U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2011–5030

In cooperation with the U.S. Environmental Protection Agency

Estimated Loads of Suspended Sediment and Selected 

Trace Elements Transported Through the Clark Fork Basin, 

Montana, in Selected Periods Before and After the Breach 

of Milltown Dam (Water Years 1985–2009)
Cover. Upper left photograph: aerial view of the Clark Fork flowing past the remnants of Milltown Dam (courtesy of Gary Matson, 

 April 2008). Background photograph: Milltown Dam prior to removal in March 2008. Photograph by John H. Lambing, 

 U.S. Geological Survey, taken May 1989.
Estimated Loads of Suspended Sediment and 

Selected Trace Elements Transported 

Through the Clark Fork Basin, Montana, in 

Selected Periods Before and After the Breach 

of Milltown Dam (Water Years 1985–2009)

By Steven K. Sando and John H. Lambing

In cooperation with the U.S. Environmental Protection Agency

Scientific Investigations Report 2011–5030

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2011

For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, 

natural hazards, and the environment, visit http://www.usgs.gov or call 1-888-ASK-USGS

For an overview of USGS information products, including maps, imagery, and publications, 

visit http://www.usgs.gov/pubprod

To order this and other USGS information products, visit http://store.usgs.gov

Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Sando, S.K., and Lambing, J.H., 2011, Estimated loads of suspended sediment and selected trace elements transported through the Clark Fork basin, Montana, in selected periods before and after the breach of Milltown Dam 

(water years 1985–2009): U.S. Geological Survey Scientific Investigations Report 2011–5030, 64 p.
iii

Contents

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

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

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

Description of the Study Area ............................................................................................................5

Hydrologic Characteristics .................................................................................................................6

Methods of Data Collection and Quality Assurance................................................................................9

Methods for Estimating Constituent Loads .............................................................................................12

Estimation of Constituent Loads for 

High-Intensity Stations.........................................................................................................13

Suspended Sediment ...............................................................................................................14

Trace Elements...........................................................................................................................16

Estimation of Constituent Loads for Low-Intensity Stations........................................................20

Estimated Loads Transported Through the Milltown Reservoir Project Area...................................25

Daily Loads...........................................................................................................................................25

Suspended Sediment ................................................................................................................26

Trace Elements...........................................................................................................................31

Annual Loads and Mass Balance ....................................................................................................32

Estimated Loads Transported Through the Clark Fork Downstream from the Milltown Reservoir 

Project Area ....................................................................................................................................39

Summary and Conclusions.........................................................................................................................50

Acknowledgments..............................................................................................................................54

References Cited..........................................................................................................................................54

Supplement 1. Methods for Estimating Censored Concentrations for Water Year 2009 

and Comparison with Methods Used in Previous Years..........................................................57

Supplement 2. Regression Equations for Estimating Suspended-Sediment and 

Trace-Element Discharge for the High-Intensity Stations.......................................................59

Supplement 3. Regression Equations for Estimating Suspended-Sediment and 

Trace-Element Discharge for the Low-Intensity Stations .......................................................61

Figures

1. Map showing location of study area.........................................................................................3

2–11. Graphs showing:

2. Daily mean streamflow during water year 2004–09 and selected 

exceedance percentiles of long-term (water years 1930–2009) daily 

mean streamflow for Clark Fork above Missoula, Mont...............................................7

3. Relations between suspended-sediment discharge and streamflow for 

water years 2008–09, with comparison to data for water years 1985–2007 ............15

4. Relations between unfiltered-recoverable copper discharge and 

suspended-sediment discharge for water years 2008–09, with 

comparison to data for water years 1985–2007............................................................18
iv

5. Relations between unfiltered-recoverable arsenic discharge and 

streamflow for water years 2008–09, with comparison to data for 

water years 1985–2007......................................................................................................19

6. Relations between suspended-sediment discharge and streamflow for 

water years 2008–09, with comparison to data for water years 2006–07 ................22

7. Relations between unfiltered-recoverable copper discharge and 

streamflow for water years 2008–09, with comparison to data for 

water years 2006–07..........................................................................................................23

8. Relations between unfiltered-recoverable arsenic discharge and 

streamflow for water years 2008–09, with comparison to data for 

water years 2006–07..........................................................................................................24

9. Daily mean streamflow and estimated daily suspended-sediment 

loads transported to and from the Milltown Reservoir project area, 

water years 2004–09..........................................................................................................27

10. Daily mean streamflow and estimated daily unfiltered-recoverable 

copper loads transported to and from the Milltown Reservoir project 

area, water years 2004–09 ...............................................................................................28

11. Daily mean streamflow and estimated daily unfiltered-recoverable 

arsenic loads transported to and from the Milltown Reservoir project 

area, water years 2004–09 ...............................................................................................29

12. Pie charts showing annual streamflow volume and estimated loads of 

suspended sediment, unfiltered-recoverable copper, and unfilteredrecoverable arsenic contributed from upstream source areas and the 

Milltown Reservoir project area for selected time periods during water 

years 1985–2009 ..........................................................................................................................37

13. Graphs showing cumulative estimated loads of suspended sediment, 

unfiltered-recoverable copper, and unfiltered-recoverable arsenic 

contributed from the Milltown Reservoir project area during the period 

June 1, 2006 (start of permanent drawdown), through water year 2009...........................40

14–15. Pie charts showing:

14. Annual streamflow and estimated loads of suspended sediment, 

unfiltered-recoverable copper, and unfiltered-recoverable arsenic 

for the Clark Fork downstream from the confluence with the 

Flathead River and contributions from upstream source areas, 

water years 2006–09..........................................................................................................45

15. Percent of fines and sand in the cumulative loads of suspended 

sediment for concurrent periodic water-quality samples for the 

combined inflow to and outflow from the main-stem reach of the 

Clark Fork from the Clark Fork above Missoula to the Clark Fork at 

St. Regis, water years 2006–09........................................................................................47
v

Tables

1. Data-collection stations and information on data-collection activities in the 

study area during water years 1985–2009 ................................................................................5

2. Annual streamflow and peak-flow characteristics for selected periods, with 

comparison to long-term annual streamflow (water years 1930–2009) and 

peak-flow (water years 1908, 1930–2009) characteristics for Clark Fork above 

Missoula, Mont. (station 12340500)............................................................................................9

3. Annual streamflow and peak-flow characteristics for selected periods, with 

comparison to long-term annual streamflow and peak-flow (water 

years 1911–23, 1929–2009) characteristics for Clark Fork at St. Regis, Mont. 

(station 12354500)........................................................................................................................10

4. Percentage of samples with censored trace-element concentrations, 

water years 2004–09...................................................................................................................13

5. Mass balance of annual streamflow and estimated loads of suspended 

sediment and unfiltered-recoverable trace elements transported to and from 

the Milltown Reservoir project area for selected periods during water 

years 1985–2009 ..........................................................................................................................34

6. Mass balance of annual streamflow and estimated loads of suspended 

sediment and unfiltered-recoverable trace elements transported to the 

Clark Fork downstream from the confluence with the Flathead River during 

water years 2006–09...................................................................................................................42

7. Estimated annual loads and mass balance of various size classes of suspended 

sediment transported to and from the main-stem reach of the Clark Fork from 

the Clark Fork above Missoula to the Clark Fork at St. Regis during water 

years 2006–09 ..............................................................................................................................48

S1.1. Regression equations for estimating unfiltered-recoverable trace-element 

concentrations for censored data for the high-intensity data-collection stations, 

water year 2009...........................................................................................................................58

S1.2. Regression equations for estimating unfiltered-recoverable trace-element 

concentrations for censored data for low-intensity data-collection stations 

in the lower Clark Fork basin, water years 2006–09..............................................................58

S2.1. Regression equations for estimating unfiltered-recoverable trace-element 

discharge for Clark Fork at Turah Bridge, near Bonner, Mont. (station 12334550), 

water year 2009...........................................................................................................................59

S2.2. Regression equations for estimating unfiltered-recoverable trace-element 

discharge for Blackfoot River near Bonner, Mont. (station 12340000), 

water year 2009...........................................................................................................................59

S2.3. Regression equations for estimating unfiltered-recoverable trace-element 

discharge for Clark Fork above Missoula, Mont. (station 12340500), 

water year 2009...........................................................................................................................60

S3.1. Regression equations for estimating suspended-sediment and unfilteredrecoverable trace-element discharge for Bitterroot River near Missoula, 

Mont. (station 12352500), water years 2006–09 .....................................................................61

S3.2. Regression equations for estimating suspended-sediment and unfilteredrecoverable trace-element discharge for Clark Fork at St. Regis, Mont. (station 

12354500), water years 2006–09 ...............................................................................................62

S3.3. Regression equations for estimating suspended-sediment and unfilteredrecoverable trace-element discharge for Flathead River near Perma, Mont. 

(station 12388700), water years 2006–09.................................................................................64
vi

Conversion Factors, Datum, Abbreviated Units, and 

Acronyms

Multiply By To obtain

acre 0.4047 hectare (ha)

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

)

cubic yard (yd3

) 0.7647 cubic meter (m3)

cubic foot per second (ft3

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

foot (ft) 0.3048 meter (m)

gallon (gal) 3.785 liter (L)

mile (mi) 1.609 kilometer (km)

ounce (oz) 28,350 milligram (mg)

ounce (oz) 28,350,000 microgram (μg)

pound, avoirdupois (lb) 0.4536 kilogram (kg) 

square mile (mi2

) 2.590 square kilometer (km2) 

ton, short (2,000 lb) 0.9072 megagram (Mg) 

ton per day 0.9072 metric ton per day

ton per year 0.9072 metric ton per year

Horizontal coordinate information is referenced to the North American Datum of 1927 (NAD 27).

Water year is 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 2009 is the period from October 1, 2008, through September 30, 2009.

Abbreviated units and symbol used in this report:

acre-ft acre-feet

ft feet

ft3/s cubic feet per second

μg/L micrograms per liter

μg/g micrograms per gram

mg/L milligrams per liter

mi miles

mi2 square miles

mm millimeter

μm micrometer

yd3 cubic yards

< less than
vii

Acronyms used in this report:

AMLE adjusted maximum likelihood estimation

ASQ arsenic discharge

CDQ cadmium discharge

CUQ copper discharge

FEQ iron discharge

LOG logarithm (base 10)

LOWESS locally weighted scatter plot smoothing

MNQ manganese discharge

NPL National Priorities List

NWQL National Water Quality Laboratory

NWIS National Water Information System

OLS ordinary least squares

p-value significance level

PBQ lead discharge Q streamflow R2 coefficient of determination

RBCF retransformation-bias-correction factor

RL reporting level

RSD relative standard deviation

SEDQ suspended-sediment discharge

SE standard error of estimate

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

ZNQ zinc discharge

Estimated Loads of Suspended Sediment and Selected 

Trace Elements Transported Through the Clark Fork Basin, 

Montana, in Selected Periods Before and After the Breach 

of Milltown Dam (Water Years 1985–2009)

By Steven K. Sando and John H. Lambing1

1

U.S. Geological Survey, retired

Abstract

Milltown Reservoir is a National Priorities List Superfund site in the upper Clark Fork basin of western Montana 

where sediments enriched in trace elements from historical 

mining and ore processing have been deposited since the 

completion of Milltown Dam in 1908. Milltown Dam was 

breached on March 28, 2008, as part of Superfund remediation 

activities to remove the dam and excavate contaminated sediment that had accumulated in Milltown Reservoir. In preparation for the breach of Milltown Dam, permanent drawdown 

of Milltown Reservoir began on June 1, 2006, and lowered 

the water-surface elevation by about 10 to 12 feet. After the 

breach of Milltown Dam, the water-surface elevation was 

lowered an additional 17 feet.

Hydrologic data-collection activities were conducted by 

the U.S. Geological Survey in cooperation with U.S. Environmental Protection Agency to estimate loads of suspended 

sediment and trace elements transported through the Clark 

Fork basin before and after the breach of Milltown Dam. This 

report presents selected results of the data-collection activities.

Daily and cumulative loads of suspended sediment and 

selected trace elements transported during water year 2009 

were estimated for three high-intensity sampling stations that 

bracket the Milltown Reservoir project area and were used 

to quantify the net gain or loss (mass balance) of suspended 

sediment and trace elements within the project area for water 

year 2009. Estimated loads and mass balance for water 

year 2009 within the project area were compared to estimated 

loads and mass balance within the project area for selected 

periods before and after the breach of Milltown Dam. Loads 

of suspended sediment and selected trace elements transported 

during water years 2006–09 also were estimated for three lowintensity sampling stations in the lower Clark Fork (downstream from Milltown Reservoir project area) and were used 

to evaluate how project area outflow loads were integrated into 

downstream transport processes.

During the period June 1, 2006 (start of permanent 

drawdown) to the end of water year 2009, the estimated 

cumulative loads contributed from within the project area were 

623,000 tons, 235 tons, and 26.8 tons of suspended sediment, 

copper, and arsenic, respectively. During the years of the most 

substantial remediation activities (that is, water year 2007, 

when permanent drawdown was conducted for the entire year, 

and water year 2008 when Milltown Dam was breached), 

constituent loads relative to streamflow were proportionately larger than other years, and the relative contributions of 

constituents from within the project area also were larger than 

other years. The relative contributions of constituents from 

source areas during water years 2006 and 2009 (before and 

after the most substantial remediation activities) generally 

were similar. Thus, in a relatively short time frame after the 

start of the most substantial remediation activities (that is, the 

period from the start of permanent drawdown on June 1, 2006, 

to the end of water year 2008, during which two complete 

annual-runoff periods occurred), constituent transport characteristics in the Clark Fork near the project area appear to be 

approaching typical conditions observed before the breach of 

Milltown Dam. However, remediation and restoration activities that occur after the end of water year 2009 might affect 

the apparent temporal pattern in constituent transport characteristics; for example, the diversion of the Clark Fork from 

the constructed bypass channel to a new channel that occurred 

in December 2010. Adjustment of the Clark Fork to a new 

geomorphic environment might alter patterns in constituent 

transport characteristics.

The effects of the Milltown Dam removal activities 

are apparent in the temporal variability of relative contributions of estimated annual suspended-sediment loads from 

source areas to the lower Clark Fork. During the years of 

the most substantial remediation activities (water years 2007 

and 2008), the estimated annual loads for the Clark Fork 

above Missoula (project area outflow) accounted for a large 

percentage (65 percent for water year 2007 and 70 percent 
2 Estimated Loads of Suspended Sediment and Trace Elements, Clark Fork Basin, Montana (Water Years 1985–2009)

for water year 2008) of the suspended-sediment load transported past the lower Clark Fork outflow (calculated as the 

sum of Clark Fork at St. Regis and Flathead River at Perma). 

During water years 2006 and 2009 (before and after the most 

substantial remediation activities), the Clark Fork above 

Missoula accounted for smaller percentages (25 percent for 

water year 2006 and 38 percent for water year 2009) of the 

suspended-sediment load transported past the lower Clark 

Fork outflow than in water years 2007–08. Also, the relative 

contributions of suspended sediment from source areas during 

water years 2006 and 2009 generally were similar. 

Temporal variability in the relative contributions of 

estimated annual copper loads from source areas to the 

lower Clark Fork downstream from the project area outflow 

generally was similar to that of estimated annual suspendedsediment loads. During the years of the most substantial remediation activities (water years 2007 and 2008), the estimated 

annual loads for the project area outflow at Clark Fork above 

Missoula accounted for a large percentage (87 percent for 

water years 2007 and 2008) of the estimated annual copper 

load transported past the lower Clark Fork outflow. During 

water years 2006 and 2009 (before and after the most substantial remediation activities), the Clark Fork above Missoula 

accounted for a smaller percentage (60 percent for water 

year 2006 and 78 percent for water year 2009) of the annual 

copper load transported past the lower Clark Fork outflow than 

in water years 2007–08.

Temporal variability in the relative contributions of 

estimated annual arsenic loads from source areas to the lower 

Clark Fork differed somewhat from those of estimated annual 

suspended-sediment and copper loads, primarily because a 

relatively large proportion of arsenic is transported in dissolved phase in the Clark Fork. The project area outflow at 

Clark Fork above Missoula contributed about 50 percent or 

more of the annual arsenic load transported past the lower 

Clark Fork outflow for all years during 2006–09. A substantial 

part of the arsenic load transported past the Clark Fork above 

Missoula was contributed from the basin upstream from Clark 

Fork at Turah Bridge in all years. 

For all years, loads of all constituents contributed from 

the basin upstream from Flathead River at Perma to the lower 

Clark Fork outflow generally were small, even though the 

Flathead River accounted for an average of 62 percent of 

the annual streamflow of the lower Clark Fork outflow. The 

interannual range in percent contribution of estimated annual 

loads from the Flathead River at Perma to the lower Clark 

Fork outflow was 5 to 19 percent for suspended sediment, 4 to 

11 percent for copper, and 17 to 26 percent for arsenic.

Introduction

Milltown Reservoir was located on the Clark Fork at the 

confluence of the Clark Fork and Blackfoot River in western 

Montana (fig. 1). The reservoir was formed by the completion 

of Milltown Dam in 1908 (U.S. Environmental Protection 

Agency, 2009) for hydroelectric power. Historical large-scale 

mining and ore processing in the upper Clark Fork basin produced large quantities of tailings enriched with trace elements 

such as cadmium, copper, lead, zinc, and arsenic. Tailings 

have been eroded and transported downstream along the Clark 

Fork, contaminating the approximately 6.6 million cubic yards 

(yd3

) of sediment that had accumulated in Milltown Reservoir (U.S. Environmental Protection Agency, 2004). Potential 

toxicity from the elevated trace-element concentrations in 

water and bed sediment led to the designation of Milltown 

Reservoir and upstream reaches of the Clark Fork as an 

extended National Priorities List (NPL) Superfund site in 1983 

(U.S. Environmental Protection Agency, 2004). 

In response to the need for a water-quality data base to 

guide resource management and remediation decisions, a 

routine data-collection program to document suspended sediment and trace elements in the upper Clark Fork basin was 

begun in March 1985 (Lambing, 1991) by the U.S. Geological 

Survey (USGS) in cooperation with the U.S. Environmental 

Protection Agency (USEPA) and has continued to present day 

(2010). The routine data-collection program is referred to in 

this report as the “long-term monitoring program.”

As part of remediation planning associated with the 

Superfund process, the USEPA issued a record of decision in 

December 2004 that included removal of Milltown Dam as 

one of the remediation activities (U.S. Environmental Protection Agency, 2004). Descriptions of activities associated 

with the removal of Milltown Dam are provided in a series of 

updates by USEPA (http://www.epa.gov/region8/superfund/

mt/milltown/updates.html, accessed October 29, 2010). On 

March 28, 2008, Milltown Dam was breached as part of the 

process to remove the dam and excavate a large portion of 

the contaminated sediment. The term “Milltown Reservoir 

project area” (hereinafter generally referred to as “project 

area”) is used to refer to the remediation-affected area during 

the periods before and after the dam breach when the environment changed from a reservoir to a free-flowing river. The 

project area (fig. 1) collectively includes the former location of 

the reservoir, plus river reaches of indefinite length upstream 

from the Clark Fork and Blackfoot River arms of the reservoir where increased erosion could occur as the result of the 

rivers adjusting to the steeper hydraulic gradient caused by the 

breach of Milltown Dam.

To accommodate various construction activities and the 

excavation of a portion of the reservoir bottom sediments 

before dam removal, the initial phase (Stage 1) of a permanent drawdown of the reservoir to a lower pool level began 

on June 1, 2006 (Lambing and Sando, 2009); thus, Milltown 

Reservoir was relatively shallow during all of water year 2007 

and part of water year 2008 before the breach of Milltown 

Dam. The Stage 1 drawdown lowered the average summer 

pool by about 10 to 12 feet (ft). Two subsequent phases of 

drawdown (Stages 2 and 3) were designed to lower the reservoir pool an additional 17 ft (for a total drawdown of about 

29 ft) to allow access for removal of the powerhouse and 
Figure 1. Location of study area.

River

Fork

Rock

Creek

Blackfoot Former Milltown Reservoir

Bitterroot

River

Clark

Flint

Creek

Little

Clark Fork

Warm Springs

 Ponds

Warm Springs Creek

Silver

Bow

Lost Creek

Mill

Willow Creek

Creek

Blackfoot

River

Creek

Blacktail

Creek

Bonner

Turah

Missoula

Drummond

Goldcreek

Anaconda

Galen

Warm Springs

Deer Lodge

Butte

DIVIDE

CONTINENTAL

Base modified from U.S. Geological Survey, 

State base map, 1966, 1:1,000,000

North American Datum of 1927 (NAD 27) 

46°

114°

113°

47°

0 10 20 MILES

0 10 20 KILOMETERS

A

A

Former 

Milltown Dam

Approximate extent of former

Milltown Reservoir and project area

Clark

Clark

River

Fork

ork

Blackfoot

Former Milltown Reservoir

F

Bonner

Turah

1

3

4

2

115°

47°

A

A

Clark

Fork

Clark

Fork

Flathead Perma River

St. Regis

Thompson 

Falls

Plains

6

7

Thompson Falls Dam

Thompson Falls

Reservoir

Helena

M O N T A N A

Study area

A

A

Streamflow-gaging station

 Continuous record

Water-quality site

 Periodic water quality

 Daily suspended sediment

EXPLANATION

Map number (table 1)

5

5

Downstream part of study area

Upstream part of study area

Introduction 3
4 Estimated Loads of Suspended Sediment and Trace Elements, Clark Fork Basin, Montana (Water Years 1985–2009)

spillway (Lambing and Sando, 2009). The Stage 2 drawdown 

occurred on March 28, 2008, when the powerhouse coffer 

dam was breached, thereby lowering the water-surface elevation by 15 ft at the former forebay foundation (http://www.

epa.gov/region8/superfund/mt/milltown/pdf/Update02Apr08.

pdf, accessed October 29, 2010). The Stage 2 drawdown is 

hereinafter referred to as “the breach of Milltown Dam.” Prior 

to the breach of Milltown Dam, a bypass channel was constructed to provide controlled routing of the Clark Fork and 

allow further excavation of contaminated reservoir sediment 

for hauling to a repository. Diversion of the Clark Fork into 

the bypass channel began on March 21, 2008 (http://www.

epa.gov/region8/superfund/mt/milltown/pdf/Update12Mar08.

pdf, accessed October 29, 2010), and was completed with the 

construction of a diversion dike on the Clark Fork upstream 

from the bypass channel on April 12, 2008 (http://www.epa.

gov/region8/superfund/mt/milltown/pdf/Update16Apr08.pdf, 

accessed October 29, 2010). The Stage 3 drawdown began 

when the spillway coffer dam was breached on March 27, 

2009, thereby lowering the water-surface elevation the final 

2 ft to the estimated pre-dam elevation at the former spillway 

(http://www.epa.gov/region8/superfund/mt/milltown/pdf/

Update01Apr09.pdf, accessed October 29, 2010).

Because of the large quantity of contaminated sediment 

that was deposited in Milltown Reservoir, there was concern 

regarding an increased potential for scour of bottom sediments 

and associated trace elements from the shallow reservoir after 

the start of the permanent drawdown, especially during high 

streamflow. To address this concern, the USGS in cooperation 

with USEPA conducted a program of intensive sampling that 

began in water year 2006 to supplement the routine sampling 

of the long-term monitoring program (Dodge and others, 

2009). The supplemental sampling was conducted during the 

April–June periods of water years 2006 and 2007 to target the 

rising limb and peak flow of the annual hydrograph, as well as 

selected periods of reservoir drawdown. The sampling primarily provided additional information on variations in water 

quality and constituent transport through Milltown Reservoir. 

The supplemental sampling continued in March–July of water 

years 2008–09 to characterize suspended-sediment and traceelement transport during (1) the final stages of the permanent 

reservoir drawdown, (2) the period immediately before and 

after the breach of Milltown Dam in water year 2008, and 

(3) the runoff period of water year 2009, about 1 year after the 

breach of Milltown Dam.

High-intensity sampling was conducted at three 

streamflow-gaging stations (hereinafter referred to as “highintensity stations”; table 1) that bracket the project area. Clark 

Fork at Turah Bridge, near Bonner (station 12334550) and 

Blackfoot River near Bonner (station 12340000) are upstream 

from the project area and represent the primary inflows to the 

project area. Clark Fork above Missoula (station 12340500) is 

downstream from the project area and represents the outflow 

from the project area. Daily loads for these stations were 

estimated by using periodic water-quality, daily streamflow, 

and daily suspended-sediment data. Annual loads for selected 

periods were determined by summing estimated daily loads 

for each site. The difference between the annual inflow and 

outflow loads were used to quantify the net gain or loss (mass 

balance) of suspended sediment and selected trace elements 

within the project area. 

Although the supplemental sampling primarily focused 

on the three high-intensity stations that bracket the Milltown Reservoir project area and the transport of suspendedsediment and trace-element loads to and from the project 

area, low-intensity sampling also was conducted at four other 

stations (hereinafter referred to as “low-intensity stations”; 

table 1). Periodic water-quality samples were collected at 

Clark Fork Bypass near Bonner (station 12334570) in water 

years 2008–09 to provide information on the erosional processes occurring within the former Milltown Reservoir as the 

river adjusted to the steeper channel gradient caused by the 

breach of Milltown Dam. The Clark Fork Bypass did not have 

a continuous streamflow gage, and flows were considered 

equivalent to those at Clark Fork at Turah Bridge. Suspendedsediment data for the periodic water-quality samples collected at Clark Fork Bypass near Bonner are compared with 

concurrently collected data at Clark Fork at Turah Bridge to 

provide information on suspended-sediment transport within 

the intervening reach. Periodic water-quality samples also 

were collected in water years 2006–09 at three low-intensity 

stations in the Clark Fork basin downstream from Clark Fork 

above Missoula (station 12340500; the outflow from the 

project area) to provide general information on the transport 

of suspended-sediment and trace-element loads to and from 

reaches of the Clark Fork between the project area outflow 

and just downstream from the confluence with the Flathead 

River (hereinafter referred to as the lower Clark Fork). The 

three low-intensity stations downstream from the project area 

outflow are Bitterroot River near Missoula (station 12352500), 

Clark Fork at St. Regis (station 12354500), and Flathead River 

at Perma (station 12388700). 

Purpose and Scope

The purpose of this report is to present selected results 

of hydrologic data-collection activities conducted by the 

U.S. Geological Survey, in cooperation with U.S. Environmental Protection Agency, to estimate loads of suspended sediment and trace elements transported through the Clark Fork 

basin before and after the breach of Milltown Dam. Daily and 

annual cumulative loads of suspended sediment and selected 

trace elements (unfiltered-recoverable cadmium, copper, iron, 

lead, manganese, zinc, and arsenic) transported during water 

year 2009 were estimated for three high-intensity stations 

that bracket the Milltown Reservoir project area (fig. 1). The 

annual estimated loads were used to quantify the mass balance 

of suspended sediment and trace elements within the project 

area for water year 2009. Estimated loads and mass balance 

within the project area for water year 2009 are compared 

to estimated loads and mass balance within the project area 
Table 1. Data-collection stations and information on data-collection activities in the study area during water years 1985–2009.

[Abbreviations: e, estimated from U.S. Geological Survey 7.5-minute topographic map. Symbols: --, no data collected. For map number location, see figure 1]

Map 

number

Station 

number Station name

Drainage 

area

 (square 

miles)

Datacollection 

intensity

 designation

Information on data-collection activities

Water years of 

continuous 

streamflow data 

collection

Water years of 

daily suspendedsediment data 

collection

Water years of 

periodic 

water-quality 

sampling data 

collection (average 

number of samples 

per year)

1 12334550 Clark Fork at Turah 

Bridge, near Bonner, Mont.

 3,641 High intensity 1986–2009 1985–2009 1985–2005 (9);

2006–09 (22)

2 12334570 Clark Fork Bypass, 

near Bonner, 

Mont.

 e3,650 Low intensity -- -- 2008–09 (14)1

3 12340000 Blackfoot River near 

Bonner, Mont.

 2,290 High intensity 1985–2009 1986–95, 2006–09 1985–2005 (5);

2006–09 (20)

4 12340500 Clark Fork above 

Missoula, Mont.

 5,999 High intensity 1985–2009 1986–2009 1990–2005 (10);

2006–09 (22)

5 12352500 Bitterroot River near 

Missoula, Mont.

 2,814 Low intensity 1989–2009 -- 2006–09 (14)1

6 12354500 Clark Fork at St. 

Regis, Mont.

 10,709 Low intensity 1985–2009 -- 2006–09 (14)1

7 12388700 Flathead River at 

Perma, Mont.

 8,795 Low intensity 1985–2009 -- 2006–09 (14)1

1

Samples collected seasonally, generally from March–July.

Introduction 5

for selected periods before and after the breach of Milltown 

Dam, including (1) long-term average annual values for water 

years 1985–2005 (representing long-term average conditions 

during the period before the start of permanent drawdown 

of Milltown Reservoir on June 1, 2006), (2) average annual 

values for 1996–97 (representing high-streamflow conditions 

during the period before the start of permanent drawdown of 

Milltown Reservoir on June 1, 2006), (3) water years 2006–07 

(representing the period between the start of permanent 

drawdown of Milltown Reservoir and the breach of Milltown 

Dam), and (4) water year 2008 (the year when Milltown Dam 

was breached). In this report, emphasis often is placed on differences in constituent transport between water year 2008 (the 

year when Milltown Dam was breached) and water year 2009 

(the first complete water year after the dam breach).

Estimated suspended-sediment and trace-element loads 

for three low-intensity stations for water years 2006–09 also 

are presented in this report to provide general information on 

the transport of suspended-sediment and trace-element loads 

to and from reaches of the lower Clark Fork downstream from 

the project area outflow. Because data-collection activities 

were less intensive for the low-intensity stations than for the 

high-intensity stations, the presentation of results for the lowintensity stations is limited and load estimates are less confident than load estimates for high-intensity stations. For the 

low-intensity stations, emphasis generally is placed on relative 

comparison of load estimates and transport patterns between 

stations and between years, rather than on the absolute magnitudes of load estimates.

The methods used to estimate loads for the high-intensity 

and low-intensity sampling stations are described. The hydrologic data, including streamflow and constituent concentrations, used to estimate loads (or presented in support of the 

load estimates and interpretive conclusions) are accessible on 

the USGS National Water Information System (NWIS) Web 

site for Montana (http://waterdata.usgs.gov/mt/nwis). Load 

estimates presented in this report represent water-column 

transport of constituents and do not include bedload transport.

Description of the Study Area

Milltown Reservoir was a small impoundment (surface 

area of about 540 acres) formed by Milltown Dam just downstream from the confluence of the Clark Fork and Blackfoot 

River (fig. 1). The Clark Fork basin upstream from Milltown Dam drains an area of about 6,000 square miles (mi2

). 

The Clark Fork basin upstream from the confluence with 

the Blackfoot River accounts for about 61 percent (about 

3,650 mi2) of the drainage area upstream from the former 

Milltown Dam. The Blackfoot River at the mouth accounts 
6 Estimated Loads of Suspended Sediment and Trace Elements, Clark Fork Basin, Montana (Water Years 1985–2009)

for about 38 percent (about 2,280 mi2) of the drainage area 

upstream from the former Milltown Dam. The former reservoir was at the downstream end of a contiguous complex of 

NPL Superfund sites (established for remediation of effects of 

large-scale mining operations) extending from the headwaters 

of Silver Bow Creek near Butte to Milltown Dam near Missoula (U.S. Environmental Protection Agency, 2004). In the 

Blackfoot River basin, numerous small-scale placer and hardrock mining operations have been active since about 1865 

(Moore and others, 1991).

Before the breach of Milltown Dam, Milltown Reservoir 

was considered a “run-of-the-river” reservoir because on a 

daily basis streamflow leaving the reservoir generally was 

about equal to the streamflow of the Clark Fork and Blackfoot River entering the reservoir (Lambing and Sando, 2009), 

where the retention time of water in the reservoir generally 

was not substantially different than a free-flowing system. 

After Milltown Dam was breached, the confluence of the 

Clark Fork and Blackfoot River transitioned from a reservoir 

environment to a free-flowing river.

The study area includes the Clark Fork basin from 

near Butte to Thompson Falls Dam (fig. 1). Data for two 

high-intensity stations upstream from the Milltown Reservoir project area (Clark Fork at Turah Bridge, near Bonner, 

station 12334550; and Blackfoot River near Bonner, station 12340000) represent the combined inflow of streamflow and constituent load to the project area; data for the 

high-intensity station Clark Fork above Missoula (station 

12340500) represent the outflow from the project area. For 

brevity, Clark Fork at Turah Bridge, near Bonner is referred 

to as Clark Fork at Turah Bridge in this report. The lowintensity station within the project area (Clark Fork Bypass 

near Bonner, station 12334570) is located about 5 river mi 

downstream from Clark Fork at Turah Bridge (table 1 and 

fig. 1). This low-intensity station is near the upstream end of a 

constructed bypass channel into which the Clark Fork started 

to be diverted on March 21, 2008, to allow the excavation 

of contaminated reservoir sediment for hauling to a repository (http://www.epa.gov/region8/superfund/mt/milltown/pdf/

Update12Mar08.pdf, accessed October 29, 2010).

Data for three low-intensity stations provide information on the transport of suspended-sediment and trace-element 

loads to and from reaches of the Clark Fork downstream from 

the project area outflow. Bitterroot River near Missoula (station 12352500) is located about 5 river mi upstream from the 

mouth of the Bitterroot River. The confluence of the Bitterroot River with the Clark Fork is located about 11 river mi 

downstream from Clark Fork above Missoula. Clark Fork 

at St. Regis (station 12354500) is located about 87 river mi 

downstream from the high-intensity station Clark Fork above 

Missoula and about 24 river mi upstream from the confluence of the Clark Fork and the Flathead River. Flathead River 

at Perma (station 12388700) is located about 11 river mi 

upstream from the mouth of the Flathead River. The confluence of the Flathead River with the Clark Fork is located about 

45 river mi upstream from Thompson Falls Dam.

The main-stem reach of the Clark Fork from Clark Fork 

above Missoula (the project area outflow) to Clark Fork at 

St. Regis serves as an accounting area downstream from the 

project area outflow for determination of annual mass balance 

of suspended sediment and selected trace elements. Massbalance estimates for this reach can be used to evaluate how 

project area outflow loads were incorporated into downstream 

transport processes. Data for the high-intensity station Clark 

Fork above Missoula and the low-intensity station Bitterroot River near Missoula represent the primary inflows to the 

reach; data for the low-intensity station Clark Fork at St. Regis 

represents the outflow from the main-stem reach above the 

Flathead River.

Suspended-sediment and trace-element loads transported 

in the Clark Fork to Thompson Falls Reservoir also are of significance in evaluating the effects of the removal of Milltown 

Dam. The combined streamflows and constituent loads of 

Clark Fork at St. Regis and Flathead River at Perma represent both the total outflow from the lower Clark Fork and the 

primary inflow to Thompson Falls Reservoir.

Hydrologic Characteristics

Streamflow magnitude is a predominant factor affecting 

the transport of suspended sediment and sediment-associated 

constituents through a drainage basin. Streamflow magnitude 

can affect the sustained delivery of constituent loads from 

the basin, as well as the capacity to locally scour sediments 

within the project area. The hydraulic energy associated with 

high flows is especially important relative to the scour of 

bottom sediments in shallow-water bodies, such as Milltown 

Reservoir during the permanent drawdown period. Increased 

hydraulic energy from high flows and the steeper hydraulic 

gradient after the breach of Milltown Dam are primary factors 

affecting erosion of remnant coffer dam materials, former reservoir bottom sediments, and channel streambed and banks.

Because streamflow can vary substantially from year to 

year, thereby affecting rates of constituent transport, comparison of streamflow during water years 2004–09 to a long-term 

period of record allows recent hydrologic conditions to be 

placed in a historical perspective. Clark Fork above Missoula 

has the longest period of continuous-streamflow record in the 

upper Clark Fork basin (80 years, water years 1930–2009) for 

comparison to streamflow during water years 2004–09. The 

variation in daily mean streamflow at Clark Fork above Missoula during water years 2004–09 is shown in figure 2, along 

with selected long-term streamflow characteristics for water 

years 1930–2009, to illustrate differences between recent and 

long-term hydrologic conditions. Water years 2004–09 are 

specifically presented in figure 2 to illustrate streamflow conditions before the start of permanent drawdown, during permanent drawdown, the day that Milltown Dam was breached, and 

the period after the breach of Milltown Dam.

The long-term streamflow characteristics for Clark Fork 

above Missoula (fig. 2) are represented by selected exceedance 
Figure 2. Daily mean streamflow during water year 2004–09 and selected exceedance percentiles of long-term (water 

years 1930–2009) daily mean streamflow for Clark Fork above Missoula, Mont.

Breach of Milltown 

Dam (March 28, 2008)

Start of permanent drawdown of 

Milltown Reservoir (June 1, 2006)

Period after breach of Milltown Dam Period between start of permanent drawdown of 

Milltown Reservoir and breach of Milltown Dam

Period before start of permanent drawdown of Milltown Reservoir

Water year 2004 Water year 2005 Water year 2006

Water year 2007 Water year 2008 Water year 2009

Oct. 2003 Apr. 2004 Oct. 2004 Apr. 2005 Oct. 2005 Apr. 2006 Oct. 2006

Oct. 2006 Apr. 2007 Oct. 2007 Apr. 2008 Oct. 2008 Apr. 2009 Oct. 2009

1 

10 

100 

1,000 

10,000 

30,000 

1 

10 

100 

1,000 

10,000 

30,000 

EXPLANATION

Daily mean streamflow during water years 2004–09

10th-exceedance percentile of long-term (water years 1930–2009) daily mean streamflow

50th-exceedance percentile (median) of long-term (water years 1930–2009) daily mean streamflow

90th-exceedance percentile of long-term (water years 1930–2009) daily mean streamflow

Streamflow, in cubic feet per second

Period between start of permanent drawdown of 

Milltown Reservoir and breach of Milltown Dam

Introduction 7

percentiles (10th, 50th, and 90th) of daily mean streamflow 

during the period of record (water years 1930–2009). An 

exceedance percentile indicates the magnitude of daily mean 

streamflow that was exceeded the given percent of time on a 

specific day of the year during the long-term period of record. 

For example, the 10th-exceedance percentile of long-term 

daily mean streamflow represents a relatively high streamflow magnitude that was exceeded only 10 percent of the 

time on that specific day of the year (for example, on all of 

the October 1 dates, October 2 dates, and so on) during the 

period of record. Exceedance percentiles are determined for 

each day of the year to produce an annual hydrograph of 

long-term daily mean streamflows representing a particular 

exceedance percentile (hereinafter referred to as percentile). 

For brevity and relative comparison of streamflow during 

water years 2004–09 to long-term streamflow, “normal” in this 

report refers to the 50th percentile, or long-term median.
8 Estimated Loads of Suspended Sediment and Trace Elements, Clark Fork Basin, Montana (Water Years 1985–2009)

Streamflow conditions associated with suspendedsediment and trace-element loads reported for 1985–97 are 

reported in Lambing (1991), Hornberger and others (1997), 

and Lambing (1998). Descriptions of streamflow conditions 

during water years 2004–08 are presented in Lambing and 

Sando (2008, 2009). Streamflow conditions during water years 

2004–08 are briefly summarized in this report to provide information on factors affecting constituent transport before the 

start of permanent drawdown, during permanent drawdown, 

and after the breach of Milltown Dam. Detailed descriptions 

of streamflow conditions during water year 2009 with specific 

comparisons to streamflow conditions during water year 2008 

also are presented.

Streamflows during water years 2004–07 generally were 

below or near normal at Clark Fork above Missoula (fig. 2). 

Although the annual peak flows generally were near normal, 

the falling limb of the runoff hydrograph generally occurred 

sooner relative to long-term conditions and typically decreased 

to below-normal streamflow during summer. Summer streamflows during water years 2004–07 at times were near or at the 

90th percentile. In water years 2006–07, the below-normal 

streamflow conditions continued into the fall. Winter streamflows were erratic in most years, possibly due to variations in 

the duration and intensity of freezing conditions or snowfall 

amounts and runoff during periods of thawing.

Streamflows at Clark Fork above Missoula during water 

year 2008 (fig. 2) generally were below normal from about 

October 2007 through April 2008. Winter streamflows again 

were erratic, but generally were below normal. Sustained runoff started in mid-April (after Milltown Dam was breached on 

March 28), and streamflows rose to near normal by mid-May 

and to substantially above normal by late May. Streamflows 

remained substantially above normal through June, but the 

departure from normal decreased in July and streamflows were 

near normal in August. Streamflows increased to above normal 

again in September.

Streamflows at Clark Fork above Missoula during water 

year 2009 (fig. 2) generally were above normal to near normal 

throughout the year. Streamflows from about October 2008 

through February 2009 generally were above normal with 

extended periods in mid-November and January when streamflows exceeded the 10th percentile. Six days in water year 

2009 (November 14–17, 19, and January 14) had the maximum recorded streamflows for those dates during the period 

of record at Clark Fork above Missoula (http://waterdata.usgs.

gov/mt/nwis accessed October 29, 2010). Although winter 

streamflows generally were above normal, there was a period 

during late December when streamflows decreased sharply 

to less than the 90th percentile. December 16 had the minimum recorded streamflow for that date during the period of 

record at Clark Fork above Missoula. Streamflows from about 

March through early June generally ranged from near normal 

to substantially above normal. There were 18 days during the 

rising limb of the runoff hydrograph when streamflow equaled 

or exceeded the 10th percentile. Starting in early June 2009, 

streamflows declined sharply to near-normal levels and 

continued at generally near-normal levels through the remainder of the water year. There were, however, several days in 

late August when streamflows exceeded the 10th percentile. 

As in water year 2008, the timing of the rising limb, peak flow, 

and falling limb of the runoff hydrograph generally was similar to long-term patterns, unlike the pronounced shift towards 

an earlier occurrence of runoff and recession noted for water 

years 2006 and 2007.

Differences in rising-limb streamflows between water 

year 2009 and water year 2008 are of particular relevance 

to differences in suspended-sediment and trace-element 

transport between the two years. Differences in rising-limb 

streamflows between water year 2009 and water year 2008 

are best discerned by comparison to the 10th, 50th, and 90th 

percentiles (fig. 2). Water year 2008 rising-limb streamflows 

started at levels near the 90th percentile and remained below 

or near the 50th percentile until mid-May when there was a 

precipitous rise to about the 10th percentile for a short period 

near the time of the annual peak flow on May 21. Water year 

2009 rising-limb streamflows generally were at or above the 

50th percentile with some extended periods near or above 

the 10th percentile including several days near the time of 

the annual peak flow on May 31. Near the peak of the water 

year 2009 runoff hydrograph, there were 8 consecutive days 

(May 25–June 1) when streamflows equaled or exceeded the 

maximum daily streamflow in water year 2008. 

Annual streamflow and peak-flow characteristics for 

selected periods (table 2) for Clark Fork above Missoula 

are compared to long-term characteristics for the period of 

continuous-streamflow records (water years 1930–2009) and 

peak-flow records (water years 1908, 1930–2009). Characteristics for the periods before the breach of Milltown Dam are 

presented for informational purposes. Characteristics for water 

years 2008–09 and comparisons to long-term characteristics 

are discussed in more detail.

Annual mean streamflow at Clark Fork above Missoula 

for water year 2008 [3,040 cubic feet per second (ft3/s)] was 

slightly larger (105 percent) than the long-term mean annual 

streamflow (2,910 ft3/s). Annual mean streamflow for water 

year 2009 (3,560 ft3

/s) was substantially larger (122 percent) 

than the long-term mean annual streamflow and also moderately larger than the water year 2008 annual mean streamflow. The annual peak flows for water year 2008 and water 

year 2009 were identical (17,500 ft3/s) and were 112 percent 

of the long-term mean annual peak flow (15,600 ft3/s), but 

only 36 percent of the long-term maximum annual peak flow 

(48,000 ft3/s), which occurred in 1908. Although the annual 

peak flows for water year 2008 and water year 2009 were 

identical, streamflows in water year 2009 remained near the 

annual peak flow magnitude for several days before and after 

the peak flow, whereas the rise to and fall from the annual 

peak flow was more precipitous in water year 2008.

Annual streamflow and peak-flow characteristics for 

the individual water years 2006–09 (table 3) for Clark Fork 

at St. Regis are compared to long-term characteristics for 

the period of continuous-streamflow records and peak-flow 
Table 2. Annual streamflow and peak-flow characteristics for selected periods, with comparison to long-term annual streamflow 

(water years 1930–2009) and peak-flow (water years 1908, 1930–2009) characteristics for Clark Fork above Missoula, Mont. 

(station 12340500).

[Abbreviations: ft3/s, cubic feet per second]

Water year(s)

Annual streamflow Annual peak flow

Annual mean 

streamflow 

(ft3

/s)

Percent of 

long-term mean 

annual streamflow

(2,910 ft3

/s)

Percent of 

long-term maximum 

annual streamflow

(5,070 ft3

/s; 1976)

Annual 

peak flow 

(ft3

/s)

Percent of 

long-term mean 

peak flow

(15,600 ft3

/s)

Percent of 

long-term maximum 

peak flow

(48,000 ft3

/s; 1908)

Period before start of permanent drawdown of Milltown Reservoir on June 1, 2006 (water years 1985–2005)

1985–20051,2 2,550 88 50 12,200 78 25

1996-971,3 4,540 156 90 22,800 146 47

Period between start of permanent drawdown of Milltown Reservoir on June 1, 2006, and 

breach of Milltown Dam on March 28, 2008 (water years 2006–07)

20064 2,480 86 49 12,900 83 27

2007 2,440 84 48 9,320 60 19

Period after breach of Milltown Dam on March 28, 2008 (water years 2008–09)

20084 3,040 105 60 17,500 112 36

2009 3,560 122 70 17,500 112 36

1

Average values for the indicated period (mean annual).

2

Average values for water years 1985–2005 represent typical conditions during the period before the start of permanent drawdown of Milltown Reservoir.

3

Average values for water years 1996–97 represent high-flow conditions during the period before the start of permanent drawdown of Milltown Reservoir.

4

Although part of the given water year occurred outside of the indicated period, the given water year is assigned to the period most representative of the 

prevailing conditions of that water year.

Methods of Data Collection and Quality Assurance 9

records (water years 1911–23, 1929–2009). Streamflow 

characteristics for Clark Fork at St. Regis provide information 

relevant to suspended-sediment and trace-element transport 

characteristics for the Clark Fork basin downstream from the 

project area outflow.

Annual mean streamflows at Clark Fork at St. Regis for 

water years 2006–07 (6,560 and 6,040 ft3

/s, respectively) were 

smaller (90 and 83 percent, respectively) than the long-term 

mean annual streamflow (7,280 ft3

/s). Annual mean streamflows for water years 2008–09 (7,610 and 7,810 ft3

/s, respectively) were similar in magnitude and were slightly larger 

(104 and 107 percent, respectively) than the long-term mean 

annual streamflow. The annual peak flow for water year 2006 

(39,600 ft3/s) was slightly larger (107 percent) than the longterm mean peak flow (37,000 ft3

/s), and the annual peak flow 

for water year 2007 (23,600 ft3

/s) was substantially smaller 

(64 percent) than the long-term mean peak flow. The annual 

peak flows for water year 2008 and water year 2009 (46,300 

and 42,700 ft3

/s, respectively) were substantially to moderately 

larger (125 and 115 percent, respectively) than the long-term 

mean annual peak flow, but were only 67 and 62 percent of the 

long-term maximum annual peak flow (68,900 ft3

/s), which 

occurred in 1948.

Methods of Data Collection and Quality 

Assurance

Methods of data collection associated with suspendedsediment and trace-element loads reported for the highintensity stations for water years 1985–97 are reported in 

Lambing (1991), Hornberger and others (1997), and Lambing (1998). Methods of data collection associated with 

suspended-sediment and trace-element loads reported for 

water years 2004–08 are presented in Lambing and Sando 

(2008, 2009). Methods of data collection for the high-intensity 

stations during water year 2009 are presented in this report.

Constituent loads for the low-intensity stations have not 

been previously estimated or reported. Data-collection activities for the low-intensity stations were restricted to water years 

2006–09, and the data-collection methods are presented in this 

report.

Water-quality samples were collected periodically for 

analysis of suspended sediment and trace elements. Routine 

samples at the three high-intensity stations bracketing the 

Milltown Reservoir project area (table 1, fig. 1) were collected 

at a frequency of 6 to 8 times during water year 2009 as part 
Table 3. Annual streamflow and peak-flow characteristics for selected periods, with comparison to long-term annual streamflow and 

peak-flow (water years 1911–23, 1929–2009) characteristics for Clark Fork at St. Regis, Mont. (station 12354500).

[Abbreviations: ft3/s, cubic feet per second]

Water 

year(s)

Annual streamflow Annual peak flow

Annual mean 

streamflow 

(ft3

/s)

Percent of long-term 

mean annual 

streamflow

(7,280 ft3

/s)

Percent of long-term 

maximum annual 

streamflow

(11,600 ft3

/s; 1997)

Annual 

peak flow 

(ft3

/s)

Percent of long-term 

mean peak flow

(37,000 ft3/s)

Percent of long-term 

maximum peak flow

(68,900 ft3/s; 1948)

Period between start of permanent drawdown of Milltown Reservoir on June 1, 2006, and 

breach of Milltown Dam on March 28, 2008 (water years 2006–07)

20061 6,560 90 57 39,600 107 57

2007 6,040 83 52 23,600 64 34

Period after breach of Milltown Dam on March 28, 2008 (water years 2008–09)

20081 7,610 104 66 46,300 125 67

2009 7,810 107 68 42,700 115 62

1

Although part of the given water year occurred outside of the indicated period, the given water year is assigned to the period most representative of the 

prevailing conditions of that water year.

10 Estimated Loads of Suspended Sediment and Trace Elements, Clark Fork Basin, Montana (Water Years 1985–2009)

of the long-term Clark Fork monitoring program (Dodge and 

others, 2009). The routine long-term samples were collected 

during both runoff and base-flow periods. An additional 

19 samples were collected at each of the three high-intensity 

stations during the runoff period of the annual hydrograph 

(March–July) of water year 2009 as part of a supplemental 

monitoring program for the lower Clark Fork basin. These 

additional samples provide better resolution of the transport 

of sediment and trace elements from the project area associated with dam-removal activities and high-flow conditions 

when most of the annual transport was expected to occur. The 

combined sampling efforts for both programs resulted in a 

sampling frequency of 25 to 27 times in water year 2009 at 

each of the high-intensity stations. 

The four low-intensity stations (table 1, fig. 1) are not 

part of the long-term monitoring program, and periodic 

water-quality samples were not collected at these sites before 

the start of the supplemental monitoring program in water 

year 2006. For three of the low-intensity stations in the lower 

Clark Fork (Bitterroot River near Missoula, Clark Fork at 

St. Regis, and Flathead River at Perma), periodic water-quality 

sampling was started in water year 2006. For Clark Fork 

Bypass near Bonner (located within the Milltown Reservoir 

project area), periodic water-quality sampling was started in 

water year 2008. The sampling frequency for each of the lowintensity stations ranged from 11 to 21 times per year during 

water years 2006–09, with samples collected seasonally during 

the runoff period (March–July).

Generally, periodic water-quality samples were collected 

by the USGS throughout the entire stream depth at multiple 

locations across the stream by using depth- and widthintegration methods described by Ward and Harr (1990), 

Edwards and Glysson (1999), and U.S. Geological Survey 

(variously dated). However, the use of standard depth- and 

width-integrating procedures for the Clark Fork Bypass were 

not practical, because the steep gradient of the channel and the 

routing of the water through the openings between the bridge 

piers resulted in areas of turbulence, eddying, and unrepresentative flow direction such that sampling locations could not be 

distributed uniformly across the stream. Thus, for the Clark 

Fork Bypass, depth-integrated water samples were collected 

at 3 to 4 locations across the stream in areas of substantial 

discharge with generally uniform and representative flow 

direction.

Sampling equipment consisted of depth-integrating 

suspended-sediment samplers (DH–81, D–74TM, and D–95), 

which were equipped with nylon nozzles and constructed of 

plastic or coated with a nonmetallic epoxy paint. Depth-integrating samplers are designed to sample the water column isokinetically, whereby velocity flow paths into the nozzle are not 

distorted and suspended particles enter the sample-collection 

bottle at the same concentration as in the surrounding water. 

The samplers collect water from the water surface to the top 

of an unsampled zone about 4 inches above the streambed; 

however, bedload materials transported below the unsampled 

zone are not sampled by depth-integrating suspended-sediment 

samplers. The combination of appropriate sampling methods 

and depth-integrating sampling equipment provides a vertically and laterally discharge-weighted composite sample of 

water and suspended-particulate matter that is representative 

of the flow passing through the cross-sectional area of the 

stream. Samples were processed onsite according to procedures described by Ward and Harr (1990), Horowitz and 

others (1994), and U.S. Geological Survey (variously dated). 

Quality-assurance procedures for processing water-quality 

samples are described by U.S. Geological Survey (variously 

dated) and Lambing (2006).
Methods of Data Collection and Quality Assurance 11

Measurements of pH, specific conductance, and water 

temperature were collected onsite during all sampling visits. 

Generally, instantaneous streamflow at the time of periodic 

water-quality sampling was determined either by direct 

measurement at the time of sampling or from stage readings applied to the stage-discharge rating table for the station 

(Rantz and others, 1982). However, because a continuous 

streamflow-gaging station was not operated at the lowintensity station Clark Fork Bypass near Bonner and streamflow could not be accurately measured at this site, streamflow 

at the time of sampling was not directly determined. Instead, 

streamflow at Clark Fork Bypass was assumed to be equal to 

the streamflow determined for Clark Fork at Turah Bridge, 

located about 5 mi upstream.

Daily mean streamflows at the sampling stations with 

continuous streamflow gages were determined by applying 

stage-discharge relations developed from periodic streamflow 

measurements to the continuous record of stage according to 

procedures described by Rantz and others (1982). The three 

high-intensity stations also were operated as daily-sediment 

stations during water year 2009. Suspended-sediment samples for the daily-sediment stations were collected at a high 

frequency (2 to 14 times per week) by local contract observers using depth-integration methods at a single location near 

midstream. The frequency of suspended-sediment sample collection by observers increased seasonally as flows increased or 

temporarily during short-duration periods of runoff. Qualityassurance procedures for generating daily records of streamflow and suspended-sediment data are described by White and 

others (1998). The quality of daily records (Rantz and others, 

1982) were rated good to excellent, except for periods of ice 

cover, which were rated poor.

Periodic water-quality samples were analyzed for 

suspended-sediment concentration and percent of suspended 

sediment finer than 0.062-mm diameter by the USGS Montana Water Science Center Sediment Laboratory (hereinafter 

referred to as Montana Sediment Laboratory) in Helena, 

Mont., according to methods described by Guy (1969) and 

Dodge and Lambing (2006). Samples collected by observers for the daily sediment stations were analyzed only for 

suspended-sediment concentration. Quality-assurance procedures used by the Montana Sediment Laboratory are described 

by Dodge and Lambing (2006).

Periodic water-quality samples also were analyzed for 

filtered (0.45-μm pore size) and unfiltered-recoverable traceelement concentrations by the USGS National Water Quality 

Laboratory (NWQL) in Denver, Colo. Filtered concentrations formerly were referred to as dissolved (Lambing, 1991; 

Lambing, 1998; Hornberger and others, 1997); unfilteredrecoverable concentrations formerly were referred to as total 

recoverable. Unfiltered-recoverable concentrations represent 

the combined dissolved and particulate fractions of the trace 

element. Unfiltered samples were first digested with dilute 

hydrochloric acid before analysis to liberate the weakly bound 

trace elements from sediment particles (Hoffman and others, 

1996). Filtered samples and the digested unfiltered samples 

then were analyzed by inductively coupled plasma–mass spectrometry (Garbarino and Struzeski, 1998). Quality-assurance 

procedures used by the NWQL are described by Friedman and 

Erdmann (1982), Jones (1987), Pritt and Raese (1995), and 

Maloney (2005).

Quality-assurance data for periodic water-quality samples 

were obtained by analysis of quality-control samples (blanks 

and replicates), which were submitted along with the environmental samples on every field trip. Analytical results for 

quality-control samples are used to evaluate the performance 

of sampling and analytical methods to ensure that results for 

environmental samples are accurate and unbiased. Qualityassurance data for the long-term Clark Fork basin sampling 

program are reported in annual data reports for previous years; 

quality-assurance data for water year 2009 are available for 

inspection (Dodge and others, 2010).

Trace-element concentrations in blank samples collected during water years 2006–09 were almost always less 

than the reporting level (RL). Values exceeding twice the RL 

were noted during data reviews to evaluate the presence of 

a consistent trend that could indicate systematic contamination. Values exceeding twice the RL were infrequent (only 

2 occurrences for zinc in about 650 individual analyses for 

all trace elements) and occurred sporadically, which indicated 

that there were no consistent trends of contamination bias that 

might affect the data for environmental samples; therefore, no 

adjustments to trace-element concentrations were made during 

water years 2006–09 based on analytical results for blanks.

Precision of trace-element concentrations was determined 

by calculating a relative standard deviation (RSD) for analytical results of replicate samples. The RSD is calculated according to the following equation (Taylor, 1987):

, (1)

where

RSD is the relative standard deviation;

S is the standard deviation; and

is the mean concentration for all replicate 

analyses.

The RSDs for all the unfiltered-recoverable trace elements used to estimate loads during water years 2006–09 

were within the 20-percent data quality objective of acceptable precision for concentrations in replicate samples, with 

the exception of iron in water year 2008 replicate samples 

and copper, lead, and manganese in water year 2009 samples. 

The RSD for the water year 2008 iron-replicate samples was 

30 percent. The high RSD resulted from poor agreement for 

a single replicate sample pair; removing the results for the 

problem replicate pair resulted in a low RSD of 6 percent. The 

RSD for the water year 2009 copper, lead, and manganese replicate samples ranged from 22 to 46 percent and resulted from 

poor agreement for a single replicate sample pair; removing 

the results for the problem replicate pair resulted in low RSDs 

ranging from 5.1 to 12 percent. Thus, the replicate samples 
12 Estimated Loads of Suspended Sediment and Trace Elements, Clark Fork Basin, Montana (Water Years 1985–2009)

indicate acceptable precision, and the infrequent problem 

replicates represent isolated occurrences rather than systematic 

sampling or analytical problems.

Methods for Estimating Constituent 

Loads

The term “load” represents the mass (commonly 

expressed as tons) of a constituent transported past a sampling 

site during a specified period of time. Loads can be computed 

for various time increments, such as instantaneous, daily, 

monthly, seasonal, or annual. Instantaneous loads represent 

the mass transported at the time of sampling, whereas daily, 

monthly, seasonal, and annual loads represent the cumulative 

mass transported over a prolonged period. The term “discharge” represents the rate at which load is transported and 

incorporates both mass and time units (commonly expressed 

as tons per day or tons per year). The measured instantaneous 

discharge of a constituent is calculated as the product of 

sample concentration and instantaneous streamflow at the time 

of sampling. Instantaneous discharge also can be expressed 

as an equivalent daily discharge that represents the total 

load transported during a day (daily load) assuming that the 

measured rate of instantaneous discharge was maintained for 

24 hours. Daily loads can be added to determine the cumulative load for specific periods within a year or an annual load 

for the entire year. 

Cumulative load estimates, such as seasonal or annual, 

generally are more informative than instantaneous loads 

measured at the time of sampling because they represent the 

total constituent mass transported over a prolonged period, 

and thereby incorporate the potentially large range of daily 

and seasonal variations. Further, load estimates associated 

with short periods (such as instantaneous or daily periods) can 

have relatively large errors, but such errors generally are about 

equally distributed in the negative and positive direction. 

Summing short-period load estimates to produce cumulative 

load estimates for a longer period generally results in smaller 

relative errors for the longer term period due to balancing of 

negative and positive errors associated with the short-period 

load estimates. Cumulative load estimates also are useful for 

evaluating differences in constituent transport among sites 

to identify source areas contributing substantial inputs on a 

sustained basis over time. Differences in cumulative loads 

transported past various locations along a stream can result 

from differences in seasonal or annual flow volumes, physical 

basin characteristics, current and historical land-use activities, and localized conditions that affect constituent supply or 

susceptibility to erosion.

Estimation of cumulative loads typically requires either 

high-frequency sampling or applying statistical relations, such 

as regression equations (Cohn and others, 1992), to a daily 

record of a hydrologically related explanatory variable to 

produce an estimated daily load. The daily loads estimated by 

such methods provide increased temporal resolution of variability within a year that can give added insight to the effects 

of streamflow variations, seasonal differences, or unique 

conditions associated with discrete events.

Methods for developing regression equations used to 

estimate constituent loads reported for the high-intensity stations for water years 1985–97 are reported in Lambing (1991), 

Hornberger and others (1997), and Lambing (1998). Methods 

for developing regression equations used to estimate constituent loads reported for water years 2004–08 are presented in 

Lambing and Sando (2008, 2009). Methods for developing 

regression equations used to estimate constituent loads for the 

high-intensity stations during water year 2009 are presented in 

this report.

Constituent loads for the low-intensity stations have not 

been previously estimated or reported. Methods for developing regression equations used to estimate constituent loads 

for the low-intensity stations during water years 2006–09 are 

presented in this report.

Regression equations used to estimate constituent 

discharges based on relations with explanatory variables 

were developed from instantaneous streamflow at the time of 

sampling and concentration data for periodic water-quality 

samples collected during all or part of water years 2004–09. 

Various forms of data transformation were examined to 

produce a linear distribution that could be fit adequately by 

an ordinary least squares (OLS) regression line (Helsel and 

Hirsch, 2002). Selection of the best data transformation was 

based on the ability to produce linear relations that were statistically significant at the 0.05 significance level (p-value <0.05) 

and that had a uniform distribution of residuals around the 

regression line. Also, the coefficient of determination (R2

) and 

standard error of estimate (SE), in percent, which are measures 

of the scatter of data points around the regression line, were 

used in conjunction with statistical significance and uniformity 

of residual distribution to evaluate the various relations and 

select the best form of regression equation. 

Some trace-element concentrations were censored 

(reported as less than the RL) for nearly all stations (table 4). 

In previous reports presenting estimated loads of trace elements for the high-intensity stations (Lambing, 1991; Lambing, 1998; Hornberger and others, 1997; Lambing and Sando, 

2008, 2009), trace-element concentrations that were censored 

were estimated by substituting one-half of the RL that was in 

effect during the data-collection period. For constituents that 

had multiple RLs during the data-collection period, one-half 

of the median RL during the period was substituted. Data 

analysis indicated that this approach for handling censored 

concentrations was suitable for meeting the study objectives 

(Lambing and Sando, 2009) and did not substantially bias 

the study results. However, load estimates for constituents 

having greater than about 30-percent censored concentrations 

for a given site were qualified as having greater uncertainty 

than constituents with no or a very small percent of censored 

concentrations.
Table 4. Percentage of samples with censored trace-element concentrations, water years 2004–09.

[Light gray shading indicates some censored values. Dark gray shading indicates greater than 50-percent censored values]

Station 

number Station name

Percent of samples with censored concentrations (and total number of samples), 

for indicated trace element

Unfilteredrecoverable 

cadmium

Unfilteredrecoverable 

copper

Unfilteredrecoverable 

iron

Unfilteredrecoverable 

lead

Unfilteredrecoverable 

manganese

Unfilteredrecoverable 

zinc

Unfilteredrecoverable 

arsenic

12334550 Clark Fork at Turah 

Bridge, near Bonner, Mont.

1.0 (103) 0.0 (103) 0.0 (103) 0.0 (103) 0.0 (103) 0.0 (103) 0.0 (103)

12334570 Clark Fork Bypass, 

near Bonner, Mont.

0.0 (38) 0.0 (38) 0.0 (38) 0.0 (38) 0.0 (38) 0.0 (38) 0.0 (38)

12340000 Blackfoot River near 

Bonner, Mont.

77.4 (93) 26.6 (94) 0.0 (94) 6.4 (94) 0.0 (94) 29.0 (93) 8.5 (94)

12340500 Clark Fork above 

Missoula, Mont.

1.0 (104) 0.0 (104) 0.0 (104) 0.0 (104) 0.0 (104) 0.0 (104) 0.0 (104)

12352500 Bitterroot River near 

Missoula, Mont.

70.9 (55) 29.1 (55) 0.0 (55) 1.8 (55) 0.0 (55) 16.7 (54) 0.0 (55)

12354500 Clark Fork at St. 

Regis, Mont.

5.4 (55) 0.0 (55) 0.0 (55) 0.0 (55) 0.0 (55) 0.0 (55) 0.0 (55)

12388700 Flathead River at 

Perma, Mont.

94.4 (54) 67.9 (53) 0.0 (54) 7.4 (54) 0.0 (54) 75.4 (53) 0.0 (54)

Methods for Estimating Constituent Loads 13

A more rigorous method of estimating censored concentrations [Adjusted Maximum Likelihood Estimation (AMLE) 

regression; Helsel and Hirsch, 2002] was used in this report 

for water year 2009 data for the high-intensity stations and 

water years 2006–09 data for the low-intensity stations. Justification for using AMLE regression for these data, detailed 

description of the application of the method, and general 

comparisons between results of the AMLE regression method 

and results of the substitution method are provided in Supplement 1 (at the back of this report). The general conclusions of 

the information presented in Supplement 1 include (1) AMLE 

regression provides the most accurate estimates of censored 

concentrations for water year 2009 data for the high-intensity 

stations and 2006–09 data for the low-intensity stations, 

and (2) differences between results of the AMLE regression 

method and results of the substitution method generally are 

relatively small and do not substantially affect comparison of 

load estimates among water years.

The effect of censored values on statistical analyses generally was minor, with the possible exception of data sets with 

greater than about 50-percent censored values (table 4; hereinafter referred to as highly censored data sets), and include 

(1) unfiltered-recoverable cadmium for Blackfoot River near 

Bonner; (2) unfiltered-recoverable cadmium for Bitterroot near 

Missoula; and (3) unfiltered-recoverable cadmium, copper, and 

zinc for Flathead River at Perma. The very large percent (94.4 

percent; table 4) of censored values for unfiltered-recoverable 

cadmium for Flathead River at Perma precluded the use of any 

method for estimating censored concentrations and no load 

estimates are presented in this report for unfiltered-recoverable 

cadmium for Flathead River at Perma. Statistical relations and 

load estimates for the highly censored data sets have a greater 

degree of uncertainty than for other data sets. However, the 

highly censored data sets still provide useful information for 

evaluating relative variability in load estimates among sites 

and years because (1) censored values generally were associated with low to moderate streamflows and did not substantially affect estimates of daily loads during high-flow periods 

when the largest constituent transport occurs; and (2) variability in trace-element concentrations among samples for the 

highly censored data sets generally was small, especially for 

samples collected during low to moderate streamflows. Thus, 

although the load estimates for the highly censored data sets 

potentially are subject to greater error, their presentation is 

warranted to provide an important indication of constituent 

transport in the Clark Fork basin.

Estimation of Constituent Loads for 

High-Intensity Stations

Methods used to estimate constituent loads for water year 

2009 were similar to those used to estimate constituent loads 

for water year 2008 (Lambing and Sando, 2009). The consistency in methodology provides a solid basis for comparison of 

load estimates between these two important years whose highflow periods occurred after the breach of Milltown Dam.
14 Estimated Loads of Suspended Sediment and Trace Elements, Clark Fork Basin, Montana (Water Years 1985–2009)

Suspended Sediment 

Daily suspended-sediment loads for water year 2009 

were estimated directly for each of the three high-intensity 

stations (fig. 1) by using high-frequency sampling of the daily 

sediment monitoring program. The concentration data from 

the high-frequency samples characterized daily temporal variations needed for developing continuous-concentration curves. 

Daily mean suspended-sediment concentrations were determined from the continuous-concentration curves according 

to methods described by Porterfield (1972). The daily mean 

suspended-sediment concentrations then were multiplied by 

the daily mean streamflows (and a units-conversion constant) 

to generate a record of daily suspended-sediment loads.

The daily suspended-sediment loads estimated for stations with daily sediment monitoring did not require development of regression equations that relate suspended-sediment 

discharges to instantaneous streamflow. However, relations 

between suspended-sediment discharge and streamflow 

(sediment-transport relations) were developed to examine 

patterns that could provide information useful in the development of regression equations for estimating trace-element 

discharges.

The suspended-sediment concentration for each periodic water-quality sample was converted to an equivalent 

suspended-sediment discharge, in tons per day, according to 

the equation:

Qsed = CsedQK, (2)

where

Qsed is suspended-sediment discharge, in tons per 

day;

Csed is suspended-sediment concentration, in 

milligrams per liter;

Q is streamflow, in cubic feet per second; and

K is a units-conversion constant (0.0027 for 

concentrations in milligrams per liter) 

to convert instantaneous suspendedsediment discharge to an equivalent daily 

suspended-sediment discharge.

After suspended-sediment concentrations were converted 

to suspended-sediment discharges, the relations between 

suspended-sediment discharge and streamflow for water 

years 2008–09 were plotted in conjunction with the data 

for water years 1985–2007 to visually identify changes in 

sediment-transport characteristics. Sediment-transport relations (fig. 3) for the three high-intensity stations during water 

years 2008–09 (predominantly after the breach of Milltown 

Dam ) were compared with pre-breach data from water years 

1985–2007 to illustrate changes in sediment transport associated with the dam breach and subsequent channel adjustment. Data for water years 2008–09 were segregated into the 

rising limb and falling limb of the runoff period of the annual 

hydrograph to provide more detailed illustration of transport 

processes associated with the dam breach and subsequent 

erosion in response to sediment supply and streamflow conditions. The data for the rising and falling limbs of water years 

2008–09 were separately fit with locally weighted scatter plot 

smoothing (LOWESS) lines (Cleveland and McGill, 1984; 

Cleveland, 1985) that show the central tendencies of the data 

distributions over the ranges of streamflows sampled during 

the two periods. The smooth lines are not regression lines and 

do not imply statistically significant relations; instead, smooth 

lines are used as a visual indication of temporal differences in 

sediment-transport characteristics at each of the three stations. 

An upward shift in the relation indicates a larger load transported for a given streamflow; a downward shift indicates a 

smaller load transported for a given streamflow. 

During water years 2008–09, sediment-transport relations for Clark Fork at Turah Bridge and Blackfoot River near 

Bonner, which are upstream from the Milltown Reservoir 

project area, generally were within the typical spread of data 

that occurred during water years 1985–2007. For both stations, there was a relatively small upward shift in the relations 

for the rising limb relative to the falling limb in both water 

year 2008 and water year 2009. This pattern reflects typical 

sediment-transport characteristics of natural streams that often 

exhibit a loop effect of sediment supply such that sediment 

discharge is greater for a given streamflow when the stream is 

rising than when it is falling (Colby, 1956).

In contrast to the two stations upstream from the project area, a large shift in the sediment-transport relation was 

apparent for Clark Fork above Missoula (fig. 3C) during the 

rising limb of water year 2008 (March 28–May 21, 2008) 

immediately after the breach of Milltown Dam. During this 

initial post-breach period, the sediment-transport relation 

generally was substantially higher than sediment transport 

for water years 1985–2007, especially at higher streamflows. 

The upward shift in the sediment-transport relation during the 

rising limb of water year 2008 presumably resulted from localized inputs of sediment from within the project area, including 

erosion of coffer dam sediment, reservoir bottom sediment, 

and sediment from the streambed and banks near the upper 

end of the former reservoir as the river adjusted to the steeper 

hydraulic gradient caused by the dam breach. The extent of 

the upward shift also likely was affected by the magnitude 

of the annual peak flow, which was relatively large in water 

year 2008 (112 percent of the long-term mean annual peak 

flow, table 2). The upward shift in the sediment-transport relation during the water year 2008 rising limb was not sustained, 

however, as indicated by a large downward shift during the 

falling limb (May 22–September 30, 2008). The very large 

downward shift of the sediment-transport relation during the 

falling limb of the post-breach period probably represented 

a diminished sediment supply after the erosion and transport 

of large quantities of sediment from the project area during the rising limb. The relation for the falling limb of water 

year 2008 diverges from the relation for the rising limb at the 

higher flows and drops to levels near the lower limits of the 

range in sediment transport for water years 1985–2007. 
Figure 3. Relations between suspended-sediment discharge and streamflow for water years 2008–09, with comparison to data for 

water years 1985–2007. A, Clark Fork at Turah Bridge, near Bonner, Mont.; B, Blackfoot River near Bonner, Mont.; C, Clark Fork above 

Missoula, Mont. Suspended-sediment discharge, in tons per day

1

10

100

1,000

10,000

1,000 10,000

Streamflow, in cubic feet per second

200

70,000

20,000

B Blackfoot River near Bonner, Mont.

A Clark Fork at Turah Bridge, near Bonner, Mont.

C Clark Fork above Missoula, Mont.

1

10

100

1,000

10,000

70,000

1

10

100

1,000

10,000

70,000

Data collected during water 

 years 1985–2007

Pre-breach period

Post-breach period

LOWESS smooth line for data

 collected during March 28, 

 2008–May 21, 2008 (water 

 year 2008 rising limb)

Data collected during 

 March 28, 2008–May 21, 2008 

 (water year 2008 rising limb)

LOWESS smooth line for data 

 collected during May 22, 

 2008–September 30, 2008 

 (water year 2008 falling limb)

Data collected during May 22, 

 2008–September 30, 2008 

 (water year 2008 falling limb)

LOWESS smooth line for data 

 collected during Septem-

 ber 30, 2008–June 1, 2009 

 (water year 2009 rising limb)

Data collected during 

 March 9, 2009–June 1, 2009 

 (water year 2009 rising limb)

LOWESS smooth line for data 

 collected during June 2, 

 2009–September 30, 2009 

 (water year 2009 falling limb)

Data collected during June 2, 

 2009–September 30, 2009 

 (water year 2009 falling limb)

EXPLANATION

Methods for Estimating Constituent Loads 15
16 Estimated Loads of Suspended Sediment and Trace Elements, Clark Fork Basin, Montana (Water Years 1985–2009)

In water year 2009, the sediment-transport relation for the 

rising limb at the Clark Fork above Missoula generally was 

substantially below the rising limb for water year 2008, even 

though streamflows for several samples collected during the 

rising limb of water year 2009 were substantially larger than 

those of water year 2008 (fig. 3C). The downward shift in the 

rising-limb relation from water year 2008 to water year 2009 

provides further evidence that sediment supply in the project 

area was substantially depleted by the end of the water year 

2008 runoff period. The relatively large variability in sediment transport among individual water samples collected at 

similar streamflows during the rising limb of water year 2009 

probably reflects the erratic streamflow characteristics of the 

water year 2009 rising limb. During the rising limb of water 

year 2009, there were several occurrences of sharp rises in 

streamflow followed by sharp declines in streamflow that were 

superimposed on the general increase in streamflow (fig. 2). 

This streamflow pattern in conjunction with typical sediment 

discharge characteristics (whereby sediment discharge generally is greater for a given streamflow when the stream is rising 

than when it is falling; Colby, 1956) probably contributed to 

the large variability in sediment transport at similar streamflows. The sediment transport measured for many individual 

water samples collected during the high-flow conditions of the 

water year 2009 rising limb generally was similar to that of 

high-flow water samples collected during water years 1985–

2007. In water year 2009, there was a downward shift of the 

falling-limb sediment-transport relation from the rising-limb 

relation, but the magnitude of the downward shift was substantially smaller in water year 2009 than in water year 2008. 

The water year 2009 falling-limb sediment-transport relation 

generally was similar to that of water year 2008 and generally 

was near the lower limits of the range in sediment transport for 

water years 1985–2007.

Trace Elements

Daily loads of unfiltered-recoverable trace elements for 

water year 2009 were estimated for each of the three highintensity stations (fig. 1) by using regression relations developed from instantaneous streamflow at the time of sampling 

and concentration data for periodic water-quality samples 

collected during all or part of water years 2004–09. Data for 

the entire 6-year period were used to develop trace-element 

transport relations for water year 2009 at Clark Fork at Turah 

Bridge and Blackfoot River near Bonner. However, because 

the remediation activities related to the removal of Milltown 

Dam resulted in large temporal variability in trace-element 

transport relations for Clark Fork above Missoula, data used to 

develop water year 2009 relations were restricted to the period 

after May 22, 2008 (start of the water year 2008 falling limb 

of the runoff period). The data were segregated into periods 

with similar transport relations to develop separate equations 

for each period, consistent with methods used for estimating 

loads for water year 2008 (Lambing and Sando, 2009).

Trace-element transport relations (fig. 4) for the three 

high-intensity stations during water years 2008–09 (after 

the breach of Milltown Dam) were compared with data for 

water years 1985–2007 (the entire period of record before 

2008) to investigate changes in trace-element transport relations associated with the dam breach and resulting erosion 

from the project area. Plots for the trace elements cadmium, 

copper, iron, lead, manganese, and zinc were constructed 

by using suspended-sediment discharge as the explanatory 

variable because unfiltered-recoverable concentrations for 

these trace elements in the Clark Fork basin generally are 

strongly associated with suspended-sediment concentrations 

(Lambing, 1991). For example, the median ratio of filtered 

(representing dissolved phase) to unfiltered copper concentration for water samples collected at Clark Fork above Missoula 

during water years 1990–2009 is about 0.18; thus, particulate 

copper concentrations that are associated with suspended 

sediment typically account for about 82 percent of unfilteredrecoverable copper concentrations. Plots for the metalloid 

trace element arsenic were constructed by using streamflow as 

the explanatory variable because concentrations of arsenic in 

the Clark Fork basin generally are predominantly transported 

in dissolved phase. For example, the median ratio of filtered 

to unfiltered arsenic concentration for water samples collected 

at Clark Fork above Missoula during water years 1990–2009 

is about 0.70; thus, filtered (dissolved phase) arsenic concentrations typically account for about 70 percent of unfilteredrecoverable arsenic concentrations and variability in arsenic 

discharges generally is strongly influenced by variability in 

streamflow. However, arsenic transport relations in the Clark 

Fork basin are more complex than the other trace elements. 

Arsenic particulate concentrations in parts of the Clark Fork 

basin can be relatively large at times; thus, during periods 

when there is large suspended-sediment transport, arsenic 

transport might be predominantly in particulate phase.

The trace-element concentration for each periodic waterquality sample was converted to an equivalent trace-element 

discharge, in tons per day, according to the equation:

Qte = CteQK, (3)

where

Qte is trace-element discharge, in tons per day;

Cte is trace-element concentration, in micrograms 

per liter;

Q is streamflow, in cubic feet per second; and

K is a units-conversion constant (0.0000027 for 

concentrations in micrograms per liter) 

to convert instantaneous trace-element 

discharge to an equivalent daily traceelement discharge.

After trace-element concentrations were converted to 

trace-element discharges, the relations between trace-element 

discharge and suspended-sediment discharge or streamflow 

for water years 2008–09 were plotted in conjunction with the 

data for water years 1985–2007 to visually identify patterns 
Methods for Estimating Constituent Loads 17

in trace-element transport characteristics. Data for water years

2008–09 were segregated into the rising limb and falling 

limb of the runoff period of the annual hydrograph to provide 

more detailed illustration of trace-element transport processes 

associated with the dam breach and changes in sediment 

supply and streamflow conditions. The data for the rising 

and falling limbs of water years 2008–09 were separately fit 

with LOWESS smooth lines showing the central tendencies 

of the data distribution over the range of sediment discharge 

or streamflows sampled during the two periods. Although 

smooth plots were examined for all the trace elements, only 

copper and arsenic are presented as examples (figs. 4 and 5, 

respectively) because they represent the greatest difference 

in chemical characteristics and transport relations among the 

trace elements; the transport relations for the other trace elements generally were similar to those for copper. The smooth 

lines are not regression lines and do not imply statistically 

significant relations, but rather are used as a visual indication 

of temporal differences in trace-element transport characteristics at each of the three high-intensity stations.

During 2008–09, copper- and arsenic-transport relations 

for Clark Fork at Turah Bridge and Blackfoot River near Bonner, which are upstream from the Milltown Reservoir project 

area, generally were consistent and within the typical spread 

of data for water years 1985–2007. As a result of the similarity 

of temporal patterns indicated by smooth plots for the different periods during water years 2008–09, transport relations 

for all the trace elements at Clark Fork at Turah Bridge and 

Blackfoot River near Bonner were considered to be equivalent 

throughout the entire regression-analysis period (water years 

2004–09). Therefore, trace-element discharge for the entire 

water year 2009 were estimated by single regression equations 

for each trace element at Clark Fork at Turah Bridge (Supplement 2, table S2.1 at the back of the report) and Blackfoot 

River near Bonner (Supplement 2, table S2.2). This approach 

for segregating the data to guide the development of single 

or multiple regression equations for estimating water year 

2009 trace-element loads for these two stations is consistent 

with the approach used for estimating water year 2008 traceelement loads (Lambing and Sando, 2009).

In contrast to the two high-intensity stations upstream 

from the project area, an upward shift in the copper- and 

arsenic-transport relations was apparent for Clark Fork above 

Missoula (figs. 4C and 5C) during the rising limb of water 

year 2008 (March 28–May 21, 2008) immediately after the 

breach of Milltown Dam. The upward shift in the coppertransport relation (based on suspended-sediment discharge) 

during the water year 2008 rising limb (after the breach of 

Milltown Dam) presumably was due to increased erosion of 

copper-enriched sediments that had accumulated in the project 

area. Examination of the copper content of the suspended 

sediment (that is, solid-phase concentration) for individual 

water samples collected before and after the breach of Milltown provides information relevant to the upward shift in the 

copper. The solid-phase copper concentration was calculated 

according to the equation:

Csp = ((Cufr-Cfr)/Css)*1000, (4)

where

Csp is trace-element solid-phase concentration, in 

micrograms per gram;

Cufr is trace-element unfiltered-recoverable 

concentration, in micrograms per liter;

Cfr is trace-element filtered-recoverable 

concentration, in micrograms per liter; and

Css is suspended-sediment concentration, in 

milligrams per liter.

The solid-phase copper concentration of the suspended 

sediment for the periodic water sample collected on March 24, 

2008 (immediately before the breach of Milltown Dam), was 

152 micrograms per gram (μg/g); the solid-phase copper concentration of the suspended sediment for the sample collected 

on March 31, 2008 (immediately after the breach of Milltown 

Dam), was 2,350 μg/g. Thus, there was a large increase in the 

solid-phase concentration of the suspended sediment for Clark 

Fork above Missoula immediately after the breach of Milltown Dam that contributed to the upward shift in the copper 

transport relation (fig. 4C).

There also was a distinct upward shift in the arsenic 

transport relation (based on streamflow) during the water year 

2008 rising limb, primarily due to an associated upward shift 

in the suspended-sediment transport relation (fig. 3C) and 

increased erosion of arsenic-enriched sediment that had accumulated in the project area. Based on data collected during 

water years 1990–2009, filtered (dissolved phase) arsenic concentrations typically (on a median basis) accounted for about 

70 percent of unfiltered-recoverable arsenic concentrations 

and variability in arsenic discharges generally was strongly 

influenced by variability in streamflow. However, during the 

water year 2008 rising-limb, arsenic transport at Clark Fork 

above Missoula was dominated by solid-phase transport due 

to the large amount of suspended sediment transported from 

the project area (fig. 3C). During the water year 2008 rising 

limb, filtered arsenic concentrations typically accounted for 

about 30 percent of unfiltered-recoverable arsenic concentrations. Thus, solid-phase arsenic concentrations associated with suspended sediment typically accounted for about 

70 percent of unfiltered-recoverable arsenic concentrations 

during the water year 2008 rising limb. During the falling 

limb of water year 2008 and continuing into water year 2009, 

the arsenic-transport relation (based on streamflow) generally 

was consistent and within the typical spread of data for water 

years 1985–2007.

The water year 2009 data for Clark Fork above Missoula were segregated relative to rising limb and falling limb 

because (1) there were adequate data in each of the periods to 

represent the relations during the specific period, and (2) this 

approach of segregating the data to develop separate regression equations for estimating trace-element loads for different 

hydrologic periods of water year 2009 for Clark Fork above 

Missoula is consistent with the approach used for estimating water year 2008 trace-element loads. Using the same 
Figure 4. Relations between unfiltered-recoverable copper discharge and suspended-sediment discharge for water years 2008–09, 

with comparison to data for water years 1985–2007. A, Clark Fork at Turah Bridge, near Bonner, Mont.; B, Blackfoot River near Bonner, 

Mont.; C, Clark Fork above Missoula, Mont.

Suspended-sediment discharge, in tons per day

Unfiltered-recoverable copper discharge, in tons per day

C Clark Fork above Missoula, Mont.

B Blackfoot River near Bonner, Mont.

A Clark Fork at Turah Bridge, near Bonner, Mont.

0.0001

0.001

0.01

0.1

1

10

40

1 10 100 1,000 10,000 70,000

0.0001

0.001

0.01

0.1

1

10

40

0.0001

0.001

0.01

0.1

1

10

40

Data collected during water 

 years 1985–2007

Pre-breach period

Post-breach period

LOWESS smooth line for data

 collected during March 28, 

 2008–May 21, 2008 (water 

 year 2008 rising limb)

Data collected during 

 March 28, 2008–May 21, 2008 

 (water year 2008 rising limb)

LOWESS smooth line for data 

 collected during May 22, 

 2008–September 30, 2008 

 (water year 2008 falling limb)

Data collected during May 22, 

 2008–September 30, 2008 

 (water year 2008 falling limb)

LOWESS smooth line for data 

 collected during Septem-

 ber 30, 2008–June 1, 2009 

 (water year 2009 rising limb)

Data collected during 

 March 9, 2009–June 1, 2009 

 (water year 2009 rising limb)

LOWESS smooth line for data 

 collected during June 2, 

 2009–September 30, 2009 

 (water year 2009 falling limb)

Data collected during June 2, 

 2009–September 30, 2009 

 (water year 2009 falling limb)

EXPLANATION

18 Estimated Loads of Suspended Sediment and Trace Elements, Clark Fork Basin, Montana (Water Years 1985–2009)
Figure 5. Relations between unfiltered-recoverable arsenic discharge and streamflow for water years 2008–09, with comparison to 

data for water years 1985–2007. A, Clark Fork at Turah Bridge, near Bonner, Mont.; B, Blackfoot River near Bonner, Mont.; C, Clark Fork 

above Missoula, Mont.

Data collected during water 

 years 1985–2007

Pre-breach period

Post-breach period

LOWESS smooth line for data

 collected during March 28, 

 2008–May 21, 2008 (water 

 year 2008 rising limb)

Data collected during 

 March 28, 2008–May 21, 2008 

 (water year 2008 rising limb)

LOWESS smooth line for data 

 collected during May 22, 

 2008–September 30, 2008 

 (water year 2008 falling limb)

Data collected during May 22, 

 2008–September 30, 2008 

 (water year 2008 falling limb)

LOWESS smooth line for data 

 collected during Septem-

 ber 30, 2008–June 1, 2009 

 (water year 2009 rising limb)

Data collected during 

 March 9, 2009–June 1, 2009 

 (water year 2009 rising limb)

LOWESS smooth line for data 

 collected during June 2, 

 2009–September 30, 2009 

 (water year 2009 falling limb)

Data collected during June 2, 

 2009–September 30, 2009 

 (water year 2009 falling limb)

EXPLANATION

Unfiltered-recoverable arsenic discharge, in tons per day0.001

0.01

0.1

1

10

0.001

0.01

0.1

1

10

200 1,000 10,000 20,000

B Blackfoot River near Bonner, Mont.

A Clark Fork at Turah Bridge, near Bonner, Mont.

C Clark Fork above Missoula, Mont.

Streamflow, in cubic feet per second

0.001

0.01

0.1

1

10

Methods for Estimating Constituent Loads 19
20 Estimated Loads of Suspended Sediment and Trace Elements, Clark Fork Basin, Montana (Water Years 1985–2009)

hydrologically based periods for data segregation ensured a 

similar treatment of data between water years 2008 and 2009 

and provided a consistent approach for comparison of load 

estimates between these two important years whose high-flow 

periods occurred after the breach of Milltown Dam.

Although LOWESS smooth lines of trace-element 

discharge relative to suspended-sediment discharge or streamflow were evaluated to identify temporal patterns useful for 

guiding data segregation in subsequent regression analysis, 

regression relations also were more broadly examined. Various combinations of variables, data segregation, and data 

transformation were tested and regression diagnostics were 

reviewed to determine the best possible data fit for estimating 

trace-element discharge. Regression relations between traceelement discharge and the two related variables—streamflow 

and suspended-sediment discharge—were examined to aid in 

the selection of the most appropriate explanatory variable in 

the regression equations. In addition to reviewing regression 

diagnostics to select the most appropriate explanatory variable, 

measured instantaneous trace-element discharges for periodic 

water-quality samples were converted to equivalent daily loads 

and plotted with estimated daily trace-element loads derived 

from various forms of regression equations. The estimated 

daily trace-element loads generated from each combination 

of data treatments then were compared to measured traceelement loads to identify the form of equation that produced 

the best matches.

In almost all instances, better relations for estimating 

water year 2009 trace-element discharge were obtained by 

using suspended-sediment discharge as the explanatory variable, presumably because of the strong association between 

suspended-sediment concentrations and unfiltered-recoverable 

concentrations of trace elements (Lambing, 1991). However, 

water year 2009 arsenic relations generally were better defined 

using streamflow as the explanatory variable. 

Regression equations for estimating unfiltered-recoverable trace-element discharge for Clark Fork at Turah Bridge, 

Blackfoot River near Bonner, and Clark Fork above Missoula 

for water year 2009, and the period of record used to develop 

the equations, are presented in Supplement 2. As part of 

regression analysis, the data were transformed to various units 

and examined to determine which form produced the best 

linear distribution and fit of a regression line. Values of both 

trace-element discharge and suspended-sediment discharge 

were transformed to base 10 logarithm (log) units to obtain 

the best linear distribution and least scatter for all relations, 

with the exception of arsenic at Clark Fork at Turah Bridge 

(Supplement 2, table S2.1), Blackfoot River near Bonner 

(Supplement 2, table S2.2), and Clark Fork above Missoula 

(Supplement 2, table S2.2; nonrising-limb periods). In the 

equations for cadmium, copper, iron, lead, manganese, and 

zinc, both trace-element discharge and suspended-sediment 

discharge were transformed to log units, and the equations are 

presented in exponential form. The equations for estimating 

unfiltered-recoverable arsenic discharge used streamflow as 

the explanatory variable, which generally was transformed 

to cube root (Q0.333) and arsenic discharge was transformed to 

log units. Single equations describe trace-element discharge 

for the entire water year 2009 at Clark Fork at Turah Bridge 

(Supplement 2, table S2.1) and Blackfoot River near Bonner 

(table Supplement 2, table S2.2). Three separate equations 

(pre-rising limb, rising limb, and falling limb) describe traceelement discharge for Clark Fork above Missoula (Supplement 2, table S2.3).

All the regression equations for estimating trace-element 

discharge (Supplement 2) for the high-intensity stations are 

statistically significant (p-value <0.001). The large R2

 values (average 0.97 and ranging from 0.94 to 1.00) and small 

SE values (average 22.8 percent and ranging from 11.4 to 

36.3 percent) indicate good transport relations for all the trace 

elements at all three stations. The averages (and ranges) of SE 

values in the equations for all three stations were 27.3 percent 

(range 22.3 to 35.1 percent) for cadmium, 26.0 percent (range 

18.8 to 36.3 percent) for copper, 18.0 percent (range 11.4 to 

26.1 percent) for iron, 26.6 percent (range 21.3 to 34.5 percent) for lead, 18.7 percent (range 16.1 to 24.1 percent) for 

manganese, 24.8 percent (range 19.8 to 29.5 percent) for zinc, 

and 18.1 percent (range 13.7 to 25.0 percent) for arsenic. The 

regression equations in Supplement 2 were applied to the 

record of either daily mean streamflow or daily suspendedsediment loads to estimate daily trace-element loads for water 

year 2009. The retransformed results were multiplied by 

retransformation-bias-correction factors to compensate for 

log-retransformation bias (Koch and Smillie, 1986) by using 

procedures described by Duan (1983); retransformation-biascorrection factors ranged from 1.00 to 1.02.

The equations for estimating unfiltered-recoverable traceelement discharge in Supplement 2 describe the relations only 

for the range of streamflow that was sampled at each station 

during the period used to develop the equation; therefore, 

extrapolation to higher streamflows might be subject to substantial error. The separate equations for estimating traceelement discharge at Clark Fork above Missoula are applicable 

only to the specific dates for the hydrologic periods indicated 

in table S2.3 of Supplement 2.

Estimation of Constituent Loads for LowIntensity Stations

Constituent loads for the low-intensity stations have 

not been previously estimated or reported. Data-collection 

activities for the low-intensity stations were restricted to water 

years 2006–09. Further, data collection for the low-intensity 

stations in each water year was restricted to the seasonal runoff period of the annual hydrograph (generally March–July), 

with no data collected during extended base-flow conditions. 

Annual loads were not estimated for the low-intensity 

station Clark Fork Bypass near Bonner (station 12334570), 

which was established within the project area in water 

year 2008 (table 1 and fig. 1, map number 2) to provide additional spatial resolution on transport characteristics in a reach 
Methods for Estimating Constituent Loads 21

of channel near the upper end of the former Milltown Reservoir. The Clark Fork Bypass near Bonner has no streamflowgaging station or daily sediment monitoring, and the channel 

configuration at the site resulted in irregular flow conditions 

that complicated the collection of representative isokinetic 

samples; thus, daily and cumulative loads are not presented 

for this site. Instead, results for periodic water-quality samples 

collected on 16 concurrent dates during water year 2008 and 

20 concurrent dates during water year 2009 at Clark Fork at 

Turah Bridge and Clark Fork Bypass near Bonner were compared to evaluate transport characteristics in a reach of channel 

near the upper end of the former Milltown Reservoir. The two 

sites are within 5 mi of each other and samples typically were 

collected within 2–3 hours of each other, which generally corresponded to the travel time through the reach.

For the three low-intensity stations (table 1 and fig. 1, 

map numbers 5–7) downstream from the project area outflow at Clark Fork above Missoula (table 1 and fig.1, map 

number 4), data-collection activities did not include highfrequency sampling for daily sediment monitoring; however, 

all three stations had continuous streamflow-gaging stations. 

Thus, to estimate suspended-sediment loads, development of 

regression equations that relate suspended-sediment discharges 

to instantaneous streamflow (sediment-transport relations) 

was required. Because all constituent loads (both suspendedsediment and trace-element loads) for the low-intensity stations were estimated using regression methods, discussion of 

the estimation methods for both suspended sediment and trace 

elements are included in this single section of this report.

Before developing the regression equations for the lowintensity stations, temporal variability in constituent-transport 

relations was evaluated using methods similar to those used 

for the high-intensity stations. Constituent-transport relations 

for the low-intensity stations are illustrated by scatter plots 

with LOWESS smooth lines of data for selected constituents 

for water years 2006–09 (figs. 6–8). Data for the rising limb 

and falling limb of water years 2008–09 were segregated, 

and fit with smooth lines to compare pre-breach and postbreach transport patterns and to maintain consistency with 

data presentation for the high-intensity stations. Although 

plots were examined for all the trace elements, only copper 

and arsenic are presented as examples (figs. 7 and 8, respectively). The plots in figures 6–8 serve as examples of the 

methods and considerations used for segregating the data for 

regression analyses.

The less intensive data-collection activities for the lowintensity stations complicated regression analyses. Sometimes, 

transport relations were somewhat to substantially different 

between the hydrologically based segregation periods, but 

there were insufficient data to adequately define the relations within a given period. In some cases, this resulted in 

the need to combine data for the different hydrologic periods 

to perform regression analysis, which contributed to larger 

estimation errors and less confidence in the regression equations for the low-intensity stations than for the high-intensity 

stations. Also, because of the lack of daily suspended-sediment 

monitoring at low-intensity stations, streamflow was used as 

the explanatory variable for all trace-element transport relations. Consequently, the presentation of load estimates for the 

low-intensity stations is more qualitative, and emphasis generally is placed on relative comparison of results and general 

transport patterns between stations and between years, rather 

than on absolute magnitude of load estimates. In conjunction 

with the limited presentation of results, less detail in discussion of methods is provided for the low-intensity stations than 

for the high-intensity stations.

Bitterroot River near Missoula represents a major tributary source area that was not influenced by activities related to 

the removal of Milltown Dam. Constituent-transport relations 

for Bitterroot River near Missoula for water years 2006–09 

were best defined by using separate regression equations for 

two different periods (Supplement 3, table S3.1 at the back of 

this report) representing (1) the water years 2006–09 rising 

limbs of the runoff periods, and (2) the water years 2006–09 

falling limbs of the runoff periods (and all other nonrisinglimb periods).

Clark Fork at St. Regis is located on the main stem 

downstream from the project area, resulting in large potential for constituent-transport relations to be strongly influenced by activities related to the removal of Milltown Dam. 

Constituent-transport relations for Clark Fork at St. Regis 

for water years 2006–09 were best defined by using separate 

regression equations for five different periods (Supplement 3, 

table S3.2) representing (1) the water years 2006–07 rising 

limbs of the runoff periods, (2) the water years 2006–07 falling limbs of the runoff periods (and all other nonrising-limb 

periods), (3) the water year 2008 rising limb of the runoff 

period, (4) the water year 2009 rising limb of the runoff 

period, and (5) the water years 2008–09 falling limbs of the 

runoff periods (and all other nonrising-limb periods).

Flathead River at Perma represents a major tributary 

source area that was not influenced by activities related to the 

removal of Milltown Dam. Constituent-transport relations for 

Flathead River at Perma for water years 2006–09 were best 

defined by using separate regression equations for three different periods (Supplement 3, table S3.3) representing (1) the 

water years 2006–07 and water year 2009 rising limbs of 

the runoff periods, (2) the water year 2008 rising limb of the 

runoff period, and (3) the water years 2006–09 falling limbs of 

the runoff periods (and all other nonrising-limb periods).

Regression equations (and associated information) for 

estimating constituent loads for water years 2006–09 at Bitterroot River near Missoula, Clark Fork at St. Regis, and Flathead River at Perma are presented in Supplement 3. As part of 

regression analysis, the data were transformed to various units 

and examined to determine which form produced the best 

linear distribution and fit of a regression line. Values for constituent discharge were transformed to log units and streamflow values were transformed to either log units, square roots 

(Q0.500), or cube roots (Q0.333) to obtain the best linear distribution and least scatter. The equations apply only to the dates for 

the individual hydrologic periods indicated in Supplement 3.
Figure 6. Relations between suspended-sediment discharge and streamflow for water years 2008–09, with 

comparison to data for water years 2006–07. A, Bitterroot River near Missoula, Mont.; B, Clark Fork at St. Regis, Mont.; 

C, Flathead River at Perma, Mont.

1

10

100

1,000

10,000

100,000

1

10

100

1,000

10,000

100,000

Data collected during water years 2006–07

Pre-breach period

EXPLANATION

1,000 10,000 100,000

1,000 10,000 100,000

LOWESS smooth line for data collected during March 9, 2008–May 21, 2008 (water year 2008 rising limb)

Data collected during March 9, 2008–May 21, 2008 (water year 2008 rising limb)

LOWESS smooth line for data collected during May 22, 2008–September 30, 2008 (water year 2008 falling limb)

Data collected during May 22, 2008–September 30, 2008 (water year 2008 falling limb)

LOWESS smooth line for data collected during March 9, 2009–June 1, 2009 (water year 2009 rising limb)

Data collected during March 9, 2009–June 1, 2009 (water year 2009 rising limb)

LOWESS smooth line for data collected during June 2, 2009–September 30, 2009 (water year 2009 falling limb)

Data collected during June 2, 2009–September 30, 2009 (water year 2009 falling limb)

Post-breach period

Suspended-sediment discharge, in tons per day

Streamflow, in cubic feet per second

A Bitterroot River near Missoula, Mont.

B Clark Fork at St. Regis, Mont.

C Flathead River at Perma, Mont.

1

10

100

1,000

10,000

100,000

100

22 Estimated Loads of Suspended Sediment and Trace Elements, Clark Fork Basin, Montana (Water Years 1985–2009)
Figure 7. Relations between unfiltered-recoverable copper discharge and streamflow for water years 2008–09, with 

comparison to data for water years 2006–07. A, Bitterroot River near Missoula, Mont.; B, Clark Fork at St. Regis, Mont.; 

C, Flathead River at Perma, Mont.

0.001

0.0001

0.01

0.1

1

10

100

0.001

0.0001

0.01

0.1

1

10

100

Data collected during water years 2006–07

Pre-breach period

EXPLANATION

LOWESS smooth line for data collected during March 9, 2008–May 21, 2008 (water year 2008 rising limb)

Data collected during March 9, 2008–May 21, 2008 (water year 2008 rising limb)

LOWESS smooth line for data collected during May 22, 2008–September 30, 2008 (water year 2008 falling limb)

Data collected during May 22, 2008–September 30, 2008 (water year 2008 falling limb)

LOWESS smooth line for data collected during March 9, 2009–June 1, 2009 (water year 2009 rising limb)

Data collected during March 9, 2009–June 1, 2009 (water year 2009 rising limb)

LOWESS smooth line for data collected during June 2, 2009–September 30, 2009 (water year 2009 falling limb)

Data collected during June 2, 2009–September 30, 2009 (water year 2009 falling limb)

Post-breach period

Streamflow, in cubic feet per second

0.001

0.0001

0.01

0.1

1

10

100

100 1,000 10,000 100,000

1,000 10,000 100,000

A Bitterroot River near Missoula, Mont.

B Clark Fork at St. Regis, Mont.

C Flathead River at Perma, Mont.

Unfiltered-recoverable copper discharge, in tons per day

Methods for Estimating Constituent Loads 23
Figure 8. Relations between unfiltered-recoverable arsenic discharge and streamflow for water years 2008–09, with 

comparison to data for water years 2006–07. A, Bitterroot River near Missoula, Mont.; B, Clark Fork at St. Regis, Mont.; 

C, Flathead River at Perma, Mont.

0.001

0.01

0.1

1

10

0.0001

Data collected during water years 2006–07

Pre-breach period

EXPLANATION

LOWESS smooth line for data collected during March 9, 2008–May 21, 2008 (water year 2008 rising limb)

Data collected during March 9, 2008–May 21, 2008 (water year 2008 rising limb)

LOWESS smooth line for data collected during May 22, 2008–September 30, 2008 (water year 2008 falling limb)

Data collected during May 22, 2008–September 30, 2008 (water year 2008 falling limb)

LOWESS smooth line for data collected during March 9, 2009–June 1, 2009 (water year 2009 rising limb)

Data collected during March 9, 2009–June 1, 2009 (water year 2009 rising limb)

LOWESS smooth line for data collected during June 2, 2009–September 30, 2009 (water year 2009 falling limb)

Data collected during June 2, 2009–September 30, 2009 (water year 2009 falling limb)

Post-breach period

Streamflow, in cubic feet per second

0.001

0.01

0.1

1

10

0.0001

0.001

0.01

0.1

1

10

0.0001

100 1,000 10,000 100,000

1,000 10,000 100,000

A Bitterroot River near Missoula, Mont.

B Clark Fork at St. Regis, Mont.

C Flathead River at Perma, Mont.

Unfiltered-recoverable arsenic discharge, in tons per day

24 Estimated Loads of Suspended Sediment and Trace Elements, Clark Fork Basin, Montana (Water Years 1985–2009)
Estimated Loads Transported Through the Milltown Reservoir Project Area 25

All the regression equations for estimating suspendedsediment discharge (Supplement 3) for the low-intensity 

stations are statistically significant (p-values <0.001). The 

relatively large R2

 values (average 0.92 and ranging from 0.81 

to 0.98) and generally moderate SE values (average 46.7 percent and ranging from 17.6 to 76.1 percent) indicate reasonably accurate transport relations for the low-intensity stations. 

The SE values for the suspended-sediment equations generally 

were less than or similar to SE values for suspended-sediment 

equations previously used to estimate suspended-sediment 

loads in the Clark Fork basin (Lambing, 1991; Hornberger 

and others, 1997; Lambing, 1998; Lambing and Sando, 2008, 

2009). However, there is substantially lower confidence in 

the suspended-sediment load estimates for the low-intensity 

stations that are based on regressions with streamflow than 

for the high-intensity stations that are determined from highfrequency daily sediment monitoring. 

All the regression equations for estimating trace-element 

discharge (Supplement 3) for the low-intensity stations are 

statistically significant (p-values ranging <0.001 to 0.003). 

The generally moderate R2

 values (average 0.88 and ranging 

from 0.40 to 0.99) and generally moderate SE values (average 

39.1 percent and ranging from 7.6 to 101.1 percent) indicate 

reasonably accurate transport relations for the low-intensity 

stations. The SE values for the trace-element equations generally were less than or similar to SE values for trace-element 

equations previously used to estimate trace-element loads in 

the Clark Fork basin at stations with similar levels of data collection (Lambing, 1991; Hornberger and others, 1997; Lambing, 1998). The averages (and ranges) of SE values in the 

equations for all of the low-intensity stations were 41.6 percent (range 19.1 to 63.1 percent) for cadmium, 45.9 percent 

(range 18.4 to 101.1 percent) for copper, 38.8 percent (range 

11.1 to 74.0 percent) for iron, 44.0 percent (range 16.4 to 

71.3 percent) for lead, 36.5 percent (range 13.6 to 69.3 percent) for manganese, 45.9 percent (range 22.1 to 69.6 percent) 

for zinc, and 21.4 percent (range 7.6 to 37.9 percent) for 

arsenic. The SEs of the trace-element equations for the lowintensity stations generally are substantially larger than for the 

high-intensity stations and indicate lower confidence in the 

trace-element load estimates for the low-intensity stations (that 

are based only on regressions with streamflow) than for the 

high-intensity stations (that are based on best-fit regressions 

with either streamflow or suspended-sediment discharge). 

The regression equations in Supplement 3 were applied to 

the record of daily mean streamflow to estimate daily constituent loads for water years 2006–09. The retransformed results 

were multiplied by retransformation-bias-correction factors to 

compensate for log-retransformation bias by using procedures 

described by Duan (1983); retransformation-bias-correction 

factors ranged from 1.00 to 1.07.

Estimated Loads Transported Through 

the Milltown Reservoir Project Area

Variability in daily loads transported past the three highintensity stations that bracket the project area illustrate the 

response of constituent transport to streamflow variations, 

ice conditions, reservoir operations, dam-removal activities, 

and discrete erosional events. Daily loads can be summed for 

individual periods to determine cumulative loads transported 

over prolonged periods to indicate the most important source 

areas contributing constituents to the river on a sustained 

basis. The cumulative net gains or losses for the periods before 

and after the breach of Milltown Dam can identify differences 

in the mass balance for distinctly different environmental settings and flow conditions. Also, the cumulative net gains or 

losses during a given year can be used to describe the annual 

mass balance of the suspended-sediment and trace-element 

loads transported through the project area. Load estimates for 

the three stations and mass balance of suspended sediment 

and trace elements within the project area for water year 2009 

are presented with comparisons to estimated loads and mass 

balance within the project area for (1) water years 1985–2005 

(average annual load and mass-balance values for all years 

combined, representing typical conditions during the period 

before the start of permanent drawdown of Milltown Reservoir on June 1, 2006); (2) water years 1996–97 (average 

annual load and mass-balance values for both years combined, 

representing high-flow conditions during the period before 

the start of permanent drawdown); (3) water years 2006–07 

(representing the period between the start of permanent 

drawdown of Milltown Reservoir and the breach of Milltown 

Dam); and (4) water year 2008, during which Milltown Dam 

was breached.

Daily Loads

Variations of estimated daily suspended-sediment, 

unfiltered-recoverable copper, and unfiltered-recoverable arsenic loads transported as inflow to and outflow from the project 

area during water years 2004–09 are shown in figures 9–11, 

respectively, along with the daily mean streamflow during 

the years. Water years 2004–09 are specifically presented in 

figures 9–11 to illustrate variability in constituent transport 

before the start of permanent drawdown, during permanent 

drawdown, on the day Milltown Dam was breached, and after 

the breach of Milltown Dam. Daily constituent loads during 

water years 2004–07 and water year 2008 were previously 

reported and discussed in detail in Lambing and Sando (2008, 

2009). Thus, in this report, variations in constituent loads for 
26 Estimated Loads of Suspended Sediment and Trace Elements, Clark Fork Basin, Montana (Water Years 1985–2009)

water years 2004–08 are briefly summarized and compared 

with variations for water year 2009, which are discussed 

in detail.

In figures 9–11, daily values for the two streamflowgaging stations upstream from the project area (Clark Fork at 

Turah Bridge and Blackfoot River near Bonner) are summed 

to represent the total inflow of streamflow and constituent 

loads from the upstream source areas to the project area. 

Values determined for Clark Fork at Turah Bridge represent 

the inflow from the upper Clark Fork basin; values determined 

for Blackfoot River near Bonner represent the inflow from 

the Blackfoot River basin. The daily values for Clark Fork 

above Missoula represent the outflow from the project area. 

Also shown are measured values of instantaneous constituent 

discharge (converted to equivalent daily loads by assuming 

the instantaneous discharge is maintained for 24 hours) for the 

periodic water-quality samples collected during water years 

2004–09 for comparison to the estimated daily loads.

Deposition of constituent load (net gain) within the 

project area is indicated by black shading of the area between 

inflow and outflow loads in figures 9–11 for days when the 

combined inflow load to the project area was greater than 

the outflow load at Clark Fork above Missoula. Conversely, 

loss of load (net loss) from the project area is indicated by 

red shading of the area between inflow and outflow loads for 

days when the outflow load at Clark Fork above Missoula was 

greater than the combined inflow load. Differences in daily 

loads transported to and from the project area illustrate discrete periods of net balance, deposition, or loss of constituent 

load within the project area. Generally, deposition of sediment 

and sediment-associated trace elements within the project area 

is more likely to occur during low streamflows when velocities are low. Loss of sediment and sediment-associated trace 

elements is more likely to occur during high streamflows when 

velocities are high; however, deposition or loss of constituents 

also can result from discrete events or localized activities not 

directly related to streamflow magnitude.

The gray-shaded area in figures 9–11 represents water 

years 2004–05 plus the first 7 months of water year 2006 

before the start of permanent drawdown of Milltown Reservoir 

on June 1, 2006. The yellow-shaded area represents the period 

between the start of permanent drawdown and the breach of 

Milltown Dam on March 28, 2008. The pink-shaded area represents the period after the breach of Milltown Dam when the 

environment of the project area changed from a reservoir to 

a free-flowing river. The color-shading representation of time 

periods shown in figures 9–11 also is used in other figures in 

this report. The Y-axes of figures 9–11 are plotted on log scale; 

thus, the visual appearance of smaller net gains or losses is 

exaggerated relative to larger net gains or losses.

The annual variability in daily streamflow (figs. 2, 9–11) 

illustrates a typical snowmelt-dominated hydrograph in which 

streamflow generally was low to moderate during the fall 

and winter, then increased for several months during spring 

runoff, which often starts in early March. During water 

years 2004–09, streamflow commonly peaked in May or early 

June and then decreased to low-flow conditions in late summer. Minimum streamflows often occurred during December 

or January when base-flow conditions occurred and ice formation further reduced streamflow. Short-lived precipitation 

events or periods of ice-melt sometimes produced substantial 

increases in streamflow during general base-flow periods, as 

evidenced by relatively large increases in streamflow during 

January 2005, November 2006, November 2008, and January 2009. Differences in daily streamflow between the combined inflow to the project area and the outflow at Clark Fork 

above Missoula were small. Minor differences between combined inflow and outflow could represent inputs from ungaged 

small tributaries in the reaches between the streamflow-gaging 

stations, loss of surface water to groundwater in the alluvium 

underlying the project area, possible small amounts of evaporative loss from the reservoir surface, or minor inaccuracies in 

streamflow records.

A detailed description of streamflow conditions during water year 2009 (with comparison to water year 2008) 

is presented in the section of this report titled “Hydrologic 

Characteristics.” In general, streamflow conditions during 

water year 2009 were above normal to near normal. Water 

year 2009 streamflow was especially high during the rising limb of the runoff period, and generally was higher and 

sustained for a longer time than water year 2008 rising-limb 

streamflow, although annual peak streamflows were the same 

for both years.

Suspended Sediment

Variations in estimated daily suspended-sediment loads 

(fig. 9) transported to and from the project area during the 

period before the start of permanent drawdown (October 1, 

2003–May 31, 2006; fig. 9) generally coincided with variations in streamflow. For most of the period, streamflows were 

below or near normal (fig. 2) and differences between the 

daily suspended-sediment loads transported to and from the 

project area generally were minor (indicating approximate net 

balance) or indicated small amounts of deposition or scour. 

However, the outflow of suspended sediment from the project 

area was substantially greater than the combined inflow 

during the temporary drawdowns in July–August 2004 and 

October–December 2005. The increase in suspended-sediment 

load transported from the project area during the temporary 

drawdowns occurred during low-flow conditions. Thus, these 

losses might have resulted from erosion of exposed shoreline sediments or localized disturbances, rather than scour of 

deeper bottom sediments (Lambing and Sando, 2008).

Consistent net loss of suspended sediment from the 

project area began with the start of permanent drawdown on 

June 1, 2006, and continued through the end of the drawdown 

period marked by the breach of Milltown Dam on March 28, 

2008. During the permanent drawdown period, streamflow 

generally was below normal (fig. 2, table 2), but the pattern of 

near continuous net loss of suspended sediment persisted for 

all streamflow conditions and, at times, the loss of sediment 
Daily mean streamflow, in cubic feet per second

Daily suspended-sediment load, in tons 

1 

10 

100 

1,000 

10,000 

30,000 

1 

10 

100 

1,000 

10,000 

30,000 

1

10

100

1,000

10,000

100,000

500,000

Outflow from Milltown Reservoir project area—Daily mean streamflow at Clark Fork above 

Missoula, Mont.

Streamflow

Suspended-sediment loads

Inflow to Milltown Reservoir project area—Daily mean streamflow (sum of daily mean streamflows 

for Clark Fork at Turah Bridge, near Bonner, Mont., and Blackfoot River near Bonner, Mont.)

Outflow from Milltown Reservoir project area—Estimated daily suspended-sediment load at 

Clark Fork above Missoula, Mont.

Inflow to Milltown Reservoir project area—Estimated daily suspended-sediment load (sum of 

estimated daily suspended-sediment loads for Clark Fork at Turah Bridge, near Bonner, Mont., 

and Blackfoot River near Bonner, Mont.)

Inflow to Milltown Reservoir project area—Periodic water-quality sample (measured instantaneous 

suspended-sediment discharge converted to an equivalent daily load)

Outflow from Milltown Reservoir project area—Periodic water-quality sample (measured 

instantaneous suspended-sediment discharge converted to an equivalent daily load)

Net loss of suspended sediment from Milltown Reservoir project area

Net gain of suspended sediment in Milltown Reservoir project area

Breach of Milltown Dam 

(March 28, 2008)

Start of permanent drawdown of Milltown Reservoir (June 1, 2006)

Temporary drawdown of 

Milltown Reservoir 

(July–August, 2004)

Temporary drawdown of 

Milltown Reservoir 

(October–December, 2005)

Period of ice formation 

followed by ice breakup

Period after breach of Milltown Dam Period between start of permanent drawdown of 

Milltown Reservoir and breach of Milltown Dam

Period before start of permanent drawdown of Milltown Reservoir

Water year 2004 Water year 2005 Water year 2006

Water year 2007 Water year 2008 Water year 2009

Oct. 2003 Apr. 2004 Oct. 2004 Apr. 2005 Oct. 2005 Apr. 2006 Oct. 2006

Oct. 2006 Apr. 2007 Oct. 2007 Apr. 2008 Oct. 2008 Apr. 2009 Oct. 2009

1

10

100

1,000

10,000

100,000

500,000

EXPLANATION

Period of ice formation 

followed by ice breakup

Period between start of permanent drawdown of 

Milltown Reservoir and breach of Milltown Dam

Figure 9. Daily mean streamflow and estimated daily suspended-sediment loads transported to and from the Milltown Reservoir project 

area, water years 2004–09.

Estimated Loads Transported Through the Milltown Reservoir Project Area 27
Figure 10. Daily mean streamflow and estimated daily unfiltered-recoverable copper loads transported to and from the Milltown 

Reservoir project area, water years 2004–09. Daily mean streamflow, in cubic feet per second

Daily unfiltered-recoverable copper load, in tons 

Outflow from Milltown Reservoir project area—Daily mean streamflow at Clark Fork above 

Missoula, Mont.

Streamflow

Unfiltered-recoverable copper loads

Inflow to Milltown Reservoir project area—Daily mean streamflow (sum of daily mean streamflows 

for Clark Fork at Turah Bridge, near Bonner, Mont., and Blackfoot River near Bonner, Mont.)

Outflow from Milltown Reservoir project area—Estimated daily unfiltered-recoverable copper load 

at Clark Fork above Missoula, Mont.

Inflow to Milltown Reservoir project area—Estimated daily unfiltered-recoverable copper load (sum 

of estimated daily unfiltered-recoverable copper loads for Clark Fork at Turah Bridge, near 

Bonner, Mont., and Blackfoot River near Bonner, Mont.)

Inflow to Milltown Reservoir project area—Periodic water-quality sample (measured instantaneous 

unfiltered-recoverable copper discharge converted to an equivalent daily load)

Outflow from Milltown Reservoir project area—Periodic water-quality sample (measured 

instantaneous unfiltered-recoverable copper discharge converted to an equivalent daily load)

Net loss of unfiltered-recoverable copper from Milltown Reservoir project area

Net gain of unfiltered-recoverable copper in Milltown Reservoir project area

EXPLANATION

0.010

0.100

1

10

100

0.001

500

0.010

0.100

1

10

100

0.001

500

1 

10 

100 

1,000 

10,000 

30,000 

1 

10 

100 

1,000 

10,000 

30,000 

Start of permanent drawdown of Milltown Reservoir (June 1, 2006)

Temporary drawdown of 

Milltown Reservoir 

(July–August, 2004)

Temporary drawdown of 

Milltown Reservoir 

(October–December, 2005)

Period of ice formation 

followed by ice breakup

Period after breach of Milltown Dam Period between start of permanent drawdown of 

Milltown Reservoir and breach of Milltown Dam

Period before start of permanent drawdown of Milltown Reservoir

Water year 2004 Water year 2005 Water year 2006

Water year 2007 Water year 2008 Water year 2009

Oct. 2003 Apr. 2004 Oct. 2004 Apr. 2005 Oct. 2005 Apr. 2006 Oct. 2006

Oct. 2006 Apr. 2007 Oct. 2007 Apr. 2008 Oct. 2008 Apr. 2009 Oct. 2009

Period of ice formation 

followed by ice breakup

Period between start of permanent drawdown of 

Milltown Reservoir and breach of Milltown Dam

Breach of Milltown Dam 

(March 28, 2008)

28 Estimated Loads of Suspended Sediment and Trace Elements, Clark Fork Basin, Montana (Water Years 1985–2009)
Figure 11. Daily mean streamflow and estimated daily unfiltered-recoverable arsenic loads transported to and from the Milltown 

Reservoir project area, water years 2004–09.

Breach of Milltown Dam 

(March 28, 2008)

Start of permanent drawdown of 

Milltown Reservoir (June 1, 2006)

Temporary drawdown of 

Milltown Reservoir 

(July–August, 2004)

Temporary drawdown of 

Milltown Reservoir 

(October–December, 2005)

Period of ice formation 

followed by ice breakup

Period after breach of Milltown Dam Period between start of permanent drawdown of 

Milltown Reservoir and breach of Milltown Dam

Period before start of permanent drawdown of Milltown Reservoir

Water year 2004 Water year 2005 Water year 2006

Water year 2007 Water year 2008 Water year 2009

Oct. 2003 Apr. 2004 Oct. 2004 Apr. 2005 Oct. 2005 Apr. 2006 Oct. 2006

Oct. 2006 Apr. 2007 Oct. 2007 Apr. 2008 Oct. 2008 Apr. 2009 Oct. 2009

Period of ice formation 

followed by ice breakup

Period between start of permanent drawdown of 

Milltown Reservoir and breach of Milltown Dam

0.01

0.1

1

0.001

10

Daily mean streamflow, in cubic feet per second

Daily unfiltered-recoverable arsenic load, in tons 

1 

10 

100 

1,000 

10,000 

30,000 

1 

10 

100 

1,000 

10,000 

30,000 

Outflow from Milltown Reservoir project area—Daily mean streamflow at Clark Fork above 

Missoula, Mont.

Streamflow

Unfiltered-recoverable arsenic loads

Inflow to Milltown Reservoir project area—Daily mean streamflow (sum of daily mean streamflows 

for Clark Fork at Turah Bridge, near Bonner, Mont., and Blackfoot River near Bonner, Mont.)

Outflow from Milltown Reservoir project area—Estimated daily unfiltered-recoverable arsenic load 

at Clark Fork above Missoula, Mont.

Inflow to Milltown Reservoir project area—Estimated daily unfiltered-recoverable arsenic load 

(sum of estimated daily unfiltered-recoverable arsenic loads for Clark Fork at Turah Bridge, near 

Bonner, Mont., and Blackfoot River near Bonner, Mont.)

Inflow to Milltown Reservoir project area—Periodic water-quality sample (measured instantaneous 

unfiltered-recoverable arsenic discharge converted to an equivalent daily load)

Outflow from Milltown Reservoir project area—Periodic water-quality sample (measured 

instantaneous unfiltered-recoverable arsenic discharge converted to an equivalent daily load)

Net loss of unfiltered-recoverable arsenic from Milltown Reservoir project area

Net gain of unfiltered-recoverable arsenic in Milltown Reservoir project area

EXPLANATION

0.01

0.1

1

0.001

10

Estimated Loads Transported Through the Milltown Reservoir Project Area 29
30 Estimated Loads of Suspended Sediment and Trace Elements, Clark Fork Basin, Montana (Water Years 1985–2009)

from the project area was substantial. The sediment losses 

from the project area likely were caused by a combination of 

factors. Primary processes likely included either erosion of 

bottom sediments in the shallow areas that were susceptible to 

velocity-induced scour during high flow or channel cutting and 

bank collapse of exposed sediment as water levels dropped 

and water drained from the material. Secondary factors might 

have included erosion of shoreline sediment by wind-induced 

waves or disturbance by construction activities (Lambing and 

Sando, 2008).

After the breach of Milltown Dam, net losses of suspended sediment from the project area were substantially 

greater than those during the permanent drawdown period 

because of generally above-normal streamflow and the 

steeper channel gradient that contributed to increased erosion 

of sediment sources within the project area. The outflow of 

suspended sediment from the project area sharply increased 

immediately after the breach of Milltown Dam, resulting in 

large daily net losses. The large post-breach net losses of 

suspended sediment from the project area continued through 

the rising limb and peak flow of water year 2008. Net losses 

of suspended sediment began to decrease after the peak flow, 

but were still relatively large during the early stages of the 

falling limb through June 2008. The net losses of suspended 

sediment continued to decrease during the falling limb and 

were relatively small by July 2008, reaching an approximate 

net balance between inflow and outflow loads by mid-August 

2008. From mid-August through September 2008, the average 

daily net loss of sediment from the project area was less than 

1 ton per day. 

In early water year 2009 (October 1–December 15, 

2008), the approximate net balance between suspendedsediment inflow and outflow loads continued with an average 

daily net loss of sediment from the project area of less than 

5 tons per day. In mid-December through late January, ice 

conditions caused a relatively large increase in net losses from 

the project area; the cumulative net loss during this period was 

about 5,100 tons of sediment for an average of about 113 tons 

per day. After this erosional event, suspended-sediment net 

losses decreased to about net balance conditions until the start 

of the rising limb of the runoff period in mid-March. During 

the water year 2009 runoff period (about March 9–July 27, 

2009) when streamflows generally were above normal, net 

losses increased substantially (although this is not readily apparent in figure 9 due to the log-scale presentation); 

the cumulative net loss during the runoff period was about 

70,500 tons of sediment for an average of about 500 tons 

per day. Average streamflow for Clark Fork above Missoula 

during the water year 2009 runoff period was 6,400 ft3

/s. In 

comparison, during the water year 2008 runoff period (about 

March 9–August 10, 2008), the cumulative net loss was about 

375,000 tons of sediment for an average of about 2,420 tons 

per day, and the average streamflow at Clark Fork above Missoula was about 5,430 ft3

/s. After the water year 2009 runoff 

period, net losses decreased to about net balance conditions 

until the end of water year 2009. During the post-runoff period 

of water year 2009, the average daily net loss was about 6 tons 

per day.

Although there were substantial net losses of suspended 

sediment from the project area during the water year 2009 runoff period, the losses were much smaller than during the water 

year 2008 runoff period, even though streamflows were higher 

in water year 2009 than in water year 2008. This pattern is 

further illustrated by comparison of suspended-sediment discharges associated with periodic water samples collected near 

the annual peak flows in water years 2008 and 2009. For water 

samples collected on May 19, 2008, at the three high-intensity 

stations that bracket the project area, the sediment discharge at 

Clark Fork above Missoula was 42,400 tons per day, and the 

combined sediment discharges for Clark Fork at Turah Bridge 

and Blackfoot River near Bonner were 9,550 tons per day. 

The –32,800 tons per day net difference (net loss) between the 

combined inflow and outflow sediment discharges occurred at 

a streamflow of 14,800 ft3

/s for Clark Fork above Missoula. 

For water samples collected on May 20, 2009, the sediment 

discharge at Clark Fork above Missoula was 17,500 tons 

per day, and the combined sediment discharges for Clark 

Fork at Turah Bridge and Blackfoot River near Bonner were 

10,000 tons per day. The -7,500 tons per day net difference 

between the combined inflow and outflow sediment discharges 

occurred at a streamflow of 15,400 ft3

/s for Clark Fork above 

Missoula. Thus, for periodic water samples collected during 

similar streamflow magnitudes and conditions, the net loss of 

suspended sediment from the project area for the water year 

2009 near-peak-flow sample set was about 23 percent of the 

net loss of suspended sediment from the project area for the 

water year 2008 near-peak-flow sample set.

Sample results for the supplemental sampling site Clark 

Fork Bypass near Bonner (fig. 1, map number 2) are useful for 

evaluating contributions from localized areas of erosion within 

the project area. A comparison of periodic water-quality data 

for samples collected on 16 concurrent dates during water year 

2008 and 20 concurrent dates during water year 2009 at Clark 

Fork at Turah Bridge and Clark Fork Bypass near Bonner 

indicates substantial differences within a short distance (about 

5 river mi) of channel between the two sites. Samples at Clark 

Fork Bypass near Bonner generally were collected within 2 

to 3 hours after samples were collected at Clark Fork at Turah 

Bridge; this time interval approximated the stream travel time, 

which provided characterization of the same hydrologic condition at both sites. With no major inflows between the two sites, 

differences in sediment transport presumably can be attributed 

to erosion from channel sources, such as the streambed and 

banks, within the intervening reach.

Differences in suspended-sediment concentrations 

between Clark Fork at Turah Bridge and Clark Fork Bypass 

near Bonner indicate a substantial input of sediment from the 

intervening channel, especially during high-flow conditions. 

The post-breach median suspended-sediment concentration 

in water year 2008 increased from 39 mg/L at Clark Fork at 

Turah Bridge to 161 mg/L at Clark Fork Bypass near Bonner 

for the 16 samples collected on concurrent dates during the 
Estimated Loads Transported Through the Milltown Reservoir Project Area 31

runoff period. In water year 2009, the median suspendedsediment concentration increased from 42 mg/L at Clark 

Fork at Turah Bridge to 78 mg/L at Clark Fork Bypass near 

Bonner for the 20 samples collected on concurrent dates during the runoff period. Thus, the water year 2009 increase in 

median suspended-sediment concentration between the two 

sites was substantially less than the water year 2008 increase, 

even though water year 2009 streamflows were higher than 

water year 2008 streamflows. These patterns indicate substantial depletion of readily erodible sediment for the evaluated 

streamflows in the reach between Clark Fork at Turah Bridge 

and Clark Fork Bypass near Bonner by the end of water 

year 2008.

Comparisons between particle-size distributions of 

suspended sediment at Clark Fork at Turah Bridge and Clark 

Fork Bypass near Bonner also provide information on differences in sediment composition and transport characteristics 

between the two sites during water years 2008 and 2009. The 

post-breach water year 2008 median percent of suspended 

sediment finer than 0.062 mm (hereinafter referred to as fines) 

for the concurrent sample sets decreased from 72 percent at 

Clark Fork at Turah Bridge to 28 percent at Clark Fork Bypass 

near Bonner. In water year 2009, the median percent fines for 

the concurrent sample sets decreased from 70 percent at Clark 

Fork at Turah Bridge to 52 percent at Clark Fork Bypass near 

Bonner. Thus, there was substantially more sand content in 

the suspended sediment at Clark Fork Bypass near Bonner 

than at Clark Fork at Turah Bridge during both water years 

2008 and 2009, but the increase in sand content at Clark Fork 

Bypass near Bonner was less in water year 2009 than in water 

year 2008. 

In both water years 2008 and 2009, the greatest inputs 

of suspended sediment from the intervening reach between 

Clark Fork at Turah Bridge and Clark Fork Bypass near Bonner occurred during the rising limb of the runoff period near 

the annual peaks. For example, the sample of May 19, 2008, 

at Clark Fork at Turah Bridge had a suspended-sediment concentration of 151 mg/L, with a size composition of 63-percent 

fines, indicating a predominance of fine sediment. In contrast, 

the sample collected about 2 hours later at Clark Fork Bypass 

near Bonner had a suspended-sediment concentration of 

3,780 mg/L (a 25-fold increase in concentration in the relatively short intervening reach) and was predominantly sand, 

with a size composition of 9-percent fines. Near the annual 

peak in water year 2009, the sample of May 26, 2009, at Clark 

Fork at Turah Bridge had a suspended-sediment concentration of 136 mg/L, with a size composition of 65-percent 

fines, indicating predominantly fine sediment. In contrast, 

the sample collected 2 hours later at Clark Fork Bypass near 

Bonner had a suspended-sediment concentration of 227 mg/L 

(a relatively small 1.7-fold increase in concentration in the 

intervening reach) and had a size composition of 51-percent 

fines. Although the sand content increased in the intervening 

reach in water year 2009, the increase was much less than in 

water year 2008 under similar hydrologic conditions.

The substantial increase during water year 2008 in 

suspended-sediment concentration and percentage of sand 

contributed from the short reach between Clark Fork at Turah 

Bridge and Clark Fork Bypass near Bonner clearly indicates 

the dynamic process of post-breach channel erosion that 

occurred during high flow in the area near the upper end of 

the Clark Fork arm of the former reservoir. The substantially smaller differences during water year 2009 between 

suspended-sediment concentration and percentage of sand 

contributed from the short reach between Clark Fork at Turah 

Bridge and Clark Fork Bypass near Bonner further indicates 

that there was substantial depletion of erodible channel materials or, possibly greater geomorphic stability, in the reach by 

the end of water year 2008.

Trace Elements

The temporal variations in daily loads for all the 

unfiltered-recoverable trace elements (cadmium, copper, iron, 

lead, manganese, zinc, and arsenic) estimated for water years 

2004–09 generally were similar. Estimated daily loads of 

copper (fig. 10) and arsenic (fig. 11) are illustrated as representative examples of trace-element transport to and from the 

project area during water years 2004–09. These two elements 

are presented as examples because they are constituents of 

concern in terms of potential toxicity and they represent the 

greatest difference in chemical characteristics and transport 

relations among the trace elements

Variations in daily copper and arsenic loads entering and 

leaving the project area generally coincided with variations 

in streamflow (figs. 10 and 11) and suspended-sediment loads 

(fig. 9); the daily loads for the other trace elements similarly 

coincided with variations in streamflow and suspendedsediment loads. The specific temporal variability in daily 

suspended-sediment loads and associated causal factors previously noted in this report generally apply to variability in daily 

trace-element loads. However, specific observations concerning daily copper and arsenic loads during high-transport periods in water year 2009 and differences in daily trace-element 

loads between water year 2008 and water year 2009 are 

presented to further characterize transport processes.

In early water year 2009 (October 1–December 15, 

2008), there was an approximate net balance between copper inflow and outflow loads within the project area, with 

an average daily net loss of about 0.001 ton per day. During 

the period of ice conditions in mid-December 2008 through 

late January 2009, the cumulative net loss of copper from 

the project area was about 2.47 tons for an average of about 

0.055 ton per day. After this erosional event, copper net losses 

decreased to about net balance conditions until the start of the 

runoff period in mid-March. During the water year 2009 runoff period (about March 9–July 27, 2009), net losses increased 

substantially; the cumulative net loss during the runoff period 

was about 25.5 tons of copper (average of about 0.181 ton 

per day). Average streamflow for Clark Fork above Missoula 

during the water year 2009 runoff period was 6,400 ft3/s. In 
32 Estimated Loads of Suspended Sediment and Trace Elements, Clark Fork Basin, Montana (Water Years 1985–2009)

comparison, during the water year 2008 runoff period (about 

March 9–August 10, 2008), which included the initial postbreach period, the cumulative net loss was about 150 tons of 

copper (average of about 0.971 ton per day), and the average streamflow at Clark Fork above Missoula was about 

5,430 ft3/s. After the water year 2009 runoff period, net losses 

persisted, but decreased until the end of water year 2009. During the post-runoff period, the average daily net loss was about 

0.012 ton per day.

Although there were substantial net losses of copper from 

the project area during the water year 2009 runoff period, the 

losses were much smaller than during the water year 2008 

runoff period, even though streamflows were higher in water 

year 2009 than in water year 2008. This pattern is further 

illustrated by comparison of copper discharges associated with 

periodic water samples collected near the peak annual flows 

in water years 2008 and 2009. For water samples collected on 

May 19, 2008, at the three high-intensity stations that bracket 

the project area, the copper discharge at Clark Fork above 

Missoula was 12.3 tons per day, and the combined copper 

discharges for Clark Fork at Turah Bridge and Blackfoot 

River near Bonner were 0.704 ton per day. The –11.6 tons per 

day net difference (net loss) between the combined inflow 

and outflow copper discharges occurred at a streamflow of 

14,800 ft3/s for Clark Fork above Missoula. For water samples 

collected on May 20, 2009, the copper discharge at Clark Fork 

above Missoula was 3.38 tons per day, and the combined copper discharges for Clark Fork at Turah Bridge and Blackfoot 

River near Bonner were 1.40 tons per day. The –1.98 tons per 

day net difference between the combined inflow and outflow 

copper discharges occurred at a streamflow of 15,400 ft3

/s for 

Clark Fork above Missoula. Thus, for periodic water samples 

collected during similar streamflow magnitudes and conditions, the net loss of copper from the project area for the water 

year 2009 near-peak-flow sample set was about 17 percent of 

the net loss of copper from the project area for the water year 

2008 near-peak-flow sample set.

In early water year 2009 (October 1–December 15, 

2008), there was an approximate net balance between arsenic 

inflow and outflow loads, with an average daily net loss of 

less than 0.001 ton per day. During the period of ice conditions in mid-December 2008 through late January 2009, the 

cumulative net loss of arsenic from the project area was about 

0.51 ton for an average of about 0.011 ton per day. After 

this erosional event, arsenic net losses decreased to about 

net balance conditions until the start of the runoff period in 

mid-March. During the water year 2009 runoff period (about 

March 9–July 27, 2009), net losses increased substantially; 

the cumulative net loss during the runoff period was about 

3.1 tons of arsenic (average of about 0.022 ton per day). 

Average streamflow for Clark Fork above Missoula during the 

water year 2009 runoff period was 6,400 ft3

/s. In comparison, 

during the water year 2008 runoff period (about March 9–

August 10, 2008), the cumulative net loss was about 11.6 tons 

of arsenic (average of about 0.075 ton per day), and the 

average streamflow at Clark Fork above Missoula was about 

5,430 ft3

/s. After the water year 2009 runoff period, net losses 

persisted but decreased until the end of water year 2009. During the post-runoff period, the average daily net loss was about 

0.010 ton per day.

Although there were substantial net losses of arsenic from 

the project area during the water year 2009 runoff period, the 

losses were much smaller than during the water year 2008 runoff period, even though streamflows were higher in water year 

2009 than in water year 2008. This pattern is further illustrated 

by comparison of arsenic discharges associated with periodic 

water samples collected near the peak annual flows in water 

years 2008 and 2009. For water samples collected on May 19, 

2008, at the three high-intensity stations that bracket the project area, the arsenic discharge at Clark Fork above Missoula 

was 1.04 tons per day, and the combined arsenic discharges for 

Clark Fork at Turah Bridge and Blackfoot River near Bonner 

were 0.210 ton per day. The –0.830 ton per day net difference 

(net loss) between the combined inflow and outflow arsenic 

discharges occurred at a streamflow of 14,800 ft3

/s for Clark 

Fork above Missoula. For water samples collected on May 20, 

2009, the arsenic discharge at Clark Fork above Missoula was 

0.473 ton per day, and the combined arsenic discharges for 

Clark Fork at Turah Bridge and Blackfoot River near Bonner 

were 0.316 ton per day. The –0.157 ton per day net difference 

between the combined inflow and outflow arsenic discharges 

occurred at a streamflow of 15,400 ft3

/s for Clark Fork above 

Missoula. Thus, for periodic water samples collected during 

similar streamflow magnitudes and conditions, the net loss of 

arsenic from the project area for the water year 2009 nearpeak-flow sample set was about 19 percent of the net loss of 

arsenic from the project area for the water year 2008 nearpeak-flow sample set.

Annual Loads and Mass Balance

Annual loads of suspended sediment and trace elements 

transported past each of the three high-intensity stations that 

bracket the Milltown Reservoir project area (table 5) were 

determined for water year 2009 by summing the estimated 

daily loads for the entire water year. Annual streamflow (in 

acre-ft) also is presented in table 5 to provide a perspective 

on streamflow conditions associated with constituent transport. Mass balance within the project area was determined 

as the difference between the combined annual inflow of 

streamflow and estimated loads from the two upstream source 

areas [basins upstream from Clark Fork at Turah Bridge 

(station 12334550) and Blackfoot River near Bonner (station 

12340000)] and the annual outflow of streamflow and estimated loads at Clark Fork above Missoula (station 12340500).

In addition to results for water year 2009, annual loads 

and mass balances are presented for selected periods during water years 1985–2008 before and after the breach of 

Milltown Dam to provide a perspective on the effects of 

hydrologic conditions and the activities associated with the 

removal of Milltown Dam (table 5). The annual-loads and 
Estimated Loads Transported Through the Milltown Reservoir Project Area 33

mass-balance data presented in table 5 include (1) average 

annual values for water years 1985–2005 (representing typical 

conditions during the period before the start of permanent 

drawdown of Milltown Reservoir on June 1, 2006); (2) average annual values for water years 1996–97 (representing highflow conditions during the period before the start of permanent 

drawdown); (3) annual values for water years 2006 and 2007 

(representing the period between the start of permanent drawdown of Milltown Reservoir and the breach of Milltown Dam; 

(4) annual values for water year 2008, during which Milltown 

Dam was breached; and (5) annual values for water year 2009 

(the first full water year after the breach of Milltown Dam).

The annual (water year) period provides a convenient 

and consistent accounting period for comparing cumulative loads to discern changes in transport characteristics. 

The annual-loads and mass-balance data are segregated into 

discrete hydrologic periods or periods representing specific 

dam-removal activities, including the permanent drawdown of 

Milltown Reservoir and the breach of Milltown Dam, which 

affected constituent transport to and from the project area. 

However, the start of permanent drawdown (June 1, 2006) 

and the breach of Milltown Dam (March 28, 2008) did not 

coincide with the start of a given water year. For comparison 

purposes, water year 2006 was assigned to the period between 

the start of permanent drawdown and the breach of Milltown 

Dam because even though part of water year 2006 occurred 

before the start of permanent drawdown, the large effect of 

the drawdown conditions dominated the constituent transport 

conditions of the water year as a whole. Likewise, water year 

2008 was assigned to the period after the breach of Milltown 

Dam because even though part of water year 2008 occurred 

before the breach of Milltown Dam, the large effect of the dam 

breach dominated the constituent transport conditions of the 

water year as a whole.

Comparison of annual mean streamflow between the 

various periods indicates substantial differences in streamflow conditions (table 2), which contributes to differences 

in constituent transport. During the period before the start 

of permanent drawdown, the mean annual streamflow of 

2,550 ft3/s for water years 1985–2005 was less (88 percent) 

than the long-term mean annual streamflow of 2,910 ft3

/s 

(table 2), but the mean annual streamflow of 4,540 ft3

/s for the 

1996–97 high-flow years was much greater (156 percent) than 

the long-term mean annual streamflow. For water years representing the period between the start of permanent drawdown 

and the breach of Milltown Dam (water years 2006–07), the 

mean annual streamflow of 2,460 ft3

/s was less (85 percent) 

than the long-term mean annual streamflow and similar to the 

mean annual streamflow for water years 1985–2005. For water 

years representing the period after the breach of Milltown 

Dam (water years 2008–09), the annual mean streamflow of 

3,040 ft3/s for water year 2008 was slightly greater (105 percent) than the long-term mean annual streamflow, and the 

annual mean streamflow of 3,560 ft3

/s for water year 2009 was 

greater (122 percent) than the long-term mean annual streamflow. The annual mean streamflows for water years 2008 and 

2009 were substantially smaller than the mean annual streamflow for water years 1996–97.

The mass balance of annual estimated loads for selected 

periods during water years 1985–2009 (table 5) provides an 

indication of variations in erosional and depositional processes 

within the project area. If less material was transported from 

the project area than entered, some of the suspended material 

entering the project area was deposited (net gain, indicated 

by positive values of mass balance). If more material was 

transported from the project area than entered, previously 

deposited sediment (and associated trace elements) within the 

project area was eroded and put into suspension, added to the 

suspended material entering the project area from upstream 

sources, and transported with the streamflow out of the project 

area (net loss, indicated by negative values of mass balance). 

The mass-balance results for the various periods indicate the 

differences in net gains or losses within the project area for 

distinctly different environmental settings and flow conditions. 

The mass-balance results and percentages of the annual 

outflow transported from the project area that were contributed 

from the two upstream source areas and from within the project area during the various periods also are illustrated by using 

pie charts (fig. 12). The annual inflow loads contributed from 

individual upstream source areas are those shown for Clark 

Fork at Turah Bridge (station 12334550) and Blackfoot River 

near Bonner (station 12340000) in table 5. The annual loads 

contributed from within the project area are those shown as 

the mass balance in table 5, which represents the net loss from 

the project area. The annual outflow load transported from the 

project area is represented as the load transported past Clark 

Fork above Missoula (station 12340500). In some periods, 

there were net gains of streamflow volume or constituent loads 

within the project area. In these cases, the magnitudes of the 

net gains are shown in boxes beside the pie charts. Generally, 

the percent values shown in parentheses in figure 12 were 

calculated as the annual load contributed from an individual 

source area divided by the annual outflow load (values shown 

in fig. 12 and table 5) during the indicated period. However, 

in years when there were net gains of constituent loads within 

the project area, the net gains within the project area had to be 

accounted for in the calculations of the percent contribution 

from an individual source area to the annual outflow load. In 

years when there were net gains within the project area, the 

contribution from an individual source area to the net gain was 

assumed to be proportional to the contribution from the individual source area to the combined inflow to the project area. 

The percent values of source-area contributions were calculated as the annual load contributed from the individual source 

area minus the proportional contribution from the individual 

source area to the net gain divided by the annual outflow load. 

In addition to the proportional contributions indicated by 

percentages, the actual annual outflow loads [shown for Clark 

Fork above Missoula (station 12340500)] and source-areacontribution loads, in tons, are shown to provide perspective 

on the relative magnitude of differences in constituent loads 

transported during the various periods.
Table 5. Mass balance of annual streamflow and estimated loads of suspended sediment and unfiltered-recoverable trace elements 

transported to and from the Milltown Reservoir project area for selected periods during water years 1985–2009.—Continued

[Abbreviations: ND, not determined. All numerical values shown are rounded to three significant figures. In some cases, the combined inflow minus the outflow 

does not exactly equal the reported mass balance due to rounding effects. When calculating combined inflow or mass balance, all values used in the preliminary 

calculation were rounded to the same decimal place as three-significant-figure rounding of the smallest number used in the calculation. The final calculated 

value then was rounded to three significant figures]

Station name and number or summation category

Annual 

streamflow 

volume 

(acre-feet)

 Estimated annual load (tons)

Suspended 

sediment Cadmium1 Copper Iron Lead Manganese

Zinc Arsenic

Period before start of permanent drawdown of Milltown Reservoir and breach of Milltown Dam (water years 1985–2005)

Average annual (water years 1985–2005)

Clark Fork at Turah Bridge, near 

Bonner, Mont. (12334550)

 856,000 52,700 ND 33.3 ND 7.02 ND 47.2 ND

Blackfoot River near Bonner, 

Mont. (12340000)

 1,000,000 40,900 ND 4.87 ND 2.39 ND 6.74 ND

Combined inflow to Milltown 

Reservoir project area (sum 

of stations 12340000 and 

12334550)

 1,860,000 93,600 ND 38.2 ND 9.41 ND 53.9 ND

Milltown Reservoir project area 

outflow (as measured at the 

Clark Fork above Missoula, 

Mont. (12340500))

 1,850,000 87,800 ND 34.1 ND 6.34 ND 56.3 ND

Mass balance for Milltown 

Reservoir project area:

net gain (+) or loss (-)2

3

+14,300 +5,750 ND +4.14 ND +3.07 ND -2.38 ND

Average annual for selected high-streamflow years (water years 1996–97)

Clark Fork at Turah Bridge, near 

Bonner, Mont. (12334550)

 1,520,000 186,000 ND 97.0 ND 16.9 ND 146 ND

Blackfoot River near Bonner, 

Mont. (12340000)

 1,740,000 142,000 ND 13.3 ND 3.05 ND 14.3 ND

Combined inflow to Milltown 

Reservoir project area (sum 

of stations 12340000 and 

12334550)

 3,260,000 328,000 ND 110 ND 19.9 ND 160 ND

Milltown Reservoir project area 

outflow (as measured at the 

Clark Fork above Missoula, 

Mont. (12340500))

 3,280,000 381,000 ND 117 ND 22.3 ND 195 ND

Mass balance for Milltown 

Reservoir project area:

net gain (+) or loss (-)2

-25,000 -53,500 ND -6.75 ND -2.40 ND -35.2 ND

34 Estimated Loads of Suspended Sediment and Trace Elements, Clark Fork Basin, Montana (Water Years 1985–2009)

The size (area) of each pie chart (fig. 12) represents the 

total outflow from the project area at Clark Fork above Missoula, with colored areas indicating the relative contributions 

from each of three source areas. For each constituent column 

(that is, streamflow, suspended sediment, copper, and arsenic), 

the size of the pie chart representing a given annual value is 

sized proportionally to the largest annual value (or average 

annual value for multiple-year periods) within the constituent group. For example, in the suspended-sediment column, 

the largest annual outflow load was 510,000 tons in water 

year 2008; thus, the size of the water year 2008 pie chart for 

suspended sediment is the largest and serves as the reference for scaling the other suspended-sediment pie charts. The 

annual suspended-sediment-outflow load in water year 2009 

was 221,000 tons, which is 43 percent of the water year 2008 

annual outflow load. Thus, the size of the water year 2009 pie 

chart for suspended sediment is 43 percent of the size of the 

water year 2008 pie chart for suspended sediment. Differences 
Table 5. Mass balance of annual streamflow and estimated loads of suspended sediment and unfiltered-recoverable trace elements 

transported to and from the Milltown Reservoir project area for selected periods during water years 1985–2009.—Continued

[Abbreviations: ND, not determined. All numerical values shown are rounded to three significant figures. In some cases, the combined inflow minus the outflow 

does not exactly equal the reported mass balance due to rounding effects. When calculating combined inflow or mass balance, all values used in the preliminary 

calculation were rounded to the same decimal place as three-significant-figure rounding of the smallest number used in the calculation. The final calculated 

value then was rounded to three significant figures]

Station name and number or summation category

Annual 

streamflow 

volume 

(acre-feet)

 Estimated annual load (tons)

Suspended 

sediment Cadmium1 Copper Iron Lead Manganese

Zinc Arsenic

Period between start of permanent drawdown of Milltown Reservoir and breach of Milltown Dam (water years 2006–07)

Water year 20064

Clark Fork at Turah Bridge, near 

Bonner, Mont. (12334550)

 794,000 33,300 0.132 18.6 555 3.23 73.5 26.7 8.60 

Blackfoot River near Bonner, 

Mont. (12340000)

 1,020,000 38,500 .0174 2.55 499 0.659 34.3 3.28 1.74 

Combined inflow to Milltown 

Reservoir project area (sum 

of stations 12340000 and 

12334550)

 1,820,000 71,800 .149 21.2 1,050 3.89 108 30.0 10.3 

Milltown Reservoir project area 

outflow (as measured at the 

Clark Fork above Missoula, 

Mont. (12340500))

 1,800,000 97,700 .255 32.7 1,300 5.57 135 52.9 11.2 

Mass balance for Milltown 

Reservoir project area:

net gain (+) or loss (-)2

3

+19,000 -25,900 -.106 -11.5 -250 -1.68 -27.0 -22.9 -0.900

Water year 2007

Clark Fork at Turah Bridge, near 

Bonner, Mont. (12334550)

 872,000 35,500 0.140 19.7 592 3.45 78.0 28.4 9.01 

Blackfoot River near Bonner, 

Mont. (12340000)

 946,000 23,000 .0160 2.03 312 0.420 24.2 2.66 1.64 

Combined inflow to Milltown 

Reservoir project area (sum 

of stations 12340000 and 

12334550)

 1,820,000 58,500 .156 21.8 904 3.87 102 31.1 10.6 

Milltown Reservoir project area 

outflow (as measured at the 

Clark Fork above Missoula, 

Mont. (12340500))

 1,770,000 188,000 .453 58.8 1,880 9.67 189 105 16.7 

Mass balance for Milltown 

Reservoir project area:

net gain (+) or loss (-)2

3

+49,000 -130,000 -.297 -37.0 -980 -5.80 -87.0 -74.0 -6.10

Estimated Loads Transported Through the Milltown Reservoir Project Area 35

in the sizes of the pie charts are proportional to differences 

in annual values between years (or average annual values 

for multiple-year periods) in the total outflow streamflow 

volume or constituent loads. Also, differences in the sizes of 

the colored pie-chart slices for a given source area are proportional to differences between years (or multiple-year periods) 

in the contributions to the project area outflow from a given 

source area.

The largest streamflow was the average annual streamflow for water years 1996–97 (3,280 thousand acre-ft). 

The next largest annual streamflows were water year 2009 

(2,580 thousand acre-ft) and water year 2008 (2,200 thousand acre-ft). Relative contributions of the two upstream 

source areas to the outflow streamflow generally were similar 

between all periods. The streamflow contribution of Clark 

Fork at Turah Bridge for the various periods ranged from 44 
Table 5. Mass balance of annual streamflow and estimated loads of suspended sediment and unfiltered-recoverable trace elements 

transported to and from the Milltown Reservoir project area for selected periods during water years 1985–2009.—Continued

[Abbreviations: ND, not determined. All numerical values shown are rounded to three significant figures. In some cases, the combined inflow minus the outflow 

does not exactly equal the reported mass balance due to rounding effects. When calculating combined inflow or mass balance, all values used in the preliminary 

calculation were rounded to the same decimal place as three-significant-figure rounding of the smallest number used in the calculation. The final calculated 

value then was rounded to three significant figures]

Station name and number or summation category

Annual 

streamflow 

volume 

(acre-feet)

 Estimated annual load (tons)

Suspended 

sediment Cadmium1 Copper Iron Lead Manganese

Zinc Arsenic

Period after breach of Milltown Dam (water years 2008–09)

Water year 20084

Clark Fork at Turah Bridge, near 

Bonner, Mont. (12334550)

 1,070,000 56,800 0.185 27.2 923 5.37 113 40.3 12.2 

Blackfoot River near Bonner, 

Mont. (12340000)

 1,100,000 61,800 .0200 3.22 743 1.04 50.9 4.37 2.07 

Combined inflow to Milltown 

Reservoir project area (sum 

of stations 12340000 and 

12334550)

 2,170,000 119,000 .205 30.4 1,670 6.41 164 44.7 14.3 

Milltown Reservoir project area 

outflow (as measured at the 

Clark Fork above Missoula, 

Mont. (12340500))

 2,200,000 510,000 1.14 188 4,640 28.9 338 290 27.5 

Mass balance for Milltown 

Reservoir project area:

net gain (+) or loss (-)2

 -30,000 -391,000 -.938 -157 -2,970 -22.5 -174 -245 -13.2

Water year 2009

Clark Fork at Turah Bridge, near 

Bonner, Mont. (12334550)

 1,290,000 82,600 0.249 39.7 1,320 7.84 154 57.6 14.6 

Blackfoot River near Bonner, 

Mont. (12340000)

 1,270,000 61,800 .0182 3.47 762 1.06 53.2 4.96 2.26 

Combined inflow to Milltown 

Reservoir project area (sum 

of stations 12340000 and 

12334550)

 2,560,000 144,000 .267 43.1 2,080 8.89 207 62.5 16.8 

Milltown Reservoir project area 

outflow (as measured at the 

Clark Fork above Missoula, 

Mont. (12340500))

 2,580,000 221,000 .478 72.1 2,530 12.3 259 111 21.1 

Mass balance for Milltown 

Reservoir project area:

net gain (+) or loss (-)2

-20,000 -76,200 -0.211 -29.0 -446 -3.43 -52.4 -48.9 -4.27

1

Estimated cadmium loads for Blackfoot River near Bonner (station 12340000) were based on regression equations developed from datasets generally having 

more than 50 percent of the values censored (reported as less than the laboratory reporting level). Thus, estimated cadmium loads for this site have greater uncertainty than loads estimated for the other trace elements and the other two sites.

2

Mass balance is the difference (net gain or loss) between the combined inflow to the Milltown Reservoir project area and the outflow transported from the 

Milltown Reservoir project area, as represented by Clark Fork above Missoula (station 12340500). Thus, a net gain (+) indicates net deposition in the Milltown 

Reservoir project area and a net loss (-) indicates net removal from the Milltown Reservoir project area.

3

During some years, daily mean streamflow for Clark Fork above Missoula (station 12340500) was less than the combined inflow, which results in a calculated net gain of water in Milltown Reservoir. Rather than an actual gain of water (net storage), this calculated difference possibly represents a loss of surface 

water to the alluvium underlying Milltown Reservoir and the river reaches between the upstream and downstream gages, plus possible small amounts of evaporative loss from the reservoir surface.

4

Although part of the given water year occurred outside of the indicated period, the given water year is assigned to the period most representative of the dominant conditions of that water year.

36 Estimated Loads of Suspended Sediment and Trace Elements, Clark Fork Basin, Montana (Water Years 1985–2009)
Figure 12. Annual streamflow volume and estimated loads of suspended sediment, unfiltered-recoverable copper, and unfilteredrecoverable arsenic contributed from upstream source areas and the Milltown Reservoir project area for selected time periods during 

water years 1985–2009.

Net gain

of 14.3 KAF

in project

area

Net gain

of 19.0 KAF 

in project 

area

Net gain

of 4.14 tons 

in project 

area

Net gain

of 49.0 KAF 

in project 

area

Net gain of 

5,750 tons 

in project 

area

Basin upstream from 

Clark Fork at Turah 

Bridge, near Bonner, 

Mont. (station 12334550)

Not

determined

Not

determined

Net gain

of 49.0 KAF 

in project 

area

2,580

1,770

2,200

1,800

1,850

3,280

221,000

188,000

510,000

97,700

87,800

381,000

21.1

16.7

27.5

11.2

34.1

117

32.7

58.8

188

72.1

56,800

(11 percent)

61,800

(12 percent)

391,000

(77 percent)

12.2

(44 percent)

2.07

(8 percent)

13.2

(48 percent)

27.2

(14 percent)

157

(84 percent)

3.22

(2 percent)

33,300

(34 percent)

38,500

(39 percent)

25,900

(27 percent)

35,500

(19 percent)

23,000

(12 percent)

130,000

(69 percent)

82,600

61,800 (37 percent)

(28 percent)

76,200

(35 percent)

8.60

(76 percent)

0.900

(8 percent)

1.74

(16 percent)

1.64

(10 percent)

9.01

(54 percent)

6.10

(36 percent)

18.6

(57 percent)

2.55

(8 percent) 11.5

(35 percent)

19.7

(34 percent)

2.03

(3 percent)

37.0

(63 percent)

14.6

(69 percent)

4.27

(20 percent)

2.26

(11 percent)

39.7

(55 percent)

3.47

(5 percent)

29.0

(40 percent)

52,700

(56 percent)

40,900

(44 percent)

186,000

(49 percent)

142,000

(37 percent)

33.3

(87 percent)

4.87

(13 percent)

97.0

(83 percent)

6.75

(6 percent)

856

(46 percent)

1,000

(54 percent)

1,520

(46 percent)

1,740

(53 percent)

794

(44 percent)

1,020

(56 percent)

872

(48 percent)

946

(52 percent)

1,070

(49 percent)

1,100

(50 percent)

30.0

(1 percent)

1,290

(50 percent)

1,270

(49 percent)

20.0

(1 percent)

53,500

(14 percent)

13.3

(11 percent)

25.0

(1 percent)

Pre-drawdown

long-term average 

annual (water 

years 1985−2005)

Period before start of permanent

drawdown of Milltown Reservoir

Period between start of permanent

drawdown of Milltown Reservoir and 

breach of Milltown Dam Period after breach of Milltown Dam

Pre-drawdown

high-flow years 

average annual 

(water years 1996−97)

Water year 20061

Water year 2007

Water year 20081

Water year 2009 

Streamflow,

[1,000 acre-feet (KAF) per year]

Suspended sediment,

(tons per year)

Arsenic,

(tons per year)

Copper,

(tons per year)

Milltown Reservoir 

project area

Net gain in Milltown

Project area indicated 

when inflow to project

area exceeded outflow

from project area

Streamflow, in acre-feet, or 

load, in tons, contributed 

from indicated source area

Basin upstream from Blackfoot River 

near Bonner, Mont. (station 12340000)

Percent of outflow streamflow or load 

transported past Clark Fork above 

Missoula, Mont. (station 12340500)

1

Although part of the given water year occurred

outside of the indicated period, the given water 

year is assigned to the period most representative

of the prevailing conditions of that water year.

Outflow transported past 

Clark Fork above Missoula 

(station 12340500)

Total pie chart represents Milltown Reservoir 

project area outflow at Clark Fork above Missoula, 

Mont. (station 12340500). 

Size (area) of pie chart is proportional to largest annual 

streamflow or load (for given constituent)

EXPLANATION

Value

(percent)

221,000

Estimated Loads Transported Through the Milltown Reservoir Project Area 37
38 Estimated Loads of Suspended Sediment and Trace Elements, Clark Fork Basin, Montana (Water Years 1985–2009)

to 50 percent of the outflow, and the streamflow contribution 

of Blackfoot River near Bonner ranged from 50 to 56 percent 

of the outflow. In all of the various periods, the net loss or gain 

of streamflow within the project area always was very minor, 

from 1 to 3 percent of the total outflow. Minor differences 

between combined inflow and outflow could represent inputs 

from ungaged small tributaries in the reaches between the 

streamflow-gaging stations, loss of surface water to groundwater in the alluvium underlying the project area, possible 

small amounts of evaporative loss from the reservoir surface, 

or minor inaccuracies in streamflow records.

Table 5 and figure 12 present a substantial amount of 

information concerning differences in suspended-sediment 

and trace-element transport characteristics, contributions from 

source areas, and mass balance within the project area for the 

various years (or multiple-year periods). Selected observations on some of the most prominent features regarding load 

magnitudes, source-area contributions, and mass balance are 

presented below.

During the pre-drawdown period (water years 1985–

2005), there was an average annual net gain of suspended sediment (5,750 tons) in the project area, but relatively substantial 

annual net loss (53,500 tons) during the high-flow years of 

1996–97 (table 5). This pattern illustrates the long-term effect 

of Milltown Dam on suspended-sediment transport. Milltown 

Reservoir was relatively shallow, providing limited storage 

capacity for sediment deposition and susceptible to bed and 

bank erosion by high-velocity streamflows or ice conditions. 

During the period 1908–2005 when the reservoir was operated 

as “run-of-the-river,” with streamflow inflows generally about 

equal to streamflow outflows on a daily basis, an approximate 

equilibrium or net balance probably had been established for 

constituent loads transported through the reservoir under longterm average streamflow conditions (for example, average 

sediment inflow loads probably were about equal to average 

sediment outflow loads). Mean annual streamflow for Clark 

Fork above Missoula during water years 1985–2005 (table 2) 

was below (88 percent) the long-term mean annual streamflow. 

During these below-normal streamflow years, below-normal 

hydraulic energies within the Milltown Reservoir resulted in 

net gain (deposition) of sediment transported into the reservoir. However, during the above-normal streamflow conditions during water years 1996–97 (which were the 3rd and 

6th largest annual mean streamflows during the 1930–2009 

period of record for Clark Fork above Missoula), the abovenormal hydraulic energies resulted in a relatively large net loss 

of sediment from the reservoir. Also, net losses in water year 

1996 were affected by scouring caused by substantial iceformation and rapid breakup.

When the “run-of-the-river” equilibrium was altered by 

the start of permanent drawdown in June 2006, net losses of 

sediment from the project area increased immediately (fig. 9). 

During the period between the start of permanent drawdown 

and the breach of Milltown Dam, the environment of Milltown Reservoir was markedly altered by the steeper hydraulic 

gradient that resulted from reducing the typical reservoir 

water level by about 10 to 12 ft. The effect of this alteration 

was substantially greater contributions of load from within 

the project area relative to pre-drawdown conditions. The 

contributions from within the project area are represented by 

the net loss determined in the mass-balance calculations. For 

example, in water year 2007 (the first full water year after the 

start of permanent drawdown), the annual contribution of suspended sediment from within the project area (130,000 tons; 

table 5, fig. 12) during below average (84 percent) streamflow 

conditions was about 2.4 times larger than the average annual 

contribution (53,500 tons) from within the project area during 

the high-flow years 1996–97. Further, water year 2007 was the 

first water year that the contribution of suspended sediment 

from within the project area to the total outflow (69 percent 

of outflow load) was larger than the combined contributions 

from the two upstream source areas represented by the basins 

upstream from Clark Fork at Turah Bridge and Blackfoot 

River near Bonner.

The activities associated with the breach of Milltown 

Dam on March 28, 2008, further altered the project area 

environment and resulted in a large increase in net losses of 

suspended sediment from the project area. The Clark Fork 

water-surface elevation was lowered by an additional 15 ft, 

and the project area essentially was changed from reservoir 

to free-flowing river conditions. The annual contribution of 

suspended sediment from within the project area during water 

year 2008 (391,000 tons) was by far the largest of any year 

during the monitoring period (77 percent of outflow load) and 

occurred during slightly above-average streamflow conditions. However, the annual contribution of suspended sediment 

from the project area during water year 2009 (76,200 tons) 

was substantially smaller than those in water years 2007 

(130,000 tons) and 2008 (391,000 tons) even though streamflow was larger in water year 2009 than in water years 2007 

and 2008. Thus, there was substantial depletion of the project 

area sediment supply by the end of water year 2008.

Interannual variability in copper transport was very similar to interannual variability in suspended-sediment transport. 

In water year 2007, the contribution of copper from the project 

area (37.0 tons) was about 5.5 times larger than the average 

annual contribution during 1996–97 (6.75 tons). The annual 

contribution of copper from the project area during water 

year 2008 (157 tons) was the largest of any year during the 

monitoring period (84 percent of the outflow load). The annual 

contribution of copper from the project area during water 

year 2009 (29.0 tons) was substantially smaller than those 

in water years 2007 (37.0 tons) and 2008 (157 tons), even 

though streamflow was larger in water year 2009 than in water 

years 2007 and 2008. 

There were distinct differences in temporal variability 

of both total outflow loads from the project area and relative contributions from source areas for arsenic compared to 

suspended sediment and copper (fig. 12). The basin upstream 

from Clark Fork at Turah Bridge accounted for the largest 

percent contribution of arsenic to the project area outflow 

from any upstream source area for all water years except 
Estimated Loads Transported Through the Clark Fork Downstream from the Milltown Reservoir Project Area 39

2008 (table 5). For all water years, contributions of arsenic 

from within the project area were smaller than the combined 

contributions from the two upstream source areas represented 

by the basins upstream from Clark Fork at Turah Bridge and 

Blackfoot River near Bonner. Differences in the patterns of 

load contributions for arsenic relative to the other trace elements probably are attributable to the greater percentage of 

arsenic that occurs in the dissolved phase relative to the other 

trace elements in the Clark Fork. Thus, the relatively small 

percentage of arsenic in the particulate phase could have 

resulted in less historical deposition within the project area 

and a smaller percentage of arsenic contributed from within 

the project area relative to the other trace elements. Also, arsenic potentially could have been mobilized from the sediment 

that was deposited in Milltown Reservoir to groundwater of 

the underlying alluvial aquifer (U.S. Environmental Protection 

Agency, 2004), thereby depleting the sediment of some of the 

original arsenic content. Annual contributions of arsenic from 

within the project area during water years 2007 and 2008 were 

6.10 and 13.2 tons, respectively. The annual contribution of 

arsenic from within the project area during water year 2009 

(4.27 tons) was smaller than water years 2007 and 2008. The 

relative reduction in the contribution of arsenic from within 

the project area between water year 2008 and water year 2009 

was substantial, but not as large as other trace elements. The 

different chemical and transport characteristics of arsenic 

relative to the other trace elements might have caused different 

spatial deposition patterns in the former Milltown Reservoir or 

different responses to the altered hydrologic conditions caused 

by dam-removal activities, and resulted in different transport 

characteristics for arsenic than for the other trace elements 

during water years 2008 and 2009.

The cumulative loads of constituents contributed from 

within the project area (shown as mass-balance net losses in 

table 5) during the remediation activities associated with the 

removal of Milltown Dam (that is, from the start of permanent 

drawdown on June 1, 2006, to the end of water year 2009) are 

presented in figure 13. The estimated cumulative loads of suspended sediment, copper, and arsenic contributed from within 

the project area during June 1, 2006–September 30, 2009 were 

623,000, 235, and 26.8 tons, respectively. Temporal variability 

in constituent transport from the project area and the effect of 

runoff or remediation activities on constituent transport are 

apparent in figure 13. During the permanent drawdown period 

(June 1, 2006–March 27, 2008), the cumulative loads of suspended sediment, copper, and arsenic contributed from within 

the project area were 172,000 tons, 56.1 tons, and 10.6 tons, 

respectively. From March 28, 2008 (when Milltown Dam was 

breached), until the end of water year 2008, the additional 

loads of suspended sediment, copper, and arsenic contributed 

from within the project area were 375,000 tons, 150 tons, and 

11.9 tons, respectively. The largest and most abrupt increase in 

loads contributed from within the project area occurred in the 

first 3 months after the breach of Milltown Dam (fig. 13). During water year 2009, the additional loads of suspended sediment, copper, and arsenic contributed from within the project 

area were 76,200 tons, 29.0 tons, and 4.27 tons, respectively. 

The much smaller contributions of constituent loads during 

water year 2009 relative to the drawdown period and the postbreach period of water year 2008 provide further evidence 

of the substantial depletion of constituent supply within the 

project area during the drawdown period and water year 2008.

During the years of the most substantial remediation 

activities (that is, water year 2007, when permanent drawdown was conducted for the entire year, and water year 2008, 

when Milltown Dam was breached) constituent loads relative 

to streamflow were proportionately larger than other years, 

and the relative contributions of constituents from within 

the project area also were larger than other years (fig. 12). 

The relative contributions of constituents from source areas 

during water years 2006 and 2009 (before and after the most 

substantial remediation activities) generally were similar. 

Thus, in a relatively short time frame after the start of the 

most substantial remediation activities (that is, the period 

from the start of permanent drawdown on June 1, 2006, 

to the end of water year 2008, during which two complete 

annual runoff periods occurred), constituent transport characteristics in the Clark Fork near the project area appear to 

be approaching typical conditions observed before the breach 

of Milltown Dam. However, remediation and restoration 

activities that occur after the end of water year 2009 might 

affect the apparent temporal pattern in constituent transport 

characteristics, for example, the diversion of the Clark Fork 

from the constructed bypass channel to a new channel that 

occurred in December 2010 (http://www.epa.gov/region8/

superfund/mt/milltown/UpdateDecember2010.pdf, accessed 

February 22, 2011). Adjustment of the Clark Fork to a new 

geomorphic environment might alter patterns in constituent 

transport characteristics.

Estimated Loads Transported Through 

the Clark Fork Downstream from the 

Milltown Reservoir Project Area

Primary objectives for the investigation of transport 

characteristics through the Clark Fork downstream from 

the Milltown Reservoir project area include (1) estimating 

annual loads and source-area contributions to the Clark Fork 

downstream from the confluence with the Flathead River 

(hereinafter referred to as the lower Clark Fork outflow), and 

(2) determining the mass balance of suspended-sediment and 

trace-element loads for an extensive main-stem reach (about 

87 mi long) of the Clark Fork between the project area outflow 

at Clark Fork above Missoula (station 12340500, fig. 1, map 

number 4) and the Clark Fork at St. Regis (station 12354500, 

fig. 1, map number 6). Annual loads of suspended sediment 

and trace elements transported past each of the three lowintensity stations downstream from the project area outflow 

(fig. 1, sites 5–7, table 6) were estimated for water years 
Figure 13. Cumulative estimated loads of suspended sediment, unfiltered-recoverable copper, and unfilteredrecoverable arsenic contributed from the Milltown Reservoir project area during the period June 1, 2006 (start of 

permanent drawdown), through water year 2009. Cumulative suspended-sediment load, in tons Cumulative unfiltered-recoverable arsenic load, in tons Cumulative unfiltered-recoverable copper load, in tons

Breach of 

Milltown Dam 

(March 28, 2008)

Water year 2006 runoff periodWater year 2007 runoff period

Water year 2008 runoff period

Water year 2009 runoff period

Drawdown period Period after breach of Milltown Dam

Oct.

2007

Apr.

2007

Oct.

2007

Apr.

2008

Oct.

2008

Apr.

2009

Oct.

2009

June

2006

Water year 2007 Water year 2008 Water year 2009

0 

100,000 

200,000 

300,000 

400,000 

500,000 

600,000 

700,000 

0 

5 

10 

15 

20 

25 

30 

0 

50 

100 

150 

200 

250 

EXPLANATION

Cumulative estimated suspended-sediment load transported from Milltown Reservoir project area

Cumulative estimated unfiltered-recoverable arsenic load transported from Milltown Reservoir project area

Cumulative estimated unfiltered-recoverable copper load transported from Milltown Reservoir project area

40 Estimated Loads of Suspended Sediment and Trace Elements, Clark Fork Basin, Montana (Water Years 1985–2009)
Estimated Loads Transported Through the Clark Fork Downstream from the Milltown Reservoir Project Area 41

2006–09 by summing the estimated daily loads for each entire 

water year. Annual streamflow (in acre-ft) also is presented in 

table 6 to provide a perspective on streamflow conditions associated with constituent transport. Most of the load estimates 

presented in table 6 for the low-intensity stations have greater 

uncertainties than load estimates for the high-intensity stations 

bracketing the project area. For the data in table 6, emphasis 

generally is placed on relative comparison of results between 

stations and years, rather than on the absolute magnitude of 

load estimates.

Mass balance of constituent loads within the 87-mi 

main-stem reach of the Clark Fork between Clark Fork above 

Missoula and Clark Fork at St. Regis was determined as the 

difference between the combined inflow of the two gaged 

upstream source areas (that is, the project area outflow at 

Clark Fork above Missoula and the Bitterroot River near 

Missoula) and the reach outflow (Clark Fork at St. Regis) for 

water years 2006–09. The two gaged source areas account for 

about 82 percent of the drainage area and about 74 percent 

of the mean annual streamflow at the Clark Fork at St. Regis. 

Mass-balance calculations for this extensive main-stem reach 

provide general information on transport of constituents 

(much of which were contributed from the Milltown Reservoir project area) in the lower Clark Fork downstream from 

the project area outflow. It is notable that there are somewhat 

substantial tributary inputs (about 18 percent of the drainage 

area and about 26 percent of the mean annual streamflow) to 

the reach that were not gaged or monitored for water quality. 

Thus, the mass-balance calculations for the main-stem reach 

reflect (1) a complete accounting of the main-stem inflows to 

the reach that represent about 56 percent of the drainage area 

and 41 percent of the mean annual inflows to the reach, (2) a 

complete accounting of the largest single tributary inflow to 

the reach (Bitterroot River) that represents about 26 percent of 

the drainage area and 33 percent of the mean annual inflows, 

and (3) an incomplete accounting of the interaction between 

net gains or losses of constituents within the Clark Fork channel and the undetermined contribution from ungaged tributary 

source areas (including the city of Missoula) that represent 

about 18 percent of the drainage area and about 26 percent 

of the mean annual inflows. Because of the uncertainty in 

distinguishing the total load input to the reach, net gains 

or losses for the reach are only generally characterized and 

detailed determination of the fate of materials from the project 

area outflow to the reach is not possible. However, the massbalance calculations for the main-stem reach provide general 

information on temporal variability in constituent transport 

in the reach and relative contributions from the two gaged 

source areas.

The annual (water year) period provides a convenient 

and consistent accounting period for comparing cumulative 

loads to discern temporal changes in transport characteristics. 

However, water-quality sampling for the low-intensity stations 

was restricted to the runoff periods of each water year. Thus, 

it was necessary to extend the regression relations developed 

using the runoff period data to substantial periods outside 

the runoff periods when no data were collected. The potential errors associated with extending the regression relations 

probably are small because (1) data collected on the fringes 

of the runoff periods (which generally were near base-flow 

conditions) probably provide a reasonably accurate representation of nonrunoff conditions in the regression relations, 

and (2) periods outside of the annual runoff period typically 

accounted for a relatively small part of the annual loads. 

During water years 2006–09, an average of about 10 percent 

of the annual suspended-sediment loads at Clark Fork above 

Missoula (a daily suspended-sediment station) was transported 

during periods outside of the annual runoff period, illustrating the relatively small percentage of constituent loads that is 

transported during nonrunoff conditions.

For water years representing the permanent drawdown 

period (water years 2006–07), the mean annual streamflow of 

6,300 ft3

/s at Clark Fork at St. Regis (table 3) was less (86 percent) than the long-term mean annual streamflow. For water 

years representing the period after the breach of Milltown 

Dam (water years 2008–09), the mean annual streamflow of 

7,710 ft3

/s was slightly greater (106 percent) than the longterm mean annual streamflow. 

The annual outflow loads transported past the Clark Fork 

downstream from the confluence with the Flathead River 

and relative contributions from the individual source areas 

are illustrated by using pie charts (fig. 14). The estimated 

percentages of the loads for Clark Fork above Missoula 

contributed from within the Milltown Reservoir project area 

also are indicated to provide information on the effect of the 

Milltown Dam removal activities on constituent transport 

characteristics downstream from the project area outflow. 

The annual inflow loads contributed from the gaged upstream 

source areas (table 6) are those shown for Clark Fork above 

Missoula (station 12340500), Bitterroot River near Missoula 

(station 12352500), and Flathead River at Perma (station 12388700). The annual loads contributed from the mainstem reach are those shown as negative values (net loss) for 

the mass balance in table 6; positive values of mass balance 

represent net gains or deposition within the main-stem reach. 

In years when there were net gains of constituent loads within 

the main-stem reach, the magnitudes of the net gains are 

shown in boxes beside the pies (fig. 14), or as positive massbalance values (table 6). The percent values of source-area 

contributions were calculated as the annual load contributed 

from an individual source area divided by the annual lower 

Clark Fork outflow load (calculated as the sum of Clark Fork 

at St. Regis and Flathead River at Perma in table 6) during 

the indicated period. In years when there were net gains of 

constituent loads within the main-stem reach, the contribution 

from an individual source area to the net gain was assumed to 

be proportional to the contribution from the individual source 

area to the combined inflow to the main-stem reach. In years 

when there were net gains, the percent values of source-area 

contributions were calculated as the annual load contributed 

from the individual source area minus the proportional contribution from the individual source area to the net gain divided 
Table 6. Mass balance of annual streamflow and estimated loads of suspended sediment and unfiltered-recoverable trace elements 

transported to the Clark Fork downstream from the confluence with the Flathead River during water years 2006–09.—Continued

[Abbreviations: ND, not determined. All numerical values shown are rounded to three significant figures. In some cases, the combined inflow minus the outflow 

does not exactly equal the reported mass balance due to rounding effects. When calculating combined inflow or mass balance, all values used in the preliminary 

calculation were rounded to the same decimal place as three-significant-figure rounding of the smallest number used in the calculation. The final calculated 

value then was rounded to three significant figures]

Station name and number or 

summation category

Annual 

streamflow 

volume 

(acre-feet)

 Estimated annual load (tons)

Suspended 

sediment Cadmium1 Copper1 Iron Lead Manganese

Zinc1 Arsenic

Period between start of permanent drawdown of Milltown Reservoir and breach of Milltown Dam (water years 2006–07)

Water year 20062

Milltown Reservoir project area 

outflow (as measured at the 

Clark Fork above Missoula, 

Mont. (12340500))

 1,800,000 97,700 0.255 32.7 1,300 5.57 135 52.9 11.2

Bitterroot River near Missoula, 

Mont. (12352500)

 1,570,000 100,000 .0210 3.33 1,180 1.12 54.6 5.65 1.02

Combined inflow to main-stem 

reach of Clark Fork (sum of 

Clark Fork above Missoula 

(12340500) and Bitterroot 

River near Missoula, Mont. 

(12352500))

 3,370,000 198,000 .276 36.0 2,480 6.69 190 58.6 12.2

Clark Fork at St. Regis, Mont. 

(12354500)

 4,750,000 331,000 .389 48.3 3,420 9.5 256 80.7 16.7

Mass balance for main-stem 

reach of Clark Fork (difference between combined 

inflow to the main-stem reach 

of the Clark Fork and the 

Clark Fork at St. Regis): net 

gain (+) or loss (-)3

 -1,380,000 -133,000 -.113 -12.3 -940 -2.81 -66.4 -22.2 -4.48

Flathead River near Perma, Mont. 

(12388700)

 9,190,000 64,500 ND4 6.02 920 1.21 68.4 6.42 5.97

Lower Clark Fork outflow (sum 

of Clark Fork at St. Regis 

(12354500) and the Flathead 

River near Perma (12388700)

 13,900,000 395,000 ND4 54.3 4,340 10.7 324 87.1 22.7

Water year 2007

Milltown Reservoir project area 

outflow (as measured at the 

Clark Fork above Missoula, 

Mont. (12340500))

 1,770,000 188,000 0.453 58.8 1,880 9.67 189 105 16.7

Bitterroot River near Missoula, 

Mont. (12352500)

 1,400,000 44,500 .0153 1.94 664 .556 32.9 3.82 0.864

Combined inflow to main-stem 

reach of Clark Fork (sum of 

Clark Fork above Missoula 

(12340500) and Bitterroot 

River near Missoula, Mont. 

(12352500))

 3,170,000 233,000 .468 60.7 2,540 10.2 222 109 17.6

Clark Fork at St. Regis, Mont. 

(12354500)

 4,370,000 163,000 .322 42.0 2,170 7.23 204 69.7 15.6

42 Estimated Loads of Suspended Sediment and Trace Elements, Clark Fork Basin, Montana (Water Years 1985–2009)
Table 6. Mass balance of annual streamflow and estimated loads of suspended sediment and unfiltered-recoverable trace elements 

transported to the Clark Fork downstream from the confluence with the Flathead River during water years 2006–09.—Continued

[Abbreviations: ND, not determined. All numerical values shown are rounded to three significant figures. In some cases, the combined inflow minus the outflow 

does not exactly equal the reported mass balance due to rounding effects. When calculating combined inflow or mass balance, all values used in the preliminary 

calculation were rounded to the same decimal place as three-significant-figure rounding of the smallest number used in the calculation. The final calculated 

value then was rounded to three significant figures]

Station name and number or 

summation category

Annual 

streamflow 

volume 

(acre-feet)

 Estimated annual load (tons)

Suspended 

sediment Cadmium1 Copper1 Iron Lead Manganese

Zinc1 Arsenic

Period between start of permanent drawdown of Milltown Reservoir and breach of Milltown Dam (water years 2006-07)—Continued

Water year 2007—Continued

Mass balance for main-stem 

reach of Clark Fork (difference between combined 

inflow to the main-stem reach 

of the Clark Fork and the 

Clark Fork at St. Regis): net 

gain (+) or loss (-)3

 -1,200,000 +70,000 +.146 +18.7 +374 +3.00 +17.9 +39.1 +1.96

Flathead River near Perma, Mont. 

(12388700)

 7,920,000 38,800 ND4 4.61 632 .888 52.0 4.89 5.16

Lower Clark Fork outflow (sum 

of Clark Fork at St. Regis 

(12354500) and the Flathead 

River near Perma (12388700)

 12,300,000 202,000 ND4 46.6 2,800 8.12 257 74.6 20.8

Period after breach of Milltown Dam (water years 2008–09)

Water year 20082

Milltown Reservoir project area 

outflow (as measured at the 

Clark Fork above Missoula, 

Mont. (12340500))

 2,200,000 510,000 1.14 188 4,640 28.9 338 290 27.5

Bitterroot River near Missoula, 

Mont. (12352500)

 1,910,000 123,000 .0246 4.14 1,320 1.31 62.4 6.39 1.20

Combined inflow to main-stem 

reach of Clark Fork (sum of 

Clark Fork above Missoula 

(12340500) and Bitterroot 

River near Missoula, Mont. 

(12352500))

 4,110,000 633,000 1.16 192 5,960 30.2 400 296 28.7

Clark Fork at St. Regis, Mont. 

(12354500)

 5,510,000 562,000 1.34 208 6,870 33.0 471 332 31.3

Mass balance for main-stem 

reach of Clark Fork (difference between combined 

inflow to the main-stem reach 

of the Clark Fork and the 

Clark Fork at St. Regis): net 

gain (+) or loss (-)3

 -1,400,000 +70,500 -.175 -15.9 -910 -2.79 -71.0 -35.6 -2.60

Flathead River near Perma, Mont. 

(12388700)

 8,720,000 87,700 ND4 8.04 1,240 1.54 79.0 8.49 6.19

Lower Clark Fork outflow (sum 

of Clark Fork at St. Regis 

(12354500) and the Flathead 

River near Perma (12388700)

 14,300,000 650,000 ND4 217 8,110 34.6 550 341 37.5

Estimated Loads Transported Through the Clark Fork Downstream from the Milltown Reservoir Project Area 43
Table 6. Mass balance of annual streamflow and estimated loads of suspended sediment and unfiltered-recoverable trace elements 

transported to the Clark Fork downstream from the confluence with the Flathead River during water years 2006–09.—Continued

[Abbreviations: ND, not determined. All numerical values shown are rounded to three significant figures. In some cases, the combined inflow minus the outflow 

does not exactly equal the reported mass balance due to rounding effects. When calculating combined inflow or mass balance, all values used in the preliminary 

calculation were rounded to the same decimal place as three-significant-figure rounding of the smallest number used in the calculation. The final calculated 

value then was rounded to three significant figures]

Station name and number or 

summation category

Annual 

streamflow 

volume 

(acre-feet)

 Estimated annual load (tons)

Suspended 

sediment Cadmium1 Copper1 Iron Lead Manganese

Zinc1 Arsenic

Period after breach of Milltown Dam (water years 2008–09)—Continued

Water year 2009

Milltown Reservoir project area 

outflow (as measured at the 

Clark Fork above Missoula, 

Mont. (12340500))

 2,580,000 221,000 0.478 72.1 2,530 12.3 259 111 21.1

Bitterroot River near Missoula, 

Mont. (12352500)

 1,950,000 134,000 .0265 4.30 1,490 1.49 70.4 7.10 1.21

Combined inflow to main-stem 

reach of Clark Fork (sum of 

Clark Fork above Missoula 

(12340500) and Bitterroot 

River near Missoula, Mont. 

(12352500))

 4,530,000 354,000 .505 76.4 4,020 13.8 330 118 22.3

Clark Fork at St. Regis, Mont. 

(12354500)

 5,660,000 551,000 .536 88.2 5,360 17.5 367 134 22.4

Mass balance for main-stem 

reach of Clark Fork (difference between combined 

inflow to the main-stem reach 

of the Clark Fork and the 

Clark Fork at St. Regis): net 

gain (+) or loss (-)3

 -1,130,000 -196,000 -.032 -11.8 -1,340 -3.71 -38.0 -15.8 -0.090

Flathead River near Perma, Mont. 

(12388700)

 6,970,000 30,300 ND4 3.88 516 .714 44.2 4.11 4.68

Lower Clark Fork outflow (sum 

of Clark Fork at St. Regis 

(12354500) and the Flathead 

River near Perma (12388700)

 12,700,000 581,000 ND4 92.1 5,870 18.2 411 138 27.1

1

Estimated loads for some trace-element and station combinations (including cadmium for stations 12340000, 12352500, and 12388700, and copper and zinc 

for station 12388700) were based on regression equations developed from datasets generally having more than 50 percent of the values censored (reported as 

less than the laboratory reporting level). Thus, estimated loads for these trace-element and station combinations have greater uncertainty than loads estimated for 

the other trace-element and station combinations.

2

Although part of the given water year occurred outside of the indicated period, the given water year is assigned to the period most representative of the 

prevailing conditions of that water year.

3

Mass balance is the difference (net gain or loss) between the combined inflow to the main-stem reach of the Clark Fork between the confluence with the Bitterroot River and Clark Fork at St. Regis and the outflow transported from the reach at Clark Fork at St. Regis (station 12354500). Thus, a net gain (+) indicates 

net deposition in the reach and a net loss (-) indicates net removal from the reach.

4

Cadmium loads for station 12388700 (and calculated values dependent on those loads) were not determined because the large percent of censored values 

(94.4 percent; table 4) precluded the use of any method for estimating censored concentrations.

44 Estimated Loads of Suspended Sediment and Trace Elements, Clark Fork Basin, Montana (Water Years 1985–2009)
Figure 14. Annual streamflow and estimated loads of suspended sediment, unfiltered-recoverable copper, and unfiltered-recoverable 

arsenic for the Clark Fork downstream from the confluence with the Flathead River and contributions from upstream source areas, 

water years 2006–09.

12,300

14,300

13,900

581,000

202,000

650,000

395,000

27.1

20.8

37.5

54.3 22.7

46.6

217

12,700 92.1

Basin upstream from Bitterroot 

River near Missoula, Mont. 

(station 12352500)

Percent of outflow streamflow or load transported 

past Clark Fork downstream from the confluence with 

the Flathead River (sum of Clark Fork at St. Regis, 

Mont., and Flathead River near Perma, Mont.)

Total transported past Clark Fork downstream from 

the confluence with the Flathead River (lower Clark 

Fork outflow; sum of Clark Fork at St. Regis, Mont., 

and Flathead River near Perma, Mont.)

Total pie chart represents outflow at Clark Fork downstream from 

the confluence with the Flathead River. Size (area) of pie chart is 

proportional to largest annual streamflow or load (for given 

constituent)

581,000

Value

(percent)

Main-stem reach of the Clark Fork between 

Clark Fork above Missoula, Mont. (station 

12340500), and Clark Fork at St. Regis, Mont. 

(station 12354500)

Basin upstream from Flathead River 

at Perma, Mont. (station 12388700)

Suspended sediment,

(tons per year)

Arsenic,

(tons per year)

Copper,

(tons per year)

Period between start of permanent drawdown of 

Milltown Reservoir and breach of Milltown Dam Period after breach of Milltown Dam

Water year 20061

Water year 2007 

Water year 20081

Water year 2009 

Net gain of 

70,000 tons 

in Reach 

Net gain of 

1.96 tons 

in Reach 

Net gain of 

18.7 tons 

in Reach 

Net gain of 

70,500 tons 

in Reach 

27.5

(73 percent)

8.04

(4 percent)

97,700

(25 percent)

64,500

(16 percent)

188,000

(65 percent)

44,500

(16 percent)

2,580

(20 percent)

11.2

(49 percent)

0.864

(4 percent)

5.16

(25 percent)

32.7

(60 percent)

1.02

3.33 (5 percent)

(6 percent)

21.1

(78 percent)

72.1

(78 percent)

11.8

(13 percent)

1,800

(13 percent)

9,190

(66 percent)

1,770

(14 percent)

2,200

(16 percent)

1,910 (13 percent)

100,000

(25 percent)

133,000

(34 percent)

4.48

(20 percent)

12.3

(23 percent)

7,920

(64 percent)

1,400

(11 percent)

8,720

(61 percent)

510,000

(70 percent)

2.6

(7 percent)

6,970

(55 percent)

1,950 (15 percent)

134,000

(23 percent)

221,000

(38 percent)

196,000

(34 percent)

0.090

(1 percent)

4.68

(17 percent)

1,130

(10 percent)

1,570 (11 percent)

1,380 (10 percent)

5.97

(26 percent)

6.02

(11 percent)

1,200 (10 percent)

38,800

(19 percent)

58.8

(87 percent)

188,000

(65 percent)

1.94

(3 percent)

4.61

(10 percent)

1,400

(10 percent)

87,700

(13 percent)

123,000

(17 percent)

6.19

(17 percent)

1.20

(3 percent)

188

(87 percent)

15.9

(7 percent)

4.14

(2 percent)

30,300

(5 percent)

1.21

(5 percent)

3.88

(4 percent)

4.30

(5 percent)

16.7

(71 percent)

Net gain in main-stem reach of the Clark Fork 

between Clark Fork above Missoula, Mont. 

(station 12340500), and Clark Fork at St. Regis, 

Mont. (station 12354500), indicated when inflow 

to reach exceeded outflow from reach

Streamflow, in acre-feet, or load, in tons, 

contributed from indicated source area

Basin upstream from Clark Fork above Missoula, 

 Mont. (Milltown Reservoir project area outflow; 

 station 12340500)—Area with dot pattern 

 indicates estimated contribution from 

 Milltown Reservoir project area

Streamflow,

[1,000 acre-feet (KAF) per year]

EXPLANATION

Net gain of 

70,500 tons 

in Reach 

1

Although part of the given water year 

occurred outside of the indicated period, 

the given water year is assigned to the 

period most representative of the 

prevailing conditions of that water year.

Estimated Loads Transported Through the Clark Fork Downstream from the Milltown Reservoir Project Area 45
46 Estimated Loads of Suspended Sediment and Trace Elements, Clark Fork Basin, Montana (Water Years 1985–2009)

by the annual lower Clark Fork outflow load. In addition to the 

percent values, the estimated total lower Clark Fork outflow 

loads, contributions from the gaged source area, and contributions from the intervening main-stem reach, in tons, are shown 

to provide perspective on the large differences in constituent 

loads transported during the various periods.

The effects of the Milltown Dam removal activities are 

apparent in the temporal variability of relative contributions of 

estimated annual suspended-sediment loads from source areas 

to the lower Clark Fork downstream from the project area 

outflow. During the years of the most substantial remediation 

activities (that is, water year 2007, when permanent drawdown was conducted for the entire year, and water year 2008 

when Milltown Dam was breached), the estimated annual 

loads for the Clark Fork above Missoula (project area outflow) 

accounted for a large percentage (65 percent for water year 

2007 and 70 percent for water year 2008) of the suspendedsediment load transported past the lower Clark Fork outflow. 

During water years 2006 and 2009 (before and after the most 

substantial remediation activities), the Clark Fork above 

Missoula accounted for substantially smaller percentages 

(25 percent for water year 2006 and 38 percent for water 

year 2009) of the suspended-sediment load transported past 

the lower Clark Fork outflow than in water years 2007–08. 

Also, the relative contributions of suspended sediment from 

source areas during water years 2006 and 2009 generally were 

similar. Thus in a relatively short time frame after the start of 

the most substantial remediation activities (that is, the period 

from the start of permanent drawdown on June 1, 2006, to the 

end of water year 2008, during which two full annual runoff 

periods occurred), relative contributions of suspended sediment from upstream source areas to the lower Clark Fork 

appear to be approaching typical conditions observed before 

the breach of Milltown Dam. However, remediation and 

restoration activities that occur after the end of water year 

2009, might affect the apparent temporal pattern in sourcearea contributions of suspended sediment to the lower Clark 

Fork, for example, the diversion of the Clark Fork from the 

constructed bypass channel to a new channel that occurred in 

December 2010 (http://www.epa.gov/region8/superfund/mt/

milltown/UpdateDecember2010.pdf, accessed February 22, 

2011). Adjustment of the Clark Fork to a new geomorphic 

environment might alter patterns in source-area contributions 

of suspended sediment to the lower Clark Fork.

The effects of the Milltown Dam removal activities 

also are apparent in the temporal variability in net gains and 

losses of suspended sediment in the main-stem reach during 

water years 2006–09. During the years of the most substantial 

remediation activities (water years 2007 and 2008), there were 

net gains of suspended sediment in the intervening mainstream reach. During water years 2006 and 2009, there were 

net losses of suspended sediment from the main-stem reach 

(fig. 14, table 6). 

Temporal variability in the particle-size composition (that 

is, the percent of fines and sand) of suspended sediment in 

the inflow to and outflow from the main-stem reach provides 

general information on the potential processes affecting variability in gain or loss of suspended sediment in the reach. The 

average annual particle-size composition for the combined 

inflow to and outflow from the main-stem reach was estimated using the particle-size composition and instantaneous 

suspended-sediment loads of concurrent periodic water-quality 

samples. The loads of fines for all concurrent samples collected during a given year for the combined inflow were 

summed to determine the cumulative load of fines for the concurrent water-quality samples for the combined inflow for the 

given year. The loads of suspended sediment for all concurrent 

samples collected during a given year for the combined inflow 

were summed to determine the cumulative load of suspended 

sediment for the concurrent water-quality samples for the 

combined inflow for the given year. The cumulative load of 

fines for the concurrent water-quality samples for the given 

year was divided by the cumulative load of suspended sediment to estimate the annual load-weighted percent fines for the 

combined inflow for the given year. The annual load-weighted 

percent sand was estimated by subtracting the annual loadweighted percent fines from 100 percent. The same procedures 

were used to estimate the annual load-weighted percent fines 

and percent sand for the outflow from the main-stem reach. 

The annual load-weighted percent fines and sand for the 

combined inflow to and outflow from the main-stem reach for 

water years 2006–09 are presented in figure 15. The estimated 

annual load-weighted percent fines and sand were multiplied 

by the annual estimated suspended-sediment loads (table 6) 

to estimate annual loads of fines and sands (table 7). These 

loads, by particle size, illustrate variability in net transport 

of fines and sand through the main-stem reach during water 

years 2006–09.

During water year 2006, streamflow at Clark Fork at 

St. Regis was below normal (table 3) and suspended sediment 

in the combined gaged inflow to the main-stem reach was 

estimated to be 53.3-percent fines, and the suspended sediment in the outflow from the main-stem reach was estimated 

to be 54.9-percent fines (fig. 15). Thus, suspended sediment in 

the inflow to and outflow from the main-stem reach consisted 

of generally similar percentages of sand and fines. There was 

an estimated net loss of both fines and sand from the mainstem reach in water year 2006 (table 7). During water year 

2007, streamflow at Clark Fork at St. Regis was below normal 

(table 3) and suspended sediment in the combined gaged 

inflow to the main-stem reach was 35.5-percent fines, and the 

suspended sediment in the outflow from the main-stem reach 

was 61.6-percent fines (fig. 15). Thus, suspended sediment 

in the combined inflow to the main-stem reach was predominantly sand (64.5 percent), and the suspended sediment in the 

outflow from the main-stem reach was predominantly fines 

(61.6 percent) in water year 2007. There was an estimated net 

loss of fines (about 17,300 tons) from the main-stem reach, but 

a larger net gain (deposition) of sand (about 87,400 tons) in 

the main-stem reach in water year 2007 (table 7). The hydraulic energies in the Clark Fork during the below normal streamflow conditions of water year 2007 probably were sufficient 
Figure 15. Percent of fines and sand in the cumulative loads of suspended sediment for concurrent 

periodic water-quality samples for the combined inflow to and outflow from the main-stem reach of 

the Clark Fork from the Clark Fork above Missoula to the Clark Fork at St. Regis, water years 2006–09. Period between start of permanent drawdown of Milltown Reservoir and breach of Milltown Dam Period after breach of Milltown Dam

Water year 20061

(12 concurrent periodic

water-quality samples)

Water year 2007 

(11 concurrent periodic

water-quality samples)

Water year 20081

(15 concurrent periodic

water-quality samples)

Water year 2009 

(17 concurrent periodic

water-quality samples)

53.3 percent

46.7 percent

54.9 percent

45.1 percent

35.5 percent

64.5 percent

61.6 percent

38.4 percent

37.6 percent

62.4 percent

55.8 percent

44.2 percent

50.4 percent

49.6 percent

52.0 percent

48.0 percent

Inflow to reach

(Clark Fork above Missoula, Mont., plus

Bitterroot River near Missoula, Mont.) 

Outflow from reach

(Clark Fork at St. Regis, Mont.)

52.0 percent

48.0 percent

Percent fines (fines refers to 

suspended sediment smaller 

than 0.062-millimeter diameter)

Percent sand (sand refers to 

suspended sediment 

0.062-millimeter diameter or larger)

1

Although part of the given water year 

occurred outside of the indicated period, 

the given water year is assigned to the 

period most representative of the 

prevailing conditions of that water year.

EXPLANATION

Estimated Loads Transported Through the Clark Fork Downstream from the Milltown Reservoir Project Area 47
Table 7. Estimated annual loads and mass balance of various size classes of suspended sediment transported to and from the main-stem reach of t

Fork above Missoula to the Clark Fork at St. Regis during water years 2006–09.[Main-stream reach refers to the Clark Fork from the Clark Fork above Missoula to the Clark Fork at St. Regis. Fines refers to suspended sediment smaller than 0.062-millimetepended sediment 0.062-millimeter diameter or larger. All numerical values shown are rounded to three significant figures. In some cases, the summation of the mass balance for sand does not exactly equal the total mass balance due to rounding effects. When calculating total mass balance, all values used in the preliminary calculation were rounded to ththree-significant-figure rounding of the smallest number used in the calculation. The final calculated value then was rounded to three significant figures. Abbreviations: USGS, U

Water 

year

Number of 

concurrent 

periodic 

waterquality 

samples

Combined inflow to main-stem reach (Clark Fork above 

Missoula, Mont., plus Bitterroot River near Missoula, Mont.)

Outflow from main-stem reach (Clark Fork at St. Regis, Mont.) Ma

reac

Clark 

th

ne

Particle-size 

composition of the 

cumulative suspendedsediment load for 

concurrent periodic 

water-quality samples1

Estimated 

annual 

suspendedsediment 

load 

(table 6)

Estimated 

annual 

fines 

load2

Estimated 

annual 

sand 

load2

Particle-size 

composition of 

cumulative 

suspended-sediment 

load for concurrent 

periodic waterquality samples1

Estimated 

annual 

suspendedsediment 

load 

(table 6)

Estimated 

annual 

fines 

load2

Estimated 

annual 

sand 

load2

Percent 

fines

Percent 

sand

Percent 

fines

Percent 

sand Fine

2006 12 53.3 46.7 198,000 105,000 92,600 54.9 45.1 331,000 182,000 149,000 -77,0

2007 11 35.5 64.5 233,000 82,700 150,000 61.6 38.4 163,000 100,000 62,600 -17,32008 15 37.6 62.4 633,000 238,000 395,000 55.8 44.2 562,000 313,000 249,000 -75,0

2009 17 50.4 49.6 354,000 178,000 176,000 52.0 48.0 551,000 286,000 264,000 -108,0

1For each station, the particle-size composition of the cumulative suspended-sediment load for concurrent water-quality samples collected during a given year 

was calculated by:

(1) multiplying the percentage of fines by the suspended-sediment load to determine the fines load for each concurrent sample at each station;(2) summing the fines loads for all of the concurrent samples for a given station and water year to determine the cumulative fines load for concurrent 

samples for a given year;

(3) summing the suspended-sediment loads for all of the concurrent samples for a given station and water year to determine the cumulative 

suspended-sediment load for concurrent samples for a given year;

(4) dividing the cumulative fines load for the concurrent samples for a given station and water year by the cumulative suspended-sediment load to 

determine the percentage of fines in the cumulative loads for concurrent samples; and

(5) subtracting the percentage of fines in the cumulative loads from 100 percent to determine the percentage of sand in the cumulative loads for 

concurrent samples for a given station and water year 

2The particle-size composition (percent fines and sand) of the cumulative annual suspended-sediment load for concurrent periodic water-quality samples was applied to the es

sediment loads (table 6) to estimate the annual loads of fines and sand.

3Mass balance is the difference (net gain or loss) between the combined inflow to the main-stem reach and the outflow transported from the main-stem reach, as represented b

Mont. (station 12354500). Thus, a net gain (+) indicates net deposition in the reach and a net loss (-) indicates net removal from the reach.
Estimated Loads Transported Through the Clark Fork Downstream from the Milltown Reservoir Project Area 49

to transport fines through the main-stem reach, but likely 

were not sufficient to transport all of the sand and resulted in 

a net gain of about 70,000 tons of suspended sediment in the 

main-stem reach during water year 2007. During water year 

2008, streamflow at Clark Fork at St. Regis was above normal 

(table 3), but as in water year 2007, the suspended sediment 

in the combined inflow to the main-stem reach was predominantly sand (62.4 percent), and the suspended sediment in the 

outflow from the main-stem reach was predominantly fines 

(55.8 percent, fig. 15). There was an estimated net loss of fines 

(about 75,000 tons) from the main-stem reach, but a large net 

gain (deposition) of sand (about 146,000 tons) in the mainstem reach (table 7). The relatively large hydraulic energy in 

the Clark Fork during water year 2008 transported much more 

sand through the reach in water year 2008 than in water year 

2007. However, because of the substantial amounts of sand 

transported into the reach after the breach of Milltown Dam, 

there was still a net gain (deposition) of about 71,000 tons 

of suspended sediment in the main-stem reach. During water 

year 2009, streamflow at Clark Fork at St. Regis (table 3) was 

above normal and suspended sediment in the combined gaged 

inflow to the main-stem reach was 50.4-percent fines, and the 

suspended sediment in the outflow from the main-stem reach 

was 52.0-percent fines (fig. 15). Thus, in water year 2009, 

the suspended sediment in the combined inflow to the mainstem reach had about equal percentages of fines and sand, but 

the suspended sediment in the outflow from the main-stem 

reach had the largest percentage of sand (48.0 percent) of any 

year (fig. 15). There was an estimated net loss of fines (about 

108,000 tons) and sand (about 88,000 tons) from the mainstem reach in water year 2009, for a net loss of 196,000 tons 

of suspended sediment in the reach.

Another factor probably contributes to the occurrence and 

magnitude of net gains (deposition) of suspended sediment in 

the main-stem reach during water years 2007 and 2008. The 

increased hydraulic energies in the Clark Fork upstream from 

the Clark Fork above Missoula that resulted from the lowering 

of the water-surface elevation during permanent drawdown 

and the breach of Milltown Dam scoured substantial amounts 

of relatively coarse bottom sediments from the project area 

and transported them in suspension. Downstream from the 

project area outflow, the additional hydraulic energies associated with the lowering of the water-surface elevation in the 

project area probably subsided to levels typical of long-term 

conditions at similar streamflow magnitudes. The relatively 

lower hydraulic energies downstream from the project area 

outflow might not have been sufficient to maintain in suspension all of the coarse material scoured from the project 

area. However, some of the coarse material might have been 

transported as bedload below the unsampled zone of the 

water-quality samplers. An increase in bedload transport relative to suspended transport in the Clark Fork downstream from 

the project area outflow would contribute to the magnitude of 

calculated net gains of suspended sediment in the main-stem 

reach. However, because bedload data were not collected, it is 

not possible to determine the effect of bedload transport on the 

net gain estimates.

Temporal variability in the relative contributions of 

estimated annual copper loads from source areas to the 

lower Clark Fork downstream from the project area outflow 

generally was similar to that of estimated annual suspendedsediment loads. During the years of the most substantial remediation activities (water years 2007 and 2008), the estimated 

annual loads for the project area outflow at Clark Fork above 

Missoula accounted for a large percentage (87 percent for 

water years 2007 and 2008) of the estimated annual copper 

load transported past the lower Clark Fork outflow. During 

water years 2006 and 2009 (before and after the most substantial remediation activities), the Clark Fork above Missoula 

accounted for a smaller percentage (60 percent for water year 

2006 and 78 percent for water year 2009) of the estimated 

annual copper load transported past the lower Clark Fork outflow than in water years 2007 and 2008.

Annual net gains and losses of copper in the main-stem 

reach of the lower Clark Fork during water years 2006–09 

varied in a pattern generally similar to those of suspended 

sediment, except during water year 2008 (fig. 14). During 

water year 2008, there was a large net gain (deposition) of suspended sediment in the main-stem reach, but there was a net 

loss of copper (fig. 14). Given the generally strong association 

between suspended-sediment transport and copper transport, it 

appears inconsistent that there would be net gains of suspended sediment in the main-stem reach in water year 2008 

but net losses of copper. However, copper (and other metallic 

trace elements) tends to be more strongly associated with fine 

suspended sediment (fines) than with coarse suspended sediment (sand) (Horowitz, 1991). For example, for the nine periodic water-quality samples collected at Clark Fork at St. Regis 

during water year 2008 that had suspended sediment with 

percent fines greater than the median percent fines (76 percent) during water years 2006–09, the average solid-phase 

copper concentration of the suspended sediment was 1,290 

μg/g. For the six periodic water-quality samples collected 

during water year 2008 that had suspended sediment with 

percent fines less than the median percent fines during water 

years 2006–09, the average solid-phase copper concentration 

of the suspended sediment was 162 μg/g. Thus, during water 

year 2008, solid-phase copper concentrations in samples with 

a greater than median proportion of fines were about 7 times 

larger than in the samples with a less than median proportion 

of fines. During water year 2008, there was an estimated net 

deposition in the main-stem reach of 146,000 tons of sand and 

an estimated net loss of 75,000 tons of fines, resulting in a net 

gain of 71,000 tons of suspended sediment (table 7). Although 

there was an overall net gain of suspended sediment in the 

main-stem reach, there actually was a net loss of fine sediment resulting in a corresponding net loss of copper, which is 

predominantly associated with fine sediment.

Temporal variability in the relative contributions of 

estimated annual arsenic loads from source areas to the lower 

Clark Fork downstream from the project area outflow differed 
50 Estimated Loads of Suspended Sediment and Trace Elements, Clark Fork Basin, Montana (Water Years 1985–2009)

somewhat from those of estimated annual suspended-sediment 

and copper loads, primarily because a relatively large proportion of arsenic is transported in dissolved phase in the 

Clark Fork. The project area outflow at Clark Fork above 

Missoula contributes about 50 percent or more of the annual 

arsenic load transported past the lower Clark Fork outflow for 

all years during 2006–09 (fig. 14). A substantial part of the 

arsenic load transported past the Clark Fork above Missoula 

was contributed from the basin upstream from Clark Fork at 

Turah Bridge in all years (fig. 12). There was large temporal variability in the contribution of arsenic from within the 

project area to the outflow at Clark Fork above Missoula. 

During the years of the most substantial remediation activities 

(water years 2007 and 2008), the project area accounted for a 

substantial percentage (about 36 percent for water year 2007 

and about 48 percent for water year 2008; fig. 12) of the estimated annual arsenic load transported past Clark Fork above 

Missoula. During water years 2006 and 2009 (before and 

after the most substantial remediation activities), the project 

area accounted for a smaller percentage (8 percent for water 

year 2006 and 20 percent for water year 2009; fig. 12) of the 

estimated annual arsenic load transported past the Clark Fork 

above Missoula than in water years 2007–08. Even though the 

percent contribution of arsenic from within the project area 

was smaller during water year 2009 than during water years 

2007 and 2008, the percent contribution of arsenic from the 

project area outflow at Clark Fork above Missoula to the lower 

Clark Fork was larger during water year 2009 than during 

any other year (fig. 14). The percent contribution of streamflow from the Clark Fork above Missoula to the lower Clark 

Fork also was larger during water year 2009 than during any 

other year. Arsenic generally occurs in dissolved phase in the 

Clark Fork and is readily transported downstream. Thus, the 

relatively larger arsenic load contributed from the project area 

outflow at Clark Fork above Missoula to the lower Clark Fork 

during water year 2009 probably is primarily attributable to 

the relatively larger streamflow contribution.

Annual net gains and losses of arsenic in the main-stem 

reach of the lower Clark Fork during water years 2006–09 

varied in a pattern generally similar to those of copper. There 

were net losses of arsenic and copper from the main-stem 

reach during water years 2006, 2008, and 2009, and net 

gains of arsenic and copper in the main-stem reach during 

water year 2007. Although there were net gains for arsenic 

and copper in the main-stem reach during water year 2007, 

the percentage of the net gains in the reach relative to the 

combined gaged inflow to the reach were somewhat different 

between arsenic and copper. For arsenic in water year 2007, 

the net gain in the reach was 11 percent of the combined gaged 

inflow to the reach. For copper in water year 2007, the net gain 

in the reach was 31 percent of the combined gaged inflow to 

the reach. The smaller percentage of net gain for arsenic than 

for copper is attributable to the typically greater proportion of 

arsenic that occurs in dissolved phase and is readily transported downstream. 

For all years, loads of all constituents contributed from 

the basin upstream from Flathead River at Perma to the lower 

Clark Fork outflow generally were small even though the 

Flathead River accounted for an average of 62 percent of the 

annual streamflow of the lower Clark Fork outflow. The interannual range in the percent contribution of estimated annual 

loads from the Flathead River at Perma to the lower Clark 

Fork outflow was 5 to 19 percent for suspended sediment, 4 to 

11 percent for copper, and 17 to 26 percent for arsenic.

Summary and Conclusions

Milltown Reservoir is a National Priorities List Superfund site in the upper Clark Fork basin of western Montana 

where sediments enriched in trace elements from historical 

mining and ore processing have been deposited since the 

completion of Milltown Dam in 1908. Milltown Dam was 

breached on March 28, 2008, as part of Superfund remediation 

activities to remove the dam and excavate contaminated sediment that had accumulated in Milltown Reservoir. In preparation for the breach of Milltown Dam, permanent drawdown of 

Milltown Reservoir began on June 1, 2006, and lowered the 

water-surface elevation by about 10 to 12 ft. After the breach 

of Milltown dam, the water-surface elevation was lowered an 

additional 17 ft.

Hydrologic data-collection activities were conducted by 

the U.S. Geological Survey in cooperation with U.S. Environmental Protection Agency to estimate loads of suspended sediment and trace elements transported through the Clark Fork 

basin before and after the breach of Milltown Dam. Highintensity sampling was conducted at three stations that bracket 

the Milltown Reservoir project area. Daily and annual loads 

of suspended sediment and selected trace elements transported 

during water year 2009 were estimated for the three highintensity stations and were used to quantify the net gain or 

loss (mass balance) of suspended sediment and trace elements 

within the project area for water year 2009. Estimated loads 

and mass balance within the project area for water year 2009 

were compared to estimated loads and mass balance within the 

project area for selected periods before and after the breach of 

Milltown Dam, including (1) long-term average annual values 

for water years 1985–2005 (representing long-term average 

conditions during the period before the start of permanent 

drawdown of Milltown on June 1, 2006); (2) average annual 

values for 1996–97 (representing high-streamflow conditions 

during the period before the start of permanent drawdown of 

Milltown on June 1, 2006); (3) water years 2006–07 (representing the period between the start of permanent drawdown 

of Milltown Reservoir and the breach of Milltown Dam); 

and (4) water year 2008, during which Milltown Dam was 

breached.

Although this study primarily focused on the transport 

of suspended-sediment and trace-element loads to and from 

the project area, low-intensity sampling also was conducted 
Summary and Conclusions 51

during water years 2006–09 at four other stations. One lowintensity station was located within the Milltown Reservoir 

project area to provide information on erosional processes that 

occurred as the river adjusted to the steeper channel gradient 

caused by the breach of Milltown Dam; this site was sampled 

during water years 2008–09. Three low-intensity stations were 

located in the Clark Fork basin downstream from the project 

area outflow to provide general information on the transport 

of suspended-sediment and trace-element loads to and from 

reaches of the Clark Fork between the project area outflow and 

just downstream from the confluence with the Flathead River; 

these sites were sampled during water years 2006–09. Loads 

of suspended sediment and selected trace elements transported 

during water years 2006–09 were estimated for the three lowintensity stations in the lower Clark Fork and were used to 

evaluate how project area outflow loads were integrated into 

downstream transport processes.

For the high-intensity stations, daily loads of suspended 

sediment were estimated by using high-frequency sampling of 

the daily sediment monitoring. For the low-intensity stations, 

suspended-sediment loads were estimated by using regression 

equations relating suspended-sediment discharge to streamflow. For all stations, daily loads of unfiltered-recoverable 

cadmium, copper, iron, lead, manganese, zinc, and arsenic 

were estimated by using regression equations relating traceelement discharge to either streamflow or suspended-sediment 

discharge. Regression equations for estimating constituent 

loads for water year 2009 were developed from instantaneous streamflow and concentration data for periodic waterquality samples collected during all or part of water years 

2004–09. For some stations, there was substantial temporal 

variability in constituent transport relations. Thus, for some 

stations, the data were segregated into hydrologically based 

periods with similar transport relations and separate equations 

were developed. 

All the regression equations used to estimate suspendedsediment and trace-element loads were statistically significant 

(p-values ranging <0.001 to 0.003). Regression equations 

for estimating water year 2009 trace-element discharge for 

the high-intensity stations have large R2

 values (average 0.97 

and ranging from 0.94 to 1.00) and small SE values (average 

22.8 percent and ranging from 11.4 to 36.3 percent) indicating good transport relations for all the trace elements at all 

three high-intensity stations. Regression equations for estimating water years 2006–09 suspended-sediment discharge for 

the low-intensity stations have moderate R2

 values (average 

0.92 and ranging from 0.81 to 0.98) and generally moderate SE values (average 46.7 percent and ranging from 17.6 

to 76.1 percent). Regression equations for estimating water 

years 2006–09 trace-element discharge for the low-intensity 

stations have moderate R2

 values (average 0.88 and ranging 

from 0.40 to 0.99) and generally moderate SE values (average 

39.1 percent and ranging from 7.6 to 101.1 percent). Thus, 

the regression equations for estimating constituent discharge 

for the low-intensity stations indicate reasonably accurate 

transport relations. The SEs of the regression equations for the 

low-intensity stations generally are substantially larger than 

for the high-intensity stations and indicate lower confidence in 

the trace-element load estimates for the low-intensity stations 

than for the high-intensity stations. Thus, for the low-intensity 

stations, emphasis generally is placed on relative comparison 

of constituent load estimates between stations and between 

years, rather than on the absolute magnitude of load estimates. 

The regression equations were applied to records of daily 

mean streamflow or daily suspended-sediment loads to estimate daily trace-element loads.

Estimates of daily loads for the high-intensity stations 

were used to evaluate constituent transport to and from the 

project area for selected periods before and after the breach 

of Milltown Dam. Variations in estimated daily suspendedsediment loads transported to and from the project area 

during the period before the start of permanent drawdown 

generally coincided with variations in streamflow. For most 

of the period, streamflows generally were below normal, and 

differences between the suspended-sediment loads transported 

to and from the project area generally were minor (indicating approximate net balance) or indicated small amounts of 

deposition or scour. Consistent net loss of suspended sediment from the project area began with the start of permanent 

drawdown on June 1, 2006, and continued through the end of 

the drawdown period marked by the breach of Milltown Dam 

on March 28, 2008. During the permanent drawdown period, 

streamflow generally was below normal, but the pattern of 

near-continuous net loss of suspended sediment persisted for 

all streamflow conditions, and, at times, the loss of sediment 

from the project area was substantial. During water year 2008, 

after the breach of Milltown Dam, net loss of suspended 

sediment from the project area was substantially greater than 

during the permanent drawdown period because of generally above-normal streamflow and the steeper gradient that 

contributed to increased erosion. The outflow of suspended 

sediment from the project area sharply increased immediately 

after the breach of Milltown Dam resulting in large daily net 

losses. The large post-breach net losses of suspended sediment from the project area continued through the rising limb 

and peak flow of water year 2008. Net losses of suspended 

sediment began to decrease after the peak flow, reaching an 

approximate net balance between inflow and outflow loads by 

mid-August 2008. During the water year 2009 runoff period 

when streamflows generally were above normal, net losses 

increased substantially; the cumulative net loss during the 

runoff period was about 70,500 tons of sediment for an average of about 500 tons per day. Average streamflow for Clark 

Fork above Missoula during the water year 2009 runoff period 

was 6,400 ft3

/s. In comparison, during the water year 2008 

runoff period, the cumulative net loss was about 375,000 tons 

of sediment for an average of about 2,420 tons per day, and 

the average streamflow at Clark Fork above Missoula was 

about 5,430 ft3

/s. After the water year 2009 runoff period, net 

losses decreased to about net balance conditions until the end 

of water year 2009. Although there were substantial net losses 

of suspended sediment from the project area during the water 
52 Estimated Loads of Suspended Sediment and Trace Elements, Clark Fork Basin, Montana (Water Years 1985–2009)

year 2009 runoff period, the losses were much smaller than 

during the water year 2008 runoff period, even though streamflows were higher in water year 2009 than in water year 2008. 

Thus, there was substantial depletion of erodible channel 

materials in the project area by the end of water year 2008.

Variations in daily trace-element loads entering and 

leaving the project area generally coincided with variations 

in streamflow and suspended-sediment loads. The specific 

temporal variability in daily suspended-sediment loads and 

associated causal factors previously noted generally apply to 

variability in daily trace-element loads; copper and arsenic 

results are described as representative examples of traceelement transport characteristics. In early water year 2009, 

there was an approximate net balance between inflow and 

outflow loads within the project area for copper and arsenic. 

During the water year 2009 runoff period, net losses of copper 

and arsenic increased substantially. The cumulative net losses 

during the runoff period were about 25.5 tons of copper (average of about 0.181 ton per day) and about 3.1 tons of arsenic 

(average of about 0.022 ton per day). In comparison, during 

the water year 2008 runoff period, the cumulative net losses 

were about 150 tons of copper (average of about 0.971 ton 

per day) and about 11.6 tons of arsenic (average of about 

0.075 ton per day). 

Annual loads and mass balances for selected periods 

during water years 1985–2009 before and after the breach of 

Milltown Dam provide perspective on the effects of the hydrologic conditions and activities associated with the removal 

of Milltown Dam. During the pre-drawdown period (water 

years 1985–2005) when Milltown Reservoir was operated 

as “run-of-the-river,” there was an average annual net gain 

of suspended sediment (5,750 tons) in the project area, but 

relatively substantial average annual net loss (53,500 tons) 

during the above average (156 percent of long-term mean 

annual streamflow) streamflow years of 1996–97. When the 

“run-of-the-river” equilibrium was altered by the start of permanent drawdown in June 2006, net losses of sediment from 

the project area increased immediately. In water year 2007 

(the first full water year after the start of permanent drawdown), the annual contribution of suspended sediment from 

within the project area (130,000 tons) during below-average 

(84 percent) streamflow conditions was about 2.4 times larger 

than the average annual contribution (53,500 tons) from within 

the project area during the high-flow years 1996–97. Further, 

water year 2007 was the first water year that the contribution 

of suspended sediment from within the project area to the 

outflow was larger than the combined contributions from the 

two upstream source areas represented by the basins upstream 

from Clark Fork at Turah Bridge and Blackfoot River near 

Bonner. The breach of Milltown Dam further altered the 

project area environment and resulted in a large increase in net 

losses of suspended sediment from the project area. 

The annual contribution of suspended sediment from 

within the project area during water year 2008 (391,000 tons) 

was the largest of any year in the monitoring period (77 

percent of the outflow load) and occurred during slightly 

above-average (105 percent) streamflow conditions. However, 

the annual contribution of suspended sediment from within 

the project area during water year 2009 (76,200 tons) was 

substantially smaller than those in water years 2007 and 2008, 

even though streamflow was larger (122 percent of average) 

in water year 2009 than in water years 2007 and 2008. Thus, 

there was substantial depletion of the project area sediment 

supply by the end of water year 2008.

Interannual variability in copper transport was similar 

to interannual variability in suspended sediment transport. In 

water year 2007, the contribution of copper from within the 

project area (37.0 tons) was about 5.5 times larger than the 

average annual contribution during 1996–97 (6.75 tons). The 

annual contribution of copper from within the project area 

during water year 2008 (157 tons) was the largest of any year 

during the monitoring period (84 percent of the outflow load). 

The annual contribution of copper from within the project area 

during water year 2009 (29.0 tons) was smaller than during 

water years 2007 and 2008 even though streamflow was larger. 

There were distinct differences in temporal variability 

of both total outflow loads from the project area and relative contributions from source areas for arsenic compared to 

suspended sediment and copper. The basin upstream from 

Clark Fork at Turah Bridge accounted for the largest percent 

contribution of arsenic to the project area outflow from any 

upstream source area for all water years except 2008. For all 

water years, contributions of arsenic from within the project 

area were smaller than the combined contributions from the 

two upstream source areas represented by the basins upstream 

from Clark Fork at Turah Bridge and Blackfoot River near 

Bonner. Differences in the patterns of load contributions 

for arsenic relative to the other trace elements probably are 

attributable to the greater percentage of arsenic that occurs 

in dissolved phase relative to the other trace elements in the 

Clark Fork. Thus, the relatively small percentage of arsenic 

in particulate phase could have resulted in less historical 

deposition within the project area, and a smaller percentage of 

arsenic contributed from within the project area relative to the 

other trace elements. The annual contribution of arsenic from 

within the project area during water year 2009 (4.27 tons) 

was smaller than during water years 2007 and 2008 (6.10 and 

13.2 tons, respectively). The relative reduction in the contribution of arsenic from within the project area between water year 

2008 and water year 2009 was substantial, but not as large as 

other trace elements. 

Cumulative loads of constituents contributed from within 

the project area during the period June 1, 2006, to the end 

of water year 2009 provide perspective on the timing and 

effect of runoff or remediation activities associated with the 

removal of Milltown Dam on constituent transport from the 

project area. During this period, the estimated cumulative 

loads of suspended sediment, copper, and arsenic contributed 

from within the project area were 623,000 tons, 235 tons, and 

26.8 tons, respectively. During the years of the most substantial remediation activities (that is, water year 2007, when 

permanent drawdown was conducted for the entire year, and 
Summary and Conclusions 53

water year 2008 when Milltown Dam was breached) constituent loads relative to streamflow were proportionately larger 

than other years, and the relative contributions of constituents 

from within the project area also were larger than other years. 

The relative contributions of constituents from source areas 

during water years 2006 and 2009 (before and after the most 

substantial remediation activities) generally were similar. 

Thus, in a relatively short time frame after the start of the most 

substantial remediation activities (that is, the period from the 

start of permanent drawdown on June 1, 2006, to the end of 

water year 2008, during which two complete annual runoff 

periods occurred), constituent transport characteristics in 

the Clark Fork near the project area appear to be approaching typical conditions observed before the breach of Milltown Dam. However, remediation and restoration activities 

that occur after the end of water year 2009 might affect the 

apparent temporal pattern in constituent transport characteristics, for example, the diversion of the Clark Fork from the 

constructed bypass channel to a new channel that occurred 

in December 2010. Adjustment of the Clark Fork to a new 

geomorphic environment might alter patterns in constituent 

transport characteristics.

Estimated loads of suspended sediment and trace elements for water years 2006–09 at the three low-intensity 

stations located in the Clark Fork basin downstream from the 

project area outflow provide general information on constituent transport characteristics in the lower Clark Fork. Primary 

objectives for the investigation of constituent transport 

characteristics in the lower Clark Fork include (1) estimating 

the annual loads and source-area contributions to the Clark 

Fork downstream from the confluence with the Flathead River 

(the lower Clark Fork outflow), and (2) determining the mass 

balance of suspended-sediment and trace-element loads for 

an extensive main-stem reach (about 87 mi long) of the Clark 

Fork between the project area outflow at Clark Fork above 

Missoula and the Clark Fork at St. Regis.

Mass balance of constituent loads within the 87-mi 

main-stem reach of the Clark Fork between Clark Fork above 

Missoula and Clark Fork at St. Regis was determined as the 

difference between the combined annual inflow of the two 

gaged upstream source areas (that is, Clark Fork above Missoula and the Bitterroot River near Missoula) and the reach 

outflow (Clark Fork at St. Regis) for water years 2006–09. It 

is notable that there are somewhat substantial tributary inputs 

(about 18 percent of the drainage area and about 26 percent of 

the mean annual discharge) to the reach that were not gaged or 

monitored for water quality. Thus, the mass-balance calculations for the main-stem reach reflect an incomplete accounting 

of the interaction between net gains or losses of constituents 

within the Clark Fork channel and the undetermined contribution from ungaged tributary source areas. As a result, detailed 

determination of the fate of materials contributed from the 

project area outflow to the reach is not possible. However, the 

mass-balance calculations for the main-stem reach provide 

general information on temporal variability in constituent 

transport in the reach and relative contributions from the two 

gaged source areas.

The effects of the Milltown Dam removal activities are 

apparent in the temporal variability in relative contributions of 

estimated annual suspended-sediment loads from source areas 

to the lower Clark Fork. During the years of the most substantial remediation activities (that is, water year 2007, when 

permanent drawdown was conducted for the entire year, and 

water year 2008, when Milltown Dam was breached), the estimated annual loads for the Clark Fork above Missoula (project 

area outflow) accounted for a large percentage (65 percent 

for water year 2007 and 70 percent for water year 2008) of 

the suspended-sediment load transported past the lower Clark 

Fork outflow. During water years 2006 and 2009 (before 

and after the most substantial remediation activities), the 

Clark Fork above Missoula accounted for smaller percentages (25 percent for water year 2006 and 38 percent for water 

year 2009) of the suspended-sediment load transported past 

the lower Clark Fork outflow than in water years 2007–08. 

Also, the relative contributions of suspended sediment from 

source areas during water years 2006 and 2009 generally were 

similar. Thus, in a relatively short time frame after the start of 

the most substantial remediation activities (that is, the period 

from the start of permanent drawdown on June 1, 2006, to the 

end of water year 2008, during which two complete annual 

runoff periods occurred), relative contributions of suspended 

sediment from upstream source areas to the lower Clark Fork 

appear to be approaching typical conditions observed before 

the breach of Milltown Dam. However, remediation and 

restoration activities that occur after the end of water year 

2009 might affect the apparent temporal pattern in source-area 

contributions of suspended sediment to the lower Clark Fork. 

Adjustment of the Clark Fork to a new geomorphic environment might alter patterns in source-area contributions of 

suspended sediment to the lower Clark Fork.

The remediation activities associated with the removal 

of Milltown Dam affected the particle-size composition of 

suspended sediment transported through the lower Clark Fork 

and the mass balance of sediment in the main-stem reach. 

During the years of the most substantial remediation activities 

(water years 2007 and 2008), suspended sediment in periodic 

water-quality samples collected from the combined inflow to 

the main-stem reach was composed of predominantly sand 

(64.5 and 62.4 percent during water years 2007 and 2008, 

respectively), and suspended sediment in the outflow from 

the reach was composed of predominantly fines (61.6 and 

55.8 percent during water years 2007 and 2008, respectively). 

The relatively large amounts of sand in the inflow resulted in 

a net gain of suspended sediment in the main-stem reach during water years 2007 and 2008. During the years before and 

after the most substantial remediation activities, suspended 

sediment in the inflow to and the outflow from the main-stem 

reach consisted of generally similar percentages of sand and 

fines. During water years 2006 and 2009, there were net losses 

of suspended sediment from the main-stem reach. 
54 Estimated Loads of Suspended Sediment and Trace Elements, Clark Fork Basin, Montana (Water Years 1985–2009)

Temporal variability in the relative contributions of 

estimated annual copper loads from source areas to the 

lower Clark Fork downstream from the project area outflow 

generally was similar to that of estimated annual suspendedsediment loads. During the years of the most substantial remediation activities (water years 2007 and 2008), the estimated 

annual loads for the project area outflow at Clark Fork above 

Missoula accounted for a large percentage (87 percent for 

water years 2007 and 2008) of the estimated annual copper 

load transported past the lower Clark Fork outflow. During 

water years 2006 and 2009 (before and after the most substantial remediation activities), the Clark Fork above Missoula 

accounted for a smaller percentage (60 percent for water year 

2006 and 78 percent for water year 2009) of the estimated 

annual copper load transported past the lower Clark Fork outflow than in water years 2007–08.

Annual net gains and losses of copper in the main-stem 

reach of the lower Clark Fork during water years 2006–09 

varied in a pattern generally similar to those of suspended 

sediment, except during water year 2008. During water year 

2008, there was a large net gain (deposition) of suspended 

sediment in the main-stem reach, but there was a net loss of 

copper. Although there was an overall net gain of suspended 

sediment in the main-stem reach, there actually was a net loss 

of fine sediment resulting in a corresponding net loss of copper, which is predominantly associated with fine sediment.

Temporal variability in the relative contributions of 

estimated annual arsenic loads from source areas to the lower 

Clark Fork downstream from the project area outflow differed 

somewhat from those of estimated annual suspended-sediment 

and copper loads, primarily because a relatively large proportion of arsenic is transported in dissolved phase in the Clark 

Fork. The project area outflow at Clark Fork above Missoula 

contributes about 50 percent or more of the arsenic load transported past the lower Clark Fork outflow for all years during 

2006–09. A substantial part of the arsenic load transported past 

the Clark Fork above Missoula was contributed from the basin 

upstream from Clark Fork at Turah Bridge in all years.

Annual net gains and losses of arsenic in the main-stem 

reach of the lower Clark Fork during water years 2006–09 

varied in a pattern generally similar to those of copper. There 

were net losses of arsenic and copper from the main-stem 

reach during water years 2006, 2008, and 2009, and net gains 

of arsenic and copper in the main-stem reach during water 

year 2007. 

For all years, loads of all constituents contributed from 

the basin upstream from Flathead River at Perma to the lower 

Clark Fork outflow generally were small even though the 

Flathead River accounted for an average of 62 percent of the 

annual streamflow of the lower Clark Fork outflow. The interannual range in the percent contribution of estimated annual 

loads from the Flathead River at Perma to the lower Clark 

Fork outflow was 5 to 19 percent for suspended sediment, 4 to 

11 percent for copper, and 17 to 26 percent for arsenic.

Acknowledgments

Special thanks are given to Russ Forba (retired), Kristine 

Edwards, and Diana Hammer of the USEPA for their support 

of this study. Special thanks also are given to Bill Bakeberg 

and Michael Kriesberg for their diligent efforts in collecting high-frequency suspended-sediment samples. Technical 

reviews by Jill Frankforter and Galen Hoogestraat improved 

the report and are greatly appreciated. Finally, special thanks 

are given to the dedicated hydrologic technicians and hydrologists of the USGS who collected the streamflow and waterquality data for this study; notable individuals include Kent A. 

Dodge, Terry L. Heinert, Craig L. Bowers, Rod R. Caldwell, 

Thomas E. Cleasby, Wayne A. Tice, and Allen L. Furlow. 

References Cited

Cleveland, W.S., 1985, The elements of graphing data: Monterey, Calif., Wadsworth Books, 323 p.

Cleveland, W.S., and McGill, R., 1984, The many faces of a 

scatterplot: Journal of the American Statistical Association, 

v. 79, p. 807–822.

Cohn, T.A., Caulder, D.L., Gilroy, E.J., Zynjuk, L.D., and 

Sommers, R.M., 1992, The validity of a simple statistical 

model for estimating fluvial constituent loads—An empirical study involving nutrient loads entering Chesapeake Bay: 

Water Resources Research, v. 28, p. 2352–2363.

Colby, B.R., 1956, Relationship of sediment discharge 

to streamflow: U.S. Geological Survey Open-File 

Report 56–27, 170 p.

Dodge, K.A., Hornberger, M.I., and Dyke, J.L., 2009, Waterquality, bed-sediment, and biological data (October 2007 

through September 2008) and statistical summaries of 

long-term data for streams in the Clark Fork basin, Montana: U.S. Geological Survey Open-File Report 2009–1178, 

139 p., accessed October 29, 2010, at http://pubs.usgs.gov/

of/2009/1178/.

Dodge, K.A., Hornberger, M.I., and Dyke, J.L., 2010, Waterquality, bed-sediment, and biological data (October 2008 

through September 2009) and statistical summaries of 

long-term data for streams in the Clark Fork basin, Montana: U.S. Geological Survey Open-File Report 2010–1267, 

137 p.

Dodge, K.A., and Lambing, J.H., 2006, Quality-assurance plan 

for the analysis of suspended sediment by the U.S. Geological Survey in Montana: U.S. Geological Survey Open-File 

Report 2006–1242, 25 p., accessed October 29, 2010, at 

http://pubs.water.usgs.gov/ofr2006-1242.
References Cited 55

Duan, N., 1983, A nonparametric retransformation method: 

Journal of the American Statistical Association, v. 78, 

no. 383, p. 605–610.

Edwards, T.K., and Glysson, G.D., 1999, Field methods for 

measurement of fluvial sediment: U.S. Geological Survey 

Techniques of Water-Resources Investigations, book 3, 

chap. C2, 89 p., accessed October 29, 2010, at http://pubs.

water.usgs.gov/twri/.

Friedman, L.C., and Erdmann, D.E., 1982, Quality assurance 

practices for the chemical and biological analyses of water 

and fluvial sediments: U.S. Geological Survey Techniques 

of Water-Resources Investigations, book 5, chap. A6, 

181 p., accessed October 29, 2010, at http://pubs.water.

usgs.gov/twri/.

Garbarino, J.R., and Struzeski, T.M., 1998, Methods of 

analysis by the U.S. Geological Survey National Water 

Quality Laboratory—Determination of elements in wholewater digests using inductively coupled plasma–optical 

emission spectrometry and inductively coupled plasma–

mass spectrometry: U.S. Geological Survey Open-File 

Report 98–165, 101 p.

Guy, H.P., 1969, Laboratory theory and methods for sediment 

analysis: U.S. Geological Survey Techniques of WaterResources Investigations, book 5, chap. C1, 58 p., accessed 

October 29, 2010, at http://pubs.water.usgs.gov/twri/.

Helsel, D.R., 2005, Nondetects and data analysis—Statistics 

for censored environmental data: Hoboken, N.J., WileyInterscience, 250 p.

Helsel, D.R., and Hirsch, R.M., 2002, Statistical methods in 

water resources: U.S. Geological Survey Techniques of 

Water-Resources Investigations, book 4, chap. A3, 524 p., 

accessed October 29, 2010, at http://pubs.water.usgs.gov/

twri/twri4a3/.

Hoffman, G.L., Fishman, M.J., and Garbarino, J.R., 1996, 

Methods of analysis by the U.S. Geological Survey 

National Water Quality Laboratory—In-bottle digestion of 

whole-water samples: U.S. Geological Survey Open-File 

Report 96–225, 28 p.

Hornberger, M.I., Lambing, J.H., Luoma, S.N., and Axtmann, 

E.V., 1997, Spatial and temporal trends of trace metals in 

surface water, bed sediment, and biota of the upper Clark 

Fork basin, Montana, 1985–95: U.S. Geological Survey 

Open-File Report 97–669, 127 p.

Horowitz, A.J., 1991, A primer on sediment-trace element 

chemistry (2nd ed.): Chelsea, Mich., Lewis Publishers, Inc., 

136 p.

Horowitz, A.J., Demas, C.R., Fitzgerald, K.K., Miller, T.L., 

and Rickert, D.A., 1994, U.S. Geological Survey protocol 

for the collection and processing of surface-water samples 

for the subsequent determination of inorganic constituents in filtered water: U.S. Geological Survey Open-File 

Report 97–669, 84 p.

Jones, B.E., 1987, Quality control manual of the U.S. Geological Survey’s National Water Quality Laboratory: U.S. Geological Survey Open-File Report 87–457, 17 p.

Koch, R.W., and Smillie, G.M., 1986, Bias in hydrologic 

prediction using log-transformed regression models: Water 

Resources Bulletin, v. 22, p. 717–723.

Lambing, J.H., 1991, Water quality and transport characteristics of suspended sediment and trace elements in streamflow 

of the upper Clark Fork basin from Galen to Missoula, 

Montana, 1985–90: U.S. Geological Survey WaterResources Investigations Report 91–4139, 73 p.

Lambing, J.H., 1998, Estimated 1996–97 and long-term 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.

Lambing, J.H., comp., 2006, Quality-assurance plan for waterquality activities of the U.S. Geological Survey Montana 

Water Science Center: U.S. Geological Survey Open-File 

Report 2006–1275, 39 p., accessed October 29, 2010, at 

http://pubs.usgs.gov/of/2006/1275.

Lambing, J.H., and Sando, S.K., 2008, Estimated loads of 

suspended sediment and selected trace elements transported 

through Milltown Reservoir in the upper Clark Fork basin, 

Montana, water years 2004–07: U.S. Geological Survey 

Scientific Investigations Report 2008–5080, 24 p., accessed 

October 29, 2010, at http://pubs.usgs.gov/sir/2008/5080.

Lambing, J.H., and Sando, S.K., 2009, Estimated loads of 

suspended sediment and selected trace elements transported 

through Milltown Reservoir project area before and after 

the breaching of Milltown Dam in the upper Clark Fork 

basin, Montana, water year 2008: U.S. Geological Survey 

Scientific Investigations Report 2009–5095, 30 p., accessed 

October 29, 2010, at http://pubs.usgs.gov/sir/2009/5095.

Maloney, T.J., ed., 2005, Quality management system, U.S. 

Geological Survey National Water Quality Laboratory: U.S. 

Geological Survey Open-File Report 2005–1263, version 

1.3, November 9, 2005, chapters and appendixes [variously 

paged], accessed October 29, 2010, at http://pubs.usgs.gov/

of/2005/1263/.
56 Estimated Loads of Suspended Sediment and Trace Elements, Clark Fork Basin, Montana (Water Years 1985–2009)

Moore, J.N., Luoma, S.N., and Peters, Donald, 1991, Downstream effects of mine effluent on an intermontane riparian 

system: Canadian Journal of Fisheries and Aquatic Sciences, v. 48, p. 222–232.

Porterfield, George, 1972, Computation of fluvial-sediment 

discharge: U.S. Geological Survey Techniques of WaterResources Investigations, book 3, chap. C3, 66 p., accessed 

October 29, 2010, at http://pubs.usgs.gov/twri/twri3-c3/.

Pritt, J.W., and Raese, J.W., 1995, Quality assurance/quality 

control manual, National Water Quality Laboratory: U.S. 

Geological Survey Open-File Report 95–443, 35 p.

Rantz, S.E., and others, 1982, Measurement and computation 

of streamflow (volumes 1 and 2): U.S. Geological Survey 

Water-Supply Paper 2175, 2 v., 631 p.

Taylor, J.K., 1987, Quality assurance of chemical measurements: Chelsea, Mich., Lewis Publishers, 328 p.

U.S. Environmental Protection Agency, 2004, Milltown Reservoir Sediments Operable Unit of the Milltown Reservoir/

Clark Fork River Superfund Site—Record of Decision, 

Part 2: U.S. Environmental Protection Agency Decision 

Summary, 141 p., accessed October 29, 2010, at http://www.

epa.gov/region8/superfund/mt/milltown/mrsrod.html.

U.S. Geological Survey, variously dated, National field manual for the collection of water-quality data: U.S. Geological Survey Techniques of Water-Resources Investigations, 

book 9, chaps. A1–A9, accessed October 29, 2010, at http://

pubs.water.usgs.gov/twri/.

Ward, J.R., and Harr, C.A., eds., 1990, Methods for collection 

and processing of surface-water and bed-material samples 

for physical and chemical analyses: U.S. Geological Survey 

Open-File Report 90–140, 71 p.

White, M.K., Shields, R.R., and Dodge, K.A., 1998, Surfacewater quality-assurance plan for the Montana District of the 

U.S. Geological Survey: U.S. Geological Survey Open-File 

Report 98–173, 54 p.
Supplement 1 57

Supplement 1. Methods for Estimating Censored Concentrations for Water 

Year 2009 and Comparison with Methods Used in Previous Years

In previous reports presenting estimated loads of trace 

elements for the high-intensity stations (Lambing, 1991; 

Lambing, 1998; Hornberger and others, 1997; Lambing and 

Sando, 2008, 2009), values for trace-element concentrations 

that were censored were estimated by substituting one-half 

of the RL (reporting limit) in effect during the data-collection 

period for purposes of plotting and analysis of statistical relations. For constituents that had multiple RLs during the datacollection period, one-half of the median RL during the period 

was substituted. Data analysis indicated that this approach for 

handling censored concentrations was suitable for meeting 

the study objectives (Lambing and Sando, 2009) and did not 

substantially bias the study results. However, load estimates 

for constituents having greater than about 30-percent censored 

concentrations for a given site were qualified as having greater 

uncertainty than constituents with no or a very small percent 

of censored concentrations.

A more rigorous method of estimating censored concentrations [Adjusted Maximum Likelihood Estimation (AMLE) 

regression; Helsel and Hirsch, 2002] was used in this report 

for water year 2009 data for the high-intensity stations and 

water years 2006–09 data for the low-intensity stations. 

Factors that contributed to the need for using AMLE regression include (1) the NWQL RL for unfiltered-recoverable 

copper (an important constituent in terms of potential toxicity and representation of constituent transport) increased 

substantially between water years 2008 and 2009 (from 1.2 

to 4 mg/L), and (2) two of the low-intensity stations (Bitterroot River near Missoula and Flathead River at Perma) for 

which load estimates are presented in this report had relatively high proportions of censored concentrations of some 

trace elements (table 4). The increase in the NWQL RL for 

unfiltered-recoverable copper in water year 2009 resulted in 

greater uncertainty in estimating the copper load for Blackfoot 

River near Bonner, a critical inflow station for determining 

the annual mass balance in the Milltown Reservoir project 

area. For example, about 20 percent of unfiltered-recoverable 

copper concentrations for Blackfoot River near Bonner were 

censored during water years 2006–08, but about 60 percent of 

the concentrations were censored during water year 2009. The 

high-streamflow conditions of water year 2009 also have the 

potential to magnify the effect of uncertainty associated with 

censored concentrations on load estimates and estimating the 

annual mass balance of copper for the project area for water 

year 2009. Two low-intensity stations (Bitterroot River near 

Missoula and Flathead River at Perma) have relatively high 

percent censored concentrations for cadmium and copper (both 

stations) and zinc (Flathead River at Perma). Load estimates 

for these two stations are critical for evaluating trace-element 

transport characteristics for the Clark Fork basin downstream 

from the project area outflow. Reducing uncertainty in estimating loads for samples with censored concentrations improves 

the accuracy of determining trace-element transport characteristics in the lower Clark Fork basin.

AMLE regression analyzes the relation between a given 

trace element with some censored concentrations (the response 

variable) and some other constituent (the explanatory variable) 

with all reported concentrations above the RL (no censored 

concentrations). The relation between the response and 

explanatory variables is determined using available sample 

pairs with reported concentrations above the RL for both the 

response and explanatory variables. Proportions of censored 

concentrations of the response variable for various reporting 

levels also are analyzed in relation to the associated explanatory-variable concentrations above the RL. These relations 

are used to determine equations that estimate the most likely 

values of the censored concentrations of the response variable 

based on the associated reported concentrations of the explanatory variable. For data sets of at least 50 uncensored sample 

pairs (which applies to all station and trace-element combinations with censored concentrations in this report), AMLE 

regression is the preferred method for estimating values of 

censored concentrations (Helsel, 2005). 

Equations developed by using AMLE regression for each 

station and trace-element combination that had some censored 

concentrations (table 4) are presented in tables S1.1 and S1.2 

for the high-intensity and low-intensity stations, respectively. 

To evaluate differences between results of the AMLE method 

and results of the substitution method, AMLE-estimated concentrations were compared to substitution-estimated concentrations for water year 2008 samples collected at Blackfoot 

River near Bonner. The average relative percent difference 

between the AMLE and substitution estimates were (1) 4 percent for 15 censored unfiltered-recoverable cadmium concentrations, (2) 8 percent for 8 censored unfiltered-recoverable 

copper concentrations, and (3) 10 percent for 8 censored 

unfiltered-recoverable zinc concentrations. The relative percent differences between the estimates of the two methods are 

relatively small.

Consideration of how the estimated censored concentrations are used to develop load estimates provides further 

perspective on the suitability of comparison of load estimates 

in this report with previously reported estimates to evaluate 

temporal variability in trace-element transport in the Clark 

Fork basin. The estimated censored concentrations are used 

in conjunction with the associated streamflow to calculate 

instantaneous trace-element discharges for the given periodic 

water-quality samples. Thus, variability in trace-element 

discharges among samples reflects variability in both constituent concentration and streamflow. Censored values generally 

are associated with low to moderate streamflows and do not 

substantially affect calculations of trace-element discharges 

for high-flow samples representative of typically higher 

(uncensored) concentrations and conditions when the largest 
58 Estimated Loads of Suspended Sediment and Trace Elements, Clark Fork Basin, Montana (Water Years 1985–2009)

Table S1.1. Regression equations for estimating unfiltered-recoverable trace-element concentrations for censored data for the highintensity data-collection stations (table 1), water year 2009.

[Abbreviations: N, number of values; p-value, significance level; SE, standard error of estimate, in percent; LOG, base 10 logarithm; AS, arsenic concentration, 

in micrograms per liter; CD, cadmium concentration, in micrograms per liter; SED, suspended-sediment concentration, in milligrams per liter; CU, copper concentration, in micrograms per liter; FE, iron concentration, in micrograms per liter; PB, lead concentration, in micrograms per liter; MN, manganese concentration, in micrograms per liter; ZN, zinc concentration, in micrograms per liter. Symbol: <, less than]

Trace element Equation N p-value SE

Clark Fork at Turah Bridge near Bonner, Mont. (station 12334550)

Equations applied to water year 2009. Equations developed using data collected water years 2004–09

Cadmium CD=0.00296(MN)0.871 103 <0.001 23.0

Blackfoot River near Bonner, Mont. (station 12340000)

Equations applied to water year 2009. Equations developed using data collected water years 2004–09

Cadmium CD=0.000367(FE)0.555 93 <0.001 65.7

Copper CU=0.200(MN)0.673 94 <.001 43.7

Lead PB=0.00123(FE)1.01 94 <.001 24.7

Zinc ZN=0.0508(FE)0.675 93 <.001 33.6

Arsenic LOG(AS)=0.0208(SED0.500)-0.00710 94 <.001 21.5

Clark Fork above Missoula, Mont. (station 12340500)

Equations applied to water year 2009. Equations developed using data collected water years 2004–09

Cadmium CD=0.000629(FE)0.853 104 <0.001 47.5

Table S1.2. Regression equations for estimating unfiltered-recoverable trace-element concentrations for censored data for lowintensity data-collection stations (table 1) in the lower Clark Fork basin, water years 2006–09.

[Abbreviations: N, number of values; p-value, significance level; SE, standard error of estimate, in percent; LOG, base 10 logarithm; CD, cadmium concentration, in micrograms per liter; SED, suspended-sediment concentration, in milligrams per liter; CU, copper concentration, in micrograms per liter; FE, iron concentration, in micrograms per liter; PB, lead concentration, in micrograms per liter; MN, manganese concentration, in micrograms per liter; ZN, zinc concentration, in micrograms per liter. Symbol: <, less than]

Trace element Equation N p-value SE

Bitterroot River near Missoula, Mont. (station 12352500)

Equations applied to and developed using data collected during water years 2006–09

Cadmium CD=0.000437(FE)0.495 55 <0.001 59.9

Copper LOG(CU)=0.0588(SED0.500)-0.273 55 <.001 21.7

Lead PB=0.000500(FE)1.10 55 <.001 19.1

Zinc ZN=0.0294(FE)0.730 54 <.001 50.9

Clark Fork at St. Regis, Mont. (station 12354500)

Equations applied to and developed using data collected during water years 2006–09

Cadmium CD=0.00116(MN)1.10 55 <0.001 50.1

Flathead River near Perma, Mont.1

 (station 12388700)

Equations applied to and developed using data collected during water years 2006–09

Copper CU=0.0505(MN)1.23 53 0.007 103

Lead PB=0.00137(FE)0.994 54 <.001 25.7

Zinc ZN=0.0178(FE)0.776 53 .005 85.9

1

Flathead River at Perma had a very large percent (94.4 percent; table 4) of censored values for unfiltered-recoverable cadmium, which precluded the development of regression equations for estimating censored concentrations. No results are presented in this report for unfiltered-recoverable cadmium for Flathead 

River at Perma.
Supplement 2 59

constituent transport occurs. Variability in trace-element 

concentrations generally is small during low to moderate 

streamflows relative to variability during higher streamflows. 

Further, the discharges for samples affected by censoring 

are combined with discharges for samples not affected by 

censoring (that is, with reported concentrations above the 

RL) to develop regression equations based on relations with 

explanatory variables. The regression equations are then used 

to develop load estimates. Thus, although AMLE regression 

is the preferred method for estimating censored concentrations and provides the most accurate estimates for water year 

2009 for the high-intensity stations and water years 2006–09 

for the low-intensity stations, differences between results of 

the AMLE regression method and results of the substitution 

method generally are relatively small and do not substantially 

affect comparison of load estimates among water years.

Supplement 2. Regression Equations for Estimating Suspended-Sediment and

Trace-Element Discharge for the High-Intensity Stations

Table S2.1. Regression equations for estimating unfiltered-recoverable trace-element discharge for Clark Fork at Turah Bridge, near 

Bonner, Mont. (station 12334550), water year 2009.

[Abbreviations: N, number of values; R2, coefficient of determination; p-value, significance level; SE, standard error of estimate, in percent; RBCF, retransformation-bias-correction factor; CDQ, cadmium discharge, in tons per day; SEDQ, suspended-sediment discharge, in tons per day; CUQ, copper discharge, in 

tons per day; FEQ, iron discharge, in tons per day; PBQ, lead discharge, in tons per day; MNQ, manganese discharge, in tons per day; ZNQ, zinc discharge, in 

tons per day; ASQ, arsenic discharge, in tons per day; Q, streamflow, in cubic feet per second. Symbol: <, less than]

Trace element Equation N R2 p-value SE RBCF

Equations applied to all of water year 2009. Equations developed using data collected during water years 2004–09

Cadmium CDQ=0.0000144(SEDQ)0.760 103 0.97 <0.001 22.3 1.00

Copper CUQ=0.00137(SEDQ)0.840 103 .97 <.001 25.2 1.01

Iron FEQ=0.0181(SEDQ)0.982 103 .99 <.001 20.6 1.00

Lead PBQ=0.000123(SEDQ)0.960 103 .97 <.001 29.7 1.01

Manganese MNQ=0.00637(SEDQ)0.813 103 .98 <.001 19.5 1.00

Zinc ZNQ=0.00177(SEDQ)0.858 103 .97 <.001 25.5 1.01

Arsenic ASQ=0.00000247(Q)1.28 103 .94 <.001 25.0 1.01

Table S2.2. Regression equations for estimating unfiltered-recoverable trace-element discharge for Blackfoot River near Bonner, 

Mont. (station 12340000), water year 2009.

[Abbreviations: N, number of values; R2, coefficient of determination; p-value, significance level; SE, standard error of estimate, in percent; RBCF, retransformation-bias-correction factor; CDQ, cadmium discharge, in tons per day; SEDQ, suspended-sediment discharge, in tons per day; CUQ, copper discharge, in 

tons per day; FEQ, iron discharge, in tons per day; PBQ, lead discharge, in tons per day; MNQ, manganese discharge, in tons per day; ZNQ, zinc discharge, in 

tons per day; LOG, base 10 logarithm; ASQ, arsenic discharge, in tons per day; Q, streamflow, in cubic feet per second. Symbol: <, less than]

Trace element Equation N R2 p-value SE RBCF

Equations applied to all of water year 2009. Equations developed using data collected during water years 2004–09

Cadmium CDQ=0.00000348(SEDQ)0.619 93 0.95 <0.001 35.1 1.01

Copper CUQ=0.000414(SEDQ)0.697 94 .95 <.001 36.3 1.01

Iron FEQ=0.0224(SEDQ)0.912 94 .99 <.001 26.1 1.01

Lead PBQ=0.0000282(SEDQ)0.926 94 .99 <.001 25.9 1.01

Manganese MNQ=0.00289(SEDQ)0.820 94 .98 <.001 24.1 1.01

Zinc ZNQ=0.000467(SEDQ)0.735 93 .97 <.001 28.2 1.01

Arsenic LOG(ASQ)=0.107(Q0.333)-3.59 94 .97 <.001 18.7 1.00
60 Estimated Loads of Suspended Sediment and Trace Elements, Clark Fork Basin, Montana (Water Years 1985–2009)

Table S2.3. Regression equations for estimating unfiltered-recoverable trace-element discharge for Clark Fork above Missoula, Mont. 

(station 12340500), water year 2009.

[Abbreviations: N, number of values; R2, coefficient of determination; p-value, significance level; SE, standard error of estimate, in percent; RBCF, retransformation-bias-correction factor; CDQ, cadmium discharge, in tons per day; SEDQ, suspended-sediment discharge, in tons per day; CUQ, copper discharge, in 

tons per day; FEQ, iron discharge, in tons per day; PBQ, lead discharge, in tons per day; MNQ, manganese discharge, in tons per day; ZNQ, zinc discharge, in 

tons per day; LOG, base 10 logarithm; ASQ, arsenic discharge, in tons per day; Q, streamflow, in cubic feet per second. Symbol: <, less than]

Trace element Equation N R2 p-value SE RBCF

Equations applied to the part of water year 2009 before the start of the rising limb of the runoff period (October 1, 2008–March 8, 2009) 

Equations developed using data collected May 23, 2008–March 8, 2009

Cadmium CDQ=0.00000850(SEDQ)0.829 11 0.99 <0.001 25.5 1.00

Copper CUQ=0.00138(SEDQ)0.823 11 .99 <.001 18.8 1.00

Iron FEQ=0.0256(SEDQ)0.888 11 1.00 <.001 11.4 1.00

Lead PBQ=0.000158(SEDQ)0.862 11 .98 <.001 34.5 1.01

Manganese MNQ=0.0139(SEDQ)0.663 11 .99 <.001 17.6 1.00

Zinc ZNQ=0.00167(SEDQ)0.863 11 .99 <.001 19.8 1.00

Arsenic LOG(ASQ)=0.103(Q0.333)-2.93 11 .98 <.001 18.4 1.00

Equations applied to and developed using data collected during the rising limb of the water year 2009 runoff period (March 9–June 1, 2009)

Cadmium CDQ=0.0000117(SEDQ)0.773 17 0.95 <0.001 28.7 1.01

Copper CUQ=0.00228(SEDQ)0.745 17 .96 <.001 25.1 1.01

Iron FEQ=0.0403(SEDQ)0.844 17 .99 <.001 16.3 1.00

Lead PBQ=0.000295(SEDQ)0.789 17 .97 <.001 21.7 1.00

Manganese MNQ=0.00665(SEDQ)0.766 17 .98 <.001 16.3 1.00

Zinc ZNQ=0.00272(SEDQ)0.779 17 .95 <.001 29.5 1.01

Arsenic ASQ=0.00193(SEDQ)0.568 17 .98 <.001 14.8 1.00

Equations applied to and developed using data collected during the falling limb of the water year 2009 

runoff period (June 2–September 30, 2009)

Cadmium CDQ=0.0000281(SEDQ)0.683 9 0.96 <0.001 24.7 1.00

Copper CUQ=0.00216(SEDQ)0.773 9 .97 <.001 24.5 1.00

Iron FEQ=0.0306(SEDQ)0.874 9 .99 <.001 15.4 1.00

Lead PBQ=0.000242(SEDQ)0.818 9 .98 <.001 21.3 1.00

Manganese MNQ=0.0142(SEDQ)0.680 9 .98 <.001 16.1 1.00

Zinc ZNQ=0.00193(SEDQ)0.850 9 .98 <.001 21.0 1.00

Arsenic LOG(ASQ)=0.0810(Q0.333)-2.50 9 .97 <.001 13.7 1.00
Supplement 3 61

Table S3.1. Regression equations for estimating suspended-sediment and unfiltered-recoverable trace-element discharge for 

Bitterroot River near Missoula, Mont. (station 12352500), water years 2006–09.

[Abbreviations: N, number of values; R2, coefficient of determination; p-value, significance level; SE, standard error of estimate, in percent; RBCF, retransformation-bias-correction factor; SEDQ, suspended-sediment discharge, in tons per day; Q, streamflow, in cubic feet per second; CDQ, cadmium discharge, in tons 

per day; LOG, base 10 logarithm; CUQ, copper discharge, in tons per day; FEQ, iron discharge, in tons per day; PBQ, lead discharge, in tons per day; MNQ, 

manganese discharge, in tons per day; ZNQ, zinc discharge, in tons per day; ASQ, arsenic discharge, in tons per day. Symbol: <, less than]

Constituent Equation N R2 p-value SE RBCF

Equations applied to and developed using data collected1

 during the rising limbs of water years 2006–09 runoff periods 

(March 21–May 21, 2006; March 3–May 14, 2007; March 9–May 21, 2008; March 9–June 1, 2009)

Suspended sediment SEDQ=0.00000265(Q)2.27 29 0.92 <0.001 66.1 1.03

Cadmium CDQ=0.000000000548(Q)1.49 29 .91 <.001 44.9 1.02

Copper LOG(CUQ)=0.0214(Q0.500)-3.28 29 .94 <.001 36.0 1.01

Iron FEQ=0.000000774(Q)1.92 29 .93 <.001 52.3 1.02

Lead PBQ=0.000000000465(Q)1.97 29 .93 <.001 52.6 1.02

Manganese MNQ=0.0181(Q)0.0187 29 .84 <.001 57.6 1.03

Zinc ZNQ=0.0000000855(Q)1.56 28 .85 <.001 66.3 1.03

Arsenic ASQ=0.000000427(Q)1.13 29 .95 <.001 23.5 1.00

Equations applied to and developed using data collected1

 during the falling limbs (and all other nonrising-limb periods) of 

water years 2006–09 (October 1, 2005–March 20, 2006; May 22, 2006–March 2, 2007; May 15, 2007–March 8, 2008; May 22, 2008–March 8, 2009; 

and June 2–September 30, 2009)

Suspended sediment SEDQ=0.000000284(Q)2.41 26 0.98 <0.001 33.8 1.01

Cadmium CDQ=0.000000000618(Q)1.41 26 .98 <.001 19.1 1.00

Copper LOG(CUQ)=0.110(Q0.333)-3.79 26 .97 <.001 20.7 1.00

Iron FEQ=0.000000768(Q)1.81 26 .99 <.001 18.5 1.00

Lead LOG(PBQ)=0.160(Q0.333)-5.35 26 .98 <.001 23.3 1.00

Manganese LOG(MNQ)=0.114(Q0.333)-2.73 26 .98 <.001 18.3 1.00

Zinc ZNQ=0.0000000358(Q)1.57 26 .96 <.001 26.3 1.01

Arsenic LOG(ASQ)=0.0790(Q0.333)-3.65 26 .98 <.001 12.4 1.00

1

In some cases, data collected immediately before or after the indicated period were included in the regression analyses if those data were determined to 

improve the regression relations.

Supplement 3. Regression Equations for Estimating Suspended-Sediment and

Trace-Element Discharge for the Low-Intensity Stations
62 Estimated Loads of Suspended Sediment and Trace Elements, Clark Fork Basin, Montana (Water Years 1985–2009)

Table S3.2. Regression equations for estimating suspended-sediment and unfiltered-recoverable trace-element discharge for Clark 

Fork at St. Regis, Mont. (station 12354500), water years 2006–09.—Continued

[Abbreviations: N, number of values; R2, coefficient of determination; p-value, significance level; SE, standard error of estimate, in percent; RBCF, retransformation-bias-correction factor; SEDQ, suspended-sediment discharge, in tons per day; Q, streamflow, in cubic feet per second; CDQ, cadmium discharge, in tons 

per day; CUQ, copper discharge, in tons per day; FEQ, iron discharge, in tons per day; PBQ, lead discharge, in tons per day; MNQ, manganese discharge, in 

tons per day; ZNQ, zinc discharge, in tons per day; ASQ, arsenic discharge, in tons per day; LOG, base 10 logarithm. Symbol: <, less than]

Constituent Equation N R2 p-value SE RBCF

Equations applied to and developed using data collected during the rising limbs of water years 2006–07 runoff periods 

(March 21–May 21, 2006; March 3–May 14, 2007)

Suspended sediment SEDQ=0.00000000445(Q)2.79 11 0.81 <0.001 76.1 1.04

Cadmium CDQ=0.000000000424(Q)1.65 11 .67 .002 63.1 1.03

Copper CUQ=0.0000000613(Q)1.64 11 .66 .002 64.5 1.03

Iron FEQ=0.00000000486(Q)2.33 11 .76 .001 74.0 1.04

Lead PBQ=0.000000000804(Q)1.92 11 .70 .001 69.1 1.03

Manganese MNQ=0.000000181(Q)1.70 11 .65 .003 69.3 1.03

Zinc ZNQ=0.0000000747(Q)1.68 11 .64 .003 69.6 1.03

Arsenic ASQ=0.00000127(Q)1.18 11 .74 .001 36.3 1.01

Equations applied to and developed using data collected during the falling limbs (and all other nonrising-limb periods) of water 

years 2006–07 (October 1, 2005–March 20, 2006; May 22, 2006–March 2, 2007; May 15, 2007–September 30, 2007)

Suspended sediment LOG(SEDQ)=0.0201(Q0.500)+0.405 10 0.97 <0.001 34.9 1.01

Cadmium LOG(CDQ)=0.0832(Q0.333)-4.83 10 .90 <.001 39.1 1.01

Copper CUQ=0.00000122(Q)1.26 10 .89 <.001 40.0 1.01

Iron LOG(FEQ)=0.123(Q0.333)-1.99 10 .97 <.001 30.9 1.01

Lead LOG(PBQ)=0.0999(Q0.333)-3.92 10 .92 <.001 43.5 1.01

Manganese LOG(MNQ)=0.0129(Q0.333)-1.63 10 .94 <.001 32.6 1.01

Zinc ZNQ=0.000000708(Q)1.36 10 .86 <.001 50.5 1.02

Arsenic ASQ=0.0000310(Q)0.829 10 .90 <.001 24.5 1.00

Equations applied to and developed using data collected during the rising limb of the water year 2008 runoff period (March 9–May 21, 2008)

Suspended sediment SEDQ=0.00000310(Q)2.19 8 0.94 <0.001 55.2 1.02

Cadmium CDQ=0.0000000120(Q)1.48 8 .89 <.001 54.1 1.02

Copper CUQ=0.00000531(Q)1.38 8 .88 <.001 52.1 1.02

Iron FEQ=0.000000679(Q)1.90 8 .94 <.001 48.6 1.02

Lead PBQ=0.000000490(Q)1.43 8 .88 <.001 53.4 1.02

Manganese MNQ=0.00000116(Q)1.58 8 .92 <.001 47.4 1.02

Zinc ZNQ=0.00000186(Q)1.53 8 .89 <.001 54.6 1.02

Arsenic ASQ=0.00000506(Q)1.14 8 .90 <.001 37.9 1.01
Supplement 3 63

Table S3.2. Regression equations for estimating suspended-sediment and unfiltered-recoverable trace-element discharge for Clark 

Fork at St. Regis, Mont. (station 12354500), water years 2006–09.—Continued

[Abbreviations: N, number of values; R2, coefficient of determination; p-value, significance level; SE, standard error of estimate, in percent; RBCF, retransformation-bias-correction factor; SEDQ, suspended-sediment discharge, in tons per day; Q, streamflow, in cubic feet per second; CDQ, cadmium discharge, in tons 

per day; CUQ, copper discharge, in tons per day; FEQ, iron discharge, in tons per day; PBQ, lead discharge, in tons per day; MNQ, manganese discharge, in 

tons per day; ZNQ, zinc discharge, in tons per day; ASQ, arsenic discharge, in tons per day; LOG, base 10 logarithm. Symbol: <, less than]

Constituent Equation N R2 p-value SE RBCF

Equations applied to and developed using data collected during the rising limb of the water year 2009 runoff period (March 9–June 1, 2009)

Suspended sediment SEDQ=0.000000120(Q)2.47 12 0.94 <0.001 56.3 1.02

Cadmium CDQ=0.00000000126(Q)1.55 12 .90 <.001 45.4 1.02

Copper CUQ=0.0000000386(Q)1.72 12 .88 <.001 56.2 1.02

Iron FEQ=0.0000000566(Q)2.09 12 .91 <.001 60.2 1.02

Lead PBQ=0.00000000195(Q)1.86 12 .85 <.001 71.3 1.03

Manganese MNQ=0.000000737(Q)1.57 12 .87 <.001 53.6 1.02

Zinc ZNQ=0.000000109(Q)1.66 12 .89 <.001 52.8 1.02

Arsenic ASQ=0.00000135(Q)1.20 12 .90 <.001 33.4 1.01

Equations applied to and developed using data collected during the falling limbs (and all other nonrising-limb periods) of water 

years 2008–09 (May 22, 2008–March 8, 2009; June 2–September 30, 2009)

Suspended sediment SEDQ=0.000000000264(Q)3.01 8 0.97 <0.001 17.6 1.00

Cadmium CDQ=0.00000000000156(Q)2.17 8 .87 <.001 25.8 1.01

Copper CUQ=0.00000000184(Q)1.96 8 .92 <.001 18.4 1.00

Iron FEQ=0.000000000712(Q)2.46 8 .98 <.001 11.1 1.00

Lead PBQ=0.0000000000296(Q)2.20 8 .95 <.001 16.4 1.00

Manganese MNQ=0.000000000927(Q)2.17 8 .96 <.001 13.6 1.00

Zinc ZNQ=0.0000000000524(Q)2.36 8 .92 <.001 22.1 1.00

Arsenic ASQ=0.000000236(Q)1.36 8 .90 <.001 13.9 1.00
64 Estimated Loads of Suspended Sediment and Trace Elements, Clark Fork Basin, Montana (Water Years 1985–2009)

Table S3.3. Regression equations for estimating suspended-sediment and unfiltered-recoverable trace-element discharge for 

Flathead River near Perma, Mont. (station 12388700), water years 2006–09.

[Abbreviations: N, number of values; R2, coefficient of determination; p-value, significance level; SE, standard error of estimate, in percent; RBCF, retransformation-bias-correction factor; SEDQ, suspended-sediment discharge, in tons per day; Q, streamflow, in cubic feet per second; ND, not determined; CDQ, cadmium discharge, in tons per day; CUQ, copper discharge, in tons per day; LOG, base 10 logarithm; FEQ, iron discharge, in tons per day; PBQ, lead discharge, 

in tons per day; MNQ, manganese discharge, in tons per day; ZNQ, zinc discharge, in tons per day; ASQ, arsenic discharge, in tons per day. Symbol: <, less 

than]

Constituent Equation N R2 p-value SE RBCF

Equations applied to and developed from data collected during the rising limbs of the water years 2006–07 and 2009 runoff periods 

(March 21–June 20, 2006; March 3–June 20, 2007; March 9–June 8, 2009)

Suspended sediment SEDQ=0.00000534(Q)1.82 33 0.83 <0.001 49.0 1.02

Cadmium ND ND ND ND ND ND

Copper CUQ=0.0000000147(Q)1.46 32 .84 <.001 37.0 1.01

Iron LOG(FEQ)=0.073(Q0.333)-1.32 33 .85 <.001 35.4 1.01

Lead PBQ=0.0000000212(Q)1.28 33 .77 <.001 41.0 1.01

Manganese MNQ=0.000000791(Q)1.31 33 .88 <.001 28.6 1.01

Zinc LOG(ZNQ)=0.00610(Q0.500)-2.32 32 .40 <.001 69.3 1.04

Arsenic ASQ=0.000000704(Q)1.05 33 .98 <.001 8.6 1.00

Equations applied to and developed using data collected during the rising limb of the water year 2008 runoff period (March 9–May 27, 2008)

Suspended sediment SEDQ=0.000000026(Q)2.43 11 0.89 <0.001 50.9 1.02

Cadmium ND ND ND ND ND ND

Copper CUQ=0.000000000154(Q)2.01 11 .66 .002 101.1 1.07

Iron FEQ=0.0000000047(Q)2.16 11 .95 <.001 28.8 1.01

Lead PBQ=0.0000000000185(Q)2.05 11 .92 <.001 35.7 1.01

Manganese MNQ=0.0000000084(Q)1.82 11 .97 <.001 19.5 1.00

Zinc ZNQ=0.000000000096(Q)2.05 11 .96 <.001 23.7 1.00

Arsenic LOG(ASQ)=0.0629(Q0.333)-3.28 11 .99 <.001 7.6 1.00

Equations applied to and developed using data collected during the falling limbs (and all other nonrising-limb periods) of water 

years 2006–09 (October 1, 2005–March 20, 2006; June 21, 2006–March 2, 2007; June 21, 2007–March 8, 2008; May 28, 2008–March 8, 2009; 

June 9–September 30, 2009)

Suspended sediment LOG(SEDQ)=0.103(Q0.333)-0.371 10 0.97 <0.001 27.5 1.01

Cadmium ND ND ND ND ND ND

Copper CUQ=0.0000000876(Q)1.27 9 .89 <.001 32.9 1.01

Iron LOG(FEQ)=0.0915(Q0.333)-1.88 10 .96 <.001 27.9 1.01

Lead PBQ=0.000000000137(Q)1.77 10 .94 <.001 33.6 1.01

Manganese LOG(MNQ)=0.0730(Q0.333)-2.52 10 .95 <.001 24.4 1.00

Zinc ZNQ=0.000640(Q)0.0114 10 .96 <.001 24.2 1.00

Arsenic LOG(ASQ)=0.00620(Q0.500)-2.50 10 .94 <.001 16.0 1.00
Publishing support provided by the:

Denver and Rolla Publishing Service Centers

For more information concerning this publication, contact:

Director, Montana Water Science Center

U.S. Geological Survey

3162 Bozeman Ave.

Helena, MT 59601

(406) 457–5900

Or visit the Montana Water Science Center Web site at: 

http://mt.water.usgs.gov/
Sando and Lambing—Estimated Loads of Suspended Sediment and Trace Elements, Clark Fork Basin, Montana (Water Years 1985–2009)—SIR 2011–5030