U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2008–5080

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

Estimated Loads of Suspended Sediment and Selected 

Trace Elements Transported through Milltown Reservoir in 

the Upper Clark Fork Basin, Montana, Water Years 2004–07
Cover photograph: Clark Fork below Milltown Dam. Photograph by John Lambing, U.S. Geological Survey, taken May 1989. 
Estimated Loads of Suspended Sediment 

and Selected Trace Elements Transported 

through Milltown Reservoir in the Upper 

Clark Fork Basin, Montana, Water Years 

2004–07

By John H. Lambing and Steven K. Sando

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

Scientific Investigations Report 2008–5080

U.S. Department of the Interior

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

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2008

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Suggested citation:

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.
iii

Contents

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

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

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

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

Hydrologic Characteristics .................................................................................................................4

Methods of Data Collection and Quality Assurance................................................................................4

Methods for Estimating Constituent Loads ...............................................................................................7

Suspended Sediment ...........................................................................................................................7

Trace Elements....................................................................................................................................10

Estimated Loads Transported through Milltown Reservoir ..................................................................15

Daily Loads...........................................................................................................................................15

Annual Loads.......................................................................................................................................16

Summary and Conclusions.........................................................................................................................21

References Cited..........................................................................................................................................23

Figures

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

2–7. Graphs showing:

2. Daily mean streamflow for the study period and selected 

exceedance percentiles of long-term daily mean streamflow 

for Clark Fork above Missoula, Mont ...............................................................................5

3. Relations between suspended-sediment discharge and streamflow 

before and after June 1, 2006, water years 2004-07 ......................................................9

4. Relations between unfiltered-recoverable copper discharge and 

suspended-sediment discharge before and after June 1, 2006, 

water years 2004–07..........................................................................................................12

5. Relations between unfiltered-recoverable iron discharge and 

suspended-sediment discharge before and after June 1, 2006, 

water years 2004–07..........................................................................................................13

6. Daily mean streamflow and estimated daily suspended-sediment loads 

transported to and from Milltown Reservoir, water years 2004–07 ..........................17

7. Daily mean streamflow and estimated daily unfiltered-recoverable 

copper loads transported to and from Milltown Reservoir, water 

years 2004–07 .....................................................................................................................18
iv

Tables

1. Annual and peak streamflow characteristics for the study period, 

the period of continuous-streamflow records, and the period of 

peak-streamflow records for Clark Fork above Missoula, Mont..........................................6

2. Equations for estimating suspended-sediment discharge for Clark Fork 

at Turah Bridge, near Bonner, Mont., Blackfoot River near Bonner, Mont., 

and Clark Fork above Missoula, Mont., water years 2004–07.............................................10

3. Equations for estimating unfiltered-recoverable trace-element discharge 

for Clark Fork at Turah Bridge, near Bonner, Mont., water years 2004–07. ......................14

4. Equations for estimating unfiltered-recoverable trace-element discharge 

for Blackfoot River near Bonner, Mont., water years 2004–07. ..........................................14

5. Equations for estimating unfiltered-recoverable trace-element discharge 

for Clark Fork above Missoula, Mont., water years 2004–07...............................................15

6. Annual mean streamflow and estimated annual loads and annual mass 

balance of suspended sediment and unfiltered-recoverable trace elements 

transported to and from Milltown Reservoir, water years 2004–07. ..................................19

Conversion Factors, Datum, Abbreviated Units, and 

Acronyms

Multiply By To obtain

acre 0.4047 hectare (ha)

cubic inch (in3) 0.01639 liter (L)

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)

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 (ton/d) 0.9072 metric ton per day

ton per year (ton/yr) 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 2007 is the period from October 1, 2006, through September 30, 2007.
v

Abbreviated water-quality units used in this report:

μg/L micrograms per liter

mg/L milligrams per liter

Acronyms used in this report:

ASQ arsenic discharge

CDQ cadmium discharge

CUQ copper discharge

FEQ iron discharge

LOG logarithm (base 10)

LOWESS locally weighted scatter plot smoothing

LRL laboratory reporting level

MNQ manganese discharge

NPL National Priorities List

NWIS National Water Information System

NWQL National Water Quality Laboratory

PBQ lead discharge

p-value significance level

R2 coefficient of determination

RBCF retransformation-bias-correction factor

RSD relative standard deviation

SE standard error of estimate

SEDQ suspended-sediment discharge

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 Milltown Reservoir 

in the Upper Clark Fork Basin, Montana, Water Years 

2004–07

By John H. Lambing and Steven K. Sando

Abstract

The purpose of this report is to present estimated daily 

and annual loads of suspended sediment and selected trace elements for water years 2004–07 at two sites upstream and one 

site downstream from Milltown Reservoir. 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 construction of Milltown Dam in 

1907. The estimated loads were used to quantify annual net 

gains and losses (mass balance) of suspended sediment and 

trace elements within Milltown Reservoir before and after 

June 1, 2006, which was the start of Stage 1 of a permanent 

drawdown of the reservoir in preparation for removal of Milltown Dam. This study was done in cooperation with the U.S. 

Environmental Protection Agency.

Daily loads of suspended sediment were estimated for 

water years 2004–07 by using either high-frequency sampling 

as part of daily sediment monitoring or regression equations relating suspended-sediment discharge to streamflow. 

Daily loads of unfiltered-recoverable arsenic, cadmium, 

copper, iron, lead, manganese, and zinc were estimated by 

using regression equations relating trace-element discharge 

to suspended-sediment discharge. Regression equations 

were developed from data for periodic water-quality samples 

collected during water years 2004–07. The equations were 

applied to daily records of either streamflow or suspendedsediment discharge to produce estimated daily loads.

Variations in daily suspended-sediment and trace-element 

loads generally coincided with variations in streamflow. For 

most of the period before June 1, 2006, differences in daily 

loads transported to and from Milltown Reservoir were minor 

or indicated small amounts of deposition; however, losses of 

suspended sediment and trace elements from the reservoir 

occurred during temporary drawdowns in July–August 2004 

and October–December 2005. After the start of Stage 1 of the 

permanent drawdown on June 1, 2006, losses of suspended 

sediment and trace elements from the reservoir persisted for 

all streamflow conditions during the entire interval of the 

Stage 1 drawdown (June 1, 2006–September 30, 2007) within 

the study period.

Estimated daily loads of suspended sediment and trace 

elements were summed for each year to produce estimated 

annual loads used to determine the annual net gains (deposition) or losses (erosion) of each constituent within Milltown 

Reservoir during water years 2004–07. During water year 

2004, there was an annual net gain of suspended sediment in 

the reservoir. The annual net gains and losses of trace elements 

were inconsistent in water year 2004, with gains occurring for 

arsenic and iron, but losses occurring for cadmium, copper, 

lead, manganese, and zinc. In water year 2005, there were 

annual net gains of suspended sediment and all the trace elements within the reservoir. In water year 2006, there were 

annual net losses of all constituents from the reservoir, likely 

as the result of sediment erosion from the reservoir during 

both a temporary drawdown in October–December 2005 

and Stage 1 of the permanent drawdown that continued after 

June 1, 2006. In water year 2007, when the Stage 1 drawdown 

was in effect for the entire year, there were large annual net 

losses of suspended sediment and trace elements from the 

reservoir. The annual net losses of constituents from Milltown Reservoir in water year 2007 were the largest of any 

year during the 2004–07 study period. In water year 2007, the 

annual net loss of suspended sediment from the reservoir was 

130,000 tons, which was more than double (about 222 percent) the combined inflow to the reservoir. The largest annual 

net losses of trace elements in water year 2007, in percent of 

the combined inflow to the reservoir, occurred for cadmium, 

copper, lead, and zinc—about 190 percent for cadmium, 

170 percent for copper, 150 percent for lead, and 238 percent 

for zinc. The large annual net losses of suspended sediment 

and trace elements in water year 2007 indicate that much of 

the constituent load transported from Milltown Reservoir 

was derived from eroded reservoir sediment, rather than from 

upstream sources.
2 Estimated loads transported through Milltown Reservoir, Montana, water years 2004-07

Introduction

Milltown Reservoir is located on the Clark Fork at the 

confluence of the Clark Fork and Blackfoot River in western 

Montana (fig. 1). The reservoir is formed by Milltown Dam, 

which was constructed in 1907 to supply 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 arsenic, cadmium, copper, lead, 

and zinc. Tailings have been eroded and transported downstream along the Clark Fork, contaminating the approximately 

6.6 million yd3 of sediment that has 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).

As part of remediation planning associated with the 

Superfund process, the U.S. Environmental Protection Agency 

(USEPA) issued a record of decision in December 2004 that 

included removal of Milltown Dam as one of the remedial 

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

accommodate various construction activities and the excavation of a portion of the reservoir bottom sediments prior 

to dam removal, the initial phase (Stage 1) of a permanent 

drawdown of the reservoir to a lower pool level began on June 

1, 2006. The Stage 1 drawdown lowered the average summer 

pool by about 10–12 ft. Two subsequent phases of drawdown 

(Stages 2 and 3) are planned to eventually 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 spillway 

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

September 2007).

Because of the large quantity of contaminated sediment 

that has been 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 Stage 1 drawdown, especially 

during high streamflow. To address this concern, the USEPA 

initiated a program of intensive sampling by the U.S. Geological Survey (USGS) during water years 2006 and 2007 to 

supplement the routine sampling of the long-term monitoring 

program (Dodge and others, 2007). The supplemental sampling was conducted during the April-June periods of high 

streamflow and during selected periods of reservoir drawdown 

to provide additional resolution on variations in water quality and constituent transport through Milltown Reservoir. The 

supplemental sampling is planned to continue through the 

completion of all three stages of the permanent drawdown and 

dam removal (U.S. Environmental Protection Agency, written 

commun., 2008).

The USGS, in cooperation with the USEPA, estimated 

daily constituent loads transported at two sites upstream 

and one site downstream from Milltown Reservoir by using 

periodic water-quality, daily streamflow, and daily suspendedsediment data. Estimated annual loads were obtained by 

summing estimated daily loads and were used to determine 

the annual mass balance of suspended sediment and selected 

trace elements transported through Milltown Reservoir during 

water years 2004–07 (October 1, 2003, through September 30, 

2007).

Purpose and Scope

The purpose of this report is to present estimated daily 

and annual loads of suspended sediment and selected trace elements (unfiltered-recoverable arsenic, cadmium, copper, iron, 

lead, manganese, and zinc) for water years 2004–07 at two 

sites upstream and one site downstream from Milltown Reservoir (fig. 1). The estimated loads are used to quantify annual 

net gains and losses (mass balance) of suspended sediment 

and trace elements within Milltown Reservoir before and after 

June 1, 2006 (start of Stage 1 of the permanent drawdown of 

the reservoir). The methods used to estimate loads and the 

characteristics of daily and annual loads transported to and 

from Milltown Reservoir are described.

The USGS has collected periodic water-quality samples, 

as well as daily streamflow and suspended-sediment data, 

at each of the three sites as part of Clark Fork monitoring 

programs (Dodge and others, 2005; Dodge and others, 2006; 

and Dodge and others, 2007). The periodic water-quality, 

daily streamflow, and daily suspended-sediment data used to 

develop regression equations and estimate loads are publicly 

accessible on the USGS National Water Information System 

(NWIS) Web site for Montana (http://waterdata.usgs.gov/mt/

nwis).

Description of the Study Area

The Clark Fork basin upstream from Milltown Dam 

(fig. 1) drains an area of about 6,000 mi2. Milltown Reservoir 

is a small impoundment of about 540 acres formed by Milltown Dam at the confluence of the Clark Fork and Blackfoot 

River. The reservoir is at the downstream end of a contiguous 

complex of NPL Superfund sites extending from the headwaters of Silver Bow Creek near Butte to Milltown Dam near 

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

Two monitoring sites upstream from Milltown Reservoir 

(Clark Fork at Turah Bridge, near Bonner, station 12334550; 

and Blackfoot River near Bonner, station 12340000) represent the combined input of streamflow and constituent load to 

the reservoir; one site downstream from Milltown Reservoir 

(Clark Fork above Missoula, station 12340500) represents 

the output from the reservoir. For brevity, Clark Fork at Turah 

Bridge, near Bonner will be referred to as Clark Fork at Turah 

Bridge in this report. Milltown Reservoir is considered a “run 

of the river” reservoir because streamflow leaving the reservoir 

is equal to the streamflow of the Clark Fork and Blackfoot 
Figure 1. Location of study area.

Missoula

Butte

Anaconda

Galen

Warm Springs

Deer Lodge

Goldcreek

Drummond

Bonner

Turah

Clark

Bitterroot

River

Black

River

Fork

Rock

Creek

Flint

Little

Blackfoot

River

DIVIDE

Clark Fork

Warm Springs

 Ponds

Warm Springs Creek

Silver

Bow

Creek

Blac

Cr

ktail

eek

CONTINENTAL

Base modified from U.S. 

Geological Survey, State 

base map, 1:1,000,000, 1966. 

North American Datum of 

1927 (NAD 27)

0 10 20 MILES

0 10 20 KILOMETERS

Streamflow site

 Continuous record

Water-quality site

 Periodic water quality

 Daily suspended sediment

EXPLANATION

12340500 Site number Lost Creek

46o

114o

113o

Mill

Willow Creek

Creek

47o

Creek

foot

Helena

M O N T A N A Map

 area

Milltown

Reservoir

Bonner

Turah

Clark Fork above

Missoula, Mont. 

(12340500)

Clark Fork at Turah Bridge,

 near Bonner (12334550)

Blackfoot River

near Bonner

(12340000)

Clark

River

Fork

Clark

Fork

Blac

Milltown kfoot

 Dam

Milltown Reservoir

Stimson Dam

 (removed in 

 November 2005)

Introduction 3
4 Estimated loads transported through Milltown Reservoir, Montana, water years 2004-07

River entering the reservoir (U.S. Environmental Protection 

Agency, written commun., 2003).

Hydrologic Characteristics

Streamflow magnitude is a predominant factor affecting 

the transport of suspended sediment and sediment-associated 

constituents through a watershed. Streamflow magnitude can 

affect both the sustained delivery of constituent loads from 

the watershed and also the capacity to locally scour bottom 

sediments within Milltown Reservoir. The hydraulic energy 

associated with peak flows is especially important relative to 

the scour of bottom sediments in shallow water bodies, such 

as Milltown Reservoir. Because streamflow can vary substantially from year to year, thereby affecting rates of constituent 

transport, comparison of streamflow during the 2004–07 study 

period to a long-term period of record allows recent hydrologic conditions to be placed in a historical perspective.

The reference station with the longest period of record 

available for comparison to streamflow during the study 

period is Clark Fork above Missoula, with 78 years (water 

years 1930–2007) of continuous-streamflow record (http://

waterdata.usgs.gov/mt/nwis). The variation in daily mean 

streamflow at Clark Fork above Missoula during water years 

2004–07 is shown in figure 2, along with selected long-term 

streamflow characteristics for water years 1930–2007 to 

illustrate differences between recent and long-term hydrologic 

conditions.

The long-term streamflow characteristics at Clark Fork 

above Missoula (fig. 2) are represented by selected exceedance 

percentiles (10th, 50th, and 90th) of daily mean streamflow during the period of record. 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 for all 

the years during the period of record to produce a long-term 

annual hydrograph of daily mean streamflows representing a 

particular exceedance percentile. For relative comparison of 

streamflow during the study period to long-term streamflow, 

“normal” is indicated by the 50th exceedance percentile, or 

median.

Streamflows during water years 2004–07 (fig. 2) generally were below or near normal at Clark Fork above Missoula. 

The departures from normal flow can be misleading when 

viewing a graph plotted on a logarithmic scale; therefore, 

the proximity of the daily mean streamflow during the study 

period to the 10th (high flow) and 90th (low flow) exceedance 

percentile lines offer additional insight to the magnitude of the 

departure. Although the annual peak flows were near normal 

in every year except 2004 (in which peak flow was below 

normal), the receding limb of the hydrograph following the 

annual peak flow occurred sooner in most years during water 

years 2004-07 relative to long-term conditions and typically 

decreased to a below-normal streamflow during the summer. 

The shift towards earlier occurrence of the falling limb is most 

notable for water years 2006 and 2007, and is matched by an 

earlier occurrence of the rising limb during the spring of 2006 

and 2007. The earlier occurrence of the rising limb resulted in 

short periods of time when streamflow exceeded the median, 

and sometimes the 10th exceedance percentile, of long-term 

streamflow. During water years 2004-07, streamflows in the 

summer decreased to below normal and at times decreased to 

magnitudes near or at the 90th exceedance percentile. Streamflows typically increased in the fall to near normal; however, 

the below-normal flows in the summer of 2006 and 2007 

continued later into the fall than in previous years. Winter 

flows 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. Despite the 

irregular winter flow conditions, a consistent pattern during 

the 2004–07 study period was that streamflows decreased to 

magnitudes substantially below the 90th exceedance percentile 

on several days during the winter in every year. Three days in 

2005 (January 5 and December 7 and 8) and one day in 2007 

(January 13) had the minimum recorded streamflows for those 

dates during the period of record at Clark Fork above Missoula (http://waterdata.usgs.gov/mt/nwis). In contrast, a large 

runoff event in November 2006 (fig. 2) resulted in three days 

(November 8–10) with the maximum recorded streamflows for 

those dates during the period of record.

Values of selected annual and peak streamflow characteristics for each year of the study period (water years 2004-07) 

and for the period of record (water years 1930–2007 for 

continuous-streamflow records; water years 1908, 1930–2007 

for peak-streamflow records) are presented for Clark Fork 

above Missoula (table 1). Annual mean streamflows for every 

year of the study period were less than the long-term mean 

annual streamflow and ranged from 70 to 86 percent. Annual 

instantaneous peak streamflows during the study period also 

were less than the long-term mean annual peak streamflow 

and ranged from 47 to 83.

Methods of Data Collection and Quality 

Assurance

Water-quality samples were collected periodically at the 

three sites near Milltown Reservoir for analysis of suspended 

sediment and trace elements. Samples were collected at a frequency of 6–8 times per year during water years 2004–07 as 

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

and others, 2007). During 2006 and 2007, additional samples 

were collected as part of a supplemental monitoring program 

for the lower Clark Fork basin to provide better resolution 
Figure 2. Daily mean streamflow for the study period (water years 2004–07) and selected exceedance percentiles of long-term 

(water years 1930–2007) daily mean streamflow for Clark Fork above Missoula, Mont.

1,000 

10,000 

30,000 

STREAMFLOW, IN CUBIC FEET PER SECOND

300 

Oct

2003

Jan

2004

Apr

2004

July

2004

Oct

2004

Jan

2005

Apr

2005

July

2005

Oct

2005

Jan

2006

Apr

2006

July

2006

Oct

2006

Oct

2007

Jan

2007

Apr

2007

July

2007

Daily mean streamflow during study period 

EXPLANATION

10th-exceedance percentile of long-term daily mean streamflow 

50th-exceedance percentile (median) of long-term daily mean streamflow 

90th-exceedance percentile of long-term daily mean streamflow 

Methods of Data Collection and Quality Assurance 5

of the potential effect of scour of bottom sediments from the 

reservoir, which resulted in a sampling frequency of about 

16–20 times per year at each site (Dodge and others, 2007).

Periodic water-quality samples were collected throughout 

the entire stream depth at multiple locations across the stream 

by using depth- and width-integration methods described by 

Ward and Harr (1990), Edwards and Glysson (1999), and U.S. 

Geological Survey (variously dated). 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 combination of sampling 

methods and depth-integrating sampling equipment provides a 

vertically and laterally discharge-weighted composite sample 

that is representative of the entire 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).

Measurements of pH, specific conductance, and water 

temperature were made onsite during all sampling visits. 

Instantaneous streamflow at the time of periodic water-quality 

sampling was determined either by direct measurement or 

from stage-discharge rating tables (Rantz and others, 1982).

 All three sites near Milltown Reservoir were operated 

as continuous streamflow-gaging stations during water years 

2004–07. Daily mean streamflow was determined by using 

the continuous record of stage and stage-discharge relations 
Table 1. Annual and peak streamflow characteristics for the study period (water years 2004–07), the period of continuous-streamflow 

records (water years 1930–2007), and the period of peak-streamflow records (water years 1908, 1930–2007) for Clark Fork above 

Missoula, Mont.

[Abbreviation: e, estimated]

Water year

Annual streamflow Peak streamflow

Annual mean 

streamflow 

(cubic feet per 

second)

Percent of 

long-term 

mean annual 

streamflow1

Long-term 

exceedance 

percentile2 for 

annual mean 

streamflow 

(percent)

Annual 

instantaneous 

peak streamflow 

(cubic feet per 

second)

Percent of 

long-term 

mean annual 

instantaneous 

peak streamflow3

Percent of 

long-term 

maximum annual 

instantaneous 

peak streamflow4

2004 2,040 70 84 7,290 47 15

2005 2,260 78 75 9,400 60 20

2006 2,480 86 65 12,900 83 27

2007 2,440 84 68 e9,320 60 19

1

 The long-term mean annual streamflow for the period of continuous-streamflow records (water years 1930–2007) is 2,900 cubic feet per second.

2

 The exceedance percentile indicates the percent of the time that the annual mean streamflow for the given year was exceeded during the long-term period of 

continuous-streamflow records (water years 1930–2007). For example, an exceedance percentile of 84 percent represents a relatively low streamflow condition 

that was exceeded 84 percent of the time during the long-term period of record.

3

 The long-term mean annual instantaneous peak streamflow for the period of peak-streamflow records (water years 1908, 1930–2007) for Clark Fork above 

Missoula, Mont. is 15,600 cubic feet per second.

4

 The long-term maximum annual instantaneous peak streamflow for the period of peak-streamflow records (water years 1908, 1930–2007) is 48,000 cubic 

feet per second, which occurred in 1908.

6 Estimated loads transported through Milltown Reservoir, Montana, water years 2004-07

developed from periodic streamflow measurements according 

to procedures described by Rantz and others (1982). Clark 

Fork at Turah Bridge and Clark Fork above Missoula also 

were operated as daily sediment stations during the entire 

period of water years 2004–07; Blackfoot River near Bonner was operated as a daily sediment station only during 

water years 2006 and 2007. Suspended-sediment samples 

for the daily-sediment stations were collected at a high 

frequency (2–14 times per week) by local contract observers 

using depth integration at a single location near midstream. 

Quality-assurance procedures for generating daily records of 

streamflow and suspended-sediment data are described by 

White and others (1998). The quality of daily records were 

rated good to excellent, except for periods of ice cover, which 

were rated poor (Rantz and others, 1982).

Periodic water-quality samples were analyzed for 

suspended-sediment concentration and percent of suspended 

sediment finer than 0.062-mm diameter (http://waterdata.

usgs.gov/mt/nwis) by the USGS Montana Water Science 

Center Sediment Laboratory (herein referred to as Montana 

Sediment Laboratory) in Helena, Mont. according to methods 

described by Guy (1969) and Dodge and Lambing (2006). 

Quality-assurance procedures used by the Montana Sediment 

Laboratory are described by Dodge and Lambing (2006).

Periodic water-quality samples were analyzed for traceelement concentrations (http://waterdata.usgs.gov/mt/nwis) 

by the USGS National Water Quality Laboratory (NWQL) in 

Denver, Colo. Concentrations of “unfiltered recoverable” (formerly referred to as “total recoverable”) trace elements were 

determined from unfiltered samples that were first digested 

with dilute hydrochloric acid (Hoffman and others, 1996) and 

then analyzed by inductively coupled plasma-mass spectrometry (Garbarino and Struzeski, 1998). Unfiltered-recoverable 

concentrations represent the combined dissolved and particulate fractions of the trace element. 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 on an ongoing basis by analysis of qualitycontrol samples (blanks and replicates), which were submitted along with routine samples on every field trip. Analytical 

results for quality-control samples are used to evaluate the 

performance of sampling and analytical methods in order to 

ensure that results for environmental samples are reliable. 

Quality-assurance data for the Clark Fork sampling program 

during water years 2004-07 are reported in Dodge and others 

(2005, 2006, 2007, and Kent A. Dodge, U.S. Geological Survey, written commun., 2008).

Trace-element concentrations in blank samples collected 

during the 2004-07 study period were almost always less than 

the laboratory reporting level (LRL). Values exceeding twice 

the LRL were noted during data reviews to evaluate the presence of a consistent trend that could indicate systematic contamination. Values exceeding twice the LRL were infrequent 

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 
Methods for Estimating Constituent Loads 7

to trace-element concentrations were made during the study 

period on the basis of analytical results for blanks.

Precision for each trace element was determined by calculation of a relative standard deviation (RSD) for analytical 

results of replicate samples. The RSDs for all trace elements 

during the study period were within the 20-percent data quality objective of acceptable precision for concentrations in 

replicate samples.

Methods for Estimating Constituent 

Loads

The term “load” represents the mass (commonly 

expressed in tons or pounds) 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 specific sampling time, 

whereas daily, monthly, seasonal, and annual loads represent 

the cumulative mass transported over a prolonged period. The 

rate of load transport is commonly expressed as “discharge,” 

which incorporates both the unit of mass and time increment 

(for example, tons per day or tons per year).

Annual loads generally are more informative than instantaneous loads measured at the time of sampling because they 

represent the cumulative transport of constituent mass during 

a year, and thereby incorporate the potentially large range of 

daily variations. Annual loads also are useful for evaluating 

differences in constituent transport among sites to identify 

source areas contributing substantial inputs. Differences in 

annual loads transported past sampling sites can result from 

differences in 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 annual loads typically requires either highfrequency sampling or the use of statistical relations, such as 

regression equations, that can be applied to a daily record of a 

related 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. 

The daily loads estimated for each day of the year then can be 

summed to provide an estimated annual load.

Equations used to mathematically describe statistical 

relations and estimate daily constituent loads were developed 

from data for periodic water-quality samples collected during 

water years 2004–07. Various forms of data transformation 

were examined to produce a linear distribution that could be 

adequately fit by an ordinary least squares regression line. 

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 residual distribution 

to evaluate the relation and select the best form of regression 

equation.

For trace-element concentrations that were censored 

(reported as less-than the LRL), one-half of the minimum LRL 

used during the study period was substituted for purposes of 

plotting and analysis of statistical relations. Where a large 

percentage of values were censored, such as cadmium for 

samples collected at Blackfoot River near Bonner, the statistical relations and estimates of daily loads have a greater degree 

of uncertainty than loads estimated by using relations developed from uncensored data. Loads estimated from datasets 

with a large percentage of censored data can still provide a 

relative measure of constituent transport, but estimates of mass 

balance are potentially subject to greater error.

Suspended Sediment

 Daily suspended-sediment loads for water years 2004–07 

were estimated for each of the three sites near Milltown 

Reservoir (fig. 1) by using either high-frequency sampling 

or statistical relations. Daily suspended-sediment loads were 

estimated directly by high-frequency sampling as part of daily 

sediment monitoring for Clark Fork at Turah Bridge and Clark 

Fork above Missoula for the entire study period of water years 

2004–07, and for Blackfoot River near Bonner for water years 

2006 and 2007. A statistical relation between suspendedsediment discharge and streamflow was used to estimate daily 

suspended-sediment loads for Blackfoot River near Bonner for 

water years 2004 and 2005.

High-frequency suspended-sediment samples for daily 

sediment monitoring were collected by a contract observer 

2–14 times per week; sampling frequency depended on 

streamflow conditions. The concentration data from observer 

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 suspendedsediment concentrations then were multiplied by the daily 

mean streamflows (and a units-conversion constant) to 

generate a record of daily suspended-sediment loads (http://

waterdata.usgs.gov/mt/nwis) for the periods of daily sediment 

monitoring.

Relations between suspended-sediment discharge and 

streamflow during water years 2004–07 were examined for all 

three sites to evaluate differences in sediment-transport characteristics between the two periods before and after June 1, 

2006 (start of Stage 1 of the permanent drawdown of Milltown 

Reservoir). Separate relations were developed for the period 

before the Stage 1 drawdown (October 1, 2003–May 31, 2006) 
8 Estimated loads transported through Milltown Reservoir, Montana, water years 2004-07

and the period during the Stage 1 drawdown (June 1, 2006–

September 30, 2007). Prior to developing the relations, the 

suspended-sediment concentration for each periodic waterquality sample was converted to an equivalent suspendedsediment discharge, in tons per day, according to the equation

Qsed = CsedQK, (1)

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 suspended-sediment 

discharge to an equivalent daily suspendedsediment discharge.

After suspended-sediment concentrations were converted 

to suspended-sediment discharges, plots of the relations 

between suspended-sediment discharge and streamflow for 

the two periods before and after June 1, 2006, were evaluated to identify changes in sediment-transport characteristics. 

Sediment-transport relations for water years 2004–07 are 

illustrated by scatter plots (fig. 3) that are fit with locally 

weighted scatter plot smoothing (LOWESS) lines (Cleveland 

and McGill, 1984; Cleveland, 1985) that show the central 

tendencies of the data distribution over the range of streamflows sampled during the two periods. The smooth lines do not 

imply statistically significant relations, but rather are used as a 

visual indication of differences in sediment-transport characteristics between the two periods.

The relations between suspended-sediment discharge 

and streamflow during water years 2004–07 for Clark Fork 

at Turah Bridge and Blackfoot River near Bonner (fig. 3A

and 3B) indicate no substantial change between the two 

periods (before and after June 1, 2006). Therefore, the 

sediment-transport relations for both periods at these two sites 

can be considered equivalent and can be described by a single 

regression equation. However, a notable shift in the sedimenttransport relation occurred at Clark Fork above Missoula 

(fig. 3C) after June 1, 2006. The relation during the Stage 1 

drawdown was generally parallel and higher than the relation for the period prior to the Stage 1 drawdown, indicating 

that after June 1, 2006, a larger amount of suspended sediment was transported for a given streamflow throughout the 

entire range of streamflow conditions. This shift in sedimenttransport characteristics presumably was the result of either 

reservoir bottom sediment being more readily scoured by 

increased velocity as the depth of the overlying water in the 

reservoir decreased, or exposed sediment being eroded by 

channel cutting or bank collapse as water levels dropped and 

water drained from the material (U.S. Environmental Protection Agency, written commun., 2008). The eroded reservoir 

sediment contained a large proportion of sand, as indicated by 

a distinct change in particle-size distribution between the two 

periods for samples collected at Clark Fork above Missoula. 

The median percent of suspended sediment finer than 0.062 

mm decreased from 84 percent in samples collected prior to 

June 1, 2006, to 42 percent for samples collected after June 1, 

2006 (http://waterdata.usgs.gov/mt/nwis). As a result of the 

shift, it was determined that separate regression equations for 

the two periods before and after the start of the Stage 1 drawdown would more accurately describe the sediment-transport 

relations at Clark Fork above Missoula.

Regression equations for estimating suspended-sediment 

discharge from streamflow (table 2) provide a mathematical description of sediment-transport characteristics for all 

three sites. Statistical relations between suspended-sediment 

discharge and streamflow for periodic water-quality samples 

were developed by using ordinary least squares regression of 

data that were transformed to produce a linear distribution. 

Values for suspended-sediment discharge were transformed 

to base-10 logarithms (log) and streamflow values were 

transformed to cube roots (Q0.333) to obtain the best linear 

distribution and least scatter. Single equations represent the 

entire 2004–07 study period for Clark Fork at Turah Bridge 

and Blackfoot River near Bonner; two equations describe the 

periods before and after June 1, 2006, for Clark Fork above 

Missoula.

The regression equations in table 2 are presented primarily for informational purposes. With the exception of water 

years 2004 and 2005 at the Blackfoot River near Bonner, 

the equations for estimating suspended-sediment discharge 

in table 2 were not used because daily sediment monitoring allowed direct estimation of daily sediment loads by 

high-frequency sampling. The equation for estimating suspended-sediment discharge for Blackfoot River near Bonner 

(table 2) was used only for water years 2004–05 when daily 

sediment monitoring was temporarily suspended. The equation was applied to the daily streamflow record to construct 

a daily record of estimated suspended-sediment loads. Daily 

suspended-sediment loads for Blackfoot River near Bonner for 

water years 2006 and 2007 were estimated directly by using 

high-frequency sampling after daily sediment monitoring was 

resumed in October 2005 (Dodge and others, 2007).

All the equations for estimating suspended-sediment 

discharge (table 2) are significant (p-value< 0.001). The 

relatively large R2 values indicate minor to moderate scatter of 

data around the regression line; SE ranged from 44.0 percent 

at Blackfoot River near Bonner to 66.2 percent at Clark Fork 

above Missoula for the period prior to the Stage 1 drawdown. 

Retransformation-bias-correction factors (RBCF) were determined for each of the equations to account for the systematic 

bias that can occur when data are transformed to log units for 

purposes of removing curvature in the distribution and then 

retransformed back to original arithmetic units to produce 

the final estimate (Koch and Smillie, 1986). To compensate 

for this log-transformation bias, retransformed results can be 

multiplied by a non-parametric bias-correction factor (Duan, 

1983). In all cases, the retransformation bias was small; 
Figure 3. Relations between suspended-sediment discharge and streamflow before and after 

June 1, 2006 (start of Stage 1 of the permanent drawdown of Milltown Reservoir), water years 

2004-07: A, Clark Fork at Turah Bridge, near Bonner, Mont.; B, Blackfoot River near Bonner, Mont.; 

and C, Clark Fork above Missoula, Mont. SUSPENDED-SEDIMENT DISCHARGE, IN TONS PER DAY

1

10

100

1,000

10,000

B

1

10

100

1,000

10,000

A

20 1,000 10,000 20,000

STREAMFLOW, IN CUBIC FEET PER SECOND

1

10

100

1,000

10,000

C

LOWESS smooth line for data collected during October 1, 2003–May 31, 2006

EXPLANATION

LOWESS smooth line for data collected during June 1, 2006–September 30, 2007 

Data collected during October 1, 2003–May 31, 2006

Data collected during June 1, 2006–September 30, 2007

Methods for Estimating Constituent Loads 9
Table 2. Equations for estimating suspended-sediment discharge for Clark Fork at Turah Bridge, near Bonner, Mont., Blackfoot River 

near Bonner, Mont., and Clark Fork above Missoula, Mont., water years 2004–07.

[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; LOGSEDQ, base 10 logarithm of suspended-sediment discharge, in tons per day; Q, streamflow, in cubic feet per second; USGS, 

U.S. Geological Survey. Symbol: <, less than]

Station Period Equation1 N R2 p-value SE RBCF

Clark Fork at Turah Bridge 

near Bonner, Mont. 

(12334550)

Oct. 1, 2003–Sept. 30, 2007 LOGSEDQ=0.248(Q0.333)–1.08 53 0.88 <0.001 64.3 1.03

Blackfoot River near Bonner, 

Mont.2 (12340000)

Oct. 1, 2003–Sept. 30, 2007 LOGSEDQ=0.250(Q0.333)–1.57 46 .96 <.001 44.0 1.02

Clark Fork above Missoula, 

Mont.3 (12340500)

Oct. 1, 2003–May 31, 2006 LOGSEDQ=0.164(Q0.333)–0.363 27 .87 <.001 66.2 1.03

Clark Fork above Missoula, 

Mont.3 (12340500)

June 1, 2006–Sept. 30, 2007 LOGSEDQ=0.179(Q0.333)–0.161 26 .91 <.001 52.7 1.02

1

 Equations are provided primarily for informational purposes. Daily suspended-sediment discharge for water years 2006–07 for Blackfoot River near Bonner, 

and for the entire study period for the other two stations were estimated directly by high-frequency sampling of the daily sediment monitoring program.

2

 Equation used to estimate daily suspended-sediment discharge for water years 2004–05 when daily sediment monitoring was temporarily suspended.

3

 Separate equations were developed for the periods before (October 1, 2003–May 31, 2006) and after (June 1, 2006–September 30, 2007) the permanent 

drawdown of Milltown Reservoir that began on June 1, 2006. Unlike the other two sites, the distinct shift in sediment-transport characteristics for Clark Fork 

above Missoula, after the start of the permanent drawdown (see fig. 3) warranted a new regression relation to mathematically describe the new conditions.

10 Estimated loads transported through Milltown Reservoir, Montana, water years 2004-07

bias-correction factors ranged from 1.02 to 1.03. The bias-correction factor used for regression-estimated daily suspendedsediment loads during water years 2004-05 at Blackfoot River 

near Bonner was 1.02 (table 2).

The equations for estimating suspended-sediment 

discharge (table 2) describe the relations only for the range 

of streamflow that was sampled at each site during water 

years 2004–07; therefore, extrapolation to higher streamflows 

might be subject to substantial error. More importantly, the 

two equations for Clark Fork above Missoula (downstream 

from Milltown Reservoir) represent two distinctly different 

sediment-transport conditions during periods before and after 

the start of the Stage 1 drawdown of Milltown Reservoir. The 

increased potential for localized erosion of reservoir sediment 

during the Stage 1 drawdown is subject to change as the permanent drawdown progresses through subsequent stages and 

various activities occur within construction areas. As a result, 

the separate equations for estimating suspended-sediment 

discharge at Clark Fork above Missoula are applicable only to 

the specific dates indicated in table 2.

developed from data for periodic water-quality samples collected during water years 2004–07. Relations between traceelement discharge and two related variables—streamflow 

and suspended-sediment discharge—were examined. Better 

relations for describing variation in trace-element discharge 

were obtained using suspended-sediment discharge as the 

explanatory variable. The better correlation is a result of the 

strong association between suspended-sediment concentrations 

and unfiltered concentrations of trace elements (Lambing, 

1991). Suspended-sediment concentrations can vary substantially for a given streamflow magnitude due to differences in 

the soils and vegetative cover of areas where runoff occurs, 

seasonal and local differences in land use, and localized natural or human disturbances that cause sediment erosion. This 

variability in sediment supply, in turn, directly influences the 

concentration of sediment-associated trace elements, which 

can result in substantial variability in trace-element loads 

transported for a given streamflow. Therefore, accurate determination of suspended-sediment loads transported through 

Milltown Reservoir is important because it provides the basis 

for estimating trace-element loads transported through the 

reservoir.

Relations between trace-element discharge and 

suspended-sediment discharge for all three sites were examined for differences in trace-element transport characteristics 

between the two periods before and after the start of the 

Stage 1 drawdown of Milltown Reservoir. Prior to developing 

Trace Elements

Daily loads of unfiltered-recoverable trace elements for 

water years 2004–07 were estimated for each of the three sites 

near Milltown Reservoir (fig. 1) by using statistical relations 
Methods for Estimating Constituent Loads 11

the relations, the trace-element concentration for each periodic 

water-quality sample was converted to an equivalent traceelement discharge, in tons per day, according to the equation

Qte = CteQK, (2)

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 for the two 

periods before and after June 1, 2006, were evaluated to 

identify changes in trace-element transport characteristics. 

Trace-element transport relations for water years 2004–07 are 

illustrated by scatter plots for unfiltered-recoverable copper 

discharge (fig. 4) and unfiltered-recoverable iron discharge 

(fig. 5) that are fit with LOWESS smooth lines (Cleveland 

and McGill, 1984; Cleveland, 1985) that show the central 

tendencies of the data distribution during the two periods. The 

smooth lines do not imply statistically significant relations, but 

rather are used as a visual indication of differences in traceelement transport characteristics between the two periods.

The relations between unfiltered-recoverable copper discharge and suspended-sediment discharge during water years 

2004–07 (fig. 4) indicate no substantial change between the 

two periods before and after June 1, 2006, at any of the three 

sites. Similar transport relations for the two periods were prevalent for all the trace elements at Clark Fork at Turah Bridge 

and Blackfoot River near Bonner; therefore, the transport 

relations for all the trace elements for both periods at these 

two sites can be considered equivalent and described by single 

regression equations for the entire 2004–07 study period. In 

contrast, the relations between unfiltered-recoverable iron 

discharge and suspended-sediment discharge at Clark Fork 

above Missoula (fig. 5C) revealed a minor, but consistent, 

downward shift during the Stage 1 drawdown compared to 

the period before the Stage 1 drawdown. The geochemical 

or physical basis for the shift in iron-transport characteristics 

is not known, and such a shift was not clearly evident for the 

other trace-element transport relations at Clark Fork above 

Missoula. Although the downward shift in iron-transport 

characteristics at Clark Fork above Missoula was small, it was 

consistent enough to warrant development of separate regression equations to more accurately describe transport during the 

two periods. Separate equations also were developed for the 

two periods for the other trace elements at Clark Fork above 

Missoula to provide a consistent approach in estimating traceelement loads at this site.

Statistical relations between trace-element discharge 

and suspended-sediment discharge were developed by using 

ordinary least squares regression of data from periodic 

water-quality samples that were transformed to produce a 

linear distribution. Values of both trace-element discharge 

and suspended-sediment discharge were transformed to log 

units to obtain the best linear distribution and least scatter. 

The resulting regression equations for estimating unfilteredrecoverable trace-element discharge for Clark Fork at Turah 

Bridge, Blackfoot River near Bonner, and Clark Fork above 

Missoula for water years 2004–07 are presented in tables 3–5, 

respectively. Single equations for each trace element represent 

the entire 2004–07 study period at Clark Fork at Turah Bridge 

and Blackfoot River near Bonner; two equations represent the 

periods before and after June 1, 2006, for Clark Fork above 

Missoula.

All the equations for estimating trace-element discharge 

(tables 3–5) are significant (p-value< 0.001). The large R2

values indicate relatively minor scatter of data around the 

regression line; however, the scatter increased slightly in the 

equations for Clark Fork above Missoula (table 5) during the 

Stage 1 drawdown compared to the period before the Stage 1 

drawdown, possibly as a result of variations in localized erosion of sediment within Milltown Reservoir. The ranges of SE 

in the equations for all three sites were 20.0–34.5 percent for 

arsenic, 26.6–44.1 percent for cadmium, 30.3–44.9 percent for 

copper, 15.6–27.7 percent for iron, 21.2–43.3 percent for lead, 

21.5–30.9 percent for manganese, and 23.6–43.8 percent for 

zinc.

The regression equations in tables 3–5 were applied to 

the daily record of estimated suspended-sediment loads to 

estimate daily trace-element loads for water years 2004–07. 

The retransformed results were multiplied by bias-correction 

factors to compensate for log-transformation bias by using 

procedures described by Duan (1983). Bias-correction factors 

were small in all cases and ranged from 1.00 to 1.02.

The equations for estimating trace-element discharge 

in tables 3–5 describe the relations only for the range of 

streamflow that was sampled at each site during water years 

2004–07; therefore, extrapolation to higher streamflows might 

be subject to substantial error. Similar to suspended-sediment 

discharge, the separate equations for the two periods before 

and after June 1, 2006, at Clark Fork above Missoula are 

applicable only to the specific dates indicated in table 5.
0.0001

0.001

0.01

0.1

1

10

0.0001

0.001

0.01

0.1

1

10

B

A

C

LOWESS smooth line for data collected during October 1, 2003–May 31, 2006

EXPLANATION

LOWESS smooth line for data collected during June 1, 2006–September 30, 2007 

Data collected during October 1, 2003–May 31, 2006

Data collected during June 1, 2006–September 30, 2007

1 10 100 1,000 10,000

SUSPENDED-SEDIMENT DISCHARGE, IN TONS PER DAY

UNFILTERED-RECOVERABLE COPPER DISCHARGE, IN TONS PER DAY 0.0001

0.001

0.01

0.1

1

10

Figure 4. Relations between unfiltered-recoverable copper discharge and suspended-sediment 

discharge before and after June 1, 2006 (start of Stage 1 of the permanent drawdown of Milltown 

Reservoir), water years 2004–07: A, Clark Fork at Turah Bridge, near Bonner, Mont.; B, Blackfoot 

River near Bonner, Mont.; and C, Clark Fork above Missoula, Mont.

12 Estimated loads transported through Milltown Reservoir, Montana, water years 2004-07
Figure 5. Relations between unfiltered-recoverable iron discharge and suspended-sediment 

discharge before and after June 1, 2006 (start of Stage 1 of the permanent drawdown of Milltown 

Reservoir), water years 2004–07: A, Clark Fork at Turah Bridge, near Bonner, Mont.; B, Blackfoot 

River near Bonner, Mont.; and C, Clark Fork above Missoula, Mont.

0.01

0.1

1

10

100

0.01

0.1

1

10

100

B

A

C

LOWESS smooth line for data collected during October 1, 2003–May 31, 2006

EXPLANATION

LOWESS smooth line for data collected during June 1, 2006–September 30, 2007 

Data collected during October 1, 2003–May 31, 2006

Data collected during June 1, 2006–September 30, 2007

UNFILTERED-RECOVERABLE IRON DISCHARGE, IN TONS PER DAY

0.01

0.1

1

10

100

1 10 100 1,000 10,000

SUSPENDED-SEDIMENT DISCHARGE, IN TONS PER DAY

Methods for Estimating Constituent Loads 13
Table 3. Equations for estimating unfiltered-recoverable trace-element discharge for Clark Fork at Turah 

Bridge, near Bonner, Mont., water years 2004–07.

[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; ASQ, arsenic discharge, in tons per day; 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; SEDQ, 

suspended-sediment discharge, in tons per day. Symbol: <, less than]

Equation N R2 p-value SE RBCF

Equations for October 1, 2003–September 30, 2007

ASQ=0.00295(SEDQ)0.531 53 0.88 <0.001 34.5 1.01

CDQ=0.0000143(SEDQ)0.765 53 .96 <.001 26.6 1.01

CUQ=0.00137(SEDQ)0.837 53 .96 <.001 30.3 1.01

FEQ=0.0161(SEDQ)1.00 53 .98 <.001 24.5 1.01

PBQ=0.0000994(SEDQ)0.994 53 .97 <.001 32.8 1.01

MNQ=0.00545(SEDQ)0.837 53 .98 <.001 21.5 1.00

ZNQ=0.00158(SEDQ)0.878 53 .96 <.001 31.2 1.01

Table 4. Equations for estimating unfiltered-recoverable trace-element discharge for Blackfoot River 

near Bonner, Mont., water years 2004–07.

[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; ASQ, arsenic discharge, in tons per day; 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; SEDQ, 

suspended-sediment discharge, in tons per day. Symbol: <, less than]

Equation N R2 p-value SE RBCF

Equations for October 1, 2003–September 30, 2007

ASQ= 0.00130(SEDQ)0.400 45 0.95 <0.001 20.0 1.00

CDQ= 0.0000101(SEDQ)0.456 46 .91 <.001 31.5 1.01

CUQ= 0.000495(SEDQ)0.668 46 .93 <.001 38.4 1.01

FEQ= 0.0178(SEDQ)0.951 46 .99 <.001 21.9 1.00

PBQ= 0.0000265(SEDQ)0.931 46 .99 <.001 21.2 1.00

MNQ= 0.00281(SEDQ)0.817 46 .98 <.001 22.6 1.00

ZNQ= 0.000726(SEDQ)0.645 46 .97 <.001 23.6 1.01

14 Estimated loads transported through Milltown Reservoir, Montana, water years 2004-07
Table 5. Equations for estimating unfiltered-recoverable trace-element discharge for Clark Fork 

above Missoula, Mont., water years 2004–07.

[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; ASQ, arsenic discharge, in tons per day; 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; 

SEDQ, suspended-sediment discharge, in tons per day. Symbol: <, less than]

Equation N R2 p-value SE RBCF

Equations for October 1, 2003–May 31, 2006

ASQ=0.00263(SEDQ)0.503 27 0.91 <0.001 27.9 1.01

CDQ=0.0000228(SEDQ)0.657 27 .93 <.001 31.1 1.01

CUQ=0.00218(SEDQ)0.704 27 .93 <.001 33.1 1.01

FEQ=0.0218(SEDQ)0.945 27 .99 <.001 15.6 1.00

PBQ=0.000163(SEDQ)0.842 27 .96 <.001 28.8 1.01

MNQ=0.00929(SEDQ)0.723 27 .95 <.001 27.0 1.01

ZNQ=0.00324(SEDQ)0.711 27 .94 <.001 29.6 1.01

Equations for June 1, 2006–September 30, 2007

ASQ=0.00133(SEDQ)0.604 26 .92 <.001 31.7 1.01

CDQ=0.0000229(SEDQ)0.672 26 .88 <.001 44.1 1.02

CUQ=0.00256(SEDQ)0.694 26 .88 <.001 44.9 1.02

FEQ=0.0233(SEDQ)0.879 26 .97 <.001 27.7 1.01

PBQ=0.000282(SEDQ)0.754 26 .90 <.001 43.3 1.01

MNQ=0.00562(SEDQ)0.752 26 .95 <.001 30.9 1.01

ZNQ=0.00337(SEDQ)0.739 26 .90 <.001 43.8 1.01

Estimated Loads Transported through Milltown Reservoir 15

Estimated Loads Transported through 

Milltown Reservoir

Patterns in daily loads transported past the three monitoring sites aid in understanding the magnitude, duration, and 

source of constituent transport in response to variations in 

streamflow or reservoir operations. The cumulative net gains 

or losses of daily loads for an entire year can then be used to 

describe an annual mass balance of suspended-sediment and 

trace-element loads transported through Milltown Reservoir 

during water years 2004–07.

Deposition of sediment in Milltown Reservoir is indicated 

where the combined inflow of suspended-sediment load to the 

reservoir plots higher than the suspended-sediment load for 

Clark Fork above Missoula. Conversely, loss of sediment from 

Milltown Reservoir is indicated where the suspended-sediment 

load at Clark Fork above Missoula plots higher than the combined inflow of suspended-sediment load. Daily variations in 

loads transported to and from the reservoir illustrate periods 

of no change, deposition, or loss of sediment (and associated 

trace elements) within Milltown Reservoir. Generally, deposition of material in the reservoir is more likely to occur during 

low streamflows when velocities are low. Conversely, loss of 

material is more likely to occur during high streamflows when 

velocities are high.

Three periods of sustained drawdown are shown in 

figure 6 to indicate when the pool in Milltown Reservoir was 

lowered substantially for several weeks or more to accommodate either routine maintenance activities or activities associated with preparations for the removal of Milltown Dam. Not 

shown are periods of short-duration or small drawdowns that 

may have occurred intermittently for various operational purposes. The first two sustained drawdowns were temporary and 

occurred during July–August 2004 and October–December 

2005. The July–August 2004 drawdown was for routine dam 

Daily Loads

Variations of estimated daily suspended-sediment loads 

transported to and from Milltown Reservoir during water 

years 2004–07 are shown in figure 6, along with the daily 

mean streamflow during the period. Values for the two sites 

upstream from the reservoir (Clark Fork at Turah Bridge and 

Blackfoot River near Bonner) are summed to represent the 

total inflow to Milltown Reservoir. The values for Clark Fork 

above Missoula represent the outflow from the reservoir. 
16 Estimated loads transported through Milltown Reservoir, Montana, water years 2004-07

maintenance, and the October–December 2005 drawdown 

was to facilitate the removal of Stimson Dam (fig. 1) on the 

Blackfoot River (U.S. Environmental Protection Agency, written commun., May 2004). The third drawdown that started on 

June 1, 2006, is Stage 1 of a permanent drawdown and is part 

of preparations to remove Milltown Dam (U.S. Environmental 

Protection Agency, written commun., July 5, 2006).

The patterns in daily streamflow (fig. 6) illustrate a typical snowmelt hydrograph in which streamflow was generally 

low to moderate during the fall and winter, then increased for 

several months during spring runoff. Streamflow commonly 

peaked in May or early June. Streamflows decreased relatively 

rapidly after the peak; the minimum streamflow of the year 

generally occurred in late summer. Short-lived precipitation 

events can produce substantial increases in streamflow at any 

time, as evidenced by the large increase in November 2006. 

The difference between the combined inflow to Milltown Reservoir and the outflow from the reservoir at Clark Fork above 

Missoula was negligible, indicating both the minimal storage 

capacity of the reservoir and accurate streamflow records 

for all three sites. The minor differences between combined 

inflow and outflow could represent a loss of surface water to 

ground water in the alluvium underlying Milltown Reservoir 

and the river reaches between the gages and the reservoir, or 

evaporative loss from the surface of the reservoir.

 Variations in estimated daily suspended-sediment loads 

transported to and from Milltown Reservoir during water years 

2004–07 (fig. 6) generally coincided with variations in streamflow. For most of the period before June 1, 2006, differences 

between the daily suspended-sediment loads transported to 

and from the reservoir were minor or indicated small amounts 

of deposition. Suspended-sediment loads spiked occasionally 

without a concurrent large increase in streamflow, possibly 

as a result of localized sediment disturbances. However, the 

outflow of suspended sediment from the reservoir 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 

out of the reservoir during the temporary drawdowns occurred 

during low-flow conditions; consequently, these losses of 

sediment from the reservoir may have resulted from erosion of 

exposed shoreline sediments or localized disturbances, rather 

than scour of deeper bottom sediments.

A prominent shift in the quantity of suspended sediment 

transported from Milltown Reservoir occurred after the start 

of the Stage 1 drawdown on June 1, 2006. During the entire 

interval of the Stage 1 drawdown (June 1, 2006–September 30, 

2007) within the study period, the daily suspended-sediment 

load transported from the reservoir was consistently larger 

than the combined daily suspended-sediment load transported 

to the reservoir (fig. 6). This pattern persisted for all streamflow conditions and, at times, the loss of sediment from the 

reservoir was substantial. The sediment losses from the reservoir likely were caused by a combination of factors. Primary 

processes likely included either erosion of bottom sediments in 

the shallow reservoir 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. Minor factors might have included erosion 

of shoreline sediment by wind-induced waves or disturbance 

by construction activities (U.S. Environmental Protection 

Agency, written commun., 2008).

The transport characteristics for all the unfilteredrecoverable trace-element loads (arsenic, cadmium, copper, 

iron, lead, manganese, and zinc) estimated for this study were 

generally similar. Therefore, variations in estimated daily 

trace-element loads transported to and from Milltown Reservoir during water years 2004–07 are illustrated using copper 

as a representative example (fig. 7). Similar to suspendedsediment loads, the daily copper loads entering and leaving the 

reservoir generally coincided with variations in streamflow. 

Because sediment transport is largely a function of streamflow 

or localized sediment erosion, and trace elements such as copper are strongly associated with sediment (Lambing, 1991), 

the temporal variations of daily trace-element loads were similar to those of daily suspended-sediment loads (fig. 6).

For most of the period before June 1, 2006, the differences between daily unfiltered-recoverable copper loads transported to and from Milltown Reservoir were small (fig. 7). 

However, losses of copper from the reservoir were notable 

during the temporary drawdowns in July–August 2004 and 

October–December 2005 (fig. 7) when losses of suspended 

sediment also occurred (fig. 6).

A prominent shift in the quantity of copper transported 

out of Milltown Reservoir (fig. 7) occurred after the start of 

the Stage 1 drawdown on June 1, 2006. During the entire interval of the Stage 1 drawdown within the study period, the daily 

copper load transported from the reservoir was consistently 

larger than the combined daily copper load transported to the 

reservoir. This pattern persisted for all streamflow conditions 

and, at times, the loss of copper from the reservoir was substantial. The increased loss of copper from the reservoir after 

June 1, 2006 (fig. 7) paralleled the increased loss of suspended 

sediment during the same period (fig. 6). A similar pattern of 

increased loss of load from the reservoir during the Stage 1 

drawdown was determined for all the trace elements.

Annual Loads

Estimated annual loads of suspended sediment and trace 

elements were determined by summing the estimated daily 

loads transported past each of the three sampling sites near 

Milltown Reservoir during water years 2004–07. The mass 

balance of annual loads was determined as the net difference 

in annual loads transported to and from the reservoir and 

was used to indicate whether a cumulative net gain or loss of 

suspended sediment and trace elements within the reservoir 

occurred during a given water year (table 6).

If less material is transported annually from Milltown 

Reservoir than entered (table 6), some of the suspended material entering the reservoir was deposited (net gain, indicated by 
Figure 6. Daily mean streamflow and estimated daily suspended-sediment loads transported to and from Milltown Reservoir, water 

years 2004–07.

1 

10 

100 

STREAMFLOW, IN CUBIC FEET PER SECOND

1

10

100

1,000

10,000

100,000

300,000

SUSPENDED-SEDIMENT LOAD, IN TONS PER DAY

Oct

2003

Jan

2004

Apr

2004

July

2004

Oct

2004

Jan

2005

Apr

2005

July

2005

Oct

2005

Jan

2006

Apr

2006

July

2006

Oct

2006

Oct

2007

Jan

2007

Apr

2007

July

2007

Streamflow

Suspended-sediment loads

EXPLANATION

Outflow from Milltown Reservoir—Daily mean streamflow at Clark Fork above Missoula, Mont.

1,000 

10,000 

30,000 

Outflow from Milltown Reservoir—Estimated daily suspended-sediment load at Clark Fork 

 above Missoula, Mont.

Inflow to Milltown Reservoir—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—Daily mean streamflow (sum of daily mean streamflows for Clark Fork 

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

Temporary drawdown 

of Milltown Reservoir 

(July–August 2004)

Temporary drawdown 

of Milltown Reservoir 

(October–

December 2005)

Start of Stage 1 of 

permanent drawdown 

of Milltown Reservoir 

(June 1, 2006)

Estimated Loads Transported through Milltown Reservoir 17
Figure 7. Daily mean streamflow and estimated daily unfiltered-recoverable copper loads transported to and from Milltown 

Reservoir, water years 2004–07.

1 

10 

100 

1,000 

30,000 

10,000 

STREAMFLOW, IN CUBIC FEET PER SECOND

0.001

0.01

0.1

1

10

30

UNFILTERED-RECOVERABLE COPPER LOAD, IN TONS PER DAY

Oct.

2003

Jan.

2004

Apr.

2004

July

2004

Oct.

2004

Jan.

2005

Apr.

2005

July

2005

Oct.

2005

Jan.

2006

Apr.

2006

July

2006

Oct.

2006

Oct.

2007

Jan.

2007

Apr.

2007

July

2007

Streamflow

Unfiltered-recoverable copper loads

EXPLANATION

Outflow from Milltown Reservoir—Daily mean streamflow at Clark Fork above Missoula, Mont.

Outflow from Milltown Reservoir—Estimated unfiltered-recoverable copper load at Clark Fork 

 above Missoula, Mont.

Inflow to Milltown Reservoir—Estimated daily unfiltered-recoverable copper load (sum of estimated 

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

Inflow to Milltown Reservoir—Daily mean streamflow (sum of daily mean streamflows for Clark Fork 

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

Temporary drawdown 

of Milltown Reservoir 

(July–August 2004)

Temporary 

drawdown of 

Milltown Reservoir 

(October–

December 2005)

Start of Stage 1 of 

permanent drawdown 

of Milltown Reservoir 

(June 1, 2006)

18 Estimated loads transported through Milltown Reservoir, Montana, water years 2004-07
Table 6. Annual mean streamflow and estimated annual loads and annual mass balance of suspended sediment and unfilteredrecoverable trace elements transported to and from Milltown Reservoir, water years 2004–07.

[Abbreviations: USGS, U.S. Geological Survey; USGS station numbers: 12334550–Clark Fork at Turah Bridge, near Bonner, Mont.; 12340000–Blackfoot River 

near Bonner, Mont.; 12340500–Clark Fork above Missoula, Mont.]

USGS station number or 

summation category

Annual 

mean 

streamflow 

(cubic feet 

per second)

Estimated annual load, in tons

Suspended 

sediment Arsenic Cadmium1 Copper Iron Lead Manganese Zinc

Water year 2004

12334550 872 14,100 6.40 0.0754 9.69 233 1.38 38.3 13.2

12340000 1,210 11,900 1.38 .0129 1.41 169 .232 14.8 1.87

Combined inflow2 2,080 26,000 7.78 .0883 11.1 402 1.61 53.1 15.1

12340500 2,040 22,400 6.66 .110 13.0 379 1.79 59.9 19.9

Mass balance: net gain (+) 

or loss (–)3

4

+40 +3,600 +1.12 –.022 –1.9 +23 –.18 –6.8 –4.8

Water year 2005

12334550 1,140 39,600 8.63 0.142 20.5 662 3.84 81.2 30.0

12340000 1,220 14,000 1.40 .0132 1.49 195 .265 16.3 1.97

Combined inflow2 2,360 53,600 10.0 .155 22.0 857 4.11 97.5 32.0

12340500 2,260 51,100 7.59 .148 18.5 783 3.11 87.3 28.5

Mass balance: net gain (+) 

or loss (–)3

4

+100 +2,500 +2.4 +.007 +3.5 +74 +1.00 +10.2 +3.5

Water year 2006

12334550 1,100 33,300 8.60 0.132 18.6 555 3.23 73.5 26.7

12340000 1,420 38,500 1.74 .0174 2.55 499 .659 34.3 3.28

Combined inflow2 2,520 71,800 10.3 .149 21.2 1,050 3.89 108 30.0

12340500 2,480 97,700 11.2 .255 32.7 1,300 5.57 135 52.9

Mass balance: net gain (+) 

or loss (–)3

4

+40 –25,900 –.9 –.106 –11.5 –250 –1.68 –27 –22.9

Water year 2007

12334550 1,200 35,500 9.01 0.140 19.7 592 3.45 78.0 28.4

12340000 1,310 23,000 1.64 .0160 2.03 312 .420 24.2 2.66

Combined inflow2 2,510 58,500 10.6 .156 21.8 904 3.87 102 31.1

12340500 2,440 188,000 16.7 .453 58.8 1,880 9.67 189 105

Mass balance: net gain (+) 

or loss (–)3

4

+70 –130,000 –6.1 –.297 –37.0 –980 –5.80 –87 –74

1

 Estimated cadmium loads for Blackfoot River near Bonner (station 12340000) were based on a regression equation developed from a dataset having a large 

percentage of censored values (values reported as less than the minimum reporting level). Thus, estimated cadmium loads for this site have greater uncertainty 

than loads estimated for the other two sites.

2

 Combined inflow is the sum of annual streamflow or load transported to Milltown Reservoir from Clark Fork at Turah Bridge, near Bonner 

(station 12334550), and Blackfoot River near Bonner (station 12340000).

3

 Mass balance is the difference (net gain or loss) between the annual combined inflow to Milltown Reservoir and the annual streamflow or load transported 

from Milltown Reservoir. Thus, a net gain (+) indicates net deposition in Milltown Reservoir and a net loss (–) indicates net removal from Milltown Reservoir.

4

 The annual mean streamflow for Clark Fork above Missoula (station 12340500) is less than the combined inflow for all years of the study period, which 

results in a calculated annual net gain of water in Milltown Reservoir. Rather than an actual gain of water, this calculated difference likely 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.

Estimated Loads Transported through Milltown Reservoir 19
20 Estimated loads transported through Milltown Reservoir, Montana, water years 2004-07

values of mass balance preceded by a “+”). If more material is 

transported annually from the reservoir than entered, previously deposited sediment (and associated trace elements) 

within the reservoir was eroded and put into suspension, added 

to the suspended material entering the reservoir from upstream 

sources, and then transported with the streamflow out of the 

reservoir (net loss, indicated by values of mass balance preceded by a “-“).

In water year 2004, there was an annual net gain of 

3,600 tons of suspended sediment in Milltown Reservoir 

(table 6), which represented about 14 percent of the combined 

inflow to the reservoir. The net deposition of suspended sediment was possibly the result of below-normal flows during 

much of the year and a relatively small annual peak flow 

(fig. 2). Annual net gains also occurred in water year 2004 

for arsenic (1.12 tons) and iron (23 tons), representing about 

14 percent and 6 percent of the combined inflow of arsenic 

and iron, respectively, to the reservoir. In contrast, annual 

net losses from the reservoir occurred in water year 2004 for 

cadmium (-0.022 ton), copper (-1.9 tons), lead (-0.18 ton), 

manganese (-6.8 tons), and zinc (-4.8 tons). These losses, in 

percent of combined inflow to the reservoir, represented about 

25 percent for cadmium, 17 percent for copper, 11 percent for 

lead, 13 percent for manganese, and 32 percent for zinc. The 

inconsistent net gains and losses of constituent loads transported through the reservoir in water year 2004 might have 

been a result of the relatively small loads, which were the 

smallest of any year during the 2004–07 study period.

In water year 2005, there was an annual net gain of 

2,500 tons of suspended sediment in Milltown Reservoir 

(table 6). The net deposition of suspended sediment in water 

year 2005 was less than in water year 2004, and represented 

only about 5 percent of the combined inflow to the reservoir. 

Annual net gains also occurred in water year 2005 for all of 

the trace elements, including arsenic (2.4 tons), cadmium 

(0.007 ton), copper (3.5 tons), iron (74 tons), lead (1.00 ton), 

manganese (10.2 tons), and zinc (3.5 tons). These amounts of 

deposited trace elements, in percent of the combined inflow to 

the reservoir, represented about 24 percent for arsenic, 5 percent for cadmium, 16 percent for copper, 9 percent for iron, 

24 percent for lead, 10 percent for manganese, and 11 percent 

for zinc. Streamflows in water year 2005 were below normal 

during the spring and late summer and may have had limited 

capacity to scour reservoir sediment; however, the peak flows 

in May and June were near normal (fig. 2). Even though 

streamflow peaks were higher in water year 2005 than 2004, 

there were no extended periods of sediment or trace-element 

loss from the reservoir in water year 2005 (figs. 6 and 7). A 

possible factor contributing to the annual net deposition of 

suspended sediment and trace elements in water year 2005 

might have been the lack of a sustained drawdown of Milltown 

Reservoir and the associated erosion of reservoir sediment that 

can occur under such conditions.

In water year 2006, there was a relatively large annual 

net loss of 25,900 tons of suspended sediment from Milltown 

Reservoir (table 6), which represented about 36 percent of the 

combined inflow to the reservoir. Streamflows were above 

or near normal during the spring (fig. 2), although little or no 

net loss of sediment from the reservoir was evident during the 

high-flow period of April-May 2006 (fig. 6). The daily net loss 

of sediment persisted after the start of the Stage 1 drawdown 

on June 1, 2006 (fig. 6). The annual net loss of sediment from 

the reservoir in water year 2006 likely resulted from erosion 

of reservoir sediment during both the temporary drawdown in 

October–December 2005 and Stage 1 of the permanent drawdown that continued after June 1, 2006.

The relatively large net loss of sediment from Milltown 

Reservoir in water year 2006 resulted in an annual net loss for 

every trace element (table 6). Annual net losses occurred in 

water year 2006 for arsenic (-0.9 ton), cadmium (-0.106 ton), 

copper (-11.5 tons), iron (-250 tons), lead (-1.68 tons), manganese (-27 tons), and zinc (-22.9 tons). Iron had the largest 

annual net loss, in tons, of all the trace elements in water year 

2006, although the loss represented only about 24 percent of 

the combined inflow to the reservoir. The largest annual net 

losses for trace elements in water year 2006, in percent of 

combined inflow to the reservoir, occurred for cadmium, copper, lead, and zinc—about 71 percent for cadmium, 54 percent for copper, 43 percent for lead, and 76 percent for zinc. 

Annual net losses for the other trace elements, in percent of 

combined inflow, represented about 9 percent for arsenic and 

25 percent for manganese.

In water year 2007, there was a large annual net loss of 

130,000 tons of suspended sediment from Milltown Reservoir 

(table 6), which was more than double (about 222 percent) 

the combined inflow of suspended sediment. The net loss of 

suspended sediment from the reservoir in water year 2007 was 

the largest loss during the 2004–07 study period and was more 

than double the annual net losses of 52,000 and 55,000 tons, 

respectively, that occurred during the high-flow conditions of 

water years 1996–97 (Lambing, 1998). The large net loss of 

suspended sediment transported from the reservoir in water 

year 2007 is notable in that the annual load (188,000 tons) was 

substantially less than the annual loads transported in water 

years 1996 (317,000 tons) and 1997 (445,000 tons) at Clark 

Fork above Missoula (Lambing, 1998). The large net loss in 

water year 2007, when the Stage 1 drawdown was in effect 

for the entire year, indicates the effect of increased erosion 

of bottom and exposed sediment resulting from lower water 

levels. Consequently, much of the suspended-sediment load 

transported from Milltown Reservoir in water year 2007 was 

derived from eroded reservoir sediment, rather than upstream 

sources.

The large annual net loss of sediment from Milltown 

Reservoir in water year 2007 resulted in a large annual net loss 

for every trace element (table 6). Annual net losses occurred in 

water year 2007 for arsenic (-6.1 tons), cadmium (-0.297 ton), 

copper (-37.0 tons), iron (-980 tons), lead (-5.80 tons), manganese (-87 tons), and zinc (-74 tons). For every trace element, 

the annual net losses in water year 2007 were the largest of 

the 2004–07 study period. The annual net losses for copper, 

lead, and zinc in water year 2007 also were substantially larger 
Summary and Conclusions 21

(double or more) than the annual net losses estimated for those 

trace elements during water years 1996–97 (Lambing, 1998). 

Similar to the loads of suspended sediment, the annual loads 

of copper, lead, and zinc transported in water year 2007 were 

substantially less than those of water years 1996–97. Iron had 

the largest annual net loss, in tons, of all the trace elements in 

water year 2007; the loss represented about 108 percent of the 

combined inflow to the reservoir. The largest annual net losses 

of trace elements in water year 2007, in percent of the combined inflow to the reservoir, occurred for cadmium, copper, 

lead, and zinc—about 190 percent for cadmium, 170 percent 

for copper, 150 percent for lead, and 238 percent for zinc. 

Annual net losses for the other trace elements, in percent of 

the combined inflow to the reservoir, also were relatively large 

and represented about 58 percent for arsenic and 85 percent 

for manganese. The large annual net losses of trace elements 

in water year 2007 indicate that much of the trace-element 

load transported from Milltown Reservoir was derived from 

eroded reservoir sediment, rather than from upstream sources.

streamflows during the study period were only 47 to 83 percent of the long-term mean annual peak streamflow.

Variations in estimated daily suspended-sediment loads 

transported to and from Milltown Reservoir during water 

years 2004–07 generally coincided with variations in streamflow. For most of the period before June 1, 2006, differences 

between the daily suspended-sediment loads transported to 

and from the reservoir were minor or indicated small amounts 

of deposition. However, the outflow of suspended sediment 

from the reservoir was substantially greater than the combined 

inflow during temporary drawdowns of the reservoir in July–

August 2004 and October–December 2005. A prominent shift 

in the quantity of suspended sediment transported from the 

reservoir occurred after the start of the Stage 1 drawdown on 

June 1, 2006. During the entire interval of the Stage 1 drawdown (June 1, 2006–September 30, 2007) within the study 

period, the daily suspended-sediment load transported from 

the reservoir was consistently larger than the combined daily 

suspended-sediment load transported to the reservoir. This pattern persisted for all streamflow conditions and, at times, the 

loss of sediment from the reservoir was substantial.

The transport characteristics were generally similar for 

all the unfiltered-recoverable trace-element loads estimated 

for this study. Therefore, variations in estimated daily traceelement loads transported to and from Milltown Reservoir 

during water years 2004–07 are illustrated using copper as 

a representative example. Similar to suspended-sediment 

loads, daily copper loads transported to and from the reservoir 

generally coincided with variations in streamflow. For most of 

the period before June 1, 2006, the differences between daily 

copper loads transported to and from the reservoir were small. 

However, losses of copper from the reservoir were notable 

during the temporary drawdowns in July–August 2004 and 

October–December 2005 when losses of suspended sediment 

also occurred.

A prominent shift in the quantity of copper transported 

from Milltown Reservoir occurred after the start of the Stage 

1 drawdown on June 1, 2006. During the entire interval of the 

Stage 1 drawdown within the study period, the daily copper 

load transported from the reservoir was consistently larger 

than the combined daily copper load transported to the reservoir. This pattern persisted for all streamflow conditions and, 

at times, the loss of copper from the reservoir was substantial. 

The increased loss of copper from the reservoir after June 1, 

2006, paralleled the increased loss of suspended sediment 

during the same period. A similar pattern of increased loss 

of load from the reservoir during the Stage 1 drawdown was 

determined for all the trace elements.

Estimated annual loads of suspended sediment and trace 

elements were determined by summing the estimated daily 

loads transported past each of the three monitoring sites near 

Milltown Reservoir during water years 2004–07. The mass 

balance of annual loads was determined as the net difference 

in annual loads transported to and from Milltown Reservoir 

and was used to indicate whether a cumulative net gain or loss 

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 

construction of Milltown Dam in 1907. Daily and annual loads 

of suspended sediment and selected trace elements transported during water years 2004–07 were estimated for two 

sites upstream and one site downstream from the reservoir, in 

cooperation with the U.S. Environmental Protection Agency. 

The estimated loads were used to quantify annual net gains 

and losses (mass balance) of suspended sediment and trace 

elements within Milltown Reservoir before and after June 1, 

2006, which was the start of the first phase (Stage 1) of a permanent drawdown of the reservoir in preparation for removal 

of Milltown Dam.

Daily loads of suspended sediment were estimated for 

water years 2004–07 by using either high-frequency sampling 

as part of daily sediment monitoring or regression equations relating suspended-sediment discharge to streamflow. 

Daily loads of unfiltered-recoverable arsenic, cadmium, 

copper, iron, lead, manganese, and zinc were estimated by 

using regression equations relating trace-element discharge 

to suspended-sediment discharge. Regression equations 

were developed from data for periodic water-quality samples 

collected during water years 2004–07. The equations were 

applied to daily records of either streamflow or suspendedsediment discharge to produce estimated daily loads.

Streamflows during water years 2004–07 generally were 

below or near normal. Annual mean streamflows during the 

study period were only 70 to 86 percent of the long-term 

mean annual streamflow. Similarly, annual instantaneous peak 
22 Estimated loads transported through Milltown Reservoir, Montana, water years 2004-07

of suspended sediment and trace elements within the reservoir 

occurred during a given water year.

In water year 2004, there was an annual net gain of 

3,600 tons of suspended sediment in Milltown Reservoir, 

which represented about 14 percent of the combined inflow 

to the reservoir. Annual net gains also occurred in water year 

2004 for arsenic (1.12 tons) and iron (23 tons), representing about 14 percent and 6 percent of the combined inflow 

of arsenic and iron, respectively, to Milltown Reservoir. In 

contrast, there were annual net losses from the reservoir in 

water year 2004 for cadmium (-0.022 ton), copper (-1.9 tons), 

lead (-0.18 ton), manganese (-6.8 tons), and zinc (-4.8 tons). 

These net losses, in percent of combined inflow, represented 

about 25 percent for cadmium, 17 percent for copper, 11 percent for lead, 13 percent for manganese, and 32 percent for 

zinc. The inconsistent net gains and losses of constituent loads 

transported through the reservoir in water year 2004 might 

have been a result of the relatively small loads, which were the 

smallest of any year during the 2004–07 study period.

In water year 2005, there was an annual net gain of 

2,500 tons of suspended sediment in Milltown Reservoir. 

The net deposition of suspended sediment in water year 2005 

was less than in water year 2004, and represented only about 

5 percent of the combined inflow to the reservoir. Annual 

net gains also occurred in water year 2005 for all of the trace 

elements, including arsenic (2.4 tons), cadmium (0.007 ton), 

copper (3.5 tons), iron (74 tons), lead (1.00 ton), manganese 

(10.2 tons), and zinc (3.5 tons). The net deposition of trace 

elements, in percent of the combined inflow to the reservoir, represented about 24 percent for arsenic, 5 percent for 

cadmium, 16 percent for copper, 9 percent for iron, 24 percent 

for lead, 10 percent for manganese, and 11 percent for zinc. A 

possible factor contributing to the annual net deposition of suspended sediment and trace elements in water year 2005 might 

have been the lack of a sustained drawdown of the reservoir.

In water year 2006, there was a relatively large annual 

net loss of 25,900 tons of suspended sediment from Milltown Reservoir, which represented about 36 percent of the 

combined inflow to the reservoir. Streamflows were above or 

near normal during the spring, although little or no net loss of 

sediment from the reservoir was evident during the high-flow 

period of April-May 2006. The daily net loss of sediment persisted after the start of the Stage 1 drawdown on June 1, 2006. 

The annual net loss of sediment from the reservoir in water 

year 2006 likely resulted from erosion of reservoir sediment 

during both the temporary drawdown in October–December 

2005 and Stage 1 of the permanent drawdown that continued 

after June 1, 2006.

The relatively large net loss of sediment from Milltown 

Reservoir in water year 2006 resulted in an annual net loss 

for every trace element. Annual net losses occurred in water 

year 2006 for arsenic (-0.9 ton), cadmium (-0.106 ton), copper 

(-11.5 tons), iron (-250 tons), lead (-1.68 tons), manganese 

(-27 tons), and zinc (-22.9 tons). Iron had the largest annual 

net loss, in tons, of all the trace elements in water year 2006, 

although the loss represented only about 24 percent of the 

combined inflow to the reservoir. The largest annual net losses 

of trace elements in water year 2006, in percent of combined 

inflow to the reservoir, occurred for cadmium, copper, lead, 

and zinc—about 71 percent for cadmium, 54 percent for copper, 43 percent for lead, and 76 percent for zinc. For the other 

trace elements, the annual net losses, in percent of combined 

inflow, represented about 9 percent for arsenic and 25 percent 

for manganese.

In water year 2007, there was a large annual net loss of 

130,000 tons of suspended sediment from Milltown Reservoir, 

which was more than double (about 222 percent) the combined inflow of suspended sediment. The annual net loss of 

suspended sediment from Milltown Reservoir in water year 

2007 was the largest loss during the 2004–07 study period 

and was more than double the annual net losses of 52,000 and 

55,000 tons, respectively, that occurred during the high-flow 

conditions of water years 1996–97. The large loss of suspended sediment from Milltown Reservoir in water year 2007 

is notable in that the annual load transported (188,000 tons) 

was substantially less than the annual loads transported in 

water years 1996 (317,000 tons) and 1997 (445,000 tons) at 

Clark Fork above Missoula. The large net loss in water year 

2007, when the Stage 1 drawdown was in effect for the entire 

year, indicates the effect of increased erosion of bottom and 

exposed sediment resulting from lower water levels. Consequently, much of the suspended-sediment load transported 

from Milltown Reservoir in water year 2007 was derived from 

eroded reservoir sediment, rather than from upstream sources.

The large annual net loss of sediment from Milltown 

Reservoir in water year 2007 resulted in a large annual net loss 

for every trace element. Annual net losses occurred in water 

year 2007 for arsenic (-6.1 tons), cadmium (-0.297 ton), copper (-37.0 tons), iron (-980 tons), lead (-5.80 tons), manganese 

(-87 tons), and zinc (-74 tons). For every trace element, the 

annual net losses in water year 2007 were the largest of the 

2004–07 study period. The annual net losses of copper, lead, 

and zinc in water year 2007 also were substantially larger 

(double or more) than the annual net losses estimated for those 

trace elements during water years 1996–97. Similar to the 

loads of suspended sediment, the loads of copper, lead, and 

zinc transported in water year 2007 were substantially less 

than those of water years 1996–97. Iron had the largest net 

loss, in tons, of all the trace elements in water year 2007; the 

loss represented about 108 percent of the combined inflow to 

the reservoir. The largest annual net losses of trace-element 

load in water year 2007, in percent of the combined inflow 

to the reservoir, occurred for cadmium, copper, lead, and 

zinc—about 190 percent for cadmium, 170 percent for copper, 

150 percent for lead, and 238 percent for zinc. Annual net 

losses of load for the other trace elements, in percent of the 

combined inflow, also were generally large and represented 

about 58 percent for arsenic and 85 percent for manganese. 

The large annual net losses in water year 2007 indicate that 

much of the trace-element load transported from Milltown 

Reservoir was derived from eroded reservoir sediment, rather 

than from upstream sources.
References Cited 23

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Lambing and Sando—Estimated Loads of Suspended Sediment and Selected Trace Elements Transported

through Milltown Reservoir in the Upper Clark Fork Basin, Montana, Water Years 2004–07

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Scientific Investigations Report 2008–5080