Prepared in cooperation with the U.S. Army Corps of Engineers

Sediment Transport in the Lower Snake and 

Clearwater River Basins, Idaho and Washington, 2008–11

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2013–5083
Cover:

Background: Barge on Lower Granite Reservoir near Clarkston, Washington, transporting grain downstream to the Columbia River. Photograph by Rhonda 

Weakland, U.S. Geological Survey, February 1, 2010. 

From left to right

Inset 1: Sampling sediment during high streamflow conditions on the Selway River, Idaho. Photograph by Doug Ott, U.S. Geological Survey, May 8, 2008.

Inset 2: Suspended-sediment sample collected from the Snake River near Anatone, Washington. Photograph by Greg Clark, U.S. Geological Survey, May 6, 2008. 
Sediment Transport in the Lower Snake and 

Clearwater River Basins, Idaho and 

Washington, 2008–11

By Gregory M. Clark, Ryan L. Fosness, and Molly S. Wood

Prepared in cooperation with the U.S. Army Corps of Engineers

Scientific Investigation Report 2013–5083

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director 

U.S. Geological Survey, Reston, Virginia: 2013

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as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Clark, G.M., Fosness, R.L., and Wood, M.S., 2013, Sediment transport in the lower Snake and Clearwater River 

Basins, Idaho and Washington, 2008–11: U.S. Geological Survey Scientific Investigations Report 2013-5083, 56 p.
iii

Contents

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

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

Purpose and Scope .............................................................................................................................5

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

Salmon River Basin...............................................................................................................................5

Snake River Downstream of Hells Canyon Dam..............................................................................5

Grande Ronde River Basin ..................................................................................................................6

Clearwater River Basin........................................................................................................................6

Palouse River Basin..............................................................................................................................7

Methods of Data Collection and Analysis .................................................................................................7

Streamflow.............................................................................................................................................8

Suspended Sediment and Bedload ...................................................................................................8

Suspended Sediment ..................................................................................................................8

Suspended-Sediment Surrogates...........................................................................................11

Bedload........................................................................................................................................13

Sediment Transport in the Lower Snake and Clearwater River Basins..............................................14

Suspended-Sediment Transport.......................................................................................................16

Suspended Sediment Delivery to Lower Granite Reservoir...............................................28

Comparison with 1972–79 Study..............................................................................................28

Suspended-Sediment Surrogates....................................................................................................33

Bedload.................................................................................................................................................41

Summary........................................................................................................................................................52

Acknowledgments.......................................................................................................................................53

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

Figures

1. Map showing lower Snake, Salmon, and Clearwater River Basins, Idaho,

Oregon, and Washington.............................................................................................................3

2. Map showing land use and location of sediment-sampling stations in the lower

Snake, Salmon, and Clearwater River Basins, Idaho, Oregon, and Washington,

2008–11 ...........................................................................................................................................4

3. Hydrographs showing streamflows at which suspended-sediment and bedload

samples were collected from the Snake River near Anatone, Washington, and

Clearwater River at Spalding, Idaho, water years 2008–11.................................................10

4. Hydrographs showing daily mean streamflow in the Snake River near Anatone,

Washington and the Clearwater River at Spalding, Idaho, 2008–11, compared

with the 30-year mean for water years 1981–2010................................................................15

5. Suspended-sediment transport curves and 95-percent prediction intervals

representing best-fit regression equations for a log-log power fit of total

suspended sediment with streamflow in the Salmon River at Whitebird, Idaho,

Snake River near Anatone, Washington, South Fork Clearwater River at Stites,

Idaho, and Clearwater River at Spalding, Idaho....................................................................17

6. Suspended-sediment transport curves representing best-fit regression

equations for total suspended-sediment concentrations and total

suspended-sediment loads at sampling stations in the lower Snake and

Clearwater River Basins, Washington and Idaho .................................................................18

7. Boxplots showing estimated total suspended-sediment loads and 95-percent

confidence intervals for stations in the lower Snake and Clearwater River

Basins, Idaho and Washington, water years 2009–11..........................................................24

8. Graph showing mean annual yield of suspended sand and fines in the lower

Snake and Clearwater River Basins, Washington and Idaho, water years 2009–11 ......25

9. Flow charts showing estimated loads of total suspended sediment, total

suspended sand, and total suspended fines transported in the lower Snake and

Clearwater River Basins, water years 2009–11 .....................................................................26

10. Graph showing suspended-sediment loads delivered monthly to Lower Granite

Reservoir from the Snake and Clearwater Rivers, Washington and Idaho, March

2008–September 2011 ................................................................................................................29

11. Suspended-sediment transport curves for concentrations and loads in the

Snake River near Anatone, Washington, and Clearwater River at Spalding,

Idaho, for data collected during water years 1972–79 and 2008–11 ..................................30

12. Suspended-sediment transport curves for concentrations and loads for

suspended fines and suspended sands, Snake River near Anatone, Washington,

water years 1972–79 and 2008–11............................................................................................31

13. Graph showing estimated annual suspended sand and fine loads, Snake River

near Anatone, Washington, and Clearwater River at Spalding, Idaho, water

years 1972–79 and 2009–11 .......................................................................................................32

14. Best-fit surrogate models for the Snake River near Anatone, Washington, using

the 1.5MHz acoustic Doppler velocity meter and the Clearwater River at

Spalding, Idaho, using the 3MHz ADVM, water years 2010–11 ..........................................34

15. Graphs showing comparison of total monthly sediment load estimated using

acoustic backscatter and LOADEST models in the Snake River near Anatone,

Washington, and Clearwater River at Spalding, Idaho, water years 2009–11 .................36

16. Graph showing comparison of suspended-sediment concentrations estimated

using acoustic backscatter and a LOADEST model, Snake River near Anatone,

Washington, water year 2011 ...................................................................................................37
v

Figures

17. Graphs showing comparison of total suspended sediment, suspended sand, and

suspended fine concentrations estimated using acoustic backscatter and a

LOADEST model during spring snowmelt runoff at Snake River near Anatone,

Washington, June 2010..............................................................................................................39

18. Graph showing comparison of total suspended-sediment concentrations

estimated using acoustic backscatter and a LOADEST model during a

hydrologic event at Clearwater River at Spalding, Idaho, April and May 2010................41

19. Flow chart showing estimated bedload transported in the lower Snake and

Clearwater River Basins, Washington and Idaho, water years 2009–11 ..........................45

20. Bedload transport curves representing best-fit regression equations for

selected stations in the lower Snake and Clearwater River Basins, Washington

and Idaho .....................................................................................................................................46

21. Bedload transport curves based on samples collected from the Lochsa and

Selway Rivers near Lowell, Idaho, during water years 2009–11, and during 1994–97....49

22. Bedload transport curves comparing data collected at Snake River near

Anatone, Washington, during water years 1972–79 and 2008–11 ......................................51

23. Bedload transport curves comparing data collected at Clearwater River at

Spalding, Idaho, water years 1972–79 and 2008–11..............................................................52

Tables

1. U.S. Geological Survey streamgaging stations where sediment samples were

collected, lower Snake and Clearwater River Basins, Washington and Idaho,

water years 2008–11.....................................................................................................................7

2. Ranges of streamflow sampled and suspended-sediment concentrations in

samples collected from the lower Snake and Clearwater River Basins,

Washington and Idaho, water years 2008–11..........................................................................9

3. Regression coefficients and coefficients of determination for models used to

estimate concentrations and loads of total suspended sediment and

suspended-sediment size fractions at stations in the lower Snake and

Clearwater River Basins, Washington and Idaho .................................................................12

4. Estimated annual, total, and mean loads of suspended sediment, suspended

sand, and suspended fines for stations in the lower Snake and Clearwater River

Basins, Washington and Idaho, water years 2009–11..........................................................19

5. Summary of acoustic surrogate and LOADEST models used to evaluate

suspended-sediment concentrations at the Snake River near Anatone,

Washington, and Clearwater River at Spalding, Idaho. ......................................................33

6. Comparison of suspended-sediment loads estimated using acoustic backscatter

and LOADEST models in the Snake River near Anatone, Washington, and

Clearwater River at Spalding, Idaho........................................................................................35

7. Comparison of suspended-sediment loads during selected hydrologic events

calculated using acoustic backscatter and a LOADEST model in the Snake River

near Anatone, Washington, and Clearwater River at Spalding, Idaho..............................38

8. Bedload data from sampling stations in the lower Snake and Clearwater River

Basins, Washington and Idaho, water years 2008–11..........................................................42

9. Best-fit regression equations for bedload from sampling stations in the lower

Snake and Clearwater River Basins, Washington and Idaho, water years 2008–11 ......46

10. Particle-size distribution for bedload samples collected from stations in the

lower Snake and Clearwater River Basins, Washington and Idaho, water years

2008–11 .........................................................................................................................................47
vi

Conversion Factors and Datums

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2) 2.590 square kilometer (km2) 

Flow rate

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

Mass

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

ton per square mile per year 

[(ton/mi2)/yr]

 0.3502 megagram per square

kilometer year [(Mg/km2)/yr]

ton per day (ton/d) 0.9072 metric ton per day

ton per month (ton/m) 0.9072 metric ton per month

ton per year (ton/yr) 0.9072 metric ton per year

SI to Inch/Pound

Multiply By To obtain

Length

millimeter (mm) 0.03937 inch (in.)

Volume

liter (L) 0.2642 gallon (gal)

Mass

gram (g) 0.03527 ounce, avoirdupois (oz)

kilogram per day (kg/d) 2.205 pound per day (lb/d)

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L).

Datums

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NADV 88).

Horizontal coordinate information is referenced to North American Datum of 1983 (NAD83).

Altitude, as used in this report, refers to distance above the vertical datum. 
Sediment Transport in the Lower Snake and 

Clearwater River Basins, Idaho and 

Washington, 2008–11

By Gregory M. Clark, Ryan L. Fosness, and Molly S. Wood

Abstract

Sedimentation is an ongoing maintenance problem for 

reservoirs, limiting reservoir storage capacity and navigation. 

Because Lower Granite Reservoir in Washington is the most 

upstream of the four U.S. Army Corps of Engineers reservoirs 

on the lower Snake River, it receives and retains the largest 

amount of sediment. In 2008, in cooperation with the U.S. 

Army Corps of Engineers, the U.S. Geological Survey began 

a study to quantify sediment transport to Lower Granite 

Reservoir. Samples of suspended sediment and bedload were 

collected from streamgaging stations on the Snake River near 

Anatone, Washington, and the Clearwater River at Spalding, 

Idaho. Both streamgages were equipped with an acoustic 

Doppler velocity meter to evaluate the efficacy of acoustic 

backscatter for estimating suspended-sediment concentrations 

and transport. In 2009, sediment sampling was extended 

to 10 additional locations in tributary watersheds to help 

identify the dominant source areas for sediment delivery to 

Lower Granite Reservoir. Suspended-sediment samples were 

collected 9–15 times per year at each location to encompass 

a range of streamflow conditions and to capture significant 

hydrologic events such as peak snowmelt runoff and rain-onsnow. Bedload samples were collected at a subset of stations 

where the stream conditions were conducive for sampling, and 

when streamflow was sufficiently high for bedload transport. 

At most sampling locations, the concentration of 

suspended sediment varied by 3–5 orders of magnitude with 

concentrations directly correlated to streamflow. The largest 

median concentrations of suspended sediment (100 and 

94 mg/L) were in samples collected from stations on the 

Palouse River at Hooper, Washington, and the Salmon River 

at White Bird, Idaho, respectively. The smallest median 

concentrations were in samples collected from the Selway 

River near Lowell, Idaho (11 mg/L), the Lochsa River 

near Lowell, Idaho (11 mg/L), the Clearwater River at 

Orofino, Idaho (13 mg/L), and the Middle Fork Clearwater 

River at Kooskia, Idaho (15 mg/L). The largest measured 

concentrations of suspended sediment (3,300 and 1,400 mg/L) 

during a rain-on-snow event in January 2011 were from 

samples collected at the Potlatch River near Spalding, Idaho, 

and the Palouse River at Hooper, Washington, respectively. 

Generally, samples collected from agricultural watersheds had 

a high percentage of silt and clay-sized suspended sediment, 

whereas samples collected from forested watersheds had a 

high percentage of sand.

During water years 2009–11, Lower Granite Reservoir 

received about 10 million tons of suspended sediment from 

the combined loads of the Snake and Clearwater Rivers. 

The Snake River accounted for about 2.97 million tons per 

year (about 89 percent) of the total suspended sediment, 

1.48 million tons per year (about 90 percent) of the suspended 

sand, and about 1.52 million tons per year (87 percent) of the 

suspended silt and clay. Of the suspended sediment transported 

to Lower Granite Reservoir, the Salmon River accounted 

for about 51 percent of the total suspended sediment, about 

56 percent of the suspended sand, and about 44 percent of the 

suspended silt and clay. About 6.2 million tons (62 percent) 

of the sediment contributed to Lower Granite Reservoir 

during 2009–11 entered during water year 2011, which was 

characterized by an above average winter snowpack and 

sustained spring runoff.

A comparison of historical data collected from the Snake 

River near Anatone with data collected during this study 

indicates that concentrations of total suspended sediment 

and suspended sand in the Snake River were significantly 

smaller during water years 1972–79 than during 2008–11. 

Most of the increased sediment content in the Snake River 

is attributable to an increase of sand-size material. During 

1972–79, sand accounted for an average of 28 percent of the 

suspended-sediment load; during 2008–11, sand accounted for 

an average of 48 percent. Historical data from the Clearwater 

River at Spalding indicates that the concentrations of total 

suspended sediment collected during 1972–79 were not 

significantly different from the concentrations measured 

during this study. However, the suspended-sand concentrations 

in the Clearwater River were significantly smaller during 

1972–79 than during 2008–11. The increase in suspended‐sand 

concentrations in the Snake and Clearwater Rivers are 

probably attributable to numerous severe forest fires that 

burned large areas of central Idaho from 1980–2010.
2 Sediment Transport in the Lower Snake and Clearwater River Basins, Idaho and Washington, 2008–11

Acoustic backscatter from an acoustic Doppler velocity 

meter proved to be an effective method of estimating 

suspended-sediment concentration and load for most 

streamflow conditions in the Snake and Clearwater Rivers. 

Models based on acoustic backscatter were able to simulate 

most of the variability in suspended-sediment concentrations 

in the Clearwater River at Spalding (coefficient of 

determination [R2]=0.93) and the Snake River near Anatone 

(R2=0.92). Acoustic backscatter seems to be especially 

effective for estimating suspended-sediment concentration 

and load over short (monthly and single storm event) and long 

(annual) time scales when sediment load is highly variable. 

However, during high streamflow events acoustic surrogate 

tools may be unable to capture the contribution of suspended 

sand moving near the bottom of the water column and thus, 

underestimate the total load of suspended sediment. 

At the stations where bedload was collected, the 

particle‐size distribution at low streamflows typically was 

unimodal with sand comprising the dominant particle size. 

At higher streamflows and during peak bedload discharge, 

the particle size typically was bimodal and was comprised 

primarily of sand and coarse gravel. About 55,000 tons of 

bedload was discharged from the Snake River to Lower 

Granite Reservoir during water years 2009–11, about 

0.62 percent of the total sediment load delivered by the Snake 

River. About 9,500 tons of bedload was discharged from the 

Clearwater River to Lower Granite Reservoir during 2009–11, 

about 0.83 percent of the total sediment load discharged by the 

Clearwater River during 2009–11.

Introduction

Since construction of the first dam on the lower Snake 

River, the U.S. Army Corps of Engineers (USACE) has 

recognized that managing sediment is an ongoing maintenance 

issue in the reservoirs on the lower Snake River. Historically, 

the USACE has used dredging to manage accumulated 

sediment and to maintain a sufficient navigational channel. 

Recently, however, the USACE determined that managing 

sediment in the upstream watersheds might be a more effective 

approach to reducing sediment accumulation in the reservoirs. 

Although the USACE does not have the authority to manage 

land outside of the reservoir project boundaries, they can 

identify and evaluate management strategies that could be 

implemented on non-USACE property to reduce sediment 

mobilization and transport (Tetra Tech, 2006).

Because Lower Granite Reservoir is the farthest upstream 

reservoir on the lower Snake River, it receives and retains 

the largest amount of sediment. Based on extensive sediment 

range surveys conducted by the USACE, Lower Granite 

Reservoir contains an estimated 75 million cubic yards of 

sediment, with average annual sediment inputs of about 

2.3 million cubic yards since impoundment in 1975 (Teasdale, 

2010). Because of the accumulated sediment, water depths 

in the navigation channel near the Ports of Lewiston, Idaho, 

and Clarkston, Washington, at times are less than the 14-ft 

authorized depth and in some places are as shallow as 8 ft. 

Because of this reduction in the depths of the channel and 

the port berthing areas, some port facilities have been forced 

to operate at reduced capacity. Sedimentation has caused 

increased safety risks for the shipping industry; increased 

risks of grounding and damage to equipment; and decreased 

efficiencies due to modified approach, loading, and unloading 

procedures (U.S. Army Corps of Engineers, 2005).

In addition to these navigation issues, deposition of 

sediment and reduced channel capacity at the confluence of the 

Snake and Clearwater Rivers have reduced the effectiveness 

of the levee system that protects the cities of Lewiston and 

Clarkston from flooding. If allowed to continue, sedimentation 

may reduce the flow capacity to a point that the Standard 

Project Flood, an estimated or hypothetical flood that may be 

expected from the most severe combination of weather and 

flow conditions that are considered reasonably characteristic 

of the geographical area, may overtop the levees in Lewiston 

(U.S. Army Corps of Engineers, 2002).

The USACE asked the U.S. Geological Survey (USGS) 

to measure sediment transport in the Snake and Clearwater 

Rivers during construction and following the completion 

of Lower Granite Dam in 1972. The USGS collected data 

from the Snake River near Anatone, Washington, and the 

Clearwater River at Spalding, Idaho, from 1972 through 1979. 

From these data, the USGS developed sediment rating curves 

to estimate potential sediment transport and deposition to 

Lower Granite Reservoir (Jones and Seitz, 1980). Suspended 

silt and sand dominated the sediment load entering the 

reservoir, whereas bedload accounted for only about 5 percent. 

Based on the calculations by Jones and Seitz (1980), the 

Snake River delivered, on average, about 1.8 million tons per 

year (80 percent) of the total sediment load entering Lower 

Granite Reservoir; the Clearwater River accounted for about 

0.47 million tons per year (about 20 percent).

In 2005, the USACE published a Notice of Intent (U.S. 

Army Corps of Engineers, 2005) stating plans to prepare 

an Environmental Impact Statement for a Programmatic 

Sediment Management Plan (PSMP) to address sediment 

management within the four lower Snake River reservoirs. 

The intent of the PSMP was to identify ways to reduce the 

amount of sediment entering the reservoirs, how to manage the 

sediment once it enters the reservoirs, and possible changes 

to structures or operations that could reduce maintenance yet 

maintain navigational access to the ports of Lewiston and 

Clarkston. By using various sediment management measures, 

the PSMP will become a comprehensive, watershed-level 

framework for managing sediment movement and deposition 

while maintaining current uses such as commercial navigation, 

irrigation withdrawals, recreation, and flow conveyance.
Introduction 3

In March 2008, as part of its PSMP, the USACE asked 

the USGS to start a second sediment-sampling program in 

the lower Snake and Clearwater River Basins (fig. 1) and 

to evaluate sediment depositional characteristics in Lower 

Granite Reservoir. In addition to conventional sampling 

for suspended sediment and bedload, the USGS equipped 

streamgages on the Snake River near Anatone, Washington 

(USGS streamgage 13334300), and the Clearwater River at 

Spalding, Idaho (streamgage 13342500) (fig. 2) with acoustic 

Doppler velocity meters (ADVMs) and other instruments 

to evaluate whether surrogate technologies for measuring 

suspended-sediment concentration could adequately estimate 

suspended-sediment transport to Lower Granite Reservoir. In 

addition to the data described in this report, the USACE also 

contracted the USGS to evaluate sediment deposition within 

Lower Granite Reservoir (Williams and others, 2012; Braun 

and others, 2012). 

Lower

Granite

Dam

Salmon River Basin

Lower

Snake River

Basin

Clearwater River Basin

Extent of Lower

Granite Reservoir

Troy

Asotin

Salmon

Lowell Anatone

Zindel

Stanley

Riggins

Spalding

Lewiston

Harpster

White

Bird

Hooper

Orofino

Kooskia

Stites

Salmon River

Snake River

Palouse River

Clearwater

Grande Ronde River

River

Lochsa River

Imnaha River

Lemhi River

Cow Creek

 Union Flat

Creek

Pahsimeroi River

Wallowa River

Lolo Creek

Potlach River

Little Salmon

 River

Rebel Flat Creek

Middle Fork

South Fork

South Fork

Middle Fork

Lapwai

Paradise Creek

Salmon River

Salmon River

North Fork

River

Clearwater River

Selway

Creek

Clearwater River

South

Fork

Palouse River

Clearwater

Snake River

Salmon River

Salmon River

Snake River

 River

Lower Granite

 Reservoir

Washington

Dworshak

Dam Bitterroot Mountains

Blue Mountains

Wallowa Mountains

Hells Canyon

Oregon

Hells Canyon 

Dam

Idaho

0 20 40 60 80 MILES

0 20 40 60 80 KILOMETERS

Base modified from U.S. Geological Survey National

Elevation Dataset, 90 meter, and National

Hydrogphay Dataset, 1:2,000,000.

118°

117°

116°

115°

114°

113°

47°

46°

45°

44°

119°

UTAH

MONTANA

NEVADA

IDAHO

OREGON

WYOMING

WASHINGTON

EXPLANATION

Basin Boundary

Figure 1. Lower Snake, Salmon, and Clearwater River Basins, Idaho, Oregon, and Washington.
4 Sediment Transport in the Lower Snake and Clearwater River Basins, Idaho and Washington, 2008–11

tac12-0737_fig02

13334000

13340000

13351000

13342500

13341570

13338500

13338100

13337120

13337000

13334300

13317000

13336500

Salmon River Basin

Lower

Snake River

Basin

Clearwater River Basin

0 20 40 60 80 MILES

0 20 40 60 80 KILOMETERS

Base modified from U.S. Geological Survey

National Hydrogphay Dataset, 1:2,000,000.

118°

117°

114°

115°

116°

113°

47°

45°

46°

44°

119°

EXPLANATION

National Land Cover Database (2006)

Water, Ice/Snow

Developed

Barren Land

Forest

Shrub/Scrub

Grassland/Herbaceous

Pasture/Cultivated Crops

Wetlands

Basin Boundary

U.S. Geological Survey gaging station location and number

Figure 2. Land use and location of sediment-sampling stations in the lower Snake, Salmon, and Clearwater River Basins, Idaho, 

Oregon, and Washington, 2008–11.
Description of Study Area 5

Purpose and Scope 

This report documents findings based on sediment data 

collected by the USGS in the lower Snake and Clearwater 

River Basins from 2008 through 2011. The purpose of this 

report is to provide information that improves the scientific 

understanding of the processes affecting sediment generation 

in the lower Snake and Clearwater River Basins and river 

hydrodynamics controlling sediment transport, deposition, 

and retention in Lower Granite Reservoir and downstream 

in the Snake River. The report provides suspended sediment 

and bedload data to (1) identify the dominant subbasins 

contributing sediment to Lower Granite Reservoir, 

(2) evaluate sediment transport in the Clearwater and lower 

Snake River Basins, and (3) quantify transport of sediment to 

Lower Granite Reservoir. Additionally, data collected using 

acoustic Doppler velocity meters (ADVMs) is summarized 

and compared with conventionally acquired sediment data 

to evaluate the efficacy of estimating suspended-sediment 

concentration (SSC) continuously by means of acoustic 

surrogates.

Description of Study Area

The drainage basin contributing sediment to Lower 

Granite Reservoir comprises an area of about 27,000 mi2

that includes the Snake River Basin from Hells Canyon Dam 

downstream to the confluence with the Clearwater River 

and the Clearwater River Basin (fig. 1). The three dams 

constituting the Hells Canyon Complex (completed in 1967) 

on the Snake River effectively trap sediment transported from 

areas upstream of the dams (Parkinson and others, 2003), 

as does Dworshak Dam (completed in 1972) on the North 

Fork Clearwater River. The Hells Canyon Complex and 

Dworshak Dam effectively eliminate about 81 and 25 percent, 

respectively, of the total drainage basin in the Snake and 

Clearwater Rivers from contributing sediment to Lower 

Granite Reservoir. The Salmon River Basin (drainage area 

of about 14,000 mi2) is about 71 percent of the remaining 

area in the Snake River Basin downstream of Hells Canyon 

Dam and about 52 percent of the total combined area of the 

Snake and Clearwater River Basins contributing sediment 

to Lower Granite Reservoir. Other contributing basins are 

the Clearwater (about 7,200 mi2 or 27 percent of the total 

area), the Grande Ronde (about 3,950 mi2 or 15 percent), and 

the Snake River Basin downstream of Hells Canyon Dam 

to Lower Granite Reservoir, but excluding the Salmon and 

Grande Ronde Basins (about 1,850 mi2 or 7 percent). The 

Palouse River has a drainage basin of about 3,280 mi2, and is a 

tributary to the Snake River about 48 mi downstream of Lower 

Granite Dam.

Salmon River Basin

The Salmon River is an unregulated, free-flowing river 

that originates in mountain ranges in Idaho and western 

Montana and flows about 410 mi through central Idaho to 

its confluence with the Snake River in lower Hells Canyon 

(fig. 1). The Salmon River derives its streamflow from several 

tributaries including the Lemhi, Pahsimeroi, Middle Fork 

Salmon, South Fork Salmon, and Little Salmon Rivers. Peak 

flows in the Salmon River generally occur in May and June 

during snowmelt runoff. Between 1975 and 2010, the Salmon 

River, on average, discharged about 10,700 ft3/s to the Snake 

River. The Salmon River contributes about 32 percent of the 

mean streamflow measured at the Snake River near Anatone 

(streamgage 13334300 [fig. 2]) and about 22 percent of the 

combined streamflow entering Lower Granite Reservoir 

from the Snake and Clearwater Rivers (U.S. Geological 

Survey, 2013).

About 90 percent of the Salmon River Basin is comprised 

of Federal lands, about 77 percent national forest managed 

by the U.S. Forest Service, and 13 percent managed by the 

Bureau of Land Management. Nearly 80 percent of the land 

cover is forest in the Salmon River Basin (fig. 2); agricultural 

and urban areas combined account for less than 3 percent 

(Tetra Tech, 2006). Key geologic features in the Salmon River 

Basin are the Idaho Batholith and Challis volcanics that tend 

to produce coarse, sandy soils that are highly erodible when 

weathered (King and others, 2004). The erodible geology, 

steep topography, and lack of hydrologic control structures 

combine to mobilize large quantities of sediment from the 

Salmon River Basin to downstream waters. Numerous forest 

fires that burned large areas of the basin between 1980 and 

2010 have further worsened the susceptibility of the Salmon 

River Basin to erosion.

Snake River Downstream of Hells Canyon Dam

The Snake River in the Hells Canyon reach incorporates 

drainages upstream of Lower Granite Reservoir and 

downstream of Hells Canyon Dam, exclusive of the Salmon 

and Grande Ronde River Basins (fig. 1). The supply of 

sediment to this reach of the Snake River is limited by 

upstream trapping in the Hells Canyon Complex of dams that 

was completed in 1967. Although sediment delivery to the 

Snake River in the Hells Canyon reach is limited, streamflow 

is generated from the entire 93,500-mi2 drainage basin, 

including the area upstream of the Hells Canyon complex. In 

the 110-mi reach of the Snake River between Hells Canyon 

Dam and Lower Granite Reservoir, the Snake River flows 

primarily north, forming first the border between Idaho and 

Oregon, and then the border between Idaho and Washington. 
6 Sediment Transport in the Lower Snake and Clearwater River Basins, Idaho and Washington, 2008–11

The primary tributary in this reach, excluding the Salmon and 

the Grande Ronde Rivers, is the Imnaha River which enters 

from the west about 55 mi downstream of Hells Canyon Dam. 

Between Hells Canyon Dam and the Imnaha, the Snake River 

runs through a deep and narrow v-shaped valley entrenched in 

erosion-resistant basalt and metamorphic bedrock (Parkinson 

and others, 2003). About 22 percent of the 1,850 mi2 area 

drained by the Snake River in the Hells Canyon reach is 

comprised of agricultural land, primarily in the lower parts 

of the drainage basin downstream of the confluence with 

the Imnaha. Forested land makes up about 47 percent of the 

drainage basin (fig. 2) (Tetra Tech, 2006). Sediment source 

studies in the Hells Canyon reach indicate that sediments 

transported in the Snake River are primarily coarse grained, 

gravel-sized materials derived from local landslides, talus 

slopes, and tributary inflows (Miller and others, 2003).

Grande Ronde River Basin

The Grande Ronde River flows about 180 mi in a 

northeasterly direction draining streams in the Blue and 

Wallowa Mountains of northeastern Oregon. Most of the 

Grande Ronde drainage basin is in Oregon with a small part 

in southeastern Washington. A major tributary to the Grande 

Ronde River is the Wallowa River. The Grande Ronde River 

flows into the Snake River about 23 mi upstream of the town 

of Asotin, Washington, and just upstream of the sampling 

location on the Snake River near Anatone (fig. 2). Because 

most of the drainage basin in the lower Grande Ronde is at an 

altitude less than 3,000 ft, runoff typically starts in April and 

lasts until late June when snowmelt occurs in higher elevation 

areas of the basin. 

About 70 percent of the Grande Ronde Basin is forested 

and about 17 percent is agricultural land (fig. 2). About 

50 percent of the basin is privately owned; most of these 

private lands lie within stream valleys along the Grande 

Ronde and Wallowa Rivers. Private lands primarily are used 

for agriculture, grazing, and forestry (Tetra Tech, 2006). The 

U.S. Forest Service manages about 47 percent of the land 

within the Grande Ronde Basin for multiple uses including 

timber production, livestock grazing, and recreation. The 

surface geology in the Grande Ronde Basin is primarily basalt 

from the Columbia River Group with a highly variable soil 

erosional capacity. In contrast to the Salmon River Basin, 

where most of the sediment is derived from natural processes, 

land-use practices probably account for a large part, if not 

most of the sediment production in the Grande Ronde Basin. 

Clearwater River Basin

The Clearwater River originates in the Bitterroot 

Mountains at the border of Idaho and Montana and flows 

westward to its confluence with the Snake River at Lewiston, 

Idaho (fig. 1). Major tributaries to the Clearwater River 

include the Lochsa and Selway Rivers, the North and South 

Forks of the Clearwater River, and the Potlatch River. 

Dworshak Dam effectively traps most of the sediment 

transported from the North Fork Clearwater River drainage 

basin prior to reaching the main stem Clearwater River. 

From 1975 through 2010, the Clearwater River contributed 

about 30 percent of the streamflow (as measured at Spalding) 

entering Lower Granite Reservoir. Similar to the Salmon 

River, streamflow in the Clearwater River typically peaks in 

May and June in response to snowmelt runoff. Highly erosive 

igneous rocks underlay a large part of the Clearwater River 

Basin (King and others, 2004). As a result, much of the basin 

is highly susceptible to erosion and subsequent sediment 

transport. Overall, cropland and pastureland make up about 

18 percent of the Clearwater River Basin (Tetra Tech, 2006). 

The intensity of agricultural activity generally increases in a 

downstream or westerly direction (fig. 2).

The Lochsa and Selway Rivers combined drain about 

46 percent of the Clearwater River Basin, draining areas that 

are essentially 100 percent forested; the U.S Forest Service 

manages more than 95 percent of the land (Tetra Tech, 2006). 

Underlain by the Idaho Batholith, the Lochsa and Selway 

drainage basins are characterized by rock that weathers deeply 

to produce coarse, sandy soils with high erosion rates if 

disturbed (King and others, 2004).

The Potlatch River is the largest tributary (drainage basin 

of about 550 mi2) to the lower Clearwater River Basin. The 

Potlatch River is a tributary to the Clearwater River about 

15 mi upstream of Lower Granite Reservoir (fig. 1). About 

57 percent of the Potlatch River drainage basin is forested, 

mostly in the northern upstream areas. Primary land-uses 

in the forested areas include timber harvest and other forest 

management practices. The downstream part of the drainage 

basin is predominantly agricultural (about 43 percent of the 

total basin area), used primarily for dryland agriculture and 

grazing (Latah Soil and Water Conservation District, 2007). 

Land-use activities in the drainage basin have resulted in 

changes in the vegetative cover, increases in soil compaction, 

and channel modifications that have resulted in a flashy 

hydrograph and rapid streamflow runoff (Latah Soil and Water 

Conservation District, 2007). Instantaneous streamflows of 

8,000 ft3/s in winter and early spring are not unusual.
Methods of Data Collection and Analysis 7

Palouse River Basin

The Palouse River drains about 3,300 mi2 of southeastern 

Washington and parts of the northern Idaho panhandle. The 

headwaters of the Palouse River originate in the forested 

mountains of northwestern Idaho; the river flows westward 

through farmland to its confluence with the Snake River about 

48 mi downstream of Lower Granite Dam (fig. 1). Major 

tributaries to the Palouse River are the South Fork Palouse 

River and Paradise, Rebel Flat, Rock, Union Flat, and Cow 

Creeks. Activities that affect water quality in the Palouse River 

Basin include dryland agriculture (67 percent of the drainage 

basin), rangeland (26 percent), timber harvest, mining, and 

urban development (Washington State Department of Ecology, 

2006). Irrigated farmland adjacent to the Palouse River and 

its tributaries comprises less than 1 percent of the land use. 

Forested land comprises about 6 percent of the drainage 

basin, primarily in the upland northern and eastern parts 

(fig. 2). Agricultural fields throughout the basin are highly 

susceptible to soil erosion from November through March 

when high intensity rainstorms can cause intensive runoff and 

soil erosion. These winter storms can deliver large quantities 

of sediment to streams throughout the Palouse River drainage 

basin (Ebbert and Roe, 1998).

Methods of Data Collection 

and Analysis

Data were collected for this study from March 2008 

through September 2011 to evaluate sediment transport in 

the lower Snake and Clearwater River Basins and sediment 

deposition in Lower Granite Reservoir. Data collection 

included measurement of streamflow, conventional sampling 

for suspended sediment and bedload at 12 sampling stations 

in the lower Snake and Clearwater River Basins, and acoustic 

surrogate sampling at two USGS streamgages: the Snake 

River near Anatone, Washington and the Clearwater River 

at Spalding, Idaho (table 1). The methods and results of 

depositional surveys conducted in Lower Granite Reservoir 

are described in Williams and others (2012) and Braun 

and others (2012), respectively. This section of the report 

documents the methods used for data collection and analysis 

of streamflow and suspended sediment and bedload. 

Table 1. U.S. Geological Survey streamgaging stations where sediment samples were collected, lower Snake and Clearwater River 

Basins, Washington and Idaho, water years 2008–11.

[Locations of stations are shown in figure 2. Effective drainage area does not include watershed area upstream of Dworshak and Hells Canyon Dams. 

Abbreviations: USGS, U.S. Geological Survey; mi2, square mile]

USGS 

gaging station 

No.

Gaging station name

Effective 

drainage area 

(mi2

)

Sampling 

period

Type of 

streamflow 

record

Number of 

suspendedsediment 

samples

Number of 

bedload 

samples

13317000 Salmon River at White Bird, Idaho 13,500 03-2009–07-2011 Continuous 42 5

13334000 Grande Ronde River at Zindel, Washington 3,940 03-2009–07-2011 Indexed 38 0

13334300 Snake River near Anatone, Washington1 19,700 04-2008–07-2011 Continuous 39 16

13336500 Selway River near Lowell, Idaho 1,910 03-2009–07-2011 Continuous 37 11

13337000 Lochsa River near Lowell, Idaho 1,180 03-2009–07-2011 Continuous 36 3

13337120 Middle Fork Clearwater River at Kooskia, Idaho 5,490 03-2009–07-2011 Indexed 38 11

13338100 South Fork Clearwater River near Harpster, Idaho 865 03-2009–07-2011 Indexed 39 15

13338500 South Fork Clearwater River at Stites, Idaho 1,150 03-2009–07-2011 Continuous 41 14

13340000 Clearwater River at Orofino, Idaho 5,580 03-2009–07-2011 Continuous 36 7

13341570 Potlatch River below Little Potlatch Creek near 

Spalding, Idaho

583 03-2009–07-2011 Continuous 37 0

13342500 Clearwater River at Spalding, Idaho1 7,140 03-2008–07-2011 Continuous 40 11

13351000 Palouse River at Hooper, Washington 2,500 03-2009–07-2011 Continuous 33 0

1

 Station was equipped with suspended sediment surrogate technology.
8 Sediment Transport in the Lower Snake and Clearwater River Basins, Idaho and Washington, 2008–11

Streamflow

Streamflow measurements at all of the sediment sampling 

stations were obtained using standard USGS methods 

(Mueller and Wagner, 2009; Turnipseed and Sauer, 2010). 

At 9 of the 12 sampling stations, streamflow was measured 

at an established streamgage using a continuous record of 

water stage calibrated to periodic onsite measurements of 

streamflow. For the three sampling stations without continuous 

streamflow record (table 1), streamflow was measured when 

sediment samples were collected, and then indexed to the 

streamflow at a nearby streamgage (or group of streamgages) 

with continuous data. The correlation between streamflow 

at the non-continuous sampling station and the continuous 

streamgage(s) was used to generate a daily mean streamflow 

record for the non-continuous sampling station over the 

period of this study. For example, instantaneous streamflow 

measurements made during sediment sample collection 

at the non-continuous sampling station on the South Fork 

Clearwater River near Harpster, Idaho (USGS station 

13338100) from March 2009 through July 31, 2011, were 

indexed to the streamflow recorded simultaneously at the 

continuous streamgage on the South Fork Clearwater River 

at Stites, Idaho (USGS streamgage 13338500) (R2=0.996). 

The resultant relation was used to estimate the daily mean 

streamflow at the South Fork Clearwater River near Harpster 

from the daily mean streamflow at South Fork Clearwater 

River at Stites for the period of study. The same methodology 

was used for two other non-continuous sampling stations; 

the Grande Ronde River at Zindel, Washington (USGS 

station 13334000) was indexed to an upstream streamgage 

with continuous record on the Grande Ronde River at Troy, 

Oregon (R2=0.976), and the Middle Fork Clearwater River 

at Kooskia, Idaho (USGS station 13337120) was indexed to 

the combined streamflow from the continuous streamgage 

on the Selway River near Lowell, Idaho (USGS streamgage 

13336500) and the Lochsa River near Lowell, Idaho (USGS 

streamgage 13337000) (R2=0.992).

Suspended Sediment and Bedload

Streams transport sediment by carrying the finer particles 

in suspension with turbulent eddies and by rolling or skipping 

coarser particles along the streambed. The discharge of 

fine-grained particles typically is controlled by the available 

supply of fine-grained sediment. These fine-grained sediments 

generally move downstream in suspension at about the same 

velocity as the water. The sediment that moves on or near 

the stream bottom by sliding, rolling, or bouncing is bedload 

(Edwards and Glysson, 1999), which increases exponentially 

with increasing streamflow. The total sediment load in a 

stream is defined as the sum of the suspended-sediment load 

plus the bedload. Because the particle size distribution of 

the suspended load is a function of streamflow, substantial 

variation typically occurs in the concentration and grain-size 

characteristics of sediments both spatially at different locations 

in a stream and temporally with changes in the magnitude 

of streamflow.

For this study, suspended sediment and bedload sampling 

started in spring 2008 at the Snake River near Anatone and 

the Clearwater River at Spalding streamgages. In spring 

2009, sample collection extended to 10 additional stations for 

suspended sediment and 7 stations for bedload to help quantify 

the sediment contributions from discrete subbasins in the 

lower Snake and Clearwater River Basins (table 1). Sample 

collection at all the stations continued through July 2011, 

following the end of the spring runoff. All sediment samples 

were collected using standard USGS methods, procedures, and 

equipment as documented by Edwards and Glysson (1999).

Suspended Sediment

Suspended-sediment samples were collected 9–15 times 

per year at each of the study stations to encompass a range 

of streamflow conditions (table 2). As illustrated in figure 3

at the Snake River near Anatone and the Clearwater River at 

Spalding, targeted suspended-sediment samples were collected 

during the ascending limb, the peak, and the descending 

limb of the snowmelt runoff hydrograph at each station 

to determine the effects of hysteresis on SSC. Significant 

hydrologic events such as a rain-on-snow event that occurred 

during January 2011 were targeted for additional sampling. 

Suspended-sediment samples were collected using 

equal-width and depth-integrating techniques. Isokinetic D-96 

collapsible-bag samplers were suspended from either a bridge 

or cableway to obtain the samples. Use of the D-96 allows for 

isokinetic sampling over a wide range of stream depths and 

velocities (Davis, 2005). For collection of suspended‐sediment 

samples, the total stream width at each station was divided into 

10 equal-width increments, and individual depth-integrated 

samples were collected at the centroid of each increment. 
Methods of Data Collection and Analysis 9

Table 2. Ranges of streamflow sampled and suspended-sediment concentrations in samples collected from the lower Snake and 

Clearwater River Basins, Washington and Idaho, water years 2008–11.

[Locations of stations are shown in figure 2. Abbreviations: USGS, U.S. Geological Survey; ft3/s, cubic foot per second; mg/L, milligram per liter; 

μm, micrometer]

USGS gaging 

station 

No.

Gaging station name

Streamflow range 

when samples 

collected

(ft3

/s)

Median 

suspendedsediment 

concentration 

(mg/L)

Range of 

suspendedsediment 

concentration 

(mg/L)

Median 

suspendedsediment 

fraction less 

than 62.5 μm 

(percent)

Range 

suspendedsediment 

concentration 

less than 

62.5 μm 

(percent)

13317000 Salmon River at White Bird, Idaho 3,970–83,900 94 3.0–600 51 14–97

13334000 Grande Ronde River at Zindel, Washington 766–25,000 30 3.0–570 84 65–97

13334300 Snake River near Anatone, Washington 14,900–155,000 55 5.0–410 62 32–94

13336500 Selway River near Lowell, Idaho 651–32,500 11 1.0–180 42 15–89

13337000 Lochsa River near Lowell, Idaho 434–21,200 11 1.0–87 51 23–84

13337120 Middle Fork Clearwater River at Kooskia, Idaho 1,140–28,000 15 1.0–99 49 23–94

13338100 South Fork Clearwater River near Harpster, Idaho 194–11,000 26 1.0–200 61 28–90

13338500 South Fork Clearwater River at Stites, Idaho 213–12,400 24 2.0–530 62 39–94

13340000 Clearwater River at Orofino, Idaho 1,580–69,500 13 2.0–240 66 29–91

13341570 Potlatch River below Little Potlatch Creek near 

Spalding, Idaho

15.0–17,000 42 3.0–3,300 92 68–99

13342500 Clearwater River at Spalding, Idaho 3,390–78,900 21 3.0–210 70 30–96

13351000 Palouse River at Hooper, Washington 48.0–7,150 100 12–1,400 95 48–99

Individual samples from each centroid were composited in 

3-L bottles until the entire cross section of the stream was 

sampled. Typically, two–three 3-L bottles were required for 

each suspended-sediment composite. Samples were sent to 

the USGS Cascade Volcano Observatory (CVO) sediment 

laboratory in Vancouver, Washington after collection where 

they were analyzed for total SSC and particle-size fraction 

less than 62.5 μm (Guy, 1969). In addition to the composited 

samples, selected suspended-sediment samples were collected 

at the Snake River near Anatone and Clearwater River at 

Spalding streamgages to document the river cross-sectional 

variability. For these samples, individual bottles were used 

to collect samples from each centroid in the cross section. 

The samples were then analyzed separately for sediment 

concentration and full-grain size analysis on the sediment size 

greater than a diameter of 62.5 μm. 

The SSC in a stream generally varies in relation to 

streamflow. Because of this variability, summary statistics 

to characterize a stream, such as a mean concentration, 

may include bias resulting from variation in the sampling 

frequency and the timing of sampling over the stream 

hydrograph or a storm event. For this study, mean flowweighted concentrations and suspended-sediment loads 

were simulated using LOADEST, a FORTRAN program for 

estimating constituent loads in streams and rivers (Runkel and 

others, 2004). LOADEST is based on a rating-curve method 

(Cohn and others, 1989, 1992; Crawford, 1991) that uses 

regression to estimate constituent load in relation to several 

predictor variables related to streamflow and time. This type 

of model has been used to estimate constituent concentrations 

for periods when sample data were not available (Gilroy 

and others, 1990), to estimate a basin flux of water-quality 

constituents (Goolsby and others, 1999), and to evaluate longterm trends in water-quality data (Smith and others, 1987). 
10 Sediment Transport in the Lower Snake and Clearwater River Basins, Idaho and Washington, 2008–11

tac12-0737_fig03

Clearwater River at Spalding, Idaho

Snake River near Anatone, Washington

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

80,000

Streamflow, in cubic feet per second

0

20,000

40,000

60,000

80,000

100,000

120,000

140,000

160,000

Daily mean streamflow

Suspended-sediment sample

Bedload sample

EXPLANATION

M A M J SJ A N DO F MJ A M J SJ A N DO F MJ A M J SJ A N DO A MMJ F J SJ A

2008 2009 2010 2011

Water year

Figure 3. Streamflows at which suspended-sediment and bedload samples were collected from the Snake River 

near Anatone, Washington, and Clearwater River at Spalding, Idaho, water years 2008–11.
Methods of Data Collection and Analysis 11

For this study, LOADEST was used to develop 

regression models for estimating the loads and flow-weighted 

concentrations of total suspended sediment (TSS) and the 

sand and fine-grained fractions of suspended sediment 

for each station for water years 2009 through 2011. For 

the streamgages on the Snake River near Anatone and the 

Clearwater River at Spalding, loads and concentrations were 

estimated for water years 2008 through 2011. The regression 

model used for the TSS and suspended-sediment size 

fractions was:

lnL I a Q b Q c T

d T

= + ( ) + ( ) +  ( )  



+  ( )  

 +

ln ln sin

cos

2 2

2

π

π ε

(1)

where

L is the suspended-sediment load, in pounds 

 per day;

I is the regression intercept;

Q is the centered streamflow, in cubic feet per

 second;

T is the centered decimal time in years from the

 beginning of the calibration period;

a, b, c, and d are regression coefficients that remain constant

 over time; and

ε is unaccounted error associated with the

 regression model.

For each model, the predictor variables in the regression 

equation were selected on the basis of Aikaike Information 

Criteria (Aikaike, 1981; Judge and others, 1985). The criteria 

are designed to achieve a good compromise between using as 

many predictor variables as possible to explain the variance 

in load while minimizing the standard error of the resulting 

estimates. Estimates of the daily constituent load for each 

station were computed using the selected model (table 3) 

and daily mean streamflow. Bias introduced by conversion 

of the logarithm of load into estimates of actual load were 

corrected using the Bradu-Mundlak method (Bradu and 

Mundlak, 1970; Cohn and others, 1989; Crawford, 1991). A 

mean flow-weighted concentration for suspended sediment at 

each station was estimated as the TSS load over a given time 

period divided by the total streamflow at the station during the 

same period.

Suspended-Sediment Surrogates

The need to measure fluvial sediment has led to the 

development of sediment surrogate technologies, particularly 

in locations where streamflow alone may not be a good 

estimator of SSC because of regulated flow, hysteresis effect 

on sediment concentration, episodic sediment sources, and 

non-equilibrium sediment transport. An effective surrogate 

technology is low-maintenance and robust over a range of 

hydrologic conditions, and measured variables can be modeled 

to SSC, sediment load, and duration of elevated levels on a 

real-time and continuous basis. Although numerous surrogate 

technologies were tested during this study (including turbidity 

and laser diffraction), only acoustic backscatter measurements 

using ADVMs is described in this report. Wood and Teasdale 

(2013) present a detailed discussion for each of the surrogate 

technologies tested during this study, and compare and 

contrast the results. 

Acoustic backscatter has been used successfully as 

a surrogate for SSC in the San Francisco Bay, California 

(Gartner, 2004), estuaries in Florida (Patino and Byrne, 

2004), Colorado River (Topping and others, 2004), Hudson 

River, New York (Wall and others, 2006), the Aegean 

Region, Turkey (Elci and others, 2009), and subtropical 

estuaries in Australia (Chanson and others, 2008). For the 

acoustic backscatter method, a device is used that emits an 

acoustic pulse into the water. Theoretically, the strength of 

the reflected pulse, called backscatter, increases when more 

particles, in this case assumed to be sediment, are present in 

the water. Acoustic instruments have shown great promise 

in sediment surrogate studies because they are robust to 

biological fouling and measure profiles of backscatter across 

a sampling volume rather than at a single point in the stream 

(Gartner and Gray, 2005). Acoustic frequencies were selected 

for this study to maximize sensitivity of backscatter to 

dominant grain size (lower frequency for the sand fraction 

and higher frequency for the fines fraction) and to minimize 

errors due to changing grain-size distribution, as described in 

Gartner (2004) and Topping and others (2004). To estimate 

the concentrations of total suspended sediment, sands, and 

fines, acoustic backscatter data were corrected for (1) beam 

spreading, (2) transmission losses due to absorption by 

water, and (3) absorption or attenuation by sediment. In this 

study, the Clearwater River at Spalding and the Snake River 

near Anatone streamgages (table 1) were instrumented with 

surrogate technologies. 

The equal-width-increment suspended-sediment samples 

collected at the lower Snake and Clearwater River gaging 

stations were used to develop the relation between SSC 

and ADVM backscatter. The sampling strategy targeted 

the ascending limb, the peak, and the descending limb 

of the snowmelt runoff hydrograph at each station. Forty 

suspended-sediment samples from the Clearwater River and 

39 samples from the lower Snake River were collected during 

the study period March 2008 to July 2011. Samples submitted 

for analysis were a composite representative of the entire cross 

section. As mentioned previously, cross-sectional variability 

in SSC in the Clearwater and Snake Rivers was evaluated 

by individually analyzing 10 separate vertically integrated 

samples from the width of the river channel. Cross-section 

variability was analyzed for four sample sets collected from 

the Clearwater River and five sample sets collected from 

the Snake River. All suspended-sediment samples collected 

from the Snake River near Anatone and Clearwater River at 

Spalding streamgages were analyzed for organic content using 

a loss-on ignition analysis (Schumacher, 2002) to evaluate the 

effect of organic matter on errors in the surrogate analysis. 
12 Sediment Transport in the Lower Snake and Clearwater River Basins, Idaho and Washington, 2008–11

Table 3. Regression coefficients and coefficients of determination for models used to estimate concentrations and loads of total 

suspended sediment and suspended-sediment size fractions at stations in the lower Snake and Clearwater River Basins, Washington 

and Idaho.

[Locations of stations are shown in figure 2. The regression equation is lnL = I +a(lnQ) + b(lnQ2) + c[sin(2πT)] + d[cos(2πT)] + ε, where L is the sediment load 

in kilograms per day; I is the regression intercept; Q is the centered stream discharge in cubic feet per second; T is the centered decimal time in years from the 

beginning of the calibration period; a, b, c, and d, are regression coefficients that remain constant over time; ε is unaccounted error associated with the regression 

model; and R2 (coefficient of determination) represents the amount of variance explained by the model. USGS, U.S. Geological Survey]

USGS 

gaging

station 

No.

Gaging station name I

Regression coefficient

R2

(percent) a b c d

Total suspended sediment

13317000 Salmon River at White Bird, Idaho 15.47 2.518 -0.079 -0.684 -0.790 89

13334000 Grande Ronde River at Zindel, Washington 11.84 2.236 0.575 -0.713 0.506 91

13334300 Snake River near Anatone, Washington 15.69 3.093 0.084 -0.540 0.160 95

13336500 Selway River near Lowell, Idaho 11.02 2.609 0.512 -0.923 -0.437 95

13337000 Lochsa River near Lowell, Idaho 10.38 2.193 0.402 -0.948 0.024 96

13337120 Middle Fork Clearwater River at Kooskia, Idaho 11.86 2.250 0.402 -0.947 -0.308 95

13338100 South Fork Clearwater River near Harpster, Idaho 12.04 2.714 0.030 -0.796 -1.610 96

13338500 South Fork Clearwater River at Stites, Idaho 11.04 2.463 0.328 -0.730 -0.442 95

13340000 Clearwater River at Orofino, Idaho 12.24 2.229 0.630 -0.831 -0.311 93

13341570 Potlatch River below Little Potlatch Creek near Spalding, Idaho 11.08 2.251 0.126 -0.143 -0.697 93

13342500 Clearwater River at Spalding, Idaho 12.57 2.047 0.590 0.461 -0.477 90

13351000 Palouse River at Hooper, Washington 12.64 1.655 0.021 -0.930 -0.210 87

Total suspended sands

13317000 Salmon River at White Bird, Idaho 14.51 3.212 -0.197 -1.098 -0.816 95

13334000 Grande Ronde River at Zindel, Washington 9.97 2.797 0.847 -0.541 -0.134 92

13334300 Snake River near Anatone, Washington 14.70 3.993 -0.465 -0.490 -0.056 97

13336500 Selway River near Lowell, Idaho 9.32 2.798 0.659 -0.755 0.394 95

13337000 Lochsa River near Lowell, Idaho 9.84 2.588 0.414 -0.798 -0.582 96

13337120 Middle Fork Clearwater River at Kooskia, Idaho 10.95 2.696 0.490 -0.558 -0.670 96

13338100 South Fork Clearwater River near Harpster, Idaho 11.30 3.190 -0.114 -0.459 -1.867 96

13338500 South Fork Clearwater River at Stites, Idaho 9.95 2.889 0.396 -0.301 -0.709 97

13340000 Clearwater River at Orofino, Idaho 10.65 2.587 0.751 -0.607 -0.003 95

13341570 Potlatch River below Little Potlatch, near Spalding, Idaho 8.49 2.156 0.191 -0.327 -0.982 97

13342500 Clearwater River at Spalding, Idaho 10.83 2.525 0.694 0.242 -0.854 92

13351000 Palouse River at Hooper, Washington 9.54 1.376 0.068 -0.727 0.148 69

Total suspended fines

13317000 Salmon River at White Bird, Idaho 15.20 2.349 -0.076 -0.831 -1.281 82

13334000 Grande Ronde River at Zindel, Washington 11.65 2.150 0.507 -0.709 0.607 90

13334300 Snake River near Anatone, Washington 15.12 2.717 0.225 -0.563 0.342 92

13336500 Selway River near Lowell, Idaho 10.89 2.377 0.321 -1.033 -0.933 92

13337000 Lochsa River near Lowell, Idaho 9.76 1.978 0.350 -1.099 0.148 93

13337120 Middle Fork Clearwater River at Kooskia, Idaho 11.28 1.981 0.353 -1.171 -0.236 91

13338100 South Fork Clearwater River near Harpster, Idaho 11.41 2.469 0.063 -0.977 -1.538 94

13338500 South Fork Clearwater River at Stites, Idaho 10.53 2.250 0.292 -0.857 -0.281 93

13340000 Clearwater River at Orofino, Idaho 11.92 1.991 0.545 -0.907 -0.376 90

13341570 Potlatch River below Little Potlatch, near Spalding, Idaho 10.95 2.270 0.120 -0.131 -0.658 92

13342500 Clearwater River at Spalding, Idaho 12.30 1.845 0.518 0.651 -0.358 86

13351000 Palouse River at Hooper, Washington 12.56 1.694 0.021 -0.928 -0.265 88
Methods of Data Collection and Analysis 13

The ADVMs at the Clearwater River at Spalding were 

co-located with the streamgage and installed about 16 ft 

upstream of the streamgage. In May 2008, the station was 

equipped with a 0.5 megahertz (MHz) SontekTM Argonaut-SL 

ADVM and 3 MHz SontekTM Argonaut-SL ADVM with 

datalogger and satellite telemetry. The ADVMs were mounted 

on an aluminum slide track that could be raised and lowered 

as needed to service equipment. The 0.5 and 3 MHz ADVMs 

measured backscatter in five discrete, equally sized cells in a 

horizontal sampling volume, 5.0–100 ft and 3.3–12 ft from 

the instrument, respectively. The sampling volume for each 

ADVM was selected based on meter frequency, availability 

of suspended material to reflect the acoustic pulse, and 

any obstructions in the beam path. The ADVMs collected 

backscatter data 2 minutes of every 15 minutes.

In April 2009, the Snake River near Anatone streamgage 

was equipped with a 0.5 MHz Sontek Argonaut-SL ADVM 

and 1.5 MHz Argonaut-SL ADVM with datalogger and 

satellite telemetry. The station was located on the left 

streambank about 1,000 ft upstream of the streamgage and the 

sediment sampling location. The 0.5 and 1.5 MHz ADVMs 

were mounted on an aluminum slide track and were configured 

to measure backscatter in five discrete, equally sized cells in a 

horizontal sampling volume, 6.6–203 and 6.6–59 ft from the 

instrument, respectively. The ADVMs collected backscatter 

data 2 minutes of every 15 minutes. 

Acoustic backscatter data must be corrected prior to 

relating the data to the mass concentration of suspended 

sediment in the water column. Factors affecting acoustic 

backscatter readings include transmission losses due to 

absorption by water, absorption or attenuation by sediment, 

and beam spreading. However, the methods for correcting 

backscatter data differ and can substantially alter estimates of 

SSC. Thus, selection of an appropriate method is important 

in the analysis of acoustic backscatter data. For this study, 

candidate methods for correcting the backscatter data were 

reviewed; methods selected for use with data collected at the 

Snake River and Clearwater River stations are documented in 

Wood and Teasdale (2013).

Following correction of the backscatter data, 

measurements were averaged over a 1-hour period bracketing 

the time that sediment samples were collected. This 

allowed concurrent measurements to relate backscatter data 

to the measured SSC at each station. Relations between 

backscatter and SSC were evaluated using ordinary least 

squares regression. All of the SSC data were log transformed 

prior to regression. Acoustic backscatter data are reported 

in a log-based scale and do not require a transformation. 

Regression models were selected based on statistical 

significance of explanatory variables (p-values) and various 

regression statistics such as coefficient of determination (R2), 

standard error, Mallow’s Cp (Ott and Longnecker, 2001), 

and prediction error sum of squares (PRESS) (Helsel and 

Hirsch, 1992) statistics. A nonparametric bias correction 

factor described in Duan (1983) was applied to each best‐fit 

regression model to correct for low bias induced by log 

transformation and subsequent re-transformation of the 

dependent variable. The factor was used to correct each value 

of SSC as well as upper and lower 95 percent confidence 

intervals estimated by a regression model.

Bedload

Unlike suspended-sediment transport, bedload often 

is not detectable. When bedload transport does occur, it is 

often extremely variable both spatially within the stream 

channel and temporally during steady streamflow conditions 

(Hubbell, 1964). During this study, bedload was collected at 

most of the stations when streamflow was sufficiently high 

to initiate bedload transport. However, at some stations, the 

stream cross-section was not conducive for collecting bedload 

because of poor channel geometry, backwater conditions, 

extreme turbulence, or a combination of these factors. Bedload 

samples were collected at only two of the three stations in 

the Snake River drainage basin upstream of lower Granite 

Reservoir. The channel bottom at the Grande Ronde River at 

Zindel, Washington, sampling station is composed of uneven 

bedrock, making the site nearly impossible to sample with the 

Helley-Smith bedload sampler. In the Clearwater River Basin, 

7 of the 8 stations were sampled at least 3 times for bedload, 

with the Lochsa River at Lowell limited to 3 samples because 

of backwater conditions from the Selway River. The Potlatch 

River was not sampled because of backwater conditions 

from the Clearwater River. Although bedload sampling was 

attempted on numerous occasions at the Palouse River station, 

bedload was never retained in the sampler. It is difficult to 

determine whether the lack of bedload in the Palouse River 

at Hooper was real or an artifact of a poor sampling location. 

The number of bedload samples collected at each station is 

listed in table 1. The timing of bedload sampling for the Snake 

River near Anatone and the Clearwater River at Spalding 

streamgages from March 2008 through July 2011 is shown 

in figure 3.

Most bedload samples were collected using a 

Helley-Smith type sampler with a 6- by 6-in. nozzle (Helley 

and Smith, 1971); a few samples were collected in 2011 using 

an Elwha style sampler with a 4- by 8-in. nozzle. An equalwidth-increment sampling method as described by Edwards 

and Glysson (1999) was used to collect bedload. This method 

involves dividing and collecting samples in the stream channel 

at evenly spaced sections to represent bedload transport 

accurately across the entire channel. When possible, samples 

were collected at 20 equally spaced intervals with duration of 

60 seconds on the stream bottom per interval. At some stations 

and at some streamflows, fewer than 20 sections were sampled 

because of extremely fast and turbulent stream conditions. 
14 Sediment Transport in the Lower Snake and Clearwater River Basins, Idaho and Washington, 2008–11

For analysis, all sectional samples were composited into one 

sample representing the entire cross section. Duplicates were 

collected during each sampling event and analyzed separately 

at the USGS CVO sediment laboratory. For analysis, the 

results for the duplicate samples are reported as an average. 

The bedload samples were analyzed for total mass and full 

phi (φ) particle size increments from φ = 4 (62.5 μm) to φ = -7 

(128 mm).

Because of the large variability in the spatial and 

temporal pattern of bedload transport, curves relating bedload 

to streamflow generally have a larger degree of uncertainty 

than do curves for suspended-sediment transport. To estimate 

bedload for use in transport curves, the following equation 

(Edwards and Glysson, 1999), was used:

Qb = × ( ) k W × M T/ (2)

where

Qb is bedload discharge, in tons per day;

k is a conversion factor specific to the sampler

orifice (0.381 for the Helley Smith and 0.141

 for the Elwha);

W is the total stream width, in feet;

M is the total mass of bedload sample, in grams

 and

T is the total time the sampler was on the bed, in

 seconds.

The best-fit equation between streamflow and bedload 

discharge was used to estimate the total bedload discharge for 

the period of interest using either the continuous or indexed 

daily streamflow record. Because of the paucity of samples at 

some stations and the variability at most of the stations, the 

bedload discharge estimates obtained during this study have 

a wide range of associated error and should not be viewed as 

precise estimates.

The grain size and statistics software GRADISTAT 

(Blott and Pye, 2001) was used to classify the particle-size 

distribution data. Emmett (1976) described the particle 

size distribution as bimodal at the Snake River near 

Anatone and the Clearwater River at Spalding streamgages. 

Standard statistical measures such as the arithmetic (normal 

distribution) or geometric (log-normal) method of moments 

are not suitable for identifying the bimodal particle size 

distribution because of non-normality. Folk and Ward (1957) 

described a graphical geometric (log-normal) method to 

determine grain-size analysis statistics. Blott and Pye (2001) 

determined this method to be the most appropriate for 

parameter-based estimation of bimodal bedload transport. 

Parameters used to describe the particle-size distribution 

in this study included sample distribution (unimodal, 

bimodal), cumulative exceedence values (D10, D50, D90), and 

cumulative percentage finer by weight. Standard deviation 

(sorting), skewness (positive or negative preference), and 

kurtosis (peakedness around the mean) were not used in this 

analysis as they were determined to be unreliable for bimodal 

bedload distributions.

Sediment Transport in the Lower Snake 

and Clearwater River Basins

Because sediment transport in streams varies in response 

to hydrologic conditions, information on streamflow in a 

historical context is critical for assessing sediment transport 

and deposition in the lower Snake and Clearwater River 

Basins. The magnitude and timing of streamflow in the lower 

Snake and Clearwater River Basins generally are determined 

by the amount of water derived from the winter snowpack 

and reservoir operations at Hells Canyon and Dworshak 

Reservoirs. As such, rivers and streams in the lower Snake and 

Clearwater River Basins typically reach peak flows in April, 

May, or June in association with spring snowmelt runoff. 

High streamflows occasionally occur in lower elevation 

basins in the autumn and winter in association with rain-onsnow events that can cause flooding and substantial transport 

of sediment. Generally, however, when precipitation and 

snowpack are below average, streamflow from runoff is lower 

than normal, and transport of sediment in streams is below 

average. In contrast, when precipitation, snowpack, and runoff 

are above average, sediment transport may be larger than 

normal. Generally, streamflows drop rapidly over the summer 

following the loss of the snowpack, and low flow typically is 

in September or October. 

From March 2008 through September 2011, streamflow 

conditions in the lower Snake and Clearwater River Basins 

as represented by the streamgages on the Snake River near 

Anatone and the Clearwater River at Spalding were quite 

variable in relation to the most recent 30-year average (fig. 4). 

Streamflow in the Snake and Clearwater Rivers during water 

years 2008, 2009, and, in particular, 2011 generally exceeded 

the 30-year average during most of the year. During 2010, 

streamflow in the Snake and Clearwater Rivers was less 

than the 30-year average for most of the year, exceeding the 

average streamflow only during the brief period of snowmelt 

runoff from late May through early June. A pronounced 

rain-on-snow event occurred during January 2011 affecting 

streamflows and sediment transport in the Potlatch River 

below Little Potlatch near Spalding (USGS streamgage 

13341570) and the Palouse River at Hooper (USGS 

streamgage 13351000). This event occurred across many of 

the lower-elevation drainage basins in the lower Snake and 

Clearwater River Basins and is evident in the hydrograph 

spike in late January 2011 for the Snake River near Anatone 

and in particular for the Clearwater River at Spalding (fig. 4).
Sediment Transport in the Lower Snake and Clearwater River Basins 15

tac12-0737_fig04

0 

10,000 

20,000 

30,000 

40,000 

50,000 

60,000 

70,000 

80,000 

0 

20,000 

40,000 

60,000 

80,000 

100,000 

120,000 

140,000 

160,000 

Clearwater River at Spalding, Idaho

Snake River near Anatone, Washington

Streamflow, in cubic feet per second

EXPLANATION

Daily mean streamflow

30-year mean, 1981–2010

M A M J SJ A N DO F MJ A M J SJ A N DO F MJ A M J SJ A N DO A MMJ F J SJ A

2008 2009 2010 2011

Water year

Figure 4. Daily mean streamflow in the Snake River near Anatone, Washington and the Clearwater River at Spalding, Idaho, 

2008–11, compared with the 30-year mean for water years 1981–2010.
16 Sediment Transport in the Lower Snake and Clearwater River Basins, Idaho and Washington, 2008–11

Suspended-Sediment Transport

The number of suspended-sediment samples collected 

varied from 33 to 42 per station during the sampling period 

(table 1). Fitted regression lines for a log-log power fit with 

associated R2 values of TSS and streamflow for selected 

stations in the lower Snake and Clearwater River Basins 

are shown in figure 5. Because there is considerable scatter 

around the regression lines and the R2 values at some 

stations were relatively low, a degree of uncertainty exists 

in the TSS load calculations based on streamflow. However, 

assuming that the streamflows in individual subbasins during 

2009–11 are representative of normal conditions, their 

relative contributions to the overall sediment load delivered 

to Lower Granite Reservoir probably are accurate. Based on 

the hydrographs for 2009–11 compared to the 30-year mean 

for the Snake River near Anatone and the Clearwater River at 

Spalding (fig. 4), this assumption probably is valid.

The concentrations and load of suspended sediment 

transported in streams were variable, spanning 3–5 orders of 

magnitude during water years 2008–11 (fig. 6). The largest 

measured concentrations of suspended sediment were in 

samples collected from the Palouse and Potlatch Rivers during 

the rain-on-snow event in January 2011. Concentrations 

of TSS, which includes both the sand and fine-grained 

fractions, in the Potlatch River ranged more than 4 orders 

of magnitude during the study from a low of 3 mg/L during 

baseflow conditions to more than 3,000 mg/L in January 2011 

during the rain-on-snow event (table 2). Samples collected 

from the Palouse River at Hooper had the largest median 

concentration of TSS (100 mg/L) and the largest suspended 

sediment fraction as fine-grained silt and clay less than 

62.5 μm in diameter (median 95 percent). Other rivers with 

a large percentage of fine-grained suspended sediment were 

the Potlatch River (median 92 percent) and the Grand Ronde 

River (median 84 percent) (table 2). In contrast to the other 

stations sampled during this study, the Palouse and Potlatch 

Rivers and, to a lesser extent, the Grande Ronde River showed 

an increase in the fraction of fine-grained sediment with 

increasing streamflow. The Palouse, Potlatch, and Grande 

Ronde Rivers all drain basins with relatively large proportions 

of agricultural activity, which probably accounts for the 

relatively large percentage of fine-grained sediment.

The second largest median TSS concentration (94 mg/L) 

was measured in samples collected at Salmon River at White 

Bird, Idaho (USGS streamgage 13317000). However, as a 

percentage of the TSS, the fine-grained fraction (median of 

51 percent) was smaller in samples collected in the Salmon 

River as compared with the Palouse, Potlatch, and Grande 

Ronde Rivers. The smallest TSS with median concentrations 

of 11, 11, 15, and 13 mg/L were measured in samples 

collected from the Selway and Lochsa Rivers, the Middle 

Fork Clearwater River at Kooskia, and the Clearwater River at 

Orofino (USGS streamgage 13340000), respectively. The TSS 

concentration in samples from the Lochsa River and the Middle 

Fork Clearwater River at Kooskia did not exceed 100 mg/L in 

any of the samples collected during this study (table 2).

Samples collected from the Selway River, Lochsa River, 

and the Middle Fork Clearwater River at Kooskia, and samples 

from the Salmon River at White Bird, had relatively small 

percentages of fine-grained sediment. The median silt and clay 

fractions at each of these stations were about 50 percent or less. 

All stations in the network, except the agriculturally affected 

Palouse, Potlatch, and Grande Ronde Rivers, exhibited an 

increase in the sand-size fraction of suspended sediment with 

an increase in streamflow. As a result, during the peak of spring 

snowmelt runoff, most of the suspended sediment transported 

in the lower Snake and Clearwater River Basins was sand-sized 

material larger than 62.5 μm.

Based on the transport curves (fig. 6B), the load of 

TSS in the Palouse River ranged from about 1 to more than 

10,000 tons per day and in the Potlatch River ranged from 

less than 0.1 to more 40,000 tons per day. However, because 

the transport curves in figure 6B represent a best-fit line 

through the data for each station, during peak runoff the actual 

concentration and load of TSS is often much larger than the 

load estimated using the sediment-transport curves. This is 

evident in the stations shown in figure 5 where the best-fit 

regression line for each station underestimates the measured 

TSS concentration at high streamflow. For instance, multiple 

samples collected from the Palouse and Potlatch Rivers 

during the January 2011 rain-on-snow event indicate that 

the suspended-sediment transport far exceeded the estimates 

based on the transport curves shown in figure 6. For the 

Palouse River, four TSS samples collected January 17–18 

at streamflows ranging from 5,670 to 7,150 ft3/s had 

concentrations between 930 and 1,400 mg/L, about twice 

the amount estimated using the transport curve values of 

490–600 mg/L, respectively. For the Potlatch River, five 

samples collected January 16–17 at streamflows ranging from 

8,760 to 17,000 ft3/s had TSS concentrations of between 

1,400 and 3,300 mg/L, more than three times the estimated 

concentrations of 420 and 940 mg/L, respectively.

Because the LOADEST program uses multiple variables 

to predict SSC and load, simulation results from the model 

generally provide better estimates than those based on transport 

curves as shown in figure 6. LOADEST was therefore used to 

develop regression models (table 3) for each station to estimate 

the load of TSS, total suspended sand, and total suspended fines 

for water years 2009–11(table 4). To determine the annual load, 

the daily load for each constituent was estimated using the 

daily mean streamflow and summed to determine the annual 

load. Regression coefficients and coefficients of determination 

(R2) for the LOADEST models at each station are given in 

table 3.
Sediment Transport in the Lower Snake and Clearwater River Basins 17

tac12-0737_fig05

C. South Fork Clearwater River at Stites, Idaho

A. Salmon River at White Bird, Idaho B. Snake River near Anatone, Washington

D. Clearwater River at Spalding, Idaho

1

10

100

1,000

100 1,000 10,000 100,000

1

10

100

1,000

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

1,000 10,000 100,000

R2 = 0.76

Streamflow, in cubic feet per second

R2 = 0.66

Total suspended-sediment concentration, in milligrams per liter 

R2 = 0.88

R2 = 0.63

EXPLANATION

95% prediction interval

Figure 5. Suspended-sediment transport curves and 95-percent prediction intervals representing best-fit regression 

equations for a log-log power fit of total suspended sediment with streamflow in the (A) Salmon River at Whitebird, 

Idaho, (B) Snake River near Anatone, Washington, (C) South Fork Clearwater River at Stites, Idaho, and (D) Clearwater 

River at Spalding, Idaho.
18 Sediment Transport in the Lower Snake and Clearwater River Basins, Idaho and Washington, 2008–11

 High R2 values indicate that the 

LOADEST models did an excellent job 

of accounting for the variability in the 

suspended- sediment loads: 30 of the 

36 models accounted for 90 percent 

or more of observed variability. The 

models account for less than 80 percent 

of the variability in the observed 

sediment load only for the Palouse 

River and only for the sand-size 

sediment fraction. Generally, the model 

simulation results showed the best fit 

(highest R2) for the sand size fraction 

and the poorest fit for the fine-grained 

fraction. A notable exception was for 

the Palouse River, where the best-fit 

model occurred for total suspended 

fines (R2=0.88) and the poorest fit 

occurred for the total suspended sands 

(R2=0.69).

For each of the stations in the 

sampling network, the estimated loads 

of total suspended sediment (fig. 7), 

suspended sands, and suspended fines 

were largest during water year 2011 

and smallest during water year 2010 

(table 4). The difference in the TSS 

load for the Palouse and Potlatch 

Rivers during water years 2010 and 

2011 was particularly notable. The 

TSS load in the Palouse River was 

about 8 times larger in 2011 than in 

2010 and in the Potlatch River was 

more than 50 times larger in 2011 than 

in 2010. The large difference in the 

sediment load between 2011 and 2010 

in the Palouse and Potlatch Rivers is 

attributable to the rain-on-snow event 

that occurred in these drainage basins 

during January 2011. In the Palouse 

River at Hooper, the TSS load during 

January 17–19, 2011, was about 

64,600 tons, roughly 19 percent of the 

2011 water year total of 335,000 tons 

and about 9 percent of the TSS load 

transported in the Palouse River during 

water years 2009–11 (table 4). During 

January 16–17, 2011, the estimated 

TSS load in the Potlatch River 

exceeded 110,000 tons, more than 

one-half of the total estimated load of 

212,000 tons for the entire 2011 water 

year and about 40 percent of the total 

TSS load transported in the Potlatch 

River during water years 2009–11 

(table 4). 

tac12-0737_fig06ab

100

10

1.0

0.1

Palouse River

at Hooper

Potlatch River

near Spalding 

South Fork 

Clearwater River 

near Harpster

Salmon River

at White Bird

Middle Fork Clearwater 

River at Kooskia 

South Fork Clearwater River

at Stites 

Grande Ronde

River at Zindel

Lochsa River

near Lowell

Selway River

near Lowell

Snake River

near Anatone

Clearwater 

River at 

Orofino

A. Total suspended sediment concentration

Streamflow, in cubic feet per second

10 100 1,000 10,000 100,000

0.1

1.0

10

100

1,000

1,000

10,000

100,000

B. Total suspended sediment load

Total suspended-sediment concentration, in milligrams per liter Total suspended-sediment load, in tons per day

Clearwater

River at

Spalding

Palouse River

at Hooper

Potlatch River

near Spalding South Fork 

Clearwater River 

near Harpster 

Salmon River

at White Bird

Middle Fork Clearwater 

River at Kooskia 

South Fork Clearwater River

at Stites 

Grande Ronde

River at Zindel

Lochsa River

near Lowell

Selway River

near Lowell

Snake River

near Anatone

Clearwater River 

at Orofino

Clearwater

River at

Spalding

Figure 6. Suspended-sediment transport curves representing best-fit 

regression equations for (A) total suspended-sediment concentrations 

and (B) total suspended-sediment loads at sampling stations in the 

lower Snake and Clearwater River Basins, Washington and Idaho.
Sediment Transport in the Lower Snake and Clearwater River Basins 19

Table 4. Estimated annual, total, and mean loads of suspended sediment, suspended sand, and suspended fines for stations in the 

lower Snake and Clearwater River Basins, Washington and Idaho, water years 2009–11.

[Locations of stations are shown in figure 2. Abbreviations: C.I., confidence interval; USGS, U.S. Geological Survey]

USGS 

gaging 

station 

No.

Gaging station name

Total suspended sediment 

(tons)

Total suspended sands

(tons)

Total suspended fines

(tons)

Estimated 

load

Lower 95 

percent

C.I. 

Upper 95 

percent

C.I.

Estimated 

load

Lower 95 

percent 

C.I.

Upper 95 

percent

C.I. 

Estimated 

load

Lower 95 

percent

C.I.

Upper 95 

percent

C.I.

Water year 2009

13317000 Salmon River at White Bird, 

Idaho

1,660,000 962,000 2,720,000 952,000 590,000 1,450,000 744,000 387,000 1,330,000

13334000 Grande Ronde River at Zindel, 

Washington

278,000 174,000 425,000 52,200 29,500 86,900 224,000 138,000 347,000

13334300 Snake River near Anatone, 

Washington

1,820,000 1,340,000 2,420,000 848,000 631,000 1,110,000 998,000 687,000 1,410,000

13336500 Selway River near Lowell, Idaho 153,000 87,700 250,000 116,000 59,600 203,000 46,600 25,400 81,400

13337000 Lochsa River near Lowell, Idaho 42,300 29,000 60,700 24,500 15,300 38,200 18,700 12,000 28,500

13337120 Middle Fork Clearwater River at 

Kooskia, Idaho

194,000 127,000 289,000 115,000 72,500 175,000 86,000 50,300 141,000

13338100 South Fork Clearwater River 

near Harpster, Idaho

44,600 31,200 63,200 22,100 14,500 32,900 22,700 15,000 34,100

13338500 South Fork Clearwater River at 

Stites, Idaho

48,800 33,400 69,400 18,700 13,200 26,100 28,900 18,600 43,100

13340000 Clearwater River at Orofino, 

Idaho

300,000 182,000 475,000 154,000 89,200 249,000 150,000 85,500 252,000

13341570 Potlatch River below Little 

Potlatch Creek, near Spalding, 

Idaho

57,800 20,600 135,000 2,840 1,460 5,120 55,500 18,600 134,000

13342500 Clearwater River at Spalding, 

Idaho

335,000 215,000 501,000 137,000 81,600 215,000 199,000 122,000 310,000

13351000 Palouse River at Hooper, 

Washington

318,000 130,000 672,000 19,600 4,950 55,500 290,000 123,000 596,000
20 Sediment Transport in the Lower Snake and Clearwater River Basins, Idaho and Washington, 2008–11

USGS 

gaging 

station 

No.

Gaging station name

Total suspended sediment 

(tons)

Total suspended sands

(tons)

Total suspended fines

(tons)

Estimated 

load

Lower 95 

percent

C.I. 

Upper 95 

percent

C.I.

Estimated 

load

Lower 95 

percent 

C.I.

Upper 95 

percent

C.I. 

Estimated 

load

Lower 95 

percent

C.I.

Upper 95 

percent

C.I.

Water Year 2010

13317000 Salmon River at White Bird, 

Idaho

1,030,000 590,000 1,700,000 524,000 321,000 811,000 476,000 242,000 876,000

13334000 Grande Ronde River at Zindel, 

Washington

144,000 73,600 258,000 46,700 18,300 99,600 109,000 55,300 195,000

13334300 Snake River near Anatone, 

Washington

1,650,000 1,130,000 2,320,000 795,000 552,000 1,110,000 855,000 543,000 1,290,000

13336500 Selway River near Lowell, Idaho 60,700 32,500 104,000 44,800 20,400 86,000 19,400 10,300 33,600

13337000 Lochsa River near Lowell, Idaho 18,300 12,700 25,800 9,720 6,120 14,800 8,810 5,660 13,200

13337120 Middle Fork Clearwater River at 

Kooskia, Idaho

81,600 53,000 121,000 44,800 27,900 68,600 38,800 22,400 63,400

13338100 South Fork Clearwater River 

near Harpster, Idaho

14,200 9,860 19,900 7,440 4,800 11,000 7,260 4,730 10,800

13338500 South Fork Clearwater River at 

Stites, Idaho

16,200 10,700 23,500 6,360 4,340 8,990 9,850 6,120 15,100

13340000 Clearwater River at Orofino, 

Idaho

143,000 82,900 232,000 71,400 38,900 121,000 75,100 40,800 130,000

13341570 Potlatch River below Little 

Potlatch Creek near Spalding, 

Idaho

4,090 982 11,700 229 93 482 3,810 844 11,300

13342500 Clearwater River at Spalding, 

Idaho

152,000 92,200 237,000 58,900 33,500 96,700 91,200 52,200 150,000

13351000 Palouse River at Hooper, 

Washington

39,800 12,700 96,700 2,960 572 9,410 35,000 11,800 82,500

Table 4. Estimated annual, total, and mean loads of suspended sediment, suspended sand, and suspended fines for stations in the 

lower Snake and Clearwater River Basins, Washington and Idaho, water years 2009–11.—Continued

[Locations of stations are shown in figure 2. Abbreviations: C.I., confidence interval; USGS, U.S. Geological Survey]
Sediment Transport in the Lower Snake and Clearwater River Basins 21

USGS 

gaging 

station 

No.

Gaging station name

Total suspended sediment 

(tons)

Total suspended sands

(tons)

Total suspended fines

(tons)

Estimated 

load

Lower 95 

percent

C.I. 

Upper 95 

percent

C.I.

Estimated 

load

Lower 95 

percent 

C.I.

Upper 95 

percent

C.I. 

Estimated 

load

Lower 95 

percent

C.I.

Upper 95 

percent

C.I.

Water year 2011

13317000 Salmon River at White Bird, 

Idaho

2,400,000 1,410,000 3,870,000 1,300,000 832,000 1,950,000 1,060,000 560,000 1,870,000

13334000 Grande Ronde River at Zindel, 

Washington

288,000 164,000 489,000 60,500 28,600 122,000 228,000 129,000 389,000

13334300 Snake River near Anatone, 

Washington

5,450,000 3,860,000 7,500,000 2,800,000 2,000,000 3,800,000 2,700,000 1,770,000 3,970,000

13336500 Selway River near Lowell, Idaho 154,000 91,500 247,000 109,000 58,500 186,000 50,500 28,200 86,300

13337000 Lochsa River near Lowell, Idaho 65,200 44,500 93,600 40,400 25,400 62,100 27,800 17,500 42,900

13337120 Middle Fork Clearwater River at 

Kooskia, Idaho

239,000 159,000 351,000 147,000 95,700 217,000 104,000 60,900 170,000

13338100 South Fork Clearwater River 

near Harpster, Idaho

87,100 53,300 142,000 46,500 26,500 79,300 41,700 23,800 72,800

13338500 South Fork Clearwater River at 

Stites, Idaho

101,000 60,600 159,000 51,300 31,000 80,900 52,900 30,200 87,200

13340000 Clearwater River at Orofino, 

Idaho

517,000 301,000 845,000 285,000 159,000 479,000 243,000 132,000 423,000

13341570 Potlatch River below Little 

Potlatch Creek near Spalding, 

Idaho

212,000 50,900 607,000 12,900 5,090 27,400 199,000 44,700 598,000

13342500 Clearwater River at Spalding, 

Idaho

662,000 394,000 1,050,000 297,000 164,000 498,000 372,000 210,000 618,000

13351000 Palouse River at Hooper, 

Washington

335,000 139,000 691,000 20,900 5,210 58,600 304,000 132,000 608,000

Table 4. Estimated annual, total, and mean loads of suspended sediment, suspended sand, and suspended fines for stations in the 

lower Snake and Clearwater River Basins, Washington and Idaho, water years 2009–11.—Continued

[Locations of stations are shown in figure 2. Abbreviations: C.I., confidence interval; USGS, U.S. Geological Survey]
22 Sediment Transport in the Lower Snake and Clearwater River Basins, Idaho and Washington, 2008–11

USGS 

gaging 

station 

No.

Gaging station name

Total suspended sediment 

(tons)

Total suspended sands

(tons)

Total suspended fines

(tons)

Estimated 

load

Lower 95 

percent

C.I. 

Upper 95 

percent

C.I.

Estimated 

load

Lower 95 

percent 

C.I.

Upper 95 

percent

C.I. 

Estimated 

load

Lower 95 

percent

C.I.

Upper 95 

percent

C.I.

Water years 2009–11

13317000 Salmon River at White Bird, 

Idaho

5,090,000 2,960,000 8,290,000 2,780,000 1,740,000 4,210,000 2,280,000 1,190,000 4,080,000

13334000 Grande Ronde River

at Zindel, Washington

710,000 412,000 1,170,000 159,400 76,400 308,000 561,000 322,000 931,000

13334300 Snake River near Anatone, 

Washington

8,920,000 6,330,000 12,200,000 4,440,000 3,180,000 6,020,000 4,550,000 3,000,000 6,670,000

13336500 Selway River near Lowell, Idaho 368,000 212,000 601,000 270,000 138,000 475,000 116,000 63,900 201,000

13337000 Lochsa River near Lowell, Idaho 126,000 86,200 180,000 74,600 46,800 115,000 55,300 35,200 84,600

13337120 Middle Fork Clearwater River at 

Kooskia, Idaho

515,000 339,000 761,000 307,000 196,000 461,000 229,000 134,000 374,000

13338100 South Fork Clearwater River 

near Harpster, Idaho

146,000 94,400 225,000 76,000 45,800 123,000 71,700 43,500 118,000

13338500 South Fork Clearwater River at 

Stites, Idaho

166,000 105,000 252,000 76,400 48,500 116,000 91,600 54,900 145,000

13340000 Clearwater River at Orofino, 

Idaho

960,000 566,000 1,550,000 510,000 287,000 849,000 468,000 258,000 805,000

13341570 Potlatch River below Little 

Potlatch Creek near Spalding, 

Idaho

274,000 72,500 754,000 16,000 6,640 33,000 258,000 64,100 743,000

13342500 Clearwater River at Spalding, 

Idaho

1,150,000 701,000 1,790,000 493,000 279,000 810,000 662,000 384,000 1,080,000

13351000 Palouse River at Hooper, 

Washington

693,000 282,000 1,460,000 43,500 10,700 124,000 629,000 267,000 1,290,000

Table 4. Estimated annual, total, and mean loads of suspended sediment, suspended sand, and suspended fines for stations in the 

lower Snake and Clearwater River Basins, Washington and Idaho, water years 2009–11.—Continued

[Locations of stations are shown in figure 2. Abbreviations: C.I., confidence interval; USGS, U.S. Geological Survey]
Sediment Transport in the Lower Snake and Clearwater River Basins 23

USGS 

gaging 

station 

No.

Gaging station name

Mean suspended sediment 

(tons)

Mean suspended sands

(tons)

Mean total suspended fines

(tons)

Estimated 

load

Lower 95 

percent

C.I. 

Upper 95 

percent

C.I.

Estimated 

load

Lower 95 

percent 

C.I.

Upper 95 

percent

C.I. 

Estimated 

load

Lower 95 

percent

C.I.

Upper 95 

percent

C.I.

Water years 2009–11

13317000Salmon River at White Bird, 

Idaho

1,696,667 986,667 2,763,333 926,667 580,000 1,403,333 760,000 396,667 1,360,000

13334000 Grande Ronde River at Zindel, 

Washington

236,667 137,333 390,000 53,133 25,467 102,667 187,000 107,333 310,333

13334300 Snake River near Anatone, 

Washington

2,973,333 2,110,000 4,066,667 1,480,000 1,060,000 2,006,667 1,516,667 1,000,000 2,223,333

13336500 Selway River near Lowell, Idaho 122,667 70,667 200,333 90,000 46,000 158,333 38,667 21,300 67,000

13337000 Lochsa River near Lowell, Idaho 42,000 28,733 60,000 24,867 15,600 38,333 18,433 11,733 28,200

13337120 Middle Fork Clearwater River at 

Kooskia, Idaho

171,667 113,000 253,667 102,333 65,333 153,667 76,333 44,667 124,667

13338100 South Fork Clearwater River 

near Harpster, Idaho

48,667 31,467 75,000 25,333 15,267 41,000 23,900 14,500 39,333

13338500 South Fork Clearwater River at 

Stites, Idaho

55,333 35,000 84,000 25,467 16,167 38,667 30,533 18,300 48,333

13340000 Clearwater River at Orofino, 

Idaho

320,000 188,667 516,667 170,000 95,667 283,000 156,000 86,000 268,333

13341570 Potlatch River below Little 

Potlatch Creek near Spalding, 

Idaho

91,333 24,167 251,333 5,333 2,213 11,000 86,000 21,367 247,667

13342500 Clearwater River at Spalding, 

Idaho

383,333 233,667 596,667 164,333 93,000 270,000 220,667 128,000 360,000

13351000 Palouse River at Hooper, 

Washington

231,000 94,000 486,667 14,500 3,567 41,333 209,667 89,000 430,000

Table 4. Estimated annual, total, and mean loads of suspended sediment, suspended sand, and suspended fines for stations in the 

lower Snake and Clearwater River Basins, Washington and Idaho, water years 2009–11.—Continued

[Locations of stations are shown in figure 2. Abbreviations: C.I., confidence interval; USGS, U.S. Geological Survey]
24 Sediment Transport in the Lower Snake and Clearwater River Basins, Idaho and Washington, 2008–11

tac12-0737_fig07

WY 2009

WY 2010

WY 2011

Estimated annual load

Upper 95 percent confidence interval

Lower 95 percent confidence interval

EXPLANATION

Total suspended-sediment load, in tons per year

10,000,000

1,000,000

100,000

10,000

1,000

Salmon River

at White Bird

Grande Ronde

River at Zindel

Snake River

near Anatone

Selway River

near Lowell

Lochsa River

near Lowell

Middle Fork

Clearwater River

at Kooskia 

Clearwater River

at Spalding

Palouse River

at Hooper

South Fork Clearwater River 

near Harpster 

South Fork Clearwater River

at Stites

Clearwater River

at Orofino

Potlatch River

near Spalding

Gaging station name

Figure 7. Estimated total suspended-sediment loads and 95-percent confidence intervals for stations in the lower 

Snake and Clearwater River Basins, Idaho and Washington, water years (WY) 2009–11.
Sediment Transport in the Lower Snake and Clearwater River Basins 25

EXPLANATION

0

20

40

60

80

100

120

140

160

Salmon River

at White Bird

Grande Ronde

River at Zindel

Snake River

near Anatone

Selway River

near Lowell

Lochsa River

near Lowell

Middle Fork

Clearwater River

at Kooskia

South Fork

Clearwater River

near Harpster

South Fork

Clearwater River

at Stites

Clearwater River

at Orofino

Potlatch River

near Spalding

Clearwater River

at Spalding

Palouse River

at Hooper

Suspended sand

Suspended fines

Mean annual yield 2009–11, in tons per square mile per year

Figure 8. Mean annual yield of suspended sand and fines in the lower Snake and Clearwater River Basins, 

Washington and Idaho, water years 2009–11.

The Salmon River as measured at White Bird contributed 

most of the TSS load in the Snake River as measured at the 

Anatone station and a large part of the TSS load entering 

Lower Granite Reservoir. However, the relative contribution 

of the Salmon River to the TSS load entering the reservoir 

varied annually during water years 2009, 2010, and 2011. 

During water year 2009, the Salmon River contributed an 

estimated 1.66 million tons of suspended sediment to the 

Snake River and accounted for more than 90 percent of the 

1.82 million tons of suspended sediment entering Lower 

Granite Reservoir from the Snake River. During water years 

2010 and 2011, the Salmon River contributed 1.03 million 

and 2.4 million tons, or about 62 and 44 percent, respectively, 

of the suspended sediment entering the reservoir from the 

Snake River drainage basin. The suspended-sediment yield 

from the Salmon River Basin, particularly the sand-sized 

fraction, was one of the largest of the basins evaluated (fig. 8). 

During the 3 years of sample collection, the mean annual 

yield of suspended sediment from the Salmon River Basin 

was about 125 tons per square mile per year [(tons/mi2)/yr] of 

which about 55 percent was sand. Overall, during water years 

2009–11, the Salmon River transported about 5.1 million tons 

of suspended sediment to the Snake River, equivalent to about 

51 percent of the TSS, about 56 percent of the suspended 

sand, and about 44 percent of the suspended fine-grained load 

entering Lower Granite Reservoir from the combined Snake 

and Clearwater Rivers (fig. 9A–C).
26 Sediment Transport in the Lower Snake and Clearwater River Basins, Idaho and Washington, 2008–11

tac12-0737_fig09ab

Palouse River at

Hooper, Washington

693 (6.9 percent)

Potlatch River near

Spalding, Idaho

274 (2.7 percent)

Lower Granite

Reservoir, Washington

10,000 (100 percent)

Clearwater River at

Spalding, Idaho

1,150 (11 percent)

Clearwater River at

Orofino, Idaho

960 (9.6 percent)

M. Fork Clearwater River

at Kooskia, Idaho

515 (5.1 percent)

Lochsa River near

Lowell, Idaho

126 (1.3 percent)

Snake River near

Anatone, Washington

8,900 (89 percent)

Grande Ronde River

at Zindel, Washington

710 (7.1 percent) Salmon River at

White Bird, Idaho

5,090 (51 percent)

S. Fork Clearwater River

near Harpster, Idaho

146 (1.5 percent)

Selway River near

Lowell, Idaho

368 (3.7 percent)

S. Fork Clearwater River

at Stites, Idaho

166 (1.7 percent)

Palouse River at

Hooper, Washington

43.5 (0.88 percent)

Potlatch River near

Spalding, Idaho

16 (0.32 percent)

Lower Granite

Reservoir, Washington

4,930 (100 percent)

Clearwater River at

Spalding, Idaho

493 (10 percent)

Clearwater River at

Orofino, Idaho

510 (10 percent)

M. Fork Clearwater River

at Kooskia, Idaho

307 (6.2 percent)

Lochsa River near

Lowell, Idaho

74.6 (1.5 percent)

Snake River near

Anatone, Washington

4,4400 (90 percent)

Grande Ronde River

at Zindel, Washington

159 (3.2 percent) Salmon River at

White Bird, Idaho

2,780 (56 percent)

S. Fork Clearwater River

near Harpster, Idaho

76.0 (1.5 percent)

Selway River near

Lowell, Idaho

270 (5.5 percent) S. Fork Clearwater River

at Stites, Idaho

76.4 (1.6 percent)

A. Total suspended sediment

B. Total suspended sand

Figure 9. Estimated loads of (A) total suspended sediment, (B) total suspended sand, and (C) total suspended fines transported in the 

lower Snake and Clearwater River Basins, water years 2009–11. Values are in thousands of tons and percentage of total load entering 

Lower Granite Reservoir during water years 2009–11. Width of each arrow is proportional to the estimated suspended sediment load.
Sediment Transport in the Lower Snake and Clearwater River Basins 27

tac12-0737_fig09c

Palouse River at

Hooper, Washington

629 (12 percent)

Potlatch River near

Spalding, Idaho

258 (5.0 percent)

Lower Granite

Reservoir, Washington

5,210 (100 percent)

Clearwater River at

Spalding, Idaho

662 (13 percent)

Clearwater River at

Orofino, Idaho

468 (9.0 percent)

M. Fork Clearwater River

at Kooskia, Idaho

229 (4.4%)

Lochsa River near

Lowell, Idaho

55.3 (1.1 percent)

Snake River near

Anatone, Washington

4,550 (87 percent)

Grande Ronde River

at Zindel, Washington

561 (11 percent) Salmon River at

White Bird, Idaho

2,280 (44 percent)

S. Fork Clearwater River

near Harpster, Idaho

71.7 (1.4 percent)

Selway River near

Lowell, Idaho

116 (2.2 percent)

S. Fork Clearwater River

at Stites, Idaho

91.6 (1.8 percent)

C. Total suspended fines

Figure 9.—Continued

The Grande Ronde River enters the Snake River just 

upstream of the Snake River near Anatone station (fig. 2). 

During water years 2009–11, the Grande Ronde River as 

measured at the Zindel station, contributed about 710,000 tons 

of suspended sediment to the Snake River (fig. 9A), about 

79 percent of which was fine-grained sediment (table 4). The 

mean annual yield of suspended sediment from the Grande 

Ronde River Basin was about 60 (tons/mi2)/yr, of which about 

47 (tons/mi2)/yr was fine-grained sediment (fig. 8). Overall, 

the TSS load in the Grande Ronde River was equivalent to 

about 7.1 percent of the suspended sediment, about 3.2 percent 

of the suspended sand, and about 11 percent of the suspended 

fines that entered Lower Granite Reservoir during water years 

2009–11 (fig. 9A–C).

A primary contributor of TSS in the Clearwater drainage 

basin, particularly the sand-size fraction, is the Selway 

River, which discharged about 368,000 tons of TSS during 

water years 2009–11. The Selway River accounted for about 

71 percent of the TSS and about 88 percent of the suspended 

sand as measured downstream in the Middle Fork Clearwater 

River at Kooskia. The Lochsa River contributed about 

126,000 tons, or about 24 percent of the TSS in the Middle 

Fork Clearwater River at Kooskia. Of the TSS discharged 

from the Selway and Lochsa Rivers, about 73 and 59 percent, 

respectively, was sand-sized. The mean annual basin yields 

of suspended sediment from the Selway and Lochsa Rivers 

were 64 and 36 (tons/mi2)/yr, respectively during 2009–11. 

Overall, the sediment load delivered from the Selway River 

drainage basin was equivalent to about 32 percent of the TSS 

and about 55 percent of the suspended sand discharged to 

Lower Granite Reservoir from the Clearwater River during 

water years 2009–11. The TSS load from the Lochsa River 

drainage basin was equivalent to about 11 percent of the TSS 

and about 15 percent of the suspended sand discharged from 

the Clearwater River Basin during water years 2009–11. As a 

percentage of the TSS load entering Lower Granite Reservoir 

from both the Snake and Clearwater Rivers, the Selway River 

accounted for only about 3.7 and 5.5 percent, and the Lochsa 

River only about 1.3 and 1.5 percent of the TSS and total 

suspended sand, respectively (fig. 9A and 9B).

Combined, the Middle Fork Clearwater River 

(as measured at Kooskia, Idaho) and the South Fork 

Clearwater River (as measured at Stites, Idaho) discharged 

about 681,000 tons of suspended sediment during water 

years 2009–11 (fig. 9A). Of this total, about 76 percent, or 

515,000 tons was from the Middle Fork Clearwater River, 

equivalent to about 45 percent of the TSS and 62 percent of 

the suspended sand entering Lower Granite Reservoir from 

the Clearwater River. TSS discharged from the South Fork 

Clearwater River was equivalent to about 14 percent of the 

TSS and 16 percent of the suspended sand entering Lower 

Granite Reservoir from the Clearwater River during water 

years 2009–11.
28 Sediment Transport in the Lower Snake and Clearwater River Basins, Idaho and Washington, 2008–11

From the confluence of the South Fork Clearwater 

River at Stites and the Middle Fork Clearwater River at 

Kooskia downstream to the station at Orofino, the Clearwater 

River accrued about 279,000 tons of suspended sediment 

during water years 2009–11 (fig. 9A). Of this accrual, 

about 54 percent was fine-grained sediment. The Potlatch 

River contributed an additional 274,000 tons of suspended 

sediment to the Clearwater River. About 94 percent of the 

suspended sediment discharged from the Potlatch River 

was fine-grained sediment. The mean annual yield of TSS 

(156 [tons/mi2]/yr) and of fine-grained suspended sediment 

(148 [tons/mi2]/yr) from the Potlatch River Basin were 

the largest of all the stations monitored during water years 

2009–11 (fig. 8). During water year 2011, the yield of TSS 

and fine-grained sediment from the Potlatch River Basin were 

about 362 and 343 (tons/mi2)/yr, respectively. Although the 

TSS load from the Potlatch River was equivalent to about 

24 percent of the TSS load entering Lower Granite Reservoir 

from the Clearwater River during water years 2009–11, the 

fine-grained sediment discharged from the Potlatch River 

was equivalent to about 39 percent of the fine-grained load 

entering the reservoir from the Clearwater River. During water 

year 2011, the load from the Potlatch River was equivalent to 

about 32 percent of the TSS and 53 percent of the fine-grained 

suspended sediment as measured in the Clearwater River at 

Spalding. During water year 2011, streamflow in the Potlatch 

River accounted for less than 1 percent of the total combined 

streamflow entering Lower Granite Reservoir from the Snake 

and Clearwater Basins. However, the TSS load and the 

fine‐grained suspended-sediment load from the Potlatch River 

were equivalent to about 3.5 and 6.5 percent, respectively 

of the total transported to Lower Granite Reservoir, most 

of which was generated during the rain-on-snow event in 

mid-January.

The mean annual yield of TSS in the Palouse 

River as measured at Hooper, Washington, was about 

92 (tons/mi2)/yr during water years 2009–11. More than 

90 percent of the sediment transported in the Palouse River 

was fine-grained sediment. Although the Palouse River 

discharges to the Snake River downstream of Lower Granite 

Reservoir, the load of suspended sediment transported in 

the Palouse River during water years 2009–11 was about 

693,000 tons, equivalent to about 6.9 percent of the total 

discharged to Lower Granite Reservoir during water years 

2009–11 (fig. 9A). The load of fine-grained sediment 

transported in the Palouse River during the same period was 

about 629,000 tons, equivalent to about 95 percent of the 

662,000 tons of fine-grained sediment transported from the 

Clearwater River to Lower Granite Reservoir (fig. 9C). 

Suspended Sediment Delivery to Lower 

Granite Reservoir

Combined, the Snake and Clearwater Rivers discharged 

about 10 million tons of suspended sediment to Lower Granite 

Reservoir during water years 2009–11 (table 4; fig. 9A). 

However, the delivery of sediment to Lower Granite Reservoir 

varied annually. About 60 percent of the 3-year total was 

discharged to the reservoir during water year 2011, which was 

characterized by a large winter snowpack, a sustained spring 

runoff, and high TSS loads in the Snake and Clearwater Rivers 

through most of the summer (fig. 10). During water years 

2009–11, the Snake River accounted for about 89 percent of 

the TSS, about 90 percent of the suspended sand, and about 

87 percent of the suspended fines entering Lower Granite 

Reservoir (figs. 9A–C). Only during water year 2010 did the 

Clearwater River account for more than 15 percent of the TSS 

load entering Lower Granite Reservoir. 

Comparison with 1972–79 Study

Comparing data from this study with data collected 

during water years 1972–79 as described in Jones and 

Seitz (1980) indicate that, on average, the Clearwater 

River contributed a larger percentage of the TSS load 

entering Lower Granite Reservoir during 1972–79 than 

during 2008–11. Jones and Seitz (1980) used sediment 

transport curves to estimate loads, and they reported that, 

on an average annual basis, the Snake River accounted for 

about 80 percent and the Clearwater River about 20 percent 

of the TSS load entering Lower Granite Reservoir during 

1972–79. To evaluate differences in the sediment transport 

characteristics between the data collected by Jones and Seitz 

and the data collected during this study, sediment transport 

curves for both study periods were compared (fig. 11). For 

the Snake River near Anatone (fig. 11A), the transport curves 

for TSS show larger concentrations and loads over the 

entire range of streamflow during 2008–11 as compared to 

1972–79. An Analysis of Covariance (ANCOVA) (Helsel and 

Hirsch, 1992) indicates that the difference in concentrations 

of TSS (p=0.006, fig. 11A) and suspended sands (p<0.001, 

fig. 12B) were significantly larger (α =0.05) during 2008–11 

as compared to 1972–79. However, the concentrations of 

suspended fines (p=0.193) were not significantly different 

(fig. 12A). The best‐fit lines on figure 11B for the Clearwater 

River at Spalding indicate that the concentrations and load 

of suspended sediment in relation to streamflow was similar 

during 1972–79 as compared to 2008–11. An ANCOVA 
Sediment Transport in the Lower Snake and Clearwater River Basins 29

tac12-0737_fig10 Suspended-sediment load, in tons per month

100

1,000

10,000

100,000

1,000,000

10,000,000

Clearwater River at Spalding, Idaho

Snake River near Anatone, Washington

EXPLANATION

M A M J SJ A N DO F MJ A M J SJ A N DO F MJ A M J SJ A N DO A MMJ F J SJ A

2008 2009 2010 2011

Water year

Figure 10. Suspended-sediment loads delivered monthly to Lower Granite Reservoir from the Snake and Clearwater 

Rivers, Washington and Idaho, March 2008–September 2011.

indicates that the difference in concentrations of TSS 

(p=0.853) and suspended fines (p=0.666) in the Clearwater 

River were not statistically significant between the two 

periods. However, the suspended-sand concentration in 

the Clearwater River at Spalding was significantly larger 

(p=0.011) during 2008–11 as compared to 1972–79. 

Using continuous streamflow records and suspendedsediment data collected during 1972–79, loads for suspended 

sand and suspended fines were estimated for the Snake River 

near Anatone and the Clearwater River at Spalding using the 

LOADEST model. The suspended sand and suspended fines 

were analyzed separately for the 1970s data to estimate the 

fractional loads during each year for 1972–79 and 2009–11 

(fig. 13). The results indicate that the TSS load entering Lower 

Granite Reservoir from the Snake River increased from an 

annual average of about 71 percent of the total in water years 

1972–79 to 89 percent in water years 2009–11. Conversely, 

the load from the Clearwater River decreased from 29 percent 

during 1972–79 to 11 percent during 2009–11.

As a proportion of the TSS load entering Lower Granite 

Reservoir from the combined Snake and Clearwater Rivers, 

the sand fraction increased from an annual average of about 

30 percent during 1972–79 to 48 percent during 2009–11. 

Most of the increase in the sand load was attributable to 

the Snake River. In the Snake River near Anatone, the 

sand fraction increased from an average of 28 percent of 

the TSS load during 1972–79 to an average of 48 percent 

during 2009–11 (fig. 13). Data for the Salmon River are not 

sufficient to estimate suspended-sediment loads for 1972–79. 
30 Sediment Transport in the Lower Snake and Clearwater River Basins, Idaho and Washington, 2008–11

tac12-0737_fig11ab

R2 = 0.86

R2

 = 0.80

R2

 = 0.63

R2

 = 0.52

R2 = 0.94

R2

 = 0.87

R2

 = 0.88

R2

 = 0.73

0.1

1.0

10

100

1,000

10,000

100,000

1,000,000

2,000 20,000 200,000

Total suspended-sediment load, in tons per day

0.1

1.0

10

100

1,000

10,000

100,000

1,000,000

0.1

1.0

10

100

1,000

10,000

100,000

1,000,000

0.1

1.0

10

100

1,000

10,000

100,000

1,000,000

10,000 100,000

Streamflow, in cubic feet per second

B. Clearwater River at Spalding, Idaho 

A. Snake River near Anatone, Washington 

Best fit line for load or 

concentration, 2008–11

Best fit line for load or 

concentration, 1972–79

Sediment load, 

water years 2008–11

Sediment load, 

water years 1972–79

Sediment concentration, 

water years 2008–11

Sediment concentration, 

water years 1972–79

EXPLANATION

Total suspended-sediment concentration, in milligraps per liter

Figure 11. Suspended-sediment transport curves for concentrations and loads in the (A) Snake River near Anatone, 

Washington, and (B) Clearwater River at Spalding, Idaho, for data collected during water years 1972–79 and 2008–11.
Sediment Transport in the Lower Snake and Clearwater River Basins 31

tac12-0737_fig12ab

R2 = 0.89

R2 = 0.81

R2 = 0.75

R2 = 0.61

R2 = 0.97

R2 = 0.85

R2 = 0.95

R2 = 0.75

10,000 100,000

Best fit line for load or 

concentration, 2008–11

Best fit line for load or 

concentration, 1972–79

Suspended-sands load, 

water years 2008–11

Suspended-sands load, 

water years 1972–79

Suspended-sands concentration, 

water years 2008–11

Suspended-sands concentration, 

water years 1972–79

0.1

1

10

100

1,000

10,000

100,000

1,000,000

Suspended-sediment load, in tons per day

0.1

1.0

10

100

1,000

10,000

100,000

1,000,000

0.1

1

10

100

1,000

10,000

100,000

1,000,000

Suspended-sediment concentration, in milligrams per liter

0.1

1.0

10

100

1,000

10,000

100,000

1,000,000

Streamflow, in cubic feet per second

A. Suspended fines

B. Suspended sands

EXPLANATION

Best fit line for load or 

concentration, 2008–11

Best fit line for load or 

concentration, 1972–79

Suspended-fines load, 

water years 2008–11

Suspended-fines load, 

water years 1972–79

Suspended-fines concentration, 

water years 2008–11

Suspended-fines concentration, 

water years 1972–79

EXPLANATION

Figure 12. Suspended-sediment transport curves for concentrations and loads for (A) suspended fines and (B) suspended 

sands, Snake River near Anatone, Washington, water years 1972–79 and 2008–11.
32 Sediment Transport in the Lower Snake and Clearwater River Basins, Idaho and Washington, 2008–11

tac12-0737_fig13

0 

500,000 

1,000,000 

1,500,000 

2,000,000 

2,500,000 

3,000,000 

1972 1973 1974 1975 1976 1977 1978 1979 2009 2010 2011 

Sand – Clearwater River at Spalding

Fines – Clearwater River at Spalding

Sand – Snake River near Anatone

Fines – Snake River near Anatone

Water year

Suspneded-sediment load, in tons per year

EXPLANATION

Figure 13. Estimated annual suspended sand and fine loads, Snake River near Anatone, Washington, and Clearwater River 

at Spalding, Idaho, water years 1972–79 and 2009–11.

However, the increase in the suspended-sand load noted in 

the Snake River near Anatone probably is attributable to 

the Salmon River. A century of fire suppression and other 

forest-management practices resulted in an increase in the 

number and severity of forest fires in central Idaho during the 

last quarter of the 20th century (Burton, 2005) and the first 

decade of the 21st century. The effect of wild fires on sediment 

mobility can be particularly dramatic in the Salmon River 

Basin and other areas of central Idaho where disturbance of 

steep drainage basins with highly erosive soils can mobilize 

large quantities of sand and gravel to streams (King and 

others, 2004). Although it is beyond the scope of this study, it 

would be insightful to obtain fluvial sediment data from select 

locations within the Salmon River Basin to identify critical 

subbasins and to quantify the magnitude of sediment delivery 

to discrete reaches of the Salmon River, the lower Snake 

River, and ultimately to Lower Granite Reservoir.
Sediment Transport in the Lower Snake and Clearwater River Basins 33

Suspended-Sediment Surrogates

Surrogate models were developed using suspended 

sediment and acoustic backscatter data collected during 

water year 2010, and data from water year 2011 were used to 

validate the models. The best surrogate models based on the 

highest R2 were developed using the 1.5MHz ADVM at the 

streamgage on the Snake River near Anatone and the 3MHz 

ADVM at the streamgage on the Clearwater River at Spalding 

(table 5; fig. 14). Separate models were developed for each 

streamgage to estimate overall SSC as well as concentrations 

of sand and fines (table 5). SSC results from water year 

2011 matched the acoustic surrogate models, on average, 

-8.3 percent at the Snake River station and +9.8 percent at the 

Clearwater River station. Major deviations from the models 

were not evident in water year 2011 except for one sample 

with a low sediment concentration at the Clearwater River 

gaging station (fig. 14).

Table 5. Summary of acoustic surrogate and LOADEST models used to evaluate suspended-sediment concentrations at the Snake 

River near Anatone, Washington, and Clearwater River at Spalding, Idaho. 

[Locations of stations are shown in figure 2. Abbreviations: R2, coefficient of determination; RPD, relative percent difference between sample and model 

results; BCF, Duan’s bias correction factor; MHz, megahertz; ABScorr, acoustic backscatter corrected for beam spreading and attenuation by water and sediment 

(dB, decibel); ln, natural log; SSC, suspended-sediment concentration (milligrams per liter); Q, streamflow (cubic feet per second); T, is the centered decimal 

time in years from the beginning of the calibration period]

Gaging 

station

name

Model

Number of 

samples 

used for 

regression; 

validation

Model R2

Average 

RPD

(percent)

Standard

error

BCF

Snake 

River near 

Anatone

1.5MHz_ABScorr 22; 9 SSC = 10[(0.0756×1.5MHz_ABScorr)-4.676]×1.048 0.92 +10 1.39 1.048

Sand concentration = 10[(0.105×1.5MHz_ABScorr)-7.636]×1.129 0.89 +24 1.73 1.129

Fines concentration = 10[(0.0615×1.5MHz_ABScorr)-3.730]×1.084 0.81 +19 1.54 1.084

LOADEST 38 ln SSC = 3.96 + 2.09(lnQ) + 0.08(lnQ 2

) - 0.54[sin(2πT )] + 0.16[cos(2πT )] 0.90

ln sand conc = 2.97 + 2.99(lnQ) - 0.46(lnQ 2

) - 0.49[sin(2πT )] - 0.06[cos(2πT )] 0.95

ln fines conc = 3.38 + 1.72(lnQ) + 0.22(lnQ 2

) - 0.56[sin(2πT )] + 0.34[cos(2πT )] 0.80

Clearwater

River at 

Spalding

3MHz_ABScorr 30; 11 SSC = 10[(0.0557*3MHz_ABScorr)-2.431]×1.040 0.93 +8.6 1.34 1.040

Sand concentration = 10[(0.0743×3MHz_ABScorr)-4.147)]×1.146 0.87 +34 1.71 1.146

Fines concentration = 10[(0.0461×3MHz_ABScorr)-2.030]×1.097 0.78 +19 1.58 1.097

LOADEST 38 ln SSC = 1.79 + 1.05(lnQ) + 0.59(lnQ 2

) + 0.46[sin(2πT )] - 0.48[cos(2πT )] 0.74

ln sand conc = 0.05 + 1.53(lnQ) + 0.69(lnQ 2

) + 0.24[sin(2πT )] - 0.85[cos(2πT )] 0.84

ln fines conc = 1.53 + 0.85(lnQ) + 0.52(lnQ 2

) + 0.65[sin(2πT )] - 0.36[cos(2πT )] 0.62
34 Sediment Transport in the Lower Snake and Clearwater River Basins, Idaho and Washington, 2008–11

Suspended-sediment loads computed using the acoustic 

surrogate models for the Snake and Clearwater Rivers were 

compared to the loads calculated using LOADEST. The 

acoustic surrogate models were developed for concentration 

only, and loads were calculated by multiplying concentration 

estimates by daily streamflow as measured at each streamgage. 

Performance of the models was compared by first evaluating 

the coefficient of determination (R2) of the acoustic surrogate 

and LOADEST concentration (not load) models. The acoustic 

surrogate models had a better correlation than the LOADEST 

concentration models at both stations for SSC and suspended 

fines and for suspended sand at the Clearwater River station 

(table 5). The LOADEST concentration model had a better 

correlation than the acoustic surrogate models for suspended 

sands at the Snake River station (table 5).

Load results for the Snake River using LOADEST and 

acoustic surrogate models were markedly different. The largest 

difference occurred in water year 2011, when the suspendedsediment load calculated using the LOADEST model was 

more than three times higher than the load calculated using the 

acoustic surrogate model (table 6). The total monthly sediment 

loads were substantially higher using the LOADEST model as 

compared to the acoustic surrogate model, particularly during 

May and June (fig. 15A). Generally, SSC at the Snake River 

station were underestimated by the acoustic surrogate model 

when concentrations were high (greater than 200 mg/L) and 

had a high percentage of sand. The unregulated Salmon River 

contributed an average 59 percent of the streamflow passing 

the Snake River at Anatone streamgage during these sampling 

events, but there was no clear relation between the percentage 

tac12-0737_fig14

y = 0.0557x - 2.431

R2

 = 0.93 

y = 0.0756x - 4.676

R2

 = 0.92 

0.0 

0.5 

1.0 

1.5 

2.0 

2.5 

3.0 

40 50 60 70 80 90 100 

Clearwater River WY2010 samples used to develop surrogate relation—3MHz ABScorr

Clearwater River WY2011 samples used to verify surrogate relation—3MHz ABScorr

Snake River WY2010 samples used to develop surrogate relation—1.5MHz ABScorr

Snake River WY2011 samples used to verify surrogate relation—1.5MHz ABScorr 

3MHz or 1.5MHz ABScorr, in decibels

Log suspended-sediment concentration, in milligrams per liter

EXPLANATION

Figure 14. Best-fit surrogate models for the Snake River near Anatone, Washington, using the 1.5MHz acoustic Doppler 

velocity meter (ADVM) and the Clearwater River at Spalding, Idaho, using the 3MHz ADVM, water years 2010–11.
Sediment Transport in the Lower Snake and Clearwater River Basins 35

Table 6. Comparison of suspended-sediment loads estimated using acoustic backscatter and LOADEST models in the Snake River 

near Anatone, Washington, and Clearwater River at Spalding, Idaho.

[Locations of stations are shown in figure 2. Total suspended-sediment load may not equal total sand load plus total fines load due to errors in individual 

models. Abbreviations: MHz, megahertz; ABScorr, acoustic backscatter corrected for beam spreading and attenuation by water and sediment (dB, decibel); 

Q, streamflow]

Gaging station 

name Water year Model

Total suspendedsediment load

(tons) 

Total suspendedsand load

(tons)

Total suspendedfines load

(tons)

Snake River near 

Anatone

2009 1.5MHz_ABScorr 1 1,460,000 709,000 789,000 

LOADEST (Q, Q 2, seasonality) 1,820,000 848,000 998,000 

2010 1.5MHz_ABScorr 1,120,000 589,000 606,000 

LOADEST (Q, Q 2, seasonality) 1,650,000 795,000 855,000 

2011 1.5MHz_ABScorr 1,740,000 701,000 1,030,000 

LOADEST (Q, Q 2, seasonality) 5,450,000 2,800,000 2,700,000 

Overall 1.5MHz_ABScorr 4,320,000 2,000,000 2,430,000 

LOADEST (Q, Q 2, seasonality) 8,920,000 4,440,000 4,550,000 

Clearwater River 

at Spalding

2009 3MHz_ABScorr 315,000 138,000 180,000 

LOADEST (Q, Q 2, seasonality) 335,000 137,000 199,000 

2010 3MHz_ABScorr 169,000 73,900 98,200 

LOADEST (Q, Q 2, seasonality) 152,000 59,000 91,200 

2011 3MHz_ABScorr 645,000 329,000 342,000 

LOADEST (Q, Q 2, seasonality) 662,000 297,000 372,000 

Overall 3MHz_ABScorr 1,130,000 541,000 620,000 

LOADEST (Q, Q 2, seasonality) 1,150,000 493,000 662,000 

1

Loads presented for the 1.5MHz_ABScorr in water year 2009 are for April–September only.

of unregulated streamflow passing the station and percent 

error in the estimated SSCs. There also was no clear relation 

between percent organic matter and percent error in the 

estimations. Some of the error in the acoustic surrogate model 

at high sand concentrations may be due to the placement of 

the ADVM in the water column. If the sand particles were 

not well-mixed in the water column during specific events or 

were transported in a layer close to the streambed, the ADVM 

would not have detected those particles because it is mounted 

approximately mid-depth in the water column. This scenario 

is likely, particularly in water year 2011, because of the large 

amount of sand transport measured in the Salmon River and 

contributed to the Snake River upstream of the surrogate 

monitoring station at Anatone.

Further examination of estimated SSCs in water year 

2011 shows that the LOADEST model produces much higher 

estimated concentrations than the sediment surrogate model on 

the descending limb of the hydrograph (fig. 16). The acoustic 

surrogate model should be a more direct measurement of 

sediment concentrations as compared to the LOADEST model 

(which is based on flow, time, and seasonality terms), at least 

during storm events. The pattern over the ascending and 

descending limbs of the hydrograph show that the LOADEST 

model is affected by hysteresis, but perhaps less so than a 

traditional transport curve based on a single streamflow term 

(Wood and Teasdale, 2013). Generally, the acoustic surrogate 

model matched more closely with the measured SSCs in 

samples collected at the Snake River near Anatone than the 

LOADEST model, except during high streamflow in May and 

June 2011 (fig. 16). Neither the acoustic or the LOADEST 

model performed well during this period, especially during 

June when the sample results were between the concentrations 

estimated by the two models. 
36 Sediment Transport in the Lower Snake and Clearwater River Basins, Idaho and Washington, 2008–11

Figure 15. Comparison of total monthly sediment load estimated using 

acoustic backscatter and LOADEST models in the (A) Snake River near 

Anatone, Washington, and (B) Clearwater River at Spalding, Idaho, water 

years 2009–11.

NOTE: Each month represents the total load that occurred during that 

month within the study period. For example, the total load for May in the 

Clearwater River is the sum of loads in May 2009, May 2010, and May 2011.

B. Clearwater River at Spalding

A. Snake River near Anatone

50,000 

100,000 

150,000 

200,000 

250,000 

300,000 

350,000 

400,000 

 500,000 

0 

 1,000,000 

 1,500,000 

 2,000,000 

 2,500,000 

 3,000,000 

 3,500,000 

 4,000,000 

Total suspended-sediment load, in tons per month

3MHz_ABScorr model

LOADEST model 

1.5MHz_ABScorr model

LOADEST model 

EXPLANATION

EXPLANATION

Oct . Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug. Sept. 

0 

Month
Sediment Transport in the Lower Snake and Clearwater River Basins 37

tac12-0737_fig16

Suspended-sediment concentration (SSC), in milligrams per liter

Streamflow, in cubic feet per second

0 

20,000 

40,000 

60,000 

80,000 

100,000 

120,000 

140,000 

160,000 

0 

50 

100 

150 

200 

250 

300 

350 

Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug. Sept.

Water year 2011

Instantaneous SSC, 1.5MHz_ABScorr model

Daily SSC, LOADEST model

Streamflow

Sediment sample

EXPLANATION

Figure 16. Comparison of suspended-sediment concentrations estimated using acoustic backscatter and a 

LOADEST model, Snake River near Anatone, Washington, water year 2011.

Model simulation results for load compared well for 

the Clearwater River station. Suspended-sediment loads at 

the Clearwater River computed by the acoustic surrogate 

and LOADEST models matched within 3–11 percent on 

an annual basis and within 2 percent over the entire study 

period (table 6). The percent differences in the Clearwater 

River were slightly higher for total suspended-sand load, 

from 1–22 percent on an annual basis. Overall, however, the 

model results compared well. The acoustic surrogate model, 

in general, produced sand load estimates using the 1.5 MHz 

ADVM that were slightly higher than the estimates generated 

using the LOADEST model (9 percent higher for the entire 

study period). Suspended fines loads, which were estimated 

using the higher frequency 3.0 MHz ADVM, compared 

within 7–10 percent on an annual basis between the two 

models, and within 7 percent for the entire study period. The 

monthly average suspended-sediment loads for 2010–11 for 

the Clearwater River also compared well between the acoustic 

surrogate and LOADEST models (fig. 15B).

For storm events, the acoustic surrogate models provide a 

more direct measure of sediment concentrations as compared 

to LOADEST or traditional transport curves that are based 

only on streamflow. Thus, the acoustic surrogates probably are 

a more accurate method for estimating suspended-sediment 

loads over a short period or when real-time response is 

needed. Additionally, LOADEST models cannot be used to 

provide real-time, instantaneous load estimates because of 

post-processing requirements. Differences in load estimates 

between the acoustic surrogate and LOADEST models were 

further examined by summing estimated loads over the 

ascending and descending limbs of the hydrograph for several 
38 Sediment Transport in the Lower Snake and Clearwater River Basins, Idaho and Washington, 2008–11

well-defined hydrologic events during the study period: 

seven events for the Snake River near Anatone and eight 

events for the Clearwater River at Spalding (table 7). Loads 

calculated using the acoustic surrogate model were higher on 

the ascending limb (negative percent difference) and lower on 

the descending limb (positive percent difference) as compared 

to loads calculated using the LOADEST model in all cases 

except for one event at the Snake River station (ascending 

limb) and two events at the Clearwater River station (one 

on the ascending limb, one on the descending limb). For all 

events combined, loads calculated using the acoustic surrogate 

model were 11–14 percent higher on the ascending limb and 

24–65 percent lower on the descending limb as compared 

to loads calculated using the LOADEST model. The highest 

calculated and measured SSC typically were higher on the 

ascending limb (especially for acoustics), but total calculated 

loads often were higher on the descending limb than the 

ascending limb due to a prolonged recession in the streamflow.

During the June 2010 spring snowmelt runoff, multiple 

suspended-sediment samples were collected and compared 

to the acoustic surrogate results to validate the relative 

performance as compared to LOADEST models. In the Snake 

and Salmon River Basins, a large pulse of sediment-laden 

water was delivered from the Salmon River to the Snake 

River during this snowmelt runoff event. This sediment pulse 

was captured in the acoustic surrogate model as a spike in 

the acoustic backscatter on June 3 (fig. 17A), but not by the 

LOADEST model, which is primarily based on changes in 

streamflow. Samples were not collected on June 3, 2010 to 

verify either model. The acoustic surrogate model showed 

a rapid decrease in SSC directly following the peak of 

the hydrograph on June 3, whereas the LOADEST model 

showed elevated SSC until well after the peak. The sample 

with high SSC collected on June 6 does not match estimates 

from either model, but the samples collected on June 4 and 

June 14 match well with the estimated SSC from the acoustic 

surrogate model. The acoustic surrogate model estimated sand 

concentrations close to all three of the sand concentrations in 

the samples collected from the Snake River (fig. 17B). The 

largest difference between model estimates and the June 6 

simulation results was for the fines concentration (fig. 17C), 

which was better represented by the LOADEST model. During 

the April 2010 hydrologic event on the Clearwater River 

(fig. 18), the sample SSC results match the acoustic surrogate 

model estimates closely, whereas the LOADEST model 

underestimates most concentrations and loads throughout the 

event (fig. 18, table 7). 

Table 7. Comparison of suspended-sediment loads during selected hydrologic events calculated using acoustic backscatter and a 

LOADEST model in the Snake River near Anatone, Washington, and Clearwater River at Spalding, Idaho.

[Locations of stations are shown in figure 2. ABScorr: Acoustic backscatter corrected for beam spreading and attenuation by water and sediment (dB, decibel). 

LOADEST: Load Estimator. Percentages in parentheses indicate negative difference. Abbreviation: NA, not applicable]

Gaging station 

name

Event

No. Dates of event

Suspended-sediment load over

ascending limb of hydrograph (tons)

Suspended-sediment load over

descending limb of hydrograph (tons)

ABScorr LOADEST Percent 

difference ABScorr LOADEST Percent 

difference

Snake River

near Anatone

NA NA–All seven events combined 729,000 632,000 (14) 862,000 1,700,000 65 

1 April 18–May 3, 2009 122,000 85,900 (35) 89,600 115,000 25 

2 May 5–11, 2009 48,600 37,400 (26) 24,400 37,800 43 

3 May 18–24, 2009 98,400 86,700 (13) 106,000 159,000 40 

4 April 20–27, 2010 15,700 13,000 (19) 12,100 13,100 8 

5 May 16–30, 2010 33,000 26,100 (23) 48,400 59,900 21 

6 June 2–19, 2010 267,000 214,000 (22) 481,000 953,000 66 

7 May 11–19, 2011 144,000 169,000 16 100,000 357,000 112 

Clearwater River

at Spalding

NA NA–All eight events combined 274,000 245,000 (11) 225,000 287,000 24 

1 May 13–24, 2008 59,400 94,500 46 13,000 22,200 52 

2 April 21–May 1, 2009 13,000 10,700 (19) 17,700 32,600 59 

3 April 16–27, 2010 5,740 2,990 (63) 3,700 3,260 (13)

4 May 14–26, 2010 14,700 6,880 (72) 7,370 10,700 37 

5 June 2–July 5, 2010 70,600 55,600 (24) 34,900 37,300 7 

6 January 15–February 2, 2011 17,600 5,520 (104) 17,800 18,600 4 

7 March 29–April 15, 2011 55,500 45,300 (20) 93,900 113,000 18 

8 May 11–20, 2011 37,400 23,400 (46) 36,200 49,600 31 
Sediment Transport in the Lower Snake and Clearwater River Basins 39

tac12-0737_fig17ab

0 

20,000 

40,000 

60,000 

80,000 

100,000 

120,000 

140,000 

160,000 

180,000 

200,000 

0 

 50 

 100 

 150 

 200 

 250 

 300 

 350 

 400 

 450 

 500 

ABScorr model

LOADEST model

Streamflow

Sample

0 

20,000 

40,000 

60,000 

80,000 

100,000 

120,000 

140,000 

160,000 

180,000 

200,000 

0 

50 

100 

150 

200 

250 

300 

350 

400 

450 

500 

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Suspended-sediment concentration, in milligrams per liter

Streamflow, in cubic feet per second

EXPLANATION

B. Suspended sand

A. Total suspended sediment

June 2010

Figure 17. Comparison of (A) total suspended sediment, (B) suspended sand, 

and (C) suspended fine concentrations estimated using acoustic backscatter and 

a LOADEST model during spring snowmelt runoff at Snake River near Anatone, 

Washington, June 2010.
40 Sediment Transport in the Lower Snake and Clearwater River Basins, Idaho and Washington, 2008–11

Overall, in the Snake River near Anatone and the 

Clearwater River at Spalding, acoustic backscatter appeared 

to better estimate the SSC and suspended-sediment load than 

the LOADEST model over short periods. This is primarily 

because acoustic backscatter (1) is not affected by hysteresis; 

(2) provides a more direct, in-situ measurement of suspended 

sediment; and (3) is better able to represent sediment sources 

from a combination of regulated and unregulated sources, 

which can be poorly represented by a model based primarily 

on changes in streamflow. However, on a monthly and (or) 

annual basis, the acoustic surrogate and LOADEST models 

produced similar sediment load estimates for the Clearwater 

River at Spalding. Which model produced better estimates of 

monthly and annual sediment loads for the Snake River near 

Anatone is inconclusive. Much of the difference between the 

model simulation results is represented on the falling limb of 

the hydrograph and at mid-range streamflows during which 

few samples were collected. However, during high streamflow 

events such as the snowmelt runoff in the Snake River during 

2011, acoustic surrogate tools may be unable to capture the 

contribution of suspended sand moving near the bottom of the 

water column and thus, may underestimate the total load of 

suspended sediment. 

It is useful to show the acoustic surrogate model 

simulation results for comparison with LOADEST models as 

a demonstration that the most desirable method for estimating 

sediment load depends on the period of interest, the type of 

sediment being transported, and the degree of mixing and (or) 

stratification of the sediment in the stream. On an annual basis, 

both the LOADEST and surrogate models appear to provide a 

good estimate of suspended-sediment loads in the Snake and 

Clearwater Rivers. However, because surrogate methods for 

estimating loads were not available in all the tributary basins 

evaluated during this study, the LOADEST simulation results 

were used for the basin-wide sediment budgets to provide a 

consistent method for comparison. 

tac12-0737_fig17c

0 

20,000 

40,000 

60,000 

80,000 

100,000 

120,000 

140,000 

160,000 

180,000 

200,000 

0 

50 

100 

150 

200 

250 

300 

350 

400 

450 

500 

Suspended-sediment concentration, in milligrams per liter

Streamflow, in cubic feet per second

C. Suspended fines

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

June 2010

ABScorr model

LOADEST model

Streamflow

Sample

EXPLANATION

Figure 17.—Continued
Sediment Transport in the Lower Snake and Clearwater River Basins 41

Figure 18. Comparison of total suspended-sediment concentrations estimated 

using acoustic backscatter and a LOADEST model during a hydrologic event at 

Clearwater River at Spalding, Idaho, April and May 2010.

tac12-0737_fig18

0 

5,000 

10,000 

15,000 

20,000 

25,000 

30,000 

35,000 

0 

10 

20 

30 

40 

50 

60 

14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 1 2

ABScorr model

LOADEST model

Streamflow

Samples

Total suspended-sediment concentration, in milligrams per liter

Streamflow, in cubic feet per second

EXPLANATION

April 2010 May 2010

Bedload

Bedload was highly variable throughout the Snake and 

Clearwater River Basins and at each station where samples 

were collected (table 8). Measured bedload ranged from zero 

during base flow conditions at all stations to a maximum of 

4,140 ton/d in the Salmon River at White Bird at a streamflow 

of 70,800 ft3/s (table 8). Stations located in upstream areas of 

a drainage basin generally had a larger percentage of the total 

sediment load as bedload (fig. 19) and a stronger correlation 

(higher R2) between streamflow and bedload discharge as 

compared to stations located downstream (table 9). High 

stream gradients and high stream velocities at upstream 

stations probably account for the higher percentage of bedload. 

Best-fit regression equations and coefficients of determination 

(R2) for the stations where bedload was collected are presented 

in table 9. Bedload transport curves are shown in figure 20. 

At most of the stations with measureable bedload, the particle 

size distribution (table 10) typically was unimodal at lower 

streamflows, with sand being the dominant particle size. At 

higher streamflows and during peak bedload discharge, the 

particle size was typically bimodal, with sand and coarse 

gravel composing the predominant particle size. 

In the Snake River Basin, the measured bedload at the 

Salmon River at White Bird ranged from 25.9 to 4,140 ton/d 

(table 8). However, only five bedload samples were collected 

from the Salmon River during this study, and the correlation 

between streamflow and bedload was relatively poor 

(R2=0.46; table 9). Three of the five samples collected from 

the Salmon River were unimodal, with 99 percent of the 

bedload composed of sand. The two other bedload samples 

from the Salmon River were bimodal and consisted of 

35–45 percent gravel (2–64 mm diameter) and 55–65 percent 

sand (0.063–2 mm diameter) (table 10). Based on the best‐fit 

regression equation (table 9), the total bedload discharge 

in the Salmon River was about 66,000 tons during water 

years 2009–11; about 1.3 percent of the total sediment load 

transported in the Salmon River at White Bird (fig. 19). 
42 Sediment Transport in the Lower Snake and Clearwater River Basins, Idaho and Washington, 2008–11

Table 8. Bedload data from sampling stations in the lower Snake and Clearwater River Basins, 

Washington and Idaho, water years 2008–11.

[Locations of stations are shown in figure 2. Sampler type: HS, Helley Smith. Abbreviations: ft, foot; ft3/s, cubic foot 

per second; ton/d, ton per day]

Date

Mean 

sample

time

Channel

width 

(ft)

Streamflow, 

instantaneous 

(ft3

/s)

Number of 

sampling 

points 

Rest time 

on bottom 

(seconds)

Sampler

type

Bedload 

discharge 

(ton/d)

Salmon River at White Bird, Idaho (13317000)

04-24-09 1030 370 32,400 10 60 HS 188

05-20-09 1332 460 70,800 10 60 HS 4,140

06-03-09 1535 445 60,400 10 60 HS 189

06-11-09 0821 400 41,000 10 60 HS 25.9

05-15-11 1245 430 55,300 7 60 HS 545

Snake River near Anatone, Washington (13334300)

05-06-08 1600 560 49,800 20 60 HS 110

06-10-08 1145 597 73,200 10 60 HS 271

07-08-08 0920 550 40,900 10 60 HS 14.2

04-16-09 0915 570 46,600 20 60 HS 16

04-23-09 1230 590 83,000 10 60 HS 214

05-20-09 1330 630 105,000 20 60 HS 89.8

06-16-09 1750 600 72,100 20 60 HS 165

05-20-10 0916 580 62,500 8 60 HS 176

06-04-10 1425 617 105,000 20 60 HS 195

06-14-10 1518 622 93,700 20 60 HS 170

04-04-11 1519 570 68,000 20 60 HS 406

04-05-11 1310 570 79,900 20 60 HS 317

05-14-11 1555 589 102,000 14 60 HS 20.4

05-16-11 1335 608 142,000 14 60 HS 346

05-17-11 1550 608 125,000 13 60 HS 183

06-21-11 1248 590 110,000 15 60 HS 491

Selway River near Lowell, Idaho (13336500)

04-22-09 1045 270 14,000 20 60 HS 17.5

05-18-09 1500 300 18,500 20 60 HS 234

05-19-09 1040 315 27,300 20 60 HS 463

05-21-09 1123 310 20,300 20 60 HS 75.2

06-09-09 1231 303 13,800 20 60 HS 72.1

05-19-10 1110 306 16,400 20 60 HS 24.9

06-04-10 1350 308 21,800 20 60 HS 440

06-15-10 1318 300 14,200 20 60 HS 29.4

05-17-11 1025 305 19,400 20 60 HS 151

05-24-11 1730 308 22,000 20 60 HS 92

06-10-11 0816 305 20,600 20 30 HS 86.1

Lochsa River near Lowell, Idaho (13337000)

05-18-11 0815 276 12,600 20 60 HS 132

06-07-11 0905 275 21,200 20 60 HS 329

06-10-11 1036 280 14,800 20 60 HS 26.0
Sediment Transport in the Lower Snake and Clearwater River Basins 43

Date

Mean 

sample

time

Channel

width 

(ft)

Streamflow, 

instantaneous 

(ft3

/s)

Number of 

sampling 

points 

Rest time 

on bottom 

(seconds)

Sampler

type

Bedload 

discharge 

(ton/d)

Middle Fork Clearwater River at Kooskia, Idaho (13337120)

04-22-09 1500 465 25,600 20 60 HS 23.6

05-18-09 1523 455 34,000 20 60 HS 336

05-19-09 1530 490 46,000 14 60 HS 222

05-21-09 1653 460 35,000 20 60 HS 258

06-09-09 1600 460 24,000 20 60 HS 13.9

05-21-10 1607 457 22,700 20 60 HS 13.5

06-04-10 1500 495 44,100 20 60 HS 163

06-16-10 1011 455 23,400 20 60 HS 33.7

05-17-11 1600 470 37,000 20 60 HS 163

05-26-11 0830 500 46,700 20 60 HS 146

06-08-11 1044 508 57,000 20 60 HS 592

South Fork Clearwater River near Harpster, Idaho (13338100)

04-08-09 1021 72 1,410 20 60 HS 0.6

04-14-09 1252 66 2,340 20 60 HS 10.3

04-22-09 0937 80 5,540 20 60 HS 16.7

05-07-09 0930 82 6,340 6 60 HS 79 

05-13-09 1230 76 4,000 20 60 HS 5.8

05-21-09 1145 80 6,070 12 60 HS 103

06-10-09 1414 63 3,190 17 60 HS 19.2

04-22-10 1507 71 2,450 20 60 HS 11.4

05-19-10 1525 72 2,830 20 60 HS 10.8

05-20-10 1216 70 2,820 20 60 HS 12.3

06-06-10 0840 80 4,650 20 60 HS 21.4

06-14-10 1328 76 3,270 20 60 HS 18.2

05-10-11 1057 75 3,900 20 60 HS 12

06-11-11 1000 82 6,750 18 60 HS 29

06-28-11 0903 71 3,470 17 60 Elwha 12.9

South Fork Clearwater River at Stites, Idaho (13338500)

04-14-09 1646 150 2,930 20 60 HS 4.4

04-22-09 1402 154 5,310 20 60 HS 30.6

05-07-09 1430 144 6,720 20 60 HS 190

05-13-09 1630 154 4,020 20 60 HS 32.6

05-21-09 1430 160 5,470 20 60 HS 155

06-10-09 0833 153 3,260 20 60 HS 18.8

04-22-10 1925 151 2,450 20 60 HS 10.2

05-19-10 1822 150 2,640 20 60 HS 10.1

05-20-10 0847 150 3,480 20 60 HS 18.4

06-05-10 1425 158 6,110 20 60 HS 208

06-15-10 1803 150 3,090 20 60 HS 31.2

05-10-11 1545 155 4,220 20 60 HS 12.8

06-10-11 1555 161 7,490 20 60 HS 603

06-28-11 1210 147 3,480 20 60 Elwha 31.2

Table 8. Bedload data from sampling stations in the lower Snake and Clearwater River Basins, 

Washington and Idaho, water years 2008–11.—Continued

[Locations of stations are shown in figure 2. Sampler type: HS, Helley Smith. Abbreviations: ft, foot; ft3/s, cubic foot 

per second; ton/d, ton per day]
44 Sediment Transport in the Lower Snake and Clearwater River Basins, Idaho and Washington, 2008–11

Date

Mean 

sample

time

Channel

width 

(ft)

Streamflow, 

instantaneous 

(ft3

/s)

Number of 

sampling 

points 

Rest time 

on bottom 

(seconds)

Sampler

type

Bedload 

discharge 

(ton/d)

Clearwater River at Orofino, Idaho (13340000)

04-23-09 0953 360 38,500 20 60 HS 2.64

05-19-09 1612 375 57,700 20 60 HS 44.4

06-02-09 1809 372 46,000 20 60 HS 378

06-04-10 0948 375 49,900 20 60 HS 381

06-10-10 0917 373 45,800 20 60 HS 217

06-06-11 1428 380 49,300 20 60 HS 754

06-28-11 1518 365 37,400 20 60 Elwha 23.9

Clearwater River at Spalding, Idaho (13342500)

05-05-08 1500 440 29,300 20 60 HS 11.9

05-19-08 1430 460 78,900 9 30 HS 841

05-28-08 1015 450 46,700 20 60 HS 149

06-11-08 1345 440 49,700 20 60 HS 155

07-07-08 1345 440 28,000 20 60 HS 6.0

04-15-09 1145 440 37,700 20 60 HS 15.2

04-23-09 1545 450 56,300 20 60 HS 43.3

05-19-09 0955 450 60,600 20 60 HS 12

06-03-09 0949 450 51,100 20 60 HS 383

05-20-10 1532 444 46,200 20 60 HS 126

06-15-11 1015 432 54,500 10 30 Elwha 21

Palouse River at Hooper, Washington (13351000)

03-24-09 1133 142 4,240 20 60 HS 36.8

01-17-11 1121 145 5,670 20 60 HS 5.1

01-18-11 1035 148 7,150 20 60 HS 2.5

Table 8. Bedload data from sampling stations in the lower Snake and Clearwater River Basins, 

Washington and Idaho, water years 2008–11.—Continued

[Locations of stations are shown in figure 2. Sampler type: HS, Helley Smith. Abbreviations: ft, foot; ft3/s, cubic foot 

per second; ton/d, ton per day]
Sediment Transport in the Lower Snake and Clearwater River Basins 45

tac12-0737_fig19

Palouse River at

Hooper, Washington

not determined Potlatch R. near

Spalding, Idaho

not determined

Lower Granite

Reservoir

64,500 (0.64 percent)

Clearwater River at

Spalding, Idaho

9,500 (0.83 percent)

Clearwater River at

Orofino, Idaho

15,000 (1.5 percent)

M. Fork Clearwater 

River at Kooskia, Idaho

16,000 (3.1 percent)

Lochsa R. near

Lowell, Idaho

not determined

Snake River near

Anatone, Washington

55,000 (0.62 percent)

Grande Ronde River

at Zindel, Washington

not determined

Salmon River at

White Bird, Idaho

66,000 (1.3 percent)

S. Fork Clearwater River

near Harpster, Idaho

4,700 (3.1 percent)

Selway River near

Lowell, Idaho

16,000 (4.1 percent)

S. Fork Clearwater River

at Stites, Idaho

14,000 (7.9 percent)

Figure 19. Estimated bedload transported in the lower Snake and Clearwater River Basins, Washington and Idaho, water years 

2009–11. Values are in tons and percentage of total sediment load (suspended and bedload) transported during water years 2009–11. 

Width of each arrow is proportional to the estimated bedload.
46 Sediment Transport in the Lower Snake and Clearwater River Basins, Idaho and Washington, 2008–11

1

10

100

1,000

10,000

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

Clearwater River

at Spalding, Idaho

Clearwater River

at Orofino, Idaho

Snake River

near Anatone,

Washington

Salmon River

at White Bird,

Idaho

Middle Fork

Clearwater River

at Kooskia,

Idaho 

Selway River

near Lowell,

Idaho

South Fork

Clearwater River

at Stites,

Idaho

South Fork 

Clearwater River 

near Harpster,

Idaho 

Bedload discharge, in tons per day

Streamflow, in cubic feet per second

Figure 20. Bedload transport curves representing best-fit regression equations for selected stations in the lower 

Snake and Clearwater River Basins, Washington and Idaho.

Table 9. Best-fit regression equations for bedload from sampling stations in the lower Snake and 

Clearwater River Basins, Washington and Idaho, water years 2008–11.

[Locations of stations are shown in figure 2. Only stations with at least five samples are shown. R2: coefficient 

of determination. Abbreviations: USGS, U.S. Geological Survey; BS, bedload discharge, in tons per day; Q, 

streamflow, in cubic feet per second]

USGS

gaging

station 

No.

Gaging station name Best-fit regression 

equation R2

13317000 Salmon River at White Bird, Idaho log BS = 5.033×10-17Q3.992 0.46

13334300 Snake River near Anatone, Washington log BS = 1.842×10-06Q1.603 0.26

13336500 Selway River near Lowell, Idaho log BS = 1.732×10-15Q3.919 0.61

13337120 Middle Fork Clearwater River at Kooskia, Idaho log BS = 8.220×10-10Q2.644 0.30

13338100 South Fork Clearwater River near Harpster, Idaho log BS = 2.639×10-15Q3.652 0.77

13338500 South Fork Clearwater River at Stites, Idaho log BS = 1.081×10-11Q3.470 0.79

13340000 Clearwater River at Orofino, Idaho log BS = 7.707×10-33Q7.312 0.30

13342500 Clearwater River at Spalding, Idaho log BS = 6.767×10-15Q3.403 0.42
Sediment Transport in the Lower Snake and Clearwater River Basins 47

Table 10. Particle-size distribution for bedload samples collected from stations in the lower Snake and Clearwater River Basins, 

Washington and Idaho, water years 2008–11.

[Locations of stations are shown in figure 2. Abbreviations: <, less than; mm, millimeter]

Date

Mean 

sample 

time

< 128 mm 

(percent)

< 63.0 mm 

(percent)

< 31.5 mm 

(percent)

< 16.0 mm 

(percent)

< 8.0 mm 

(percent)

< 4.0 mm 

(percent)

< 2.0 mm 

(percent)

< 1.0 mm 

(percent)

 < 0.50 mm 

(percent)

< 0.25 mm 

(percent)

< 0.125 mm 

(percent)

< 0.063 mm 

(percent)

Salmon River at White Bird, Idaho (13317000)

04-24-09 1030 100 100 100 100 100 100 99 94 23 1 0 0

05-20-09 1332 100 96 84 67 61 57 55 52 18 2 0 0

06-03-09 1535 100 100 83 74 69 66 65 61 12 3 0 0

06-11-09 0821 100 100 100 100 100 99 98 94 7 0 0 0

05-15-11 1245 100 100 100 100 100 100 99 97 55 5 1 0

Snake River near Anatone, Washington (13334300)

05-06-08 1600 100 100 66 54 53 53 52 50 36 1 0 0

06-10-08 1145 100 100 88 83 69 64 63 60 45 1 0 0

07-08-08 0920 100 100 100 100 100 100 99 96 73 4 0 0

04-16-09 0915 100 100 100 98 90 82 75 63 3 0 0 0

04-23-09 1230 100 81 32 28 27 26 24 21 2 0 0 0

05-20-09 1330 100 100 100 100 100 99 96 88 46 6 1 0

06-16-09 1750 100 100 90 71 56 46 40 32 6 0 0 0

05-20-10 0916 100 100 78 71 67 63 59 54 4 0 0 0

06-04-10 1425 100 100 82 74 63 59 57 52 26 3 0 0

06-14-10 1518 100 82 82 76 71 67 63 58 4 0 0 0

04-04-11 1519 100 69 34 15 10 9 8 7 1 0 0 0

04-05-11 1310 100 60 29 11 4 3 3 2 0 0 0 0

05-14-11 1555 100 100 17 17 14 13 13 12 10 1 0 0

05-16-11 1335 100 100 89 60 35 22 17 13 6 0 0 0

05-17-11 1550 100 77 34 29 19 12 8 6 0 0 0 0

06-21-11 1248 100 90 29 9 5 2 0 0 0 0 0 0

Selway River near Lowell, Idaho (13336500)

04-22-09 1045 100 100 100 90 88 87 85 70 3 0 0 0

05-18-09 1500 100 100 100 98 98 97 96 68 3 0 0 0

05-19-09 1040 100 86 71 63 61 59 57 38 4 0 0 0

05-21-09 1123 100 83 66 48 38 34 33 26 1 0 0 0

06-09-09 1231 100 100 100 99 92 85 76 35 0 0 0 0

05-19-10 1110 100 100 100 100 100 100 98 82 7 0 0 0

06-04-10 1350 100 91 83 81 80 78 75 48 2 0 0 0

06-15-10 1318 100 100 92 89 88 86 84 53 1 0 0 0

05-17-11 1025 100 100 100 94 89 85 80 50 14 1 0 0

05-24-11 1730 100 100 86 82 80 78 75 49 14 0 0 0

06-10-11 0816 100 100 56 51 50 49 46 29 1 0 0 0

Lochsa River near Lowell, Idaho (13337000)

05-18-11 0815 100 82 66 56 45 38 33 20 6 1 0 0

06-07-11 0905 100 82 81 79 76 74 70 45 4 0 0 0

06-10-11 1036 100 100 82 54 43 38 33 20 6 0 0 0

Middle Fork Clearwater River at Kooskia, Idaho (13337120)

04-22-09 1500 100 100 100 100 100 99 97 79 3 0 0 0

05-18-09 1523 100 73 25 16 15 14 14 11 1 0 0 0

05-19-09 1530 100 100 57 49 48 47 45 37 15 1 0 0

05-21-09 1653 100 100 99 95 91 87 84 65 3 0 0 0

06-09-09 1600 100 100 100 100 97 92 86 59 2 0 0 0

05-21-10 1607 100 100 100 100 97 91 88 68 4 0 0 0

06-04-10 1500 100 100 99 97 95 93 90 74 10 0 0 0

06-16-10 1011 100 100 100 99 95 90 86 59 1 0 0 0

05-17-11 1600 100 72 55 44 40 37 36 33 13 1 0 0

05-26-11 0830 100 100 82 78 71 66 63 58 28 2 0 0

06-08-11 1044 100 73 65 63 61 57 54 46 5 0 0 0
48 Sediment Transport in the Lower Snake and Clearwater River Basins, Idaho and Washington, 2008–11

Date

Mean 

sample 

time

< 128 mm 

(percent)

< 63.0 mm 

(percent)

< 31.5 mm 

(percent)

< 16.0 mm 

(percent)

< 8.0 mm 

(percent)

< 4.0 mm 

(percent)

< 2.0 mm 

(percent)

< 1.0 mm 

(percent)

 < 0.50 mm 

(percent)

< 0.25 mm 

(percent)

< 0.125 mm 

(percent)

< 0.063 mm 

(percent)

South Fork Clearwater River near Harpster, Idaho (13338100)

04-08-09 1021 100 100 100 100 100 99 95 83 34 5 0 0

04-14-09 1252 100 100 100 96 92 89 85 62 2 0 0 0

04-22-09 0937 100 100 80 78 75 70 66 44 2 0 0 0

05-07-09 0930 100 100 100 96 93 89 83 58 6 1 0 0

05-13-09 1230 100 100 100 98 87 81 76 57 13 1 0 0

05-21-09 1145 100 100 98 95 89 83 72 43 1 0 0 0

06-10-09 1414 100 100 86 82 80 75 68 39 1 0 0 0

04-22-10 1507 100 100 100 100 100 98 96 71 2 0 0 0

05-19-10 1525 100 100 100 100 99 96 89 50 2 0 0 0

05-20-10 1216 100 100 100 96 91 87 82 53 2 0 0 0

06-06-10 0840 100 100 100 99 95 87 73 37 1 0 0 0

06-14-10 1328 100 100 100 94 87 78 69 42 1 0 0 0

05-10-11 1057 100 100 100 100 100 99 96 81 53 10 0 0

06-11-11 1000 100 100 93 88 84 80 73 51 2 0 0 0

06-28-11 0903 100 100 100 100 98 94 85 49 1 0 0 0

South Fork Clearwater River at Stites, Idaho (13338500)

04-14-09 1646 100 100 90 90 90 90 86 64 6 1 0 0

04-22-09 1402 100 100 81 71 70 68 66 53 5 0 0 0

05-07-09 1430 100 97 71 56 45 36 33 22 1 0 0 0

05-13-09 1630 100 100 100 90 79 71 65 45 1 0 0 0

05-21-09 1430 100 100 94 80 70 61 53 35 1 0 0 0

06-10-09 0833 100 100 90 70 53 40 31 17 1 0 0 0

04-22-10 1925 100 100 100 64 56 52 48 34 1 0 0 0

05-19-10 1822 100 100 100 86 75 68 63 45 2 0 0 0

05-20-10 0847 100 100 83 79 78 76 72 57 3 0 0 0

06-05-10 1425 100 91 74 60 52 45 40 29 1 0 0 0

06-15-10 1803 100 94 94 83 69 61 53 31 1 0 0 0

05-10-11 1545 100 100 100 92 87 84 82 72 40 4 0 0

06-10-11 1555 100 74 52 39 31 24 19 13 5 0 0 0

06-28-11 1210 100 100 77 58 52 46 39 19 0 0 0 0

Clearwater River at Orofino, Idaho (13340000)

04-23-09 0953 100 100 100 86 78 74 72 62 5 1 0 0

05-19-09 1612 100 100 21 12 11 11 11 10 1 0 0 0

06-02-09 1809 100 94 49 23 12 10 10 9 0 0 0 0

06-04-10 0948 100 86 51 40 34 31 29 25 2 0 0 0

06-10-10 0917 100 91 52 37 31 25 22 18 0 0 0 0

06-06-11 1428 100 66 36 25 18 12 10 9 1 0 0 0

06-28-11 1518 100 100 77 74 70 65 61 57 2 0 0 0

Clearwater River at Spalding, Idaho (13342500)

05-05-08 1500 100 100 100 100 99 99 98 98 74 3 1 1

05-19-08 1430 100 38 33 33 33 33 33 32 26 3 0 0

05-28-08 1015 100 100 97 95 95 94 93 91 51 1 0 0

06-11-08 1345 100 100 49 31 29 28 28 27 18 0 0 0

07-07-08 1345 100 100 100 100 100 100 99 96 76 2 0 0

04-15-09 1145 100 100 100 88 87 86 85 82 1 0 0 0

04-23-09 1545 100 100 67 43 41 41 40 37 17 2 0 0

05-19-09 0955 100 100 100 86 85 85 83 76 15 2 0 0

06-03-09 0949 100 47 14 6 5 5 5 5 0 0 0 0

05-20-10 1532 100 49 29 27 27 26 26 25 2 0 0 0

06-15-11 1015 100 100 100 100 100 100 100 99 78 7 0 0

Table 10. Particle-size distribution for bedload samples collected from stations in the lower Snake and Clearwater River Basins, 

Washington and Idaho, water years 2008–11.—Continued

[Locations of stations are shown in figure 2. Abbreviations: <, less than; mm, millimeter]
Sediment Transport in the Lower Snake and Clearwater River Basins 49

n = 72, r2 = 0.84

n = 72, r2 = 0.78

0.1

1

10

100

1,000

1,000 10,000 100,000

Bedload, in tons per day 

Streamflow, in cubic feet per second

Lochsa River samples, 2009–11

Lochsa River samples, 1994–97

Selway River samples, 2009–11

Selway River samples, 1994–97

EXPLANATION

Figure 21. Bedload transport curves based on samples collected from the Lochsa and Selway Rivers near 

Lowell, Idaho, during water years 2009–11, and during 1994–97 by King and others (2004).

The measured bedload discharge in the Snake River near 

Anatone ranged from 14.2 to 491 ton/d based on 16 samples 

collected during 2008–11 (table 8). Based on a composite 

sample of all bedload sediment collected at Anatone, and 

using the GRADISTAT statistical software (Blott and Pye, 

2001), the particle-size distribution was bimodal, with coarse 

gravel (16–31.5 mm) accounting for 25.7 percent of the 

bedload material and medium sand (0.25–0.50 mm) making 

up 22.0 percent (table 10). Notably absent in the bedload 

collected from the Snake River near Anatone was small gravel. 

Applying the best-fit relation between streamflow and bedload 

discharge (table 9) to determine the transport in the Snake 

River for water years 2009–11 indicates about 55,000 tons 

of bedload, or about 0.62 percent of the total amount of 

sediment discharged from the Snake River to Lower Granite 

Reservoir (fig. 19).

In the Clearwater River Basin, bedload samples were 

collected from the Lochsa and Selway Rivers 3 and 11 times, 

respectively. At both stations, the particle-size distribution was 

bimodal, with medium sand and course gravel being the two 

dominant sizes (table 10). A comparison of the bedload data 

collected during this study from the Lochsa and Selway Rivers 

with data collected during 1994–97 by the U.S. Forest Service 

at the same sampling locations (King and others, 2004) is 

shown in figure 21.
50 Sediment Transport in the Lower Snake and Clearwater River Basins, Idaho and Washington, 2008–11

Although the range in streamflow at which bedload 

samples were collected was larger during the 1994–97 

study, at streamflows of similar magnitude, the bedload 

data collected during 2009–11 corresponded well with the 

1994–97 data. King and others (2004) determined that bedload 

discharge constituted less than 4 percent of the total sediment 

discharged from the Lochsa and Selway Rivers. Bedload was 

not estimated for the Lochsa River during 2009–11 because of 

the paucity of samples collected during this study. However, 

using the best-fit regression for the data collected from the 

Selway River during 2009–11, bedload constituted about 

4 percent of the total sediment discharge from the Selway 

River (fig. 19). 

Bedload samples were collected at the South Fork 

Clearwater River near Harpster, and South Fork Clearwater 

River at Stites 15 and 14 times, respectively during 2009–11 

(table 10). Streamflows at Harpster ranged from 1,410 to 

6,750 ft3/s, and bedload discharge ranged from 0.6 to 

103 ton/d. At the Stites station, instantaneous streamflows 

ranged from 2,450 to 7,490 ft3/s and bedload ranged from 

4.4 to more than 600 ton/d (table 8). Based on these data, it 

is apparent that although the South Fork Clearwater River 

at Stites is only 15 mi downstream of the Harpster station, 

at high streamflow the bedload transported at Stites is about 

one order of magnitude larger than at Harpster (fig. 20). The 

particle-size distribution of the bedload at both South Fork 

Clearwater River stations was bimodal, with the dominant size 

classes being medium-sized sand and medium-sized gravel. 

Using the equations from table 9, the bedload at Harpster 

constituted about 3.1 percent of the total sediment discharge 

during water years 2009–11, whereas at Stites the bedload 

constituted about 7.9 percent (figure 19).

Bedload samples were collected in the middle and lower 

sections of the Clearwater River at the Middle Fork Clearwater 

River at Kooskia, Clearwater River at Orofino, and Clearwater 

River at Spalding stations. Bedload discharge and bedload as a 

percentage of the total sediment load decreased from upstream 

to downstream through this reach of the Clearwater River 

(fig. 19) in response to a decrease in the stream gradient. The 

Middle Fork Clearwater River at Kooskia was sampled 11 

times during 2009–11 at streamflows ranging from 22,700 to 

57,000 ft3/s (table 8). Bedload at Kooskia ranged from 13.5 to 

592 ton/d, with an estimated bedload transport of 16,000 tons 

during water years 2009–11, approximately equivalent to 

the bedload discharge from the Selway River (fig. 19). The 

composited particle size distribution at Kooskia was bimodal, 

with medium sand (45 percent) and coarse gravel (12 percent) 

being the two dominant sizes. Bedload at the Kooskia 

station constituted about 3.1 percent of the total sediment 

load discharged from the Middle Fork Clearwater River as 

measured at Kooskia (fig. 19).

Bedload samples were collected seven times during 

2009–11 at Clearwater River at Orofino at streamflows 

ranging from 37,400 to 57,700 ft3/s. Bedload at Orofino 

ranged from 2.64 to 754 tons/d (table 8). The total calculated 

transport at Orofino was about 15,000 tons during water 

years 2009–11, about one-half of the combined bedload 

discharged from the South Fork and Middle Fork Clearwater 

Rivers (fig. 19). Downstream of Orofino at the Clearwater 

River at Spalding station, the total bedload during water 

years 2009–11 was about 9,500 tons, less than 1 percent of 

the total sediment transport at the station during that period. 

The reach of the Clearwater River from Orofino to Spalding 

probably transports less sediment (both suspended load and 

bedload) than it did historically because of the construction 

of Dworshak Reservoir, which essentially negates sediment 

delivery to the main stem Clearwater River from the North 

Fork Clearwater River. The total bedload for 2009–11 in the 

Clearwater River at Spalding was about 9,500 tons, about 

15 percent of the total bedload discharged to Lower Granite 

Reservoir from the combined Clearwater and Snake Rivers. 

Overall, bedload accounted for only about 0.64 percent of the 

total sediment load entering Lower Granite Reservoir during 

water years 2009–11 (fig. 19).

Bedload data collected from the Snake River near 

Anatone and the Clearwater River at Spalding during 1972–79 

(Jones and Seitz, 1980) indicated that historically, bedload 

was larger and comprised a larger part of the overall sediment 

load. Based on 63 samples collected during 1972–79 from 

the Snake River near Anatone at streamflows ranging from 

27,500 to 161,000 ft3/s, the mean bedload was about 450 ton/d 

and ranged from less than 1.0 to about 5,600 ton/d. In 

comparison, the mean bedload based on 16 samples collected 

at Anatone during this study was about 130 ton/d and ranged 

from 14.2 to 491 ton/d (table 8). Although the magnitude of 

streamflows sampled during both study periods were similar, 

the mean bedload was about 3.5 times higher during 1972–79. 

Jones and Seitz (1980) reported that bedload comprised 

about 4 percent of the overall sediment load transported in 

the Snake River during 1972–79, whereas results from this 

study indicate that bedload comprised less than 1 percent of 

the sediment load. Although the relation between streamflow 

and bedload was relatively poor for the 1972–79 (R2=0.44) 

and 2008–11 data (R2=0.26), there is an apparent shift in the 

bedload transport curve, indicating less bedload in the Snake 

River near Anatone during 2008–11 (fig. 22). The reason for 

this apparent shift is difficult to determine given the increase 

in the transport of suspended sand during 2008–11. Additional 

bedload data collected during 2008-11 from the Snake River 

near Anatone would be helpful to verify the apparent decrease 

in bedload in the Snake River.
Sediment Transport in the Lower Snake and Clearwater River Basins 51

tac12-0737_fig22

R2 = 0.44

R2

 = 0.26

10

100

1,000

10,000

20,000 200,000

Bedload, in tons per day

Streamflow, in cubic feet per second

2008–11 best-fit line

1972–79 best-fit line

2008–11 samples

1972–79 samples

EXPLANATION

Figure 22. Bedload transport curves comparing data collected at Snake River near Anatone, Washington, during 

water years 1972–79 and 2008–11.

Similarly, a comparison of data from 1972–79 and 

2008–11 indicated a decrease in bedload in the Clearwater 

River at Spalding (fig. 23). The mean bedload during 

1972–79 based on 78 samples collected at the Spalding station 

was about 120 ton/d, ranging from less than 1.0 to about 

3,700 ton/d. The mean bedload based on 11 samples collected 

during 2008–11 was about 54 ton/d, ranging from about 6.0 to 

840 ton/d (table 8). Based on this comparison, bedload in the 

Clearwater River during 1972–79 was markedly larger (mean 

of about 2.2 times) at roughly equivalent stream discharge. 

Jones and Seitz (1980) reported that, similar to the Snake 

River, during 1972–79 bedload comprised about 4 percent 

of the total sediment load in the Clearwater River. The 

2008–11 data indicate that bedload was less than 1 percent of 

the sediment load in the Clearwater River at Spalding. The 

relation between streamflow and bedload was better (higher 

R2) for the Clearwater River than the Snake River for both 

sampling periods (figs. 22 and 23).
52 Sediment Transport in the Lower Snake and Clearwater River Basins, Idaho and Washington, 2008–11

tac12-0737_fig23

R2 = 0.70

R2

 = 0.42

1

10

100

1,000

10,000

10,000 100,000

Bedload, in tons per day

Streamflow, in cubic feet per second

2008–11 best-fit line

1972–79 best-fit line

2008–11 samples

1972–79 samples

EXPLANATION

Figure 23. Bedload transport curves comparing data collected at Clearwater River at Spalding, Idaho, water years 

1972–79 and 2008–11.

Summary

Since Lower Granite Dam was completed in 1975, 

about 75 million cubic yards of sediment have accumulated 

in Lower Granite Reservoir, an average annual accumulation 

of about 2.3 million cubic yards through 2008. In 2008, the 

U.S. Geological Survey (USGS), in cooperation with the U.S. 

Army Corps of Engineers (USACE), conducted sediment 

sampling in the Snake River near Anatone, Washington, and 

the Clearwater River at Spalding, Idaho, to quantify sediment 

loading to Lower Granite Reservoir and to evaluate sediment 

depositional characteristics in the reservoir. In 2009, sediment 

sampling was extended to 10 additional sampling stations 

in the lower Snake and Clearwater River basins to help 

identify the subbasins contributing most of the sediment being 

delivered to Lower Granite Reservoir. 

Of the stations sampled during 2009–11, the largest 

measured total suspended sediment (TSS) concentrations 

were in samples collected from the Potlatch (3,300 mg/L) and 

Palouse (1,400 mg/L) Rivers during a rain-on-snow event that 

occurred in January 2011. The largest median concentration of 

TSS (100 mg/L) and the largest suspended sediment fraction 

as fine-grained silt and clay (median of 95 percent) were in 

samples collected from the Palouse River. Other rivers with a 

large percentage of fine-grained suspended sediment were the 

Potlatch (median of 92 percent) and the Grand Ronde (median 

of 84 percent) Rivers. The Palouse, Potlatch, and Grande 

Ronde Rivers all drain basins with relatively large proportions 

of agricultural activity. The Selway and Lochsa Rivers, the 

Middle Fork Clearwater River at Kooskia, and the Clearwater 

River at Orofino generally had small concentrations of TSS 

with medians of 11, 11, 15, and 13 mg/L, respectively.
Acknowledgments 53

During water years 2009–11, Lower Granite Reservoir 

received about 10 million tons of suspended sediment from 

the combined loads of the Snake and Clearwater Rivers. About 

60 percent of sediment entering the reservoir during 2009–11 

occurred during water year 2011, which was characterized 

by a large winter snowpack followed by sustained spring 

runoff and high suspended-sediment transport during most 

of the summer. Of the sediment load entering the reservoir, 

the Snake River accounted for about 89 percent of the TSS, 

about 90 percent of the suspended sand, and about 87 percent 

of the suspended fines. The Salmon River contributed about 

51 percent of the TSS, about 56 percent of the suspended sand, 

and about 44 percent of the suspended fines transported to 

Lower Granite Reservoir. 

A comparison of historical sediment data collected 

from the Snake River with data collected during this study 

indicated that concentrations of TSS and suspended sand 

were significantly larger during 2008–11 compared to 

1972–79, whereas the concentrations of suspended fines were 

not. In the Snake River, the sand fraction increased from 

an average of 28 percent of the TSS load during 1972–79 

to an average of 48 percent during 2009–11. The increase 

in the suspended-sand load in the Snake River is probably 

attributable to numerous severe forest fires that burned 

large areas of central Idaho from 1980–2010. Additional 

fluvial‐sediment monitoring in the Salmon River Basin 

would be helpful to identify source areas and to quantify 

the magnitude of sediment delivery to discrete reaches 

of the Salmon River, the lower Snake River, and Lower 

Granite Reservoir. 

In the Clearwater River, data collected during this study 

indicated that the TSS and suspended fines concentrations 

during 1972–79 were not significantly different from the 

concentrations during 2008–11. However, the concentrations 

of suspended sand in the Clearwater River were significantly 

larger during 2008–11. The increase in the sand load in the 

Clearwater River may also be attributable to forest fire activity 

in areas of the basin with highly erodible soils.

Suspended-sediment surrogate models developed using 

acoustic Doppler velocity meters (ADVMs) effectively 

estimated suspended-sediment concentrations (SSCs) and 

loads for most streamflow conditions in the lower Snake and 

Clearwater Rivers. At both stations (lower Snake River near 

Anatone and Clearwater River at Spalding) instrumented 

with ADVMs, surrogate models developed using acoustic 

backscatter had a better correlation than LOADEST models 

for SSC and suspended fines. Surrogate models also had a 

better correlation for suspended sands at the Spalding station 

on the lower Clearwater River. Over short (monthly and 

storm event) and long (annual) time scales when sediment 

concentrations and loads are highly variable, acoustic 

backscatter appears to provide better estimates of sediment 

concentration and load than do traditional sediment transport 

curves based solely on streamflow. Because acoustic 

backscatter is not affected by hysteresis, it provides a direct, 

in-situ measurement of suspended sediment, and is effective 

in representing sediment sources from a combination of 

regulated and unregulated sources. On a monthly and (or) 

annual basis, the acoustic surrogate and LOADEST models 

appear to produce similar sediment load estimates for the 

Clearwater River at Spalding. However, estimates of SSCs 

and loads in the Snake River differed substantially between 

the acoustic surrogate and LOADEST models. The largest 

difference occurred in water year 2011, when the suspendedsediment load calculated using the LOADEST model was 

more than three times higher than the suspended-sediment 

load calculated using the acoustic surrogate model. The largest 

discrepancies between the LOADEST and acoustic surrogate 

models occur primarily during high streamflow, when the SSC 

and the suspended sand fraction are large. This discrepancy 

may be a result of acoustic surrogate tools being unable to 

capture the contribution of suspended sand moving near the 

bottom of the water column.

Bedload accounted for less than 1 percent of the total 

sediment load entering Lower Granite Reservoir from the 

Snake and Clearwater Rivers. The estimated bedload in 

the Salmon River at White Bird during 2009–11 was about 

66,000 tons, the largest amount of the stations sampled. The 

lower Snake River basin, which includes the Salmon River, 

had the second largest measured bedload with a total of 

55,000 tons during 2009–11, about 0.62 percent of the total 

sediment load entering Lower Granite Reservoir from the 

Snake River. The estimated bedload in the Clearwater River at 

Spalding was only about 9,500 tons, roughly 0.83 percent of 

the total sediment load transported to Lower Granite Reservoir 

from the Clearwater River. 

Acknowledgments

The USGS wishes to thank the U.S. Army Corps of 

Engineers, Walla Walla District for providing funding for this 

project. Special thanks go to USGS field personnel from the 

Idaho Water Science Center Boise and Post Falls Field Offices 

who were responsible for most of the data collection.
54 Sediment Transport in the Lower Snake and Clearwater River Basins, Idaho and Washington, 2008–11

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Publishing support provided by the U.S. Geological Survey 

Publishing Network, Tacoma Publishing Service Center

For more information concerning the research in this report, contact the 

Director, Idaho Water Science Center

U.S. Geological Survey 

230 Collins Road

Boise, Idaho 83702

http://id.water.usgs.gov
Clark and others— Sediment Transport in the Lower Snake and Clearwater River Basins, Idaho and Washington—Scientific Investigations Report 2013–5083