Trends and Constituent-Transport Analysis Results 37

Figure 10. Flow-adjusted fitted trends for selected constituents for sampling sites in reach 9, extending from Clark Fork at Turah Bridge 

near Bonner, Montana (sampling site 20), to Clark Fork above Missoula, Montana (sampling site 22), water years 1996–2015.

A B

10

9.0 8.3 8.2 7.9

6.8

6.1 6.1 5.6 6.6

13

12

8.8

9.5

12

6.4

[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]

EXPLANATION

Reach inflow—Clark Fork near 

Turah Bridge (sampling site 

20, fig. 1, table 1)

Reach outflow—Clark Fork 

above Missoula (sampling 

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

25

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 unfiltered-recoverable arsenic

concentration, in micrograms per liter

1995 2000 2005 2010 2015

1 2 3 4

Period

4.2

3.3

3.9

3.0

7.4

9.2

9.9

6.0

7.7

6.9 6.3

15

4.8

25

3.0

6.4

4.9

4.0
38 Water-Quality Trends and Constituent-Transport Analysis for Selected Sampling Sites

Copper

Trend results indicate that FACs of unfiltered-recoverable 

copper decreased at the sampling sites from the start of period 

1 through the end of period 4 (tables 6 and 7); the decreases 

ranged from large for one sampling site (Silver Bow Creek at 

Warm Springs [sampling site 8]) to moderate for two sampling 

sites (Clark Fork near Galen [sampling site 11] and Clark Fork 

above Missoula [sampling site 22]) to small for four sampling 

sites (Clark Fork at Deer Lodge [sampling site 14], Clark Fork 

at Goldcreek [sampling site 16], Clark Fork near Drummond 

[sampling site 18], and Clark Fork at Turah Bridge [sampling 

site 20]). For period 4 (water years 2011–15), the most notable 

changes indicated for the Milltown Reservoir/Clark Fork River 

Superfund Site in the upper Clark Fork Basin were statistically 

significant decreases in FACs of unfiltered-recoverable copper 

for sampling sites 8 and 22. For all other sampling sites, the 

period 4 changes in FACs of unfiltered-recoverable copper 

were not statistically significant. 

Arsenic

Trend results indicate that FACs of unfiltered-recoverable 

arsenic decreased at the sampling sites from the start of 

period 1 through the end of period 4 (tables 6 and 7); the 

decreases ranged from minor for six sampling sites (sampling 

sites 8–20) to small for one sampling site (sampling site 22). 

For period 4 (water years 2011–15), the most notable changes 

indicated for the Milltown Reservoir/Clark Fork River 

Superfund Site in the upper Clark Fork Basin were statistically significant decreases in FACs of unfiltered-recoverable 

arsenic for sampling site 8 and near statistically significant 

decreases for sampling site 22; the p-value (0.012) for the 

period 4 decrease for sampling site 22 is not statistically significant but is only slightly larger than the selected alpha level 

(0.01 in this report). For all other sampling sites, the period 4 

changes in FACs of unfiltered-recoverable arsenic were not 

statistically significant.

Suspended Sediment

Trend results indicate that FACs of suspended sediment decreased at the sampling sites from the start of period 

1 through the end of period 4 (tables 6 and 7); the decreases 

ranged from moderate for one sampling site (sampling site 8) 

to small for six sampling sites (sampling sites 11–22). For 

period 4 (water years 2011–15), the changes in FACs of 

suspended sediment were not statistically significant for any 

sampling sites.

Overview of Water-Quality Trend Results

The most notable changes in water quality in period 4 

were indicated for Silver Bow Creek at Warm Springs (sampling site 8; reach 4 inflow) and Clark Fork above Missoula 

(sampling 22; reach 9 outflow). Trend results for sampling 

site 8 indicated more substantial changes than most other sampling sites; the decreases in specific conductance, unfilteredrecoverable copper, unfiltered-recoverable zinc, and unfilteredrecoverable arsenic were statistically significant (fig. 5 and 

3–1; tables 6 and 3–1). The most extensive remediation 

activities in the upper Clark Fork Basin have been conducted 

in the Silver Bow Creek Basin upstream from the reach 4 

inflow (sampling site 8). Sando and others (2014) noted that 

among the most notable changes indicated in the upper Clark 

Fork Basin during water years 1996–2010 were moderate to 

large decreases in FACs and loads of copper and suspended 

sediment in Silver Bow Creek upstream from Warm Springs. 

The period 4 (water years 2011–15) statistically significant 

decreases in FACs of unfiltered-recoverable copper and zinc 

provide indication that FACs of metallic contaminants continued to substantially decline at sampling site 8.

The removal of the former Milltown Dam, which was 

located between Clark Fork at Turah Bridge (sampling site 20; 

reach 9 inflow) and Clark Fork above Missoula (sampling 

site 22; reach 9 outflow), in 2008 was an important remediation activity in the upper Clark Fork Basin and strongly 

affected water-quality trends and transport characteristics 

within reach 9. As such, detailed discussion of trends is presented for reach 9. During periods 1 and 2, the former Milltown Dam was in place, and large amounts of contaminated 

sediments were retained in the former Milltown Reservoir in 

reach 9; however, the contaminated sediments largely were 

unavailable for mobilization and transport because of backwater effects of the former Milltown Dam (Sando and Lambing, 2011). Remediation activities preparing for the removal of 

the former Milltown Dam started in period 2 but were focused 

early in period 3 and included physical removal of large 

amounts of contaminated sediments; however, substantial 

amounts of contaminated sediments still remained in the Clark 

Fork channel and flood plain in reach 9. With the removal of 

the former Milltown Dam in 2008, the remaining contaminated sediments in reach 9 became more available for mobilization and transport than before the dam removal. Because 

of the substantial effect of the intentional breach of Milltown 

Dam on March 28, 2008, for sampling 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 statistically significant increase in FACs of unfilteredrecoverable copper is indicated for period 3A for sampling 

site 22 (117 percent, from 6.9 to 15 μg/L; table 7). The 

temporary increase in FACs is associated with activities that 

prepared for the removal of the Milltown Dam, including 

construction of roads and facilities, reservoir level drawdowns, 

and physical removal of large amounts of contaminated 

sediments, which likely increased mobilization of sediments 

enriched in trace elements (Sando and Lambing, 2011). After 

the intentional breach, statistically significant decreases were 

indicated for unfiltered-recoverable copper for period 3B 

(-58 percent, from 15 to 6.3 μg/L) and period 4 (-52 percent, 

from 6.3 to 3.0 μg/L). For unfiltered-recoverable arsenic, an 
Water-Quality Trends and Constituent-Transport Analysis Results 39

increase in FACs is indicated for period 3A (23 percent, from 

3.9 to 4.8 μg/L). After the intentional breach, a decrease is 

indicated for unfiltered-recoverable arsenic for period 3B 

(-17 percent, from 4.8 to 4.0 μg/L) and a near statistically 

significant decrease is indicated for period 4 (-25 percent, 

from 4.0 to 3.0 μg/L; p-value of 0.012). For suspended 

sediment, a statistically significant increase is indicated for 

period 3A (172 percent, from 9.2 to 25 mg/L). After the 

intentional breach, a statistically significant decrease for 

suspended sediment is indicated for period 3B (-60 percent, 

from 25 to 9.9 mg/L), and a decrease is indicated for period 4 

(-39 percent, from 9.9 to 6.0 mg/L). For period 4 (water years 

2011–15), trend results for the reach 9 outflow (sampling 

site 22) indicate more substantial changes than most other 

sampling sites; decreases in unfiltered-recoverable copper, 

unfiltered-recoverable zinc, and filtered arsenic were statistically significant. The p-value (0.012) for the period 4 decrease 

in FACs of unfiltered-recoverable arsenic for sampling site 22 

is not statistically significant but is only slightly larger than the 

selected alpha level (0.01 in this report).

The somewhat high streamflow conditions of period 4 

promoted mobilization of trace-element contaminants from 

the former Milltown Reservoir, thus decreasing within-reach 

source materials and resulting in lower FACs. The substantial decreases in FACs of unfiltered-recoverable copper for 

period 3B continued in period 4. Comparison of the period 4 

fitted trends for unfiltered-recoverable copper between the 

reach 9 inflow (sampling site 20) and the reach 9 outflow 

(sampling site 22) indicates large deviation from the start of 

to the end of period 4 (fig. 10A) and provides evidence of 

continued effects of the removal of the former Milltown Dam. 

Deviations in fitted trends between the period 4 reach inflow 

and reach outflow also are apparent for unfiltered-recoverable 

arsenic (fig. 10B) and suspended sediment (fig. 10C); however, 

the deviations are not as strong for those constituents as for 

unfiltered-recoverable copper.

Constituent-Transport Analysis Results

Estimated normalized loads are presented in 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 

are presented in table 8. Balance calculations for the transport analysis (that is, differences between reach inflows and 

reach outflows) are presented in tables 4–1 through 4–6 for 

reaches 4–9, respectively, in appendix 4. The transport balance calculations indicate within-reach changes in estimated 

normalized loads and allow assessment of temporal changes 

in relative contributions from upstream source areas to loads 

transported past each reach outflow. 

Hydrologic characteristics of the source areas (geometric mean streamflow; table 8) and balance results for 

the transport analysis are illustrated by using pie charts that 

show source-area information and load contributions to reach 

outflow. Pie charts illustrating temporal patterns in estimated 

normalized loads for all data-summary reaches are presented 

in figures 11–13 for unfiltered-recoverable copper, unfilteredrecoverable arsenic, and suspended sediment, respectively. 

The pie charts provide a side-by-side graphical summary 

for evaluating spatial and temporal variability in constituent 

transport relative to streamflow contributions in the Milltown 

Reservoir/Clark Fork River Superfund Site in the upper Clark 

Fork Basin. The estimated normalized loads (hereinafter 

referred to as “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 11–13, geometric mean streamflows (water 

years 1996–2015) for each reach are shown across the top 

of each figure, with the size (area) of each pie chart being 

proportional to the geometric mean streamflow for Clark 

Fork above Missoula (sampling site 22; reach 9 outflow). Pie 

charts that illustrate the constituent-transport analysis results 

for each reach for periods 1–4 are shown below the pie charts 

representing geometric mean streamflows. Pie charts illustrating loads are sized proportionally to the period 1 reach 9 

outflow load. The period 1 reach 9 outflow load was selected 

as an index for sizing the pie charts because it represents the 

total load transported from the Milltown Reservoir/Clark Fork 

River Superfund Site somewhat near the start of remediation activities. As such, the period 1 reach 9 outflow load is a 

useful index in evaluating effects of remediation in the upper 

Clark Fork Basin.

Figure 11 presents pie charts representing loads for 

unfiltered-recoverable copper and serves as an example 

for explaining the presentation of the constituent-transport 

analysis results. The size (area) of each loads pie chart represents the total outflow from the reach, with colored areas 

indicating relative contributions from each of the two source 

areas; that is, (1) the reach inflow and (2) the intervening 

drainage between the reach inflow and outflow (or withinreach sources). The left-hand column of the load pie charts 

presents results for reach 4 for periods 1–4. The period 1 

load transported past the reach 4 outflow (sampling site 11) 

is 3.7 kilograms per day (kg/d), which is 13 percent of the 

period 1 load transported past the reach 9 outflow (29 kg/d 

at sampling site 22 shown in right-hand column); thus, the 

size of the period 1 reach 4 pie chart is 13 percent of the size 

of the period 1 reach 9 pie chart. The blue-colored part of 

the period 1 reach 4 pie chart represents the load (1.9 kg/d) 

transported past the reach 4 inflow (sampling site 8). The 

orange-colored part of the period 1 reach 4 pie chart represents 

the total within-reach change in load (that is, net mobilization 

from all within-reach sources including groundwater inflow, 

tributaries, the main-stem channel, and flood plain). The total 

within-reach change in load (1.8 kg/d) was calculated by 

subtracting the reach inflow (1.9 kg/d) from the reach outflow (3.7 kg/d). In figure 11, results for reach 9 are not shown 

for period 3 because of effects of the removal of the former 
40 Water-Quality Trends and Constituent-Transport Analysis for Selected Sampling Sites

Milltown Dam and difficulties in presenting those results in 

conjunction with results for other reaches. 

Constituent-transport analysis results are described for 

copper, arsenic, and suspended sediment in the following 

subsections. 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 a 

reach is proportionally much larger than the contribution of 

streamflow from within a reach, the given reach is indicated to 

be an important disproportionate source of constituent loading. 

Conversely, if the contribution of a constituent from within a 

reach is proportionally smaller than or similar to the contribution of streamflow from within a reach, the given reach is not 

indicated to be an important disproportionate source of constituent loading and generally acts as a flow-through reach.

Table 8. Drainage area and streamflow information relevant to the transport analysis for data-summary reaches in the Milltown 

Reservoir/Clark Fork River Superfund Site in the upper Clark Fork Basin, Montana, water years 1996–2015.

[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 foot per second]

Abbreviated sampling site name (table 1) 

and number or summation category

Drainage area, 

in square miles

Geometric mean 

streamflow, 

water years 

1996–2015, 

in ft3/s

Reach 4

[extending about 2 river miles from Silver Bow Creek at Warm Springs (sampling site 8, fig. 1, table 1)

to Clark Fork near Galen (sampling site 11, fig. 1, table 1)]

Inflow

Silver Bow Creek at Warm Springs (sampling site 8) 473 64

Outflow

Clark Fork near Galen (sampling site 11) 651 118

Within-reach change—outflow (sampling site 11) minus inflow (sampling site 8)

(contributions from all within-reach sources, including groundwater inflow and tributaries) 178 54

Reach 5

[extending about 21 river miles from Clark Fork near Galen (sampling site 11, fig. 1, table 1)

to Clark Fork at Deer Lodge (sampling site 14, fig. 1, table 1)]

Inflow

Clark Fork near Galen (sampling site 11) 651 118

Outflow

Clark Fork at Deer Lodge (sampling site 14) 995 208

Within-reach change—outflow (sampling site 14) minus inflow (sampling site 11)

(contributions from all within-reach sources, including groundwater inflow and tributaries) 344 90

Reach 6

[extending about 26 river miles from Clark Fork at Deer Lodge (sampling site 14, fig. 1, table 1)

to Clark Fork at Goldcreek (sampling site 16, fig. 1, table 1)]

Inflow

Clark Fork at Deer Lodge (sampling site 14) 995 208

Outflow

Clark Fork at Goldcreek (sampling site 16) 1,704 406

Within-reach change—outflow (sampling site 16) minus inflow (sampling site 14)

(contributions from all within-reach sources, including groundwater inflow and tributaries) 709 198
Water-Quality Trends and Constituent-Transport Analysis Results 41

Abbreviated sampling site name (table 1) 

and number or summation category

Drainage area, 

in square miles

Geometric mean 

streamflow, 

water years 

1996–2015, 

in ft3/s

Reach 7

[extending about 31 river miles from Clark Fork at Goldcreek (sampling site 16, fig. 1, table 1)

to Clark Fork near Drummond (sampling site 18, fig. 1, table 1)]

Inflow

Clark Fork at Goldcreek (sampling site 16) 1,704 406

Outflow

Clark Fork near Drummond (sampling site 18) 2,501 589

Within-reach change—outflow (sampling site 18) minus inflow (sampling site 16)

(contributions from all within-reach sources, including groundwater inflow and tributaries) 797 183

Reach 8

[extending about 34 river miles from Clark Fork near Drummond (sampling site 18, fig. 1, table 1)

to Clark Fork at Turah Bridge (sampling site 20, fig. 1, table 1)]

Inflow

Clark Fork near Drummond (sampling site 18) 2,501 589

Outflow

Clark Fork at Turah Bridge (sampling site 20) 3,641 1,060

Within-reach change—outflow (sampling site 20) minus inflow (sampling site 18)

(contributions from all within-reach sources, including groundwater inflow and tributaries) 1,140 470

Reach 9

[extending about 9 river miles from Clark Fork at Turah Bridge (sampling site 20, fig. 1, table 1)

to Clark Fork above Missoula (sampling site 22, fig. 1, table 1)]

Inflow

Clark Fork at Turah Bridge (sampling site 20) 3,641 1,060

Outflow

Clark Fork above Missoula (sampling site 22) 5,999 2,100

Within-reach change—outflow (sampling site 22) minus inflow (sampling site 20) 

(contributions from all within-reach sources, including groundwater inflow and tributaries) 2,358 1,040

Table 8. Drainage area and streamflow information relevant to the transport analysis for data-summary reaches in the Milltown 

Reservoir/Clark Fork River Superfund Site in the upper Clark Fork Basin, Montana, water years 1996–2015.—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 foot per second]
42 Water-Quality Trends and Constituent-Transport Analysis for Selected Sampling Sites

Figure 11. Pie charts representing geometric mean streamflow and estimated normalized unfiltered-recoverable copper loads contributed from reach inflow and within-reach sources for data-summary reaches for selected periods.

Reach 9 Reach 4 Reach 5 Reach 6 Reach 7 Reach 8

1.9

1.8

6454

11890

208198

406

183

589470

1,0601,040

1.4

1.8

1.2

2.0

0.94

1.8

3.7

9.8

3.1

9.0

3.2

9.4

2.7

9.4

13

5.4

12

4.6

13

2.2

12

2.7

19

4.6

17

4.1

15

4.3

15

2.9

24

1.6

21

1.5

19

2.3

18

3.2

25

3.7

22

7.7

21

2.2

29 25 24 19 13 3.7

22 30 21 17 12 3.1

21 19 15 13 3.2

21 18 15 12 2.7 23

118 2,100 1,060 589 406 208

Geometric mean streamflow, in cubic feet per second (water years 1996–2015)

EXPLANATION

[estimated normalized load pie charts]

EXPLANATION

[streamflow pie charts]

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 1996–2015 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." 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.

Reach 9 pie chart for period 3 not shown because of effects of removal of the former Milltown Dam and difficulties in presentation in conjunction with results for other reaches.

Reach inflow = 21 kg/dWithin-reach change = 32 kg/dReach outlfow = 54 kg/d

Estimated normalized unfiltered-recoverable copper load,1 in kilograms per day (kg/d)

Water years1996–2000(period 1)Water years2001–5(period 2)Water years2006–10(period 3)

Circular pie chart represents geometricmean streamflowat reach outflow Size (area) of circular pie charts is proportional to geometric mean streamflow for Clark Fork above Missoula, Montana(sampling site 22)

Water years2011–15(period 4)

Value

Estimated normalized load, in kilograms per day Value

—Load 

 transported  past reach outflow,  in kilograms per day

Total within-reach change in load (net mobilization from all within-reach sources)

Reach inflow

Value—

Value

Geometric meanstreamflow, in cubicfeet per second

All within-reach sources Geometric mean streamflow,

in cubic feet per second,at reach outflow

Reach inflow

Circular pie charts represent loadstransported pastreach outflow Size (area) of circular pie charts is proportional to period 1 estimated normalized load for Clark Fork above Missoula, Montana (sampling site 22) 
Water-Quality Trends and Constituent-Transport Analysis Results 43 Figure 12. Pie charts representing geometric mean streamflow and estimated normalized unfiltered-recoverable arsenic loads contributed from reach inflow and within-reach sources for data-summary reaches for selected periods.

Reach 9 Reach 4 Reach 5 Reach 6 Reach 7 Reach 8

6454

11890

208198

406

183

589470

1,0601,040

3.4

0.78

3.5

0.70

3.6

0.31

0.46 3.3

4.2

3.5

4.2

3.4

3.9

3.5

3.8

3.3

7.7

3.5

7.6

2.6

7.4

2.8

7.0

2.8

11

5.2

10

5.0

10

4.8

9.9

4.6

16

0.49

15

0.58

15

1.5

14

1.3

17

2.5

16

2.6

16

5.9

16

2.1

19

18

22

17 16 11 7.7 4.2

16 15 10 7.6 4.2

16 15 10 7.4 3.9

16 14 9.9 7.0 3.8 18

118 2,100 1,060 589 406 208

Geometric mean streamflow, in cubic feet per second (water years 1996–2015)

EXPLANATION

[estimated normalized load pie charts]

EXPLANATION

[streamflow pie charts]

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 1996–2015 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." 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.

Circular pie chart represents geometricmean streamflowat reach outflow Size (area) of circular pie charts is proportional to geometric mean streamflow for Clark Fork above Missoula, Montana(sampling site 22)

Value

Estimated normalized load, in kilograms per day Value—Load 

 transported  past reach outflow,  in kilograms per day

Total within-reach change in load (net mobilization from all within-reach sources)

Reach inflow

Value—

Value

Geometric meanstreamflow, in cubicfeet per second

All within-reach sources Geometric mean streamflow,

in cubic feet per second,at reach outflow

Reach inflow

Circular pie charts represent loadstransported pastreach outflow Size (area) of circular pie charts is proportional to period 1 estimated normalized load for Clark Fork above Missoula, Montana (sampling site 22) 

Estimated normalized unfiltered-recoverable arsenic load,1 in kilograms per day (kg/d)

Water years1996–2000(period 1)Water years2001–5(period 2)Water years2006–10(period 3)Water years2011–15(period 4)
44 Water-Quality Trends and Constituent-Transport Analysis for Selected Sampling Sites

Figure 13. Pie charts representing geometric mean streamflow and estimated normalized suspended-sediment loads contributed from reach inflow and within-reach sources for data-summary reaches for selected periods.

Reach 9 Reach 4 Reach 5 Reach 6 Reach 7 Reach 8

6454

11890

208198

406

183

589470

1,0601,040

920670850670570860460820

1,600

6,700

1,500

5,700

1,400

5,800

1,300

5,500

8,300

7,500

7,200

5,000

7,200

3,200

6,800

4,900

16,000

10,000

12,000

8,300

10,000

10,000

12,000

9,100

26,000

6,300

21,000

5,900

21,000

5,800

21,000

6,900

33,000

6,000

28,000

12,000

26,000

16,000

39,000

42,000

33,000 26,000 16,000 8,300 1,600

26,000 21,000 12,000 7,200 1,500

27,000 21,000 10,000 7,200 1,400

28,000 21,000 12,000 6,800 1,300 40,000

118 2,100 1,060 589 406 208

Geometric mean streamflow, in cubic feet per second (water years 1996–2015)

EXPLANATION

[estimated normalized load pie charts]

EXPLANATION

[streamflow pie charts]

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 1996–2015 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." 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.

Circular pie chart represents geometricmean streamflowat reach outflow Size (area) of circular pie charts is proportional to geometric mean streamflow for Clark Fork above Missoula, Montana(sampling site 22)

Value

Estimated normalized load, in kilograms per day Value

—Load 

 transported  past reach outflow,  in kilograms per day

Total within-reach change in load (net mobilization from all within-reach sources)

Reach inflow

Value—

Value

Geometric meanstreamflow, in cubicfeet per second

All within-reach sources Geometric mean streamflow,

in cubic feet per second,at reach outflow

Reach inflow

Circular pie charts represent loadstransported pastreach outflow Size (area) of circular pie charts is proportional to period 1 estimated normalized load for Clark Fork above Missoula, Montana (sampling site 22) 

Reach 9 pie chart for period 3 not shown because of effects of removal of the former Milltown Dam and difficulties in presentation in conjunction with results for other reaches.

Reach inflow = 27,000 kg/dWithin-reach change = 56,000 kg/dReach outlfow = 83,000 kg/d

Estimated normalized suspended-sediment load,1 in kilograms per day (kg/d)

Water years1996–2000(period 1)

Water years2001–5(period 2)

Water years2006–10(period 3)Water years2011–15(period 4)
Water-Quality Trends and Constituent-Transport Analysis Results 45

Copper

The transport-analysis results indicate that outflow loads 

of unfiltered-recoverable copper decreased from the center of 

period 1 through the center of period 4 for all reaches (fig. 11). 

The largest decrease was for the reach 4 outflow load (about 

-27 percent, from 3.7 to 2.7 kg/d). The decrease in the reach 

4 outflow load (sampling site 11) largely was because of a 

substantial decrease (-50 percent, from 1.9 to 0.94 kg/d) in the 

reach 4 inflow load (sampling site 8), with little change indicated for within-reach sources. The smallest decrease was for 

the reach 5 outflow load (about -8 percent from 13 to 12 kg/d). 

Decreases in outflow loads for the other reaches (reaches 6–9) 

ranged from about -16 to -25 percent. 

Contributions of unfiltered-recoverable copper from reach 

4 sources were proportionally similar to or slightly larger 

than streamflow contributions from within reach 4 (fig. 11, 

tables 8 and 4–1) for all periods, and thus reach 4 is somewhat 

indicated to be a disproportionate source of copper loading. 

However, the period 4 net mobilization from sources within 

reach 4 (1.8 kg/d) was only about 8 percent of the period 4 

reach 9 outflow load (Clark Fork above Missoula, sampling 

site 22; 23 kg/d). Contributions of unfiltered-recoverable copper from reach 5 sources were proportionally much larger than 

streamflow contributions from within reach 5 for all periods; 

the period 4 net mobilization from sources within reach 5 

(9.4 kg/d) accounted for a substantial part (about 41 percent) 

of the period 4 reach 9 outflow load. Thus, reach 5 is indicated 

to be an important disproportionate source of copper loading. 

Contributions of unfiltered-recoverable copper from sources 

within the other reaches (reaches 6–9) were proportionally 

smaller than the within-reach streamflow contributions.

The removal of the former Milltown Dam in 2008 warrants more detailed discussion of transport analysis results 

for reach 9. The segregation of period 3 into periods 3A and 

3B for the reach 9 outflow (sampling site 22) is not directly 

incorporated into the transport analysis for reach 9; thus, the 

transport-analysis balance calculations for period 3 reflect 

the net changes in transport characteristics before and after 

the removal of the former Milltown Dam. For unfilteredrecoverable copper (fig. 11), the reach 9 outflow load (sampling site 22) decreased by about 21 percent from the center 

of period 1 (29 kg/d) to the center of period 4 (23 kg/d). Net 

mobilization from sources within reach 9 increased between 

periods 1 and 2 and also between periods 2 and 3 (fig. 11). 

Net mobilization from sources within reach 9 substantially 

decreased between periods 3 and 4. Net mobilization from 

sources within reach 9 were proportionally larger than 

streamflow contributions from within reach 9 for period 3 but 

were proportionally smaller than streamflow contributions 

for the other periods. Net mobilization from sources within 

reach 9 were smaller for period 4 (2.2 kg/d) than for period 1 

(3.7 kg/d).

Arsenic

The transport-analysis results indicate that outflow loads 

of unfiltered-recoverable arsenic decreased from the center of 

period 1 through the center of period 4 for all reaches (fig. 12). 

Decreases in outflow loads for the reaches ranged from about 

-5 to -12 percent. Temporal decreases in unfiltered-recoverable 

arsenic were smaller than copper and suspended sediment, 

which probably reflects the dispersion and solubility characteristics of arsenic.

At the upstream end of the Milltown Reservoir/Clark 

Fork River Superfund site, the reach 4 inflow load is a 

disproportionate source of arsenic loading, with the inflow 

load being proportionally larger than the streamflow (fig. 12, 

tables 8 and 4–1). Contributions of unfiltered-recoverable 

arsenic from reach 4 sources were proportionally smaller 

than streamflow contributions from within reach 4 for all 

periods. Downstream from reach 4, contributions of unfilteredrecoverable arsenic from sources within reaches 5 and 7 were 

proportionally similar to within-reach streamflow contributions. Contributions of unfiltered-recoverable arsenic from 

sources within the other reaches (reaches 6, 8, and 9) were 

proportionally smaller than the within-reach streamflow 

contributions.

For unfiltered-recoverable arsenic (fig. 12), the reach 9 

outflow load (sampling site 22) decreased by about 5 percent 

from the center of period 1 (19 kg/d) to the center of period 4 

(18 kg/d). Net mobilization from sources within reach 9 

increased between periods 2 and 3 (fig. 12). Net mobilization 

from sources within reach 9 substantially decreased between 

periods 3 and 4. Contributions of unfiltered-recoverable 

arsenic from reach 9 sources were proportionally smaller than 

streamflow contributions from within reach 9 for all periods. 

Net mobilization from sources within reach 9 were slightly 

smaller for period 4 (2.1 kg/d) than for period 1 (2.5 kg/d).

Suspended Sediment

The transport-analysis results indicate that outflow loads 

of suspended sediment decreased from the center of period 

1 through the center of period 4 for reaches 4–8 but slightly 

increased for reach 9 (fig. 13). Decreases in outflow loads for 

reaches 6–8 ranged from about -15 to -25 percent. 

Contributions of suspended sediment from reach 4 

sources were proportionally similar to or slightly larger than 

streamflow contributions from within reach 4 (fig. 13, tables 8 

and 4–1) for all periods, and thus, reach 4 is somewhat indicated to be a disproportionate source of suspended-sediment 

loading. However, the period 4 net mobilization from sources 

within reach 4 (820 kg/d) was only about 2 percent of the 

period 4 reach 9 outflow load (Clark Fork above Missoula, 

sampling site 22; 40,000 kg/d). Contributions of suspended 

sediment from reach 5 sources were proportionally much 

larger than streamflow contributions from within reach 5; 

the period 4 net mobilization from sources within reach 5 

(5,500 kg/d) accounted for about 14 percent of the period 4 
46 Water-Quality Trends and Constituent-Transport Analysis for Selected Sampling Sites

reach 9 outflow load. Thus, reach 5 is indicated to be a disproportionate source of suspended-sediment loading. Downstream 

from reach 5, contributions of sediment from sources within 

reach 7 were proportionally similar to within-reach streamflow contributions; the period 4 net mobilization from sources 

within reach 7 (9,100 kg/d) accounted for about 23 percent of 

the period 4 reach 9 outflow load. Contributions of suspended 

sediment from sources within the other reaches (reaches 6, 

8, and 9) were proportionally smaller than the within-reach 

streamflow contributions.

For suspended sediment (fig. 13), the reach 9 outflow 

load (sampling site 22) increased by about 3 percent from 

the center of period 1 (39,000 kg/d) to the center of period 4 

(40,000 kg/d). Net mobilization from sources within reach 9 

increased between periods 1 and 2 and also between periods 2 

and 3 (fig. 13). Net mobilization from sources within reach 9 

substantially decreased between periods 3 and 4. Net mobilization from sources within reach 9 was proportionally larger 

than streamflow contributions from within reach 9 for period 

3 but was proportionally smaller than streamflow contributions for the other periods. Net mobilization from sources 

within reach 9 were larger for period 4 (12,000 kg/d) than 

for period 1 (6,000 kg/d). The increase in net mobilization 

of suspended sediment from sources within reach 9 between 

periods 1 and 4 is in contrast to decreases in net mobilization 

of unfiltered-recoverable copper and arsenic between periods 1 

and 4. A possible explanation for this pattern might relate to 

flood-plain disturbance and placement of uncontaminated fill 

in the flood plain associated with remediation activities. The 

artificially installed uncontaminated fill might be more available for mobilization than sediment within the former Milltown Reservoir during period 1. 

Overview of Constituent-Transport Analysis 

Results

At the upstream end of the Milltown Reservoir/Clark 

Fork River Superfund site, the reach 4 inflow had substantial decreases from the center of period 1 to the center of 

period 4 in unfiltered-recoverable copper and suspendedsediment loads (about -50 percent for both constituents), but 

the reach 4 inflow accounts for small parts of the streamflow 

(about 3 percent), unfiltered-recoverable copper load (about 

4 percent), and suspended-sediment load (about 1 percent) of 

the reach 9 outflow in period 4 (figs. 11 and 13). The reach 4 

inflow is a disproportionate source of unfiltered-recoverable 

arsenic and accounts for about 18 percent of the reach 9 

outflow load in period 4 (fig. 12). Some downstream reaches 

(including reaches 5 and 7) have within-reach contributions of 

unfiltered-recoverable arsenic that are proportionally similar 

to streamflow contributions and also substantially contribute 

to the reach 9 outflow load. For all reaches, temporal changes 

for unfiltered-recoverable arsenic loads are smaller than for 

unfiltered-recoverable copper and suspended-sediment loads.

Reach 5 is a large source of unfiltered-recoverable copper 

and suspended sediment, which strongly affects downstream 

transport of those constituents (figs. 11 and 13). Mobilization 

of unfiltered-recoverable copper and suspended sediment from 

flood-plain tailings and the streambed of the Clark Fork and its 

tributaries within reach 5 results in a 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, unfiltered-recoverable copper loads in the Clark Fork 

increased by a factor of about 4 and suspended-sediment loads 

increased by a factor of about 5, whereas streamflow increased 

by a factor of slightly less than 2 (fig. 11). For period 4 (water 

years 2011–15), unfiltered-recoverable copper and suspendedsediment loads sourced from within reach 5 accounted for 

about 41 and 14 percent, respectively, of the loads at Clark 

Fork above Missoula (sampling site 22), whereas streamflow 

sourced from within the reach accounted for about 4 percent 

of the streamflow at sampling site 22. During water years 

1996–2015, decreases in unfiltered-recoverable copper and 

suspended-sediment loads (fig. 11 and 13) for the reach 5 

outflow and for sources within reach 5 generally were proportionally smaller than for most other reaches.

For the reaches downstream from reach 5 (reaches 6–8), 

contributions of copper loads sourced from within the reaches 

were proportionally smaller than contributions of streamflow 

sourced from within the reaches (fig. 11); thus, the lower 

reaches contributed proportionally much less than reach 5 

to unfiltered-recoverable copper loading in the Clark Fork. 

Although substantial decreases in unfiltered-recoverable 

copper and suspended-sediment loads were indicated for the 

reach 4 inflow (sampling site 8), those substantial decreases 

were not translated to the downstream reaches (reaches 5–8). 

The effect of reach 5 as a large source of unfilteredrecoverable copper and suspended sediment, in combination 

with little temporal change in those constituents for the reach 5 

outflow, contributes to this pattern.

For unfiltered-recoverable copper, unfiltered-recoverable 

arsenic, and suspended sediment, contributions from within 

reach 8 generally increased between periods 2 and 4; this 

pattern is in contrast to patterns for most other reaches. 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, backwater effects of 

the dam during high-flow conditions might have extended far 

enough upstream to affect the hydraulic gradient at the reach 8 

outflow (sampling site 20) and also affect the transport of 

materials from reach 8. After the removal of the former Milltown Dam, the hydraulic gradient at sampling site 20 might 

have steepened and promoted transport of materials from 

reach 8 during high streamflow conditions.

With the removal of the former Milltown Dam in 2008, 

substantial amounts of contaminated sediments that remained 

in the Clark Fork channel and flood plain in reach 9 became 

more available for mobilization and transport than before 

the dam removal. Net mobilization of unfiltered-recoverable 
Summary and Conclusions 47

copper, unfiltered-recoverable arsenic, and suspended sediment from sources within reach 9 substantially decreased 

between periods 3 and 4. Net mobilization of unfilteredrecoverable copper and arsenic from sources within reach 9 

is smaller for period 4 than for period 1 when the former 

Milltown Dam was in place, providing evidence that contaminant source materials have been substantially reduced in 

reach 9. However, net mobilization of suspended sediment 

from sources within reach 9 were slightly larger for period 4 

than for period 1. A possible explanation for this pattern might 

relate to flood-plain disturbance and placement of uncontaminated fill in the flood plain associated with remediation 

activities. The artificially installed uncontaminated fill might 

be more available for mobilization than sediment within the 

former Milltown Reservoir during period 1.

Summary and Conclusions

This report characterizes temporal trends in flow-adjusted 

concentrations (filtered and unfiltered) of mining-related 

contaminants and assesses those trends in the context of 

source areas and transport of those contaminants through the 

Milltown Reservoir/Clark Fork River Superfund Site in the 

upper Clark Fork Basin in Montana. The Milltown Reservoir/

Clark Fork River Superfund Site extends about 123 river miles 

from the outlet of Warm Springs Ponds on Silver Bow Creek 

to the outlet of the former Milltown Reservoir near Missoula. 

Trend analysis was done on specific conductance, selected 

trace elements (arsenic, copper, and zinc), and suspended sediment by using a joint time-series model (TSM) for concentration and streamflow for seven sampling sites for water years 

1996–2015. The most upstream site included in trend analysis 

is Silver Bow Creek at Warm Springs, Montana (sampling 

site 8), and the most downstream site is Clark Fork above Missoula, Montana (sampling site 22), which is just downstream 

from the former Milltown Dam.

During the extended history of mining in the upper Clark 

Fork Basin in Montana, 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 near Butte, and the milling and 

smelting operations near Anaconda. Extensive deposition of 

mining wastes in the Silver Bow Creek and Clark Fork channels and flood plains had substantial effects on water quality. 

Federal Superfund remediation activities in the upper Clark 

Fork Basin began in 1983 and have included substantial remediation near Butte and removal of the former Milltown Dam. 

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

Survey (USGS) in 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 (2016). A previous study analyzed 

the monitoring data and characterized flow-adjusted trends in 

mining-related contaminants for 22 sampling sites in the upper 

Clark Fork Basin for 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). An update of 

flow-adjusted water-quality trends for the monitoring data was 

needed for seven sampling sites to provide timely information 

for the 2016 5-year review for the Milltown Reservoir/Clark 

Fork River Superfund Site. 

The TSM was used to detect trends in flow-adjusted concentrations (FACs). The intent of flow-adjustment is to identify and remove streamflow-related variability in concentration 

and thereby enhance the capability to detect trends independent from effects of climatic variability. To provide temporal 

resolution of changes in water quality, trend analysis was conducted on four sequential 5-year periods: period 1 (water years 

1996–2000), period 2 (water years 2001–5), period 3 (water 

years 2006–10), and period 4 (water years 2011–15). Because 

of the substantial effect of the intentional breach of Milltown 

Dam on March 28, 2008, for Clark Fork above Missoula (sampling 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 was applied as consistently as 

possible among sampling 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. 

In conjunction with the trend analysis, estimated normalized constituent loads were calculated and presented in the 

framework of a constituent-transport analysis to assess the 

temporal trends in FACs in the context of sources and transport. The transport analysis allows assessment of temporal 

changes in relative contributions from upstream source areas 

to loads transported past each reach outflow. 

Trend results are presented for all constituents investigated; however, emphasis is placed on copper, arsenic, and 

suspended sediment. Trend results were considered statistically significant when the statistical probability level (p-value) 

was less than 0.01. 

Trend results indicate that FACs of unfiltered-recoverable 

copper decreased at the sampling sites from the start of 

period 1 through the end of period 4; the decreases ranged 

from large for one sampling site (Silver Bow Creek at Warm 

Springs [sampling site 8]) to moderate for two sampling sites 

(Clark Fork near Galen, Montana [sampling site 11] and Clark 

Fork above Missoula [sampling site 22]) to small for four 

sampling sites (Clark Fork at Deer Lodge, Montana [sampling 

site 14], Clark Fork at Goldcreek, Montana [sampling site 16], 

Clark Fork near Drummond, Montana [sampling site 18], and 

Clark Fork at Turah Bridge near Bonner, Montana [sampling 

site 20]). For period 4 (water years 2011–15), the most notable 

changes indicated for the Milltown Reservoir/Clark Fork 

River Superfund Site in the upper Clark Fork Basin were statistically significant decreases in FACs and loads of unfilteredrecoverable copper for sampling sites 8 and 22. For all other 

sampling sites, the period 4 changes in FACs of unfilteredrecoverable copper were not statistically significant.
48 Water-Quality Trends and Constituent-Transport Analysis for Selected Sampling Sites

Trend results indicate that FACs of unfiltered-recoverable 

arsenic decreased at the sampling sites from the start of 

period 1 through the end of period 4; the decreases ranged 

from minor (sampling sites 8–20) to small (sampling site 22). 

For period 4 (water years 2011–15), the most notable changes 

indicated for the Milltown Reservoir/Clark Fork River Superfund Site in the upper Clark Fork Basin were statistically significant decreases in FACs and loads of unfiltered-recoverable 

arsenic for sampling site 8 and near statistically significant 

decreases (p-value of 0.012) for sampling site 22. For all other 

sampling sites, the period 4 changes in FACs of unfilteredrecoverable arsenic were not statistically significant.

Trend results indicate that FACs of suspended sediment 

decreased at the sampling sites from the start of period 1 

through the end of period 4; the decreases ranged from 

moderate (sampling site 8) to small (sampling sites 11–22). 

For period 4 (water years 2011–15), the changes in FACs of 

suspended sediment were not statistically significant for any 

sampling sites.

The reach of the Clark Fork from Galen to Deer Lodge 

is a large source of metallic contaminants and suspended 

sediment, which strongly affects downstream transport of 

those constituents. Mobilization of unfiltered-recoverable 

copper and suspended sediment from flood-plain tailings and 

the streambed of the Clark Fork and its tributaries within the 

reach results in a contribution of those constituents that is 

proportionally much larger than the contribution of streamflow 

from within the reach. Within the reach, unfiltered-recoverable 

copper loads increased by a factor of about 4 and suspendedsediment loads increased by a factor of about 5, whereas 

streamflow increased by a factor of slightly less than 2. For 

period 4 (water years 2011–15), unfiltered-recoverable copper and suspended-sediment loads sourced from within the 

reach accounted for about 41 and 14 percent, respectively, of 

the loads at Clark Fork above Missoula (sampling site 22), 

whereas streamflow sourced from within the reach accounted 

for about 4 percent of the streamflow at sampling site 22. 

During water years 1996–2015, decreases in FACs and loads 

of unfiltered-recoverable copper and suspended sediment for 

the reach generally were proportionally smaller than those for 

most other reaches.

Unfiltered-recoverable copper loads sourced within the 

reaches of the Clark Fork between Deer Lodge and Turah 

Bridge near Bonner were proportionally smaller than contributions of streamflow sourced from within the reaches; 

these reaches contributed proportionally much less to copper 

loading in the Clark Fork than the reach between Galen and 

Deer Lodge. Although substantial decreases in FACs and 

loads of unfiltered-recoverable copper and suspended sediment were indicated for Silver Bow Creek at Warm Springs 

(sampling site 8), those substantial decreases were not 

translated to downstream reaches between Deer Lodge and 

Turah Bridge near Bonner. The effect of the reach of the Clark 

Fork from Galen to Deer Lodge as a large source of copper 

and suspended sediment, in combination with little temporal 

change in those constituents for the reach, contributes to this 

pattern.

With the removal of the former Milltown Dam in 2008, 

substantial amounts of contaminated sediments that remained 

in the Clark Fork channel and flood plain in reach 9 became 

more available for mobilization and transport than before 

the dam removal. After the removal of the former Milltown 

Dam, the Clark Fork above Missoula (sampling site 22) 

had statistically significant decreases in FACs of unfilteredrecoverable copper in period 3B (March 28, 2008, through 

water year 2010) that continued in period 4 (water years 

2011–15). Also, decreases in FACs of unfiltered-recoverable 

arsenic and suspended sediment were indicated for period 4 

at this site. The decrease in FACs of unfiltered-recoverable 

copper for sampling site 22 during period 4 was proportionally much larger than the decrease for the Clark Fork at Turah 

Bridge near Bonner (sampling site 20). Net mobilization of 

unfiltered-recoverable copper, unfiltered-recoverable arsenic, 

and suspended sediment from sources within reach 9 substantially decreased between periods 3 and 4. Net mobilization of 

unfiltered-recoverable copper and arsenic from sources within 

reach 9 were smaller for period 4 than for period 1 when the 

former Milltown Dam was in place, providing evidence that 

contaminant source materials have been substantially reduced 

in reach 9. However, net mobilization of suspended sediment 

from sources within reach 9 were slightly larger for period 4 

than for period 1. A possible explanation for this pattern might 

relate to flood-plain disturbance and placement of uncontaminated fill in the flood plain associated with remediation 

activities. The artificially installed uncontaminated fill might 

be more available for mobilization than sediment within the 

former Milltown Reservoir during period 1.

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A3, 510 p.

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average annual loads for suspended sediment and selected 

trace metals in streamflow of the upper Clark Fork Basin 

from Warm Springs to Missoula, Montana: U.S. Geological 

Survey Water-Resources Investigations Report 98–4137, 35 

p., accessed November 19, 2012, at http://pubs.er.usgs.gov/

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https://deq.mt.gov/Land/fedsuperfund/cfr.

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metal concentrations in streams—Occurrence and possible 

causes: Water Resources Research, v. 39, 17 p.

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biogeochemical processes and their effect on the aqueous 

chemistry of streams—A review: Chemical Geology, v. 283, 

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through the Clark Fork Basin, Montana, in selected periods 

before and after the breach of Milltown Dam (water years 

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upper Clark Fork Basin, Montana, water years 1996–2010: 

U.S. Geological Survey Scientific Investigations Report 

2013–5217, 162 p., with appendixes. 

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units and standard errors in percent: U.S. Geological Survey 

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review report, Silver Bow Creek/Butte Area Superfund Site 

with emphasis on the Warm Springs Ponds performance 

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Appendixes
52 Water-Quality Trends and Constituent-Transport Analysis for Selected Sampling Sites

Appendix 1—Summary Information Relating to Quality-Control Data

Summary information is presented relating to qualitycontrol data. Results for quality-control equipment blank and 

replicate samples collected during water years 1993–2015 

are summarized in table 1–1. Spike recoveries for laboratoryspiked deionized-water blank samples collected during water 

years 1993–2015 are presented in table 1–2. Spike recoveries 

for laboratory-spiked stream-water blank samples collected 

during water years 1993–2015 are presented in table 1–3. For 

reference, aquatic-life standards (based on median hardness 

for water years 2011–15, Montana Department of Environmental Quality, 2012) are presented in table 1–4. 

Evaluation of long-term spike-recovery data is particularly relevant to the long-term trend analysis. Spike-recoveries during water years 1993–2015 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 1993–99 of 99.1 percent), 

whereas after about water year 2000, spike recoveries mostly 

were less than 100 percent (mean annual spike recovery 

for water years 2000–15 of 94.3 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). The potential effects of temporal 

changes in spike recoveries on trend results were evaluated in 

exploratory analyses, as described in appendix 2.
Appendixes 53

Table 1–1. Summary information relating to quality-control samples (field equipment blank and replicate samples) collected at sampling sites in the upper Clark Fork Basin, Montana, water years 1993–2015.[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 level; RSD, relative standard deviation; μS/cm, microsiemen per centimeter at 25 degrees Celsius; NA, not applicable; μg/L, microgram per liter; mg/L, milligram per liter]

Constituent or property, 

units of measurement

Summary information for field blank samples

Summary information 

for field replicate 

samples

Number of 

field blank 

samples

Number of field 

blank samples 

with detected 

concentrations 

greater than the 

LRL at the time 

of analysis

Percentage 

of field blank 

samples with 

detected 

concentrations 

greater than the 

LRL at the time 

of analysis

Maximum 

detected 

concentration 

for field blank 

samples

Median 

concentration 

in field blank 

samples with 

detected 

concentrations 

greater than the 

LRL at the time of 

analysis

SRL used in 

application 

of the 

time-series 

model

Percentage of 

detections in 

blank samples at 

concentrations 

greater than the 

SRL used in the 

application of 

the time-series 

model

Number 

of field 

replicate 

pairs

RSD, 1

in percent

Specific conductance, μS/cm NA NA NA NA NA NA NA 162 0.1Cadmium, filtered, μg/L 193 5 2.6 0.337 0.071 NA NA 179 13.4Cadmium, unfiltered-recoverable, μg/L 189 1 0.5 0.010 0.010 NA NA 180 4.5Copper, filtered,2 μg/L 192 15 7.8 3.6 0.50 1.0 1.0 182 12.4Copper, unfiltered-recoverable,2 mg/L 189 11 5.8 3.0 1.0 1.0 2.1 180 9.0Iron, filtered, μg/L 189 4 2.1 5.9 4.8 NA NA 171 9.8Iron, unfiltered-recoverable, μg/L 185 10 5.4 35.6 7.0 NA NA 178 5.5Lead, filtered, μg/L 193 6 3.1 0.600 0.101 NA NA 178 11.0Lead, unfiltered-recoverable, μg/L 189 10 5.3 0.16 0.05 NA NA 180 16.3Manganese, filtered, μg/L 188 22 11.7 0.62 0.36 NA NA 183 5.7Manganese, unfiltered-recoverable, μg/L 185 10 5.4 0.3 0.2 NA NA 180 5.8Zinc, filtered, μg/L 191 39 20.4 6.2 0.9 NA NA 181 9.6Zinc, unfiltered-recoverable,2 μg/L 187 20 10.7 3.4 1.4 2.0 2.7 181 9.0Arsenic, filtered,2 μg/L 193 1 0.5 0.1 0.1 1.0 0.0 182 5.4Arsenic, unfiltered-recoverable,2 μg/L 189 3 1.6 0.1 0.1 1.0 0.0 181 6.8Suspended sediment,2 mg/L NA NA NA NA NA 1 NA 170 9.1

1RSD is calculated according to the following equation (Taylor, 1987):

RSD S

X

= 

where 

RSD is the relative standard deviation; 

S is the standard deviation; and 

X is the mean concentration for all replicate analyses.

2Property or constituent was analyzed for temporal trends.
54 Water-Quality Trends and Constituent-Transport Analysis for Selected Sampling Sites Table 1–2. Summary information relating to quality-control samples (laboratory-spiked deionized-water blank samples) collected at sampling sites in the upper Clark Fork Basin, Montana, water years 1993–2015. [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 Cadmium, 

F

Cadmium,

UFR

Copper, 

F

Copper, 

UFR

Iron, 

F

Iron, 

UFR

Lead, 

F

Lead, 

UFR

Manganese, 

F

Manganese, 

UFR

Zinc, 

F

Zinc, 

UFR

Arsenic, 

F

Arsenic, 

UFR

Mean spike recovery, in percent (values in parentheses indicate 95 percent confidence intervals)

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

(94.1, 98.5)

94

(89.6, 98.4)

102.6

(95.8, 109)

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

(91.5, 114)

100.6

(95.6, 106)

109.3

(104, 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)

102.3

(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

(97.1, 106)

103.9

(99.1, 109)

106.8

(103, 110)

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

(82.8, 92.8)

89.7

(77.1, 102)

104.1

(101, 107)

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

(86.4, 99.0)

93.9

(87.8, 100)

106.1

(104, 108)

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

(86.9, 116)

91.5

(87.3, 95.7)

105.4

(99.2, 112)

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

(88.9, 97.7)

108.9

(95.4, 122)

102.9

(97.8, 108)

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

(90.0, 115)

101.6

(95.3, 108)

101.4

(95.1, 108)

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

(82.7, 116)

99.2

(92.3, 106)

97.7

(86.6, 109)

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

(91.8, 105)

105.1

(95.8, 114)

97.9

(93.0, 103)

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

(93.2, 105)

87.9

(71.3, 104)

96.6

(78.5, 115)

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

(96.3, 104)

101

(75, 127)

102

(93.6, 110)

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

(83.5, 109)

97.4

(95.5, 99.3)

101

(90.7, 111)

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

(90.1, 100)

95.2

(89.2, 101)

98.5

(94.7, 102)

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

(96.5, 110)

105

(96.6, 114)

102

(95.2, 109)

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

(96, 111)

103

(89.2, 117)

102

(93.9, 110)

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

(88.5, 98.1)

101

(92.3, 110)

97

(94.9, 99.1)

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

(89.6, 113)

105

(96.7, 113)

102

(89.7, 114)
Appendixes 55

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

Arsenic, 

F

Arsenic, 

UFR

Mean spike recovery, in percent (values in parentheses indicate 95 percent confidence intervals)—Continued

2011 105

(97.9, 111)

95.7

(92.4, 99)

96.2

(89.4, 103)

93.9

(91.6, 96.2)

111

(89.3, 132)

107

(98.2, 117)

106

(98.8, 113)

99.8

(98.4, 101)

101

(97.0, 104)

98.9

(97.8, 100)

108

(94.3, 122)

96.1

(92.2, 100)

105

(102, 109)

94.7

(90.2, 99.3)

2012 102

(93.2, 112)

101

(95.1, 108)

98.4

(93.1, 104)

100

(92.5, 107)

105

(102, 108)

106

(96.2, 117)

102

(96.8, 106)

103

(98.4, 107)

105

(101, 110)

101

(95.4, 106)

103

(96.5, 109)

100

(94.9, 106)

98.1

(90.4, 106)

101

(94.3, 108)

2013 96.3

(92.4, 100)

96.6

(92.9, 100)

92.4

(87, 97.9)

96.3

(92.6, 100)

103

(95.5, 111)

105

(98.2, 112)

97.5

(92.3, 103)

99.9

(97.1, 103)

98.1

(92.3, 104)

98.5

(94.8, 102)

98.6

(90.9, 106)

95.2

(91.7, 98.7)

98

(93.1, 103)

99.3

(96, 103)

2014 99.4

(95.1, 104)

101

(99.0, 104)

98.1

(91.0, 105)

100

(98.8, 102)

103

(95.8, 111)

103

(99.7, 106)

102

(100, 104)

103

(100, 107)

99.2

(91.6, 107)

100

(97.7, 103)

110

(103, 117)

101

(97.1, 104)

94.7

(87.6, 102)

102

(99.0, 105)

2015 98.7

(95.2, 102)

102

(94, 110)

101

(93.9, 108)

105

(90.1, 120)

103

(95.3, 111)

103

(101, 106)

99.7

(96.7, 103)

100

(89.6, 111)

104

(99.2, 108)

110

(97.5, 122)

104

(99.5, 109)

118

(103, 132)

97.3

(89.2, 105)

99.8

(91.7, 108)

Table 1–2. Summary information relating to quality-control samples (laboratory-spiked deionized-water blank samples) collected at sampling sites in the upper Clark Fork Basin, Montana, water years 1993–2015.—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]
56 Water-Quality Trends and Constituent-Transport Analysis for Selected Sampling Sites Table 1–3. Summary information relating to quality-control samples (laboratory-spiked stream-water samples) collected at sampling sites in the upper Clark Fork Basin, Montana, water years 1993–2015. [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 Cadmium, 

F

Cadmium,

UFR

Copper, 

F

Copper, 

UFR

Iron, 

F

Iron, 

UFR

Lead, 

F

Lead, 

UFR

Manganese, 

F

Manganese, 

UFR

Zinc, 

F

Zinc, 

UFR

Arsenic, 

F

Arsenic, 

UFR

Mean spike recovery, in percent (values in parentheses indicate 95 percent confidence intervals)

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

(92.2, 99.2)

95.2

(92.0, 98.3)

99.9

(96.5, 103)

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

(95.4, 117)

97.3

(90.4, 104)

106.9

(101, 113)

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

(99.1, 103)

103.8

(94.6, 113)

102.2

(97.1, 107)

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

(74.8, 124)

105.9

(94.4, 117)

102.8

(96.0, 110)

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

(74.4, 111)

93.3

(73.5, 113)

107.1

(99.9, 114)

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

(85.6, 112)

90.1

(85.5, 94.7)

104

(95.8, 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

(91.1, 101)

105.2

(97.5, 113)

103.6

(96.4, 111)

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

(91.1, 104)

102.5

(97.5, 108)

98.9

(87.8, 110)

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

(75.9, 118)

102.8

(95.1, 110)

100.1

(96.7, 104)

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

(89.9, 102)

105.8

(97.1, 114)

99.9

(86.0, 114)

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

(87.5, 98.5)

94.6

(80.2, 109)

108.6

(100, 117)

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

(91, 97.8)

97.3

(86.9, 108)

112

(106, 118)

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

(80.8, 102)

103

(98.3, 107)

104

(101, 108)

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

(87.8, 94.6)

96.5

(89.0, 104)

99.1

(94.9, 103)

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

(89.2, 107)

106

(100, 113)

102

(98.2, 106)

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

(93.1, 98.2)

100

(90.2, 110)

101

(98.5, 104)

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

(80.4, 99.2)

106

(97.7, 114)

100

(89.6, 111)

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

(81.3, 107)

106

(96.0, 116)

102

(90.1, 113)
Appendixes 57

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

Arsenic, 

F

Arsenic, 

UFR

Mean spike recovery, in percent (values in parentheses indicate 95 percent confidence intervals)—Continued

2011 104

(99.2, 109)

93.9

(91.5, 96.3)

96.6

(79.9, 113)

88.3

(85.4, 91.2)

108

(92.0, 124)

101

(85.2, 117)

104

(98.8, 110)

96.5

(94.5, 98.4)

98.2

(92.2, 104)

91.3

(88.3, 94.2)

102

(90.2, 114)

86.7

(80.7, 92.7)

106

(101, 111)

94.7

(90.5, 99.0)

2012 107

(104, 110)

98.8

(91.9, 106)

94

(90.9, 97)

93.9

(87.2, 101)

108

(102, 114)

100

(98.6, 102)

102

(97.9, 107)

101

(96.3, 105)

101

(97.7, 104)

95.5

(88, 103)

102

(95.2, 109)

89.8

(82.4, 97.2)

104

(101, 106)

97.5

(91.8, 103)

2013 94.8

(90.4, 99.3)

91.3

(87, 95.7)

90.9

(86, 95.8)

90

(87.5, 92.4)

102

(94.8, 110)

101

(92.6, 110)

101

(92.8, 108)

96.7

(92.3, 101)

97.2

(95.4, 99)

93

(84.9, 101)

99.5

(92, 107)

84.1

(79.5, 88.7)

99.5

(91.2, 108)

94.9

(91, 98.8)

2014 103

(95.6, 110)

95.5

(92.0, 99.0)

96.6

(90.1, 103)

93.8

(89.8, 97.8)

97.6

(92.7, 103)

101

(92.7, 109)

100

(96.7, 103)

99.7

(94.9, 104)

97.1

(90.4, 104)

94.8

(89.3, 100)

101

(94.2, 108)

88.9

(82.7, 94.6)

92.4

(82.7, 102)

97.7

(93.5, 102)

2015 104

(97.6, 111)

106

(96.6, 115)

97.4

(92.3, 102)

97.8

(92.9, 103)

93.5

(83.2, 104)

104

(101, 106)

103

(101, 105)

106

(96.0, 115)

102

(98.3, 105)

101

(92.0, 110)

93.8

(86.2, 101)

98.1

(88.9, 107)

96.8

(86.5, 107)

104

(87.3, 121)

Table 1–3. Summary information relating to quality-control samples (laboratory-spiked stream-water samples) collected at sampling sites in the upper Clark Fork Basin, Montana, water years 1993–2015.—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]
58 Water-Quality Trends and Constituent-Transport Analysis for Selected Sampling Sites

Table 1–4. Aquatic-life standards (based on median hardness for water years 2011–15) for selected sampling sites in the Milltown 

Reservoir/Clark Fork River Superfund Site 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]

Sampling 

site 

number 

(fig. 1, 

table 1)

Abbreviated sampling site name 

(table 1)

Aquatic-life standards (Montana Department of Environmental Quality, 2012), 

in micrograms per liter

Median 

hardness for 

water years 

2011–15, in 

milligrams per 

liter as CaCO3

Cadmium Copper Lead Zinc

Acute Chronic Acute Chronic Acute Chronic Acute Chronic

8 Silver Bow Creek at Warm Springs 170 3.66 0.401 23.1 14.7 160 6.25 188 188

11 Clark Fork near Galen 164 3.53 0.390 22.3 14.2 153 5.97 182 182

14 Clark Fork at Deer Lodge 200 4.32 0.452 26.9 16.9 197 7.69 216 216

15 Clark Fork near Garrison 202 4.36 0.456 27.2 17.0 199.8 7.79 217 217

16 Clark Fork at Goldcreek 165 3.54 0.391 22.4 14.3 154 6.00 183 183

18 Clark Fork near Drummond 190 4.09 0.435 25.6 16.1 184 7.18 206 206

20 Clark Fork at Turah Bridge 132 2.82 0.331 18.1 11.8 116 4.51 151 151

22 Clark Fork above Missoula 109 2.33 0.288 15.2 10.0 91 3.55 129 129
Appendixes 59

Figure 1–1. Spike recoveries for laboratory-spiked deionized-water blank samples, water years 1993–2015. A, copper, 

filtered; B, copper, unfiltered-recoverable; C, arsenic, filtered; D, arsenic, unfiltered-recoverable.

1990 1995 2000 2005 2010 2015

Water year (October–September)

A. Copper, filtered 

B. Copper, unfiltered-recoverable

C. Arsenic, filtered

D. Arsenic, unfiltered-recoverable

75

150

125

100

50

75

150

125

100

50

75

150

125

100

50

75

150

125

100

50

Spike recovery, in percent

95 percent confidence interval about the mean spike recovery

Mean spike recovery 

EXPLANATION
60 Water-Quality Trends and Constituent-Transport Analysis for Selected Sampling Sites

Figure 1–2. Spike recoveries for laboratory-spiked stream-water samples, water years 1993–2015. A, copper, filtered; 

B, copper, unfiltered-recoverable; C, arsenic, filtered; D, arsenic, unfiltered-recoverable.

1990 1995 2000 2005 2010 2015

Water year (October–September)

A. Copper, filtered 

B. Copper, unfiltered-recoverable

C. Arsenic, filtered

D. Arsenic, unfiltered-recoverable

75

150

125

100

50

75

150

125

100

50

75

150

125

100

50

75

150

125

100

50

Spike recovery, in percent

95 percent confidence interval about the mean spike recovery

Mean spike recovery 

EXPLANATION
Appendixes 61

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

This appendix presents somewhat detailed information on 

theoretical and computational aspects of the time-series model 

(TSM). Also, specific aspects of the application of the TSM in 

this study are described.

Theoretical and Computational Information

The theory and parameter estimation for the TSM are 

described in detail in Vecchia (2005). In the TSM, log-transformed concentration data are partitioned into several components according to equation 1:

log(C) = MC + ANNC + SEASC + TREND + HFVC (1)

where

log denotes 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 1, ANNC, SEASC, and HFVC terms represent natural 

variability in concentration for different timescales. The term 

ANNC is an estimate of the interannual variability in concentration that can be attributed to long-term variability in streamflow. The term 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 the 

relative composition of runoff among groundwater, overland 

flow, and subsurface flow (Vecchia, 2005).

The term 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. 

The term SEASC is quantified by relating seasonal means (for 

the 30-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 snowaccumulation 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. 

The term HFVC is an estimate of the variability in concentration for timescales that are smaller than the seasonal 

timescale (timescales 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. The 

term 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 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).

The term 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 that can change 

the amount of suspended sediment or trace elements that reach 

the stream. The term TREND consists of piecewise monotonic 

trends during specified trend-analysis periods. The overall 

significance of TREND (determined by using the generalized 

likelihood ratio principle; appendix 1 of Vecchia, 2005) specifies whether there were any significant changes during any of 

the specified trend-analysis periods. If TREND was determined 

to be nonsignificant for a given sampling-site and constituent 

combination, the trends for all of the specified trend-analysis 

periods were considered nonsignificant, and p-values were 

not reported. If TREND was determined to be significant for 

a given sampling-site and constituent combination, the slope 

coefficient (γ; appendix 1 of Vecchia, 2005) 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 

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 

in 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 
62 Water-Quality Trends and Constituent-Transport Analysis for Selected Sampling Sites

period and the geometric mean concentration at the start of the 

period and is determined by the equation

%∆FAC = − 100( ) 10 1 γ , (2)

where

%∆FAC is the percentage 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.” By using equation 1, the flow-adjusted concentration is defined as

FAC = log(C) – ANNC – SEASC = MC + TREND + HFVC (3)

where FAC is the flow-adjusted value, as the base-10 logarithm of the original units of measurement. The FACs defined 

by equation 3 are analogous to FACs defined in other 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 streamflowrelated variability (Vecchia, 2005). Time-series plots showing 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 1–10, days 11–20, 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 water-quality 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 (4)

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 the high-frequency streamflow variability, 

computed as log(Q) – MQ – ANNQ – SEASQ

(dimensionless). 

The water-quality time-series model (equation 1) is 

directly tied to the streamflow time-series model because the 

streamflow anomalies (ANNQ and SEASQ from equation 4) 

are used as predictor variables for concentration (equation 1). 

For example, ANNC is assumed to equal a constant coefficient 

(estimated from the TSM) 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. 

Specific Aspects of the Application of the TimeSeries Model in this Study

The TSM residuals for each sampling-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 sampling-site and constituent combinations and 

practical considerations to keep the trend periods comparable 

among sampling 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 short-term (about 1–2 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 all cases the 

more complicated models did not substantially affect the overall descriptions of the trends and also did not change the general findings and conclusions of this report. Thus, the reported 

TSM results were judged to provide acceptable fits representative of linearity through nearly all of the range in FACs for 
Appendixes 63

a given sampling-site and constituent combination. Standard 

errors of estimates (SEEs) for the TSM analyses are presented 

in table 2–1. In this report, SEEs are expressed in percent and 

were converted from log units by using procedures described 

by Tasker (1978). Mean SEEs for all trace elements combined 

range from 20.8 to 50.7 percent. Mean SEEs for unfilteredrecoverable copper and arsenic concentrations are 48.3 and 

27.3 percent, respectively. Mean SEE for suspended-sediment 

concentration (65.2 percent) is substantially higher than mean 

SEEs for trace elements. The SEEs indicate reasonably accurate definition of concentration and streamflow relations for 

the purpose of trend analysis; however, a higher mean SEE for 

suspended sediment than mean SEEs for trace elements indicates lower confidence in results. For each sampling-site and 

constituent combination, the fit of the TSM can be assessed 

by examination of the fitted trends in relation to FACs that are 

shown in figures 3–1 through 3–7 in appendix 3. The distribution of FACs about the fitted trend lines shows the extent 

to which the residuals might exhibit nonconstant variance or 

unresolved trends. 

Application of the TSM in this study generally followed 

the methods applied by Sando and others (2014) who reported 

water-quality trends for 22 sampling sites in the upper Clark 

Fork Basin for water years 1996–2010. However, two factors 

might contribute to differences between Sando and others 

(2014) and this study: (1) this study included additional data 

collected after the study period of Sando and others (2014), 

and (2) this study included preliminary dummy trend periods that were inserted prior to period 1. The additional data 

after the study period of Sando and others (2014) represent 

an increase of about 25 percent and provide improvement in 

definition of concentration and streamflow relations used in 

determining FACs. Also, during exploratory analysis for this 

study, close scrutiny of the fitted trends reported by Sando 

and others (2014) indicated that in some cases the fitted 

trend values at the start of period 1 (1996) were not precisely 

centered at the median FAC at the start of period 1. In this 

study, dummy trend periods were inserted before period 1 

to more precisely center the 1996 fitted trend values at the 

median FAC. The combination of the two factors (inclusion 

of additional data and insertion of preliminary dummy trends) 

sometimes resulted in generally minor differences in the fitted 

trend lines between this report and Sando and others (2014). 

The trend results of this report supersede the trend results of 

Sando and others (2014).

Exploratory analyses were conducted to investigate two 

ancillary factors that might affect trend results, including 

potential effects of (1) temporal changes in spike recoveries (as discussed in appendix 1) and (2) diel cycling of trace 

elements. The potential effects of temporal changes in spike 

recoveries (as discussed in appendix 1) on trend results were 

evaluated by using two approaches: (1) exploratory trend 

analysis with inclusion of a step trend in the trend model and 

(2) exploratory trend analysis on constituent concentrations 

adjusted based on annual mean spike recoveries. For the 

exploratory step-trend approach, a step trend for the period 

water years 1996–99 was included in the TSM model for 

each sampling-site and constituent combination, in addition 

to including trends for periods 1–4. 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 

step-trend analysis indicated that among all sampling-site and 

constituent combinations, statistically significant step trends 

were infrequently detected (less than 20 percent of analyses). 

In all cases of statistically significant step trends, the difference in the percent change from the start of period 1 to the 

end of period 4 between the exploratory analysis including 

the step trend and the reported analysis without the step trend 

was less than 5 percent. Thus, it was concluded that temporal 

changes in spike recoveries did not have a substantial effect on 

the overall trend results and the study objectives of evaluating relative spatial and temporal changes in FACs in the upper 

Clark Fork Basin as a whole. For the exploratory spikerecovery adjustment approach, constituent concentrations for 

each year were adjusted by multiplying the concentrations 

times the annual mean spike recovery for laboratory-spiked 

stream-water samples; then exploratory trend analysis was 

done. Results of the exploratory spike-recovery adjustment 

analysis were similar to the results for the exploratory steptrend approach and resulted in the same general conclusion 

that temporal differences in spike recoveries had minor effects 

on trend results.

An important consideration in trend analysis for trace 

elements is potential effects of diel cycling in trace-element 

concentrations. Complex biogeochemical processes affected 

by the daily solar photocycle produce regular and dynamic 

changes in many physical and chemical characteristics of 

streams (Nimick and others, 2011). In some streams (including 

some of the sampling sites in this study), the biogeochemical 

processes can result in diel variability in trace-element concentrations (Nimick and others, 2003). 

Diel cycling in trace-element concentrations has the 

potential to affect trend results if (1) there is strong diel 

cycling for a given sampling-site and constituent combination 

and (2) there is a systematic temporal bias in the dataset with 

respect to the time of day of sampling. During exploratory 

analysis, potential effects of diel cycling on the trend results 

were quantitatively evaluated by including decimal day (time 

of sampling) as an ancillary variable in the trend models. The 

decimal day variable indicates the strength of diel cycling for 

a given sampling-site and constituent combination and also 

allows evaluation of the effect of temporal variability in time 

of sampling on the trend results. Although some samplingsite and constituent combinations had statistically significant 

diel cycling, in no case did the inclusion of the decimal day 

variable in trend models provide substantially different trend 

results from the reported results. Thus, potential effects on 

trend results of diel cycling of trace elements were determined 

to be minor; however, it should be noted that samples were 

collected during daylight hours and diel variations in the night 

cannot be evaluated.
64 Water-Quality Trends and Constituent-Transport Analysis for Selected Sampling Sites

Table 2–1. Statistical summaries of standard errors of estimates for the trend models.

[SEE, standard error of estimate]

Constituent or property Number of sites for which 

trend results are reported

SEE, in percent

Minimum Mean Maximum

Specific conductance 7 8.2 11.0 13.1

Copper, filtered 7 24.6 31.6 37.4

Copper, unfiltered-recoverable 7 38.3 48.3 60.7

Zinc, unfiltered-recoverable 7 41.0 50.7 65.7

Arsenic, filtered 7 15.2 20.8 26.7

Arsenic, unfiltered-recoverable 7 21.8 27.3 34.0

Suspended sediment 7 57.4 65.2 80.5
Appendixes 65

Appendix 3—Trend-Analysis Results

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 tables 3–1 (for most sampling sites) 

and 3–2 (for Clark Fork above Missoula, Montana [sampling 

site 22]). Detailed trend results are graphically presented in 

figures 3–1 through 3–7. The detailed graphical presentations 

in appendix 3 present fitted trends for all constituents and 

allow evaluation of the fitted trends for a given sampling site 

in conjunction with FACs. 
66 Water-Quality Trends and Constituent-Transport Analysis for Selected Sampling Sites Table 3–1. Flow-adjusted trend results for selected water-quality constituents and properties for selected sampling sites in the Milltown Reservoir/Clark Fork River Superfund Site in the upper Clark Fork Basin, Montana, water years 1996–2015. [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 percentage change. Gray shading 

indicates statistical significance at 

p-value less than 0.01. 

p-value, statistical probability level; SEE, standard error of estimate; <, less than; NR, not reported]

Constituent or property

Number 

of 

samples

Total percentage 

change for water 

years 1996–2000 

(period 1)

Total percentage 

change for water 

years 2001–5 

(period 2)

Total percentage 

change for water 

years 2006–10 

(period 3)

Total percentage 

change for water 

years 2011–15 

(period 4)

p-value for 

overall trend 

analysis1

SEE, 

in percent

Percentage of values 

affected by recensoring 

at study reporting level 

used in the application of 

the time-series model2

Silver Bow Creek at Warm Springs, Montana (sampling site 8, fig. 1, table 1)

Specific conductance 186 -1 (0.645) -3 (0.226) 2 (0.380) -13 (<0.001) <0.001 10.5 0.0Copper, filtered 186 -48 (<0.001) -12 (0.187) -8 (0.427) -24 (0.023) <0.001 32.9 0.0Copper, unfiltered-recoverable 186 -38 (<0.001) -14 (0.105) -12 (0.246) -28 (0.005) <0.001 38.3 0.0Zinc, unfiltered-recoverable 178 -54 (<0.001) -47 (<0.001) 16 (0.112) -37 (<0.001) <0.001 45.0 4.5Arsenic, filtered 186 1 (0.902) 5 (0.449) 5 (0.481) -18 (0.015) 0.002 24.8 0.0Arsenic, unfiltered-recoverable 186 -1 (0.907) 5 (0.303) 1 (0.894) -16 (0.004) 0.002 24.5 0.0Suspended sediment 188 17 (0.450) -27 (0.072) -40 (0.010) 15 (0.515) <0.001 65.9 0.0

Clark Fork near Galen, Montana (sampling site 11, fig. 1, table 1)

Specific conductance 217 1 (0.134) -8 (NR3

) 7 (NR3

) -12 (NR3

) 0.027 12.7 0.0

Copper, filtered 215 -45 (<0.001) -5 (0.593) -17 (0.085) 4 (0.759) <0.001 28.4 0.0Copper, unfiltered-recoverable 213 -31 (<0.001) 7 (0.527) -5 (0.702) -24 (0.035) <0.001 44.4 0.0Zinc, unfiltered-recoverable 205 -56 (<0.001) -31 (0.003) 30 (0.060) -39 (0.001) <0.001 41.0 4.8Arsenic, filtered 215 -8 (0.332) 12 (0.165) -21 (0.014) 11 (0.303) <0.001 26.7 0.0Arsenic, unfiltered-recoverable 215 -3 (0.708) 3 (0.741) -17 (0.082) 13 (0.294) 0.005 29.4 0.0Suspended sediment 229 12 (0.494) -19 (0.211) 8 (0.678) -25 (0.168) 0.002 60.0 0.0

Clark Fork at Deer Lodge, Montana (sampling site 14, fig. 1, table 1)

Specific conductance 264 1 (0.747) -4 (0.089) -2 (0.419) 1 (0.860) <0.001 11.2 0.0Copper, filtered 231 -16 (0.003) 6 (0.400) -12 (0.087) 8 (0.397) <0.001 28.9 0.0Copper, unfiltered-recoverable 229 -22 (0.019) 5 (0.661) 1 (0.963) -8 (0.595) <0.001 52.7 0.0Zinc, unfiltered-recoverable 227 -37 (<0.001) -2 (0.850) -7 (0.560) -13 (0.334) <0.001 54.0 0.9Arsenic, filtered 231 -3 (0.501) 17 (<0.001) -16 (<0.001) 4 (0.540) 0.001 15.6 0.0Arsenic, unfiltered-recoverable 230 -8 (0.184) 7 (0.308) -12 (0.114) 2 (0.828) 0.357 27.0 0.0Suspended sediment 281 -17 (0.121) -8 (0.555) 8 (0.643) -17 (0.294) 0.001 80.5 0.0
Appendixes 67

Constituent or property

Number 

of 

samples

Total percentage 

change for water 

years 1996–2000 

(period 1)

Total percentage 

change for water 

years 2001–5 

(period 2)

Total percentage 

change for water 

years 2006–10 

(period 3)

Total percentage 

change for water 

years 2011–15 

(period 4)

p-value for 

overall trend 

analysis1

SEE, 

in percent

Percentage of values 

affected by recensoring 

at study reporting level 

used in the application of 

the time-series model2

Clark Fork at Goldcreek, Montana (sampling site 16, fig. 1, table 1)

Specific conductance 186 -2 (0.372) -3 (0.063) -2 (0.317) 0 (0.972) <0.001 9.9 0.0Copper, filtered 185 -20 (0.003) 13 (0.046) -12 (0.077) 3 (0.752) 0.002 24.6 0.0Copper, unfiltered-recoverable 185 -5 (0.688) -18 (0.036) -6 (0.564) 7 (0.569) 0.002 44.0 0.0Zinc, unfiltered-recoverable 183 -25 (0.015) -37 (<0.001) 24 (0.103) -14 (0.349) <0.001 43.8 1.7Arsenic, filtered 186 -13 (NR3) 8 (0.048) -3 (0.548) -4 (0.365) 0.026 15.2 0.0Arsenic, unfiltered-recoverable 186 -17 (NR3) 3 (0.582) -4 (0.522) -3 (0.616) 0.086 21.8 0.0Suspended sediment 187 15 (0.396) -51 (<0.001) 54 (0.012) -17 (0.352) <0.001 58.4 0.0

Clark Fork near Drummond, Montana (sampling site 18, fig. 1, table 1)

Specific conductance 186 0 (0.535) -2 (0.018) -3 (<0.001) 6 (<0.001) <0.001 11.3 0.0Copper, filtered 183 0 (0.991) 10 (0.037) -24 (<0.001) 13 (0.194) 0.013 33.5 0.0Copper, unfiltered-recoverable 184 -13 (0.219) -9 (0.369) -10 (0.408) -5 (0.730) 0.002 47.6 0.0Zinc, unfiltered-recoverable 182 -48 (<0.001) -18 (0.067) 12 (0.437) -23 (0.147) <0.001 50.7 2.2Arsenic, filtered 186 -6 (0.093) 4 (0.107) -11 (NR3) 3 (0.378) 0.907 15.9 0.0Arsenic, unfiltered-recoverable 186 -15 (0.001) 3 (0.398) -6 (0.171) 0 (0.930) 0.003 23.9 0.0Suspended sediment 187 -24 (0.134) -20 (0.174) 29 (0.190) -23 (0.242) 0.065 65.6 0.0

Clark Fork at Turah Bridge near Bonner, Montana (sampling site 20, fig. 1, table 1)

Specific conductance 259 -5 (<0.001) -2 (0.378) 3 (0.184) -2 (0.502) <0.001 13.1 0.0Copper, filtered 228 -23 (<0.001) 9 (0.357) -6 (0.525) -20 (0.077) <0.001 35.0 0.0Copper, unfiltered-recoverable 227 -13 (0.073) -8 (0.385) -1 (0.920) -4 (0.762) 0.002 50.3 0.0Zinc, unfiltered-recoverable 219 -36 (<0.001) -32 (0.005) 52 (0.004) -31 (0.026) <0.001 55.1 5.0Arsenic, filtered 229 -5 (<0.001) 5 (0.002) 3 (0.435) -16 (0.002) <0.001 21.6 0.0Arsenic, unfiltered-recoverable 229 -10 (0.051) -1 (0.879) 9 (0.258) -16 (0.052) 0.204 30.3 0.0Suspended sediment 284 -13 (0.222) -25 (0.059) 36 (0.067) -21 (0.246) 0.002 57.4 0.0

1Determination of and distinction between p-value for individual trend period and p-value for overall trend analysis are discussed in the section of this report “Appendix 2—Summary of the Time-Series Model as Applied 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 “General Description of the Time-Series Model.”3Results not reported because of nonsignificant overall trend analysis (p-value greater than 0.01).

Table 3–1. Flow-adjusted trend results for selected water-quality constituents and properties for selected sampling sites in the Milltown Reservoir/Clark Fork River Superfund Site in the upper Clark Fork Basin, Montana, water years 1996–2015.—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 percentage change. Gray shading indicates statistical significance at p-value less than 0.01. p-value, statistical probability level; SEE, standard error of estimate; <, less than; NR, not reported]
68 Water-Quality Trends and Constituent-Transport Analysis for Selected Sampling Sites Table 3–2. Flow-adjusted trend results for selected water-quality constituents and properties for Clark Fork above Missoula, Montana (sampling site 22), water years 1996–2015. [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 percentage change. Gray shading 

indicates statistical significance at 

p-value less than 0.01. 

p-value, statistical probability level; SEE, standard error of estimate; <, less than; NR, not reported]

Constituent or property Number of samples

Total 

percentage 

change for 

water years 

1996–2000 

(period 1)

Total 

percentage 

change for 

water years 

2001–5 

(period 2)

Total 

percentage 

change for 

October 1, 2005–

March 27, 2008 

(period 3A)

Total 

percentage 

change for 

March 28, 2008–

September 30, 

2010 

(period 3B)

Total 

percentage 

change for 

water years 

2011–15

(period 4)

p-value for 

overall trend 

analysis1

SEE, 

in percent

Percentage of values 

affected by 

recensoring at study 

reporting level used in 

the application of the 

time-series model2

Clark Fork above Missoula, Montana (sampling site 22, fig.1, table 1)

Specific conductance 227 0 (0.840) -2 (0.250) 1 (0.585) 4 (0.101) -7 (NR3

) 0.161 8.2 0.0

Copper, filtered 206 -25 (0.006) 25 (0.057) 13 (0.357) -21 (0.089) -27 (0.032) <0.001 37.4 0.0Copper, unfiltered-recoverable 205 -23 (0.035) 41 (0.017) 120 (<0.001) -59 (<0.001) -52 (0.002) <0.001 60.7 0.0Zinc, unfiltered-recoverable 186 -49 (<0.001) 43 (0.082) 192 (<0.001) -65 (<0.001) -52 (0.003) <0.001 65.7 8.5Arsenic, filtered 207 -15 (0.005) 14 (0.033) 10 (0.171) -3 (0.664) -24 (<0.001) <0.001 26.1 0.0Arsenic, unfiltered-recoverable 207 -21 (0.006) 16 (0.110) 25 (0.036) -17 (0.099) -25 (0.012) <0.001 34.0 0.0Suspended sediment 250 -4 (0.796) 25 (0.242) 168 (<0.001) -60 (<0.001) -40 (0.032) <0.001 68.7 0.0

1Determination of and distinction between 

p-value for individual trend period and 

p-value for overall trend analysis are discussed in the section of this report “Appendix 2—Summary of the Time-Series 

Model as Applied 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 “General Description of the Time-Series Model.”3Results not reported because of nonsignificant overall trend analysis (p-value greater than 0.01).
Appendixes 69

Figure 3–1. Flow-adjusted fitted trends for selected water-quality constituents and properties for Silver Bow Creek at Warm Springs, 

Montana (sampling site 8), water years 1996–2015.

[Graph included as a place holder to assist in

comparisons.]

10

100

1

1,000

10

100

1

1,000

100

1,000

10

100

10

100

1

1,000

10

100

1

1,000

10

100

1

1,000

1

1,000

Water year (October–September)

199019952000200520102015199019952000200520102015

1 2 3 4 1 2 3 4

Period Period

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, microsiemen 

per centimeter at 25 degrees 

Celsius; p-value, statistical 

probability level]

446

521

Flow-adjusted 

concentration (FAC) 

determined by 

using the time-series 

model

Flow-adjusted fitted 

trend determined 

by using the timeseries 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

Flow-adjusted values, in indicated units of measurement

521 514 501 513 446

8.9 4.6 4.1 3.8 2.9

19 19 20 21 17

5.3 6.3 4.6 2.7 3.1

9.3 7.9 7.0 5.0

22 22 23 23 19

35 16 8.4 9.8 6.1

15

Specific conductance (μS/cm) Suspended sediment (milligram per liter)

Unfiltered-recoverable

copper (microgram per liter)

Unfiltered-recoverable

zinc (microgram per liter)

Filtered copper (microgram per liter)

Filtered arsenic 

(microgram per liter)

Unfiltered-recoverable

arsenic (microgram per liter)
70 Water-Quality Trends and Constituent-Transport Analysis for Selected Sampling Sites

Figure 3–2. Flow-adjusted fitted trends for selected water-quality constituents and properties for Clark Fork near Galen, Montana 

(sampling site 11), water years 1996–2015.

[Graph included as a place holder to assist in

comparisons.]

10

100

1

1,000

10

100

1

1,000

100

1,000

10

100

1

10

100

1

1,000

10

100

1

1,000

10

100

1

1,000

1,000

Water year (October–September)

199019952000200520102015199019952000200520102015

1 2 3 4 1 2 3 4

Period Period

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, microsiemen 

per centimeter at 25 degrees 

Celsius; p-value, statistical 

probability level]

11

447

Flow-adjusted 

concentration (FAC) 

determined by 

using the time-series 

model

Flow-adjusted fitted 

trend determined 

by using the timeseries 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

Flow-adjusted values, in indicated units of measurement

447 454 415 443 388

7.6 4.2 4.0 3.3 3.4

12 11 13 10 11

5.2 5.8 4.7 5.1 3.8

11 11 11 8.1

15 14 15 12 14

30 13 9.0 12 7.1

15

Specific conductance (μS/cm) Suspended sediment (milligram per liter)

Unfiltered-recoverable

copper (microgram per liter)

Unfiltered-recoverable

zinc (microgram per liter)

Filtered copper (microgram per liter)

Filtered arsenic 

(microgram per liter)

Unfiltered-recoverable

arsenic (microgram per liter)
Appendixes 71

Figure 3–3. Flow-adjusted fitted trends for selected water-quality constituents and properties for Clark Fork at Deer Lodge, Montana 

(sampling site 14), water years 1996–2015.

[Graph included as a place holder to assist in

comparisons.]

10

100

1

1,000

10

100

1

1,000

100

1,000

10

100

1

10

100

1

1,000

10

100

1

1,000

10

100

1

1,000

1,000

Water year (October–September)

199019952000200520102015199019952000200520102015

1 2 3 4 1 2 3 4

Period Period

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, microsiemen

per centimeter at 25 degrees 

Celsius; p-value, statistical 

probability level]

5.8

479

Flow-adjusted 

concentration (FAC) 

determined by 

using the time-series 

model

Flow-adjusted fitted 

trend determined 

by using the timeseries 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

Flow-adjusted values, in indicated units of measurement

479 482 463 454 456

6.9 5.8 6.1 5.4 5.8

11 11 13 11 11

18 15 14 15 12

23 24 25 23

16 14 15 14 14

39 24 24 22 19

30

Specific conductance (μS/cm) Suspended sediment (milligram per liter)

Unfiltered-recoverable

copper (microgram per liter)

Unfiltered-recoverable

zinc (microgram per liter)

Filtered copper (microgram per liter)

Filtered arsenic 

(microgram per liter)

Unfiltered-recoverable

arsenic (microgram per liter)
72 Water-Quality Trends and Constituent-Transport Analysis for Selected Sampling Sites

Figure 3–4. Flow-adjusted fitted trends for selected water-quality constituents and properties for Clark Fork at Goldcreek, Montana 

(sampling site 16), water years 1996–2015.

[Graph included as a place holder to assist in

comparisons.]

10

100

1

1,000

10

100

1

1,000

100

1,000

10

100

1

10

100

1

1,000

10

100

1,000

10

100

1

1

1,000

1,000

Water year (October–September)

199019952000200520102015199019952000200520102015

1 2 3 4 1 2 3 4

Period Period

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, microsiemen 

per centimeter at 25 degrees 

Celsius; p-value, statistical 

probability level]

8.3

425

Flow-adjusted 

concentration (FAC) 

determined by 

using the time-series 

model

Flow-adjusted fitted 

trend determined 

by using the timeseries 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

Flow-adjusted values, in indicated units of measurement

425 418 406 398 398

4.8 3.8 4.3 3.8 3.9

9.4 8.2 8.8 8.6 8.2

15 17 8.3 13 11

19 15 14 15

12 10 10 10 9.7

27 20 13 15 13

19

Specific conductance (μS/cm) Suspended sediment (milligram per liter)

Unfiltered-recoverable

copper (microgram per liter)

Unfiltered-recoverable

zinc (microgram per liter)

Filtered copper (microgram per liter)

Filtered arsenic 

(microgram per liter)

Unfiltered-recoverable

arsenic (microgram per liter)
Appendixes 73

Figure 3–5. Flow-adjusted fitted trends for selected water-quality constituents and properties for Clark Fork near Drummond, Montana 

(sampling site 18), water years 1996–2015.

[Graph included as a place holder to assist in

comparisons.]

10

100

1

1,000

10

100

1

1,000

100

1,000

10

100

1

10

100

1

1,000

10

100

1

1,000

10

100

1

1,000

1,000

Water year (October–September)

199019952000200520102015199019952000200520102015

1 2 3 4 1 2 3 4

Period Period

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, microsiemen 

per centimeter at 25 degrees 

Celsius; p-value, statistical 

probability level]

434

461

Flow-adjusted 

concentration (FAC) 

determined by 

using the time-series 

model

Flow-adjusted fitted 

trend determined 

by using the timeseries 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

Flow-adjusted values, in indicated units of measurement

461 459 449 434 461

3.9 3.9 4.3 3.3 3.7

9.6 9.0 9.4 8.4 8.6

21 16 13 16 13

15 14 13 12

12 10 11 10 10

36 19 15 17 13

17

Specific conductance (μS/cm) Suspended sediment (milligram per liter)

Unfiltered-recoverable

copper (microgram per liter)

Unfiltered-recoverable

zinc (microgram per liter)

Filtered copper (microgram per liter)

Filtered arsenic 

(microgram per liter)

Unfiltered-recoverable

arsenic (microgram per liter)
74 Water-Quality Trends and Constituent-Transport Analysis for Selected Sampling Sites

Figure 3–6. Flow-adjusted fitted trends for selected water-quality constituents and properties for Clark Fork at Turah Bridge near 

Bonner, Montana (sampling site 20), water years 1996–2015.

[Graph included as a place holder to assist in

comparisons.]

10

100

1

1,000

10

100

1

1,000

100

1,000

10

100

1

10

100

1

1,000

10

100

1

1,000

10

100

1

1,000

1,000

Water year (October–September)

199019952000200520102015199019952000200520102015

1 2 3 4 1 2 3 4

Period Period

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, microsiemen 

per centimeter at 25 degrees 

Celsius; p-value, statistical 

probability level]

330

347

Flow-adjusted 

concentration (FAC) 

determined by 

using the time-series 

model

Flow-adjusted fitted 

trend determined 

by using the timeseries 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

Flow-adjusted values, in indicated units of measurement

347 330 324 334 327

3.3 2.5 2.8 2.6 2.1

13 12 8.8 12 9.5

9.0 8.3 8.2 7.9

6.8 6.1 6.1 6.6 5.6 5.4 5.1 5.4 5.5 4.7

21 13 9.2 14 9.7

10

Specific conductance (μS/cm) Suspended sediment (milligram per liter)

Unfiltered-recoverable

copper (microgram per liter)

Unfiltered-recoverable

zinc (microgram per liter)

Filtered copper (microgram per liter)

Filtered arsenic 

(microgram per liter)

Unfiltered-recoverable

arsenic (microgram per liter)
Appendixes 75

Figure 3–7. Flow-adjusted fitted trends for selected water-quality constituents and properties for Clark Fork above Missoula, Montana 

(sampling site 22), water years 1996–2015.

A B

[Graph included as a place holder to assist in

comparisons.]

10

100

1

1,000

10

100

1

1,000

100

1,000

10

100

1

10

100

1

1,000

10

100

1

1,000

10

100

1

1,000

1,000

Water year (October–September)

199019952000200520102015199019952000200520102015

1 2 3 4 1 2 3 4

Period Period

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, microsiemen 

per centimeter at 25 degrees 

Celsius; p-value, statistical 

probability level]

25

277

Flow-adjusted 

concentration (FAC) 

determined by 

using the time-series 

model

Flow-adjusted fitted 

trend determined 

by using the timeseries 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

Flow-adjusted values, in indicated units of measurement

277 275 270 273 283 265

2.3 1.7 2.1 2.4 1.9 1.4

7.7 7.4

9.2

6.0

25 9.9

4.9

6.9

15

6.3

3.0

4.2 3.3 3.9 4.8 4.0 3.0

14 7.2 10

30

10 5.0

3.3 2.8 3.2 3.6 3.4 2.6

6.4

Specific conductance (μS/cm) Suspended sediment (milligram per liter)

Unfiltered-recoverable

copper (microgram per liter)

Unfiltered-recoverable

zinc (microgram per liter)

Filtered copper (microgram per liter)

Filtered arsenic 

(microgram per liter)

Unfiltered-recoverable

arsenic (microgram per liter)
76 Water-Quality Trends and Constituent-Transport Analysis for Selected Sampling Sites

Appendix 4—Transport-Analysis Balance Calculations for Data-Summary Reaches

Balance calculations for the transport analysis (that is, 

differences between reach inflows and reach outflows) are presented in tables 4–1 through 4–6 for reaches 4–9, respectively, 

in appendix 4. The transport balance calculations indicate 

within-reach changes in estimated normalized loads and allow 

assessment of temporal changes in relative contributions from 

upstream source areas to loads transported past each reach 

outflow.
Appendixes 77

Table 4–1. Constituent-transport analysis balance calculations for sampling sites in reach 4, extending from Silver Bow Creek at Warm 

Springs, Montana (sampling site 8), to Clark Fork near Galen, Montana (sampling site 11), for selected periods, water years 1996–2015.

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

Abbreviated sampling site name (table 1) and number or summation category

 Estimated normalized load,1

in kilograms per day

Unfilteredrecoverable 

copper

Unfilteredrecoverable 

arsenic

Suspended 

sediment

Water years 1996–2000 (period 1)

Inflow

Silver Bow Creek at Warm Springs (sampling site 8) 1.9 3.4 920

Outflow

Clark Fork near Galen (sampling site 11) 3.7 4.2 1,600

Total within-reach change in load—outflow (sampling site 11) minus inflow (sampling site 8) 

(positive values indicate net mobilization from all within-reach sources including groundwater 

inflow, tributaries, the main-stem channel, and flood plain)

1.8 0.78 670

Water years 2001–5 (period 2)

Inflow

Silver Bow Creek at Warm Springs (sampling site 8) 1.4 3.5 850

Outflow

Clark Fork near Galen (sampling site 11) 3.1 4.2 1,500

Total within-reach change in load—outflow (sampling site 11) minus inflow (sampling site 8) 

(positive values indicate net mobilization from all within-reach sources including groundwater 

inflow, tributaries, the main-stem channel, and flood plain)

1.8 0.70 670

Water years 2006–10 (period 3)

Inflow

Silver Bow Creek at Warm Springs (sampling site 8) 1.2 3.6 570

Outflow

Clark Fork near Galen (sampling site 11) 3.2 3.9 1,400

Total within-reach change in load—outflow (sampling site 11) minus inflow (sampling site 8) 

(positive values indicate net mobilization from all within-reach sources including groundwater 

inflow, tributaries, the main-stem channel, and flood plain)

2.0 0.31 860

Water years 2011–15 (period 4)

Inflow

Silver Bow Creek at Warm Springs (sampling site 8) 0.94 3.3 460

Outflow

Clark Fork near Galen (sampling site 11) 2.7 3.8 1,300

Total within-reach change in load—outflow (sampling site 11) minus inflow (sampling site 8) 

(positive values indicate net mobilization from all within-reach sources including groundwater 

inflow, tributaries, the main-stem channel, and flood plain)

1.8 0.46 820

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 1996–2015 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.
78 Water-Quality Trends and Constituent-Transport Analysis for Selected Sampling Sites

Table 4–2. Constituent-transport analysis balance calculations for sampling sites in reach 5, extending from Clark Fork near Galen, 

Montana (sampling site 11), to Clark Fork at Deer Lodge, Montana (sampling site 14), for selected periods, water years 1996–2015.

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

Abbreviated sampling site name (table 1) and number or summation category

 Estimated normalized load,1

in kilograms per day

Unfilteredrecoverable 

copper

Unfilteredrecoverable 

arsenic

Suspended 

sediment

Water years 1996–2000 (period 1)

Inflow

Clark Fork near Galen (sampling site 11) 3.7 4.2 1,600

Outflow

Clark Fork at Deer Lodge (sampling site 14) 13 7.7 8,300

Total within-reach change in load—outflow (sampling site 14) minus inflow (sampling site 11) 

(positive values indicate net mobilization from within-reach sources including groundwater 

inflow, tributaries, the main-stem channel, and flood plain)

9.8 3.5 6,700

Water years 2001–5 (period 2)

Inflow

Clark Fork near Galen (sampling site 11) 3.1 4.2 1,500

Outflow

Clark Fork at Deer Lodge (sampling site 14) 12 7.6 7,200

Total within-reach change in load—outflow (sampling site 14) minus inflow (sampling site 11) 

(positive values indicate net mobilization from within-reach sources including groundwater 

inflow, tributaries, the main-stem channel, and flood plain)

9.0 3.4 5,700

Water years 2006–10 (period 3)

Inflow

Clark Fork near Galen (sampling site 11) 3.2 3.9 1,400

Outflow

Clark Fork at Deer Lodge (sampling site 14) 13 7.4 7,200

Total within-reach change in load—outflow (sampling site 14) minus inflow (sampling site 11) 

(positive values indicate net mobilization from within-reach sources including groundwater 

inflow, tributaries, the main-stem channel, and flood plain)

9.4 3.5 5,800

Water years 2011–15 (period 4)

Inflow

Clark Fork near Galen (sampling site 11) 2.7 3.8 1,300

Outflow

Clark Fork at Deer Lodge (sampling site 14) 12 7.0 6,800

Total within-reach change in load—outflow (sampling site 14) minus inflow (sampling site 11) 

(positive values indicate net mobilization from within-reach sources including groundwater 

inflow, tributaries, the main-stem channel, and flood plain)

9.4 3.3 5,500

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 1996–2015 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.
Appendixes 79

Table 4–3. Constituent-transport analysis balance calculations for sampling sites in reach 6, extending from Clark Fork at Deer Lodge, 

Montana (sampling site 14), to Clark Fork at Goldcreek, Montana (sampling site 16), for selected periods, water years 1996–2015.

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

Abbreviated sampling site name (table 1) and number or summation category

 Estimated normalized load,1

in kilograms per day

Unfilteredrecoverable 

copper

Unfilteredrecoverable 

arsenic

Suspended 

sediment

Water years 1996–2000 (period 1)

Inflow

Clark Fork at Deer Lodge (sampling site 14) 13 7.7 8,300

Outflow

Clark Fork at Goldcreek (sampling site 16) 19 11 16,000

Total within-reach change in load—outflow (sampling site 16) minus inflow (sampling site 14) 

(positive values indicate net mobilization from all within-reach sources including groundwater 

inflow, tributaries, the main-stem channel, and flood plain)

5.4 3.5 7,500

Water years 2001–5 (period 2)

Inflow

Clark Fork at Deer Lodge (sampling site 14) 12 7.6 7,200

Outflow

Clark Fork at Goldcreek (sampling site 16) 17 10 12,000

Total within-reach change in load—outflow (sampling site 16) minus inflow (sampling site 14) 

(positive values indicate net mobilization from all within-reach sources including groundwater 

inflow, tributaries, the main-stem channel, and flood plain)

4.6 2.6 5,000

Water years 2006–10 (period 3)

Inflow

Clark Fork at Deer Lodge (sampling site 14) 13 7.4 7,200

Outflow

Clark Fork at Goldcreek (sampling site 16) 15 10 10,000

Total within-reach change in load—outflow (sampling site 16) minus inflow (sampling site 14) 

(positive values indicate net mobilization from all within-reach sources including groundwater 

inflow, tributaries, the main-stem channel, and flood plain)

2.2 2.8 3,200

Water years 2011–15 (period 4)

Inflow

Clark Fork at Deer Lodge (sampling site 14) 12 7.0 6,800

Outflow

Clark Fork at Goldcreek (sampling site 16) 15 9.9 12,000

Total within-reach change in load—outflow (sampling site 16) minus inflow (sampling site 14) 

(positive values indicate net mobilization from all within-reach sources including groundwater 

inflow, tributaries, the main-stem channel, and flood plain)

2.7 2.8 4,900

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 1996–2015 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.
80 Water-Quality Trends and Constituent-Transport Analysis for Selected Sampling Sites

Table 4–4. Constituent-transport analysis balance calculations for sampling sites in reach 7, extending from Clark Fork at Goldcreek, 

Montana (sampling site 16), to Clark Fork near Drummond, Montana (sampling site 18), for selected periods, water years 1996–2015.

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

Abbreviated sampling site name (table 1) and number or summation category

 Estimated normalized load,1

in kilograms per day

Unfilteredrecoverable 

copper

Unfilteredrecoverable 

arsenic

Suspended 

sediment

Water years 1996–2000 (period 1)

Inflow

Clark Fork at Goldcreek (sampling site 16) 19 11 16,000

Outflow

Clark Fork near Drummond (sampling site 18) 24 16 26,000

Total within-reach change in load—outflow (sampling site 18) minus inflow (sampling site 16) 

(positive values indicate net mobilization from all within-reach sources including groundwater 

inflow, tributaries, the main-stem channel, and flood plain)

4.6 5.2 10,000

Water years 2001–5 (period 2)

Inflow

Clark Fork at Goldcreek (sampling site 16) 17 10 12,000

Outflow

Clark Fork near Drummond (sampling site 18) 21 15 21,000

Total within-reach change in load—outflow (sampling site 18) minus inflow (sampling site 16) 

(positive values indicate net mobilization from all within-reach sources including groundwater 

inflow, tributaries, the main-stem channel, and flood plain)

4.1 5.0 8,300

Water years 2006–10 (period 3)

Inflow

Clark Fork at Goldcreek (sampling site 16) 15 10 10,000

Outflow

Clark Fork near Drummond (sampling site 18) 19 15 21,000

Total within-reach change in load—outflow (sampling site 18) minus inflow (sampling site 16) 

(positive values indicate net mobilization from all within-reach sources including groundwater 

inflow, tributaries, the main-stem channel, and flood plain)

4.3 4.8 10,000

Water years 2011–15 (period 4)

Inflow

Clark Fork at Goldcreek (sampling site 16) 15 9.9 12,000

Outflow

Clark Fork near Drummond (sampling site 18) 18 14 21,000

Total within-reach change in load—outflow (sampling site 18) minus inflow (sampling site 16) 

(positive values indicate net mobilization from all within-reach sources including groundwater 

inflow, tributaries, the main-stem channel, and flood plain)

2.9 4.6 9,100

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 1996–2015 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.
Appendixes 81

Table 4–5. Constituent-transport analysis balance calculations for sampling sites in reach 8, extending from Clark Fork near 

Drummond, Montana (sampling site 18), to Clark Fork at Turah Bridge near Bonner, Montana (sampling site 20), for selected periods, 

water years 1996–2015.

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

Abbreviated sampling site name (table 1) and number or summation category

 Estimated normalized load,1

in kilograms per day

Unfilteredrecoverable 

copper

Unfilteredrecoverable 

arsenic

Suspended 

sediment

Water years 1996–2000 (period 1)

Inflow

Clark Fork near Drummond (sampling site 18) 24 16 26,000

Outflow

Clark Fork at Turah Bridge (sampling site 20) 25 17 33,000

Total within-reach change in load—outflow (sampling site 20) minus inflow (sampling site 18) 

(negative values indicate net accumulation in reach channel; positive values indicate net 

mobilization from all within-reach sources including groundwater inflow, tributaries, the 

main-stem channel, and flood plain)

1.6 0.49 6,300

Water years 2001–5 (period 2)

Inflow

Clark Fork near Drummond (sampling site 18) 21 15 21,000

Outflow

Clark Fork at Turah Bridge (sampling site 20) 22 16 26,000

Total within-reach change in load—outflow (sampling site 20) minus inflow (sampling site 18)

(negative values indicate net accumulation in reach channel; positive values indicate net 

mobilization from all within-reach sources including groundwater inflow, tributaries, the 

main-stem channel, and flood plain)

1.5 0.58 5,900

Water years 2006–10 (period 3)

Inflow

Clark Fork near Drummond (sampling site 18) 19 15 21,000

Outflow

Clark Fork at Turah Bridge (sampling site 20) 21 16 27,000

Total within-reach change in load—outflow (sampling site 20) minus inflow (sampling site 18) 

(negative values indicate net accumulation in reach channel; positive values indicate net 

mobilization from all within-reach sources including groundwater inflow, tributaries, the 

main-stem channel, and flood plain)

2.3 1.5 5,800

Water years 2011–15 (period 4)

Inflow

Clark Fork near Drummond (sampling site 18) 18 14 21,000

Outflow

Clark Fork at Turah Bridge (sampling site 20) 21 16 28,000

Total within-reach change in load—outflow (sampling site 20) minus inflow (sampling site 18) 

(negative values indicate net accumulation in reach channel; positive values indicate net 

mobilization from all within-reach sources including groundwater inflow, tributaries, the 

main-stem channel, and flood plain)

3.2 1.3 6,900

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 1996–2015 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.
82 Water-Quality Trends and Constituent-Transport Analysis for Selected Sampling Sites

Table 4–6. Constituent-transport analysis balance calculations for sampling sites in reach 9, extending from Clark Fork at Turah Bridge 

near Bonner, Montana (sampling site 20), to Clark Fork above Missoula, Montana (sampling site 22), for selected periods, water years 

1996–2015.

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

Abbreviated sampling site name (table 1) and number or summation category

 Estimated normalized load,1

in kilograms per day

Unfilteredrecoverable 

copper

Unfilteredrecoverable 

arsenic

Suspended 

sediment

Water years 1996–2000 (period 1)

Inflow

Clark Fork at Turah Bridge (sampling site 20) 25 17 33,000

Outflow

Clark Fork above Missoula (sampling site 22) 29 19 39,000

Total within-reach change in load—outflow (sampling site 22) minus inflow (sampling site 20) 

(positive values indicate net mobilization from all within-reach sources including groundwater 

inflow, tributaries, the main-stem channel, and flood plain)

3.7 2.5 6,000

Water years 2001–5 (period 2)

Inflow

Clark Fork at Turah Bridge (sampling site 20) 22 16 26,000

Outflow

Clark Fork above Missoula (sampling site 22) 30 18 42,000

Total within-reach change in load—outflow (sampling site 22) minus inflow (sampling site 20) 

(positive values indicate net mobilization from all within-reach sources including groundwater 

inflow, tributaries, the main-stem channel, and flood plain)

7.7 2.6 16,000

Water years 2006–10 (period 3)

Inflow

Clark Fork at Turah Bridge (sampling site 20) 21 16 27,000

Outflow

Clark Fork above Missoula (sampling site 22) 54 22 83,000

Total within-reach change in load—outflow (sampling site 22) minus inflow (sampling site 20) 

(positive values indicate net mobilization from all within-reach sources including groundwater 

inflow, tributaries, the main-stem channel, and flood plain)

32 5.9 56,000

Water years 2011–15 (period 4)

Inflow

Clark Fork at Turah Bridge (sampling site 20) 21 16 28,000

Outflow

Clark Fork above Missoula (sampling site 22) 23 18 40,000

Total within-reach change in load—outflow (sampling site 22) minus inflow (sampling site 20) 

(positive values indicate net mobilization from all within-reach sources including groundwater 

inflow, tributaries, the main-stem channel, and flood plain)

2.2 2.1 12,000

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 1996–2015 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.
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U.S. Geological Survey 

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(406) 457–5900

Or visit the Wyoming-Montana Water Science Center Web site at: 

http://wy-mt.water.usgs.gov/
Sando and Vecchia—Water-Quality Trends and Constituent-Transport Analysis for Selected Sampling Sites—Scientific Investigations Report 2016–5100

ISSN 2328-0328 (online)

http://dx.doi.org/10.3133/sir20165100