Annual Trace-Metal Load Estimates and Flow-Weighted 

Concentrations of Cadmium, Lead, and Zinc, in the 

Spokane River Basin, Idaho and Washington, 1999-2004

Scientific Investigations Report 2006-5188

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

U.S. Department of the Interior

U.S. Geological Survey

Annual Trace-Metal Load Estimates and 

Flow-Weighted Concentrations of Cadmium, 

Lead, and Zinc, in the Spokane River Basin, 

Idaho and Washington, 1999-2004

By Mary M. Donato

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

Scientific Investigations Report 2006-5188

U.S. Department of the Interior

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

DIRK A. KEMPTHORNE, Secretary

U.S. Geological Survey

P. Patrick Leahy, Acting Director

U.S. Geological Survey, Reston, Virginia: 2006

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Suggested citation:

Donato, M.M., 2006, Annual trace-metal load estimates and flow-weighted concentrations of cadmium, lead, and zinc 

in the Spokane River Basin, Idaho and Washington, 1999-2004: U.S. Geological Survey Scientific Investigations Report 

2006-5188, 38 p.
iii

Contents

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

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

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

Method For Estimating Trace-Metal Loads ...............................................................................................7

Model Input.....................................................................................................................................................7

Streamflow.............................................................................................................................................7

Trace-Metal Concentrations...............................................................................................................8

Regression Statistics.....................................................................................................................................8

Estimated Trace-Metal Loads, 1999-2004 ................................................................................................12

Cadmium Loads...................................................................................................................................12

Lead Loads...........................................................................................................................................12

Zinc Loads............................................................................................................................................12

Spatial Patterns in Metal Loads and Proportions .........................................................................16

Comparison with Previous Load Estimates ....................................................................................16

Flow-Weighted Concentrations of Trace Metals, 1999-2004................................................................19

Estimated Trace-Metal Loads and Flow-Weighted Concentrations, 1991-2004................................23

Comparison with 1999-2004 Estimates ............................................................................................23

Temporal Trends in Long-Term Metal Load Estimates..................................................................33

Summary........................................................................................................................................................35

References Cited..........................................................................................................................................36

Figures

Figure 1. Map showing locations of streamflow-gaging stations and Bunker Hill Box,

including Operable Units 1 and 2, in the Spokane and Coeur d’Alene River

basins, Idaho and Washington ............................................................ 3

Figure 2. Map showing locations of streamflow-gaging stations on and near the

South Fork Coeur d’Alene River, Idaho ................................................... 4

Figure 3. Diagram showing relative locations and hydrologic relationships among 10

streamflow-gaging stations in the Spokane River basin, Idaho and Washington ... 6

Figure 4. Graphs showing relation between trace-metal concentrations and

streamflow at U.S. Geological Survey streamflow-gaging station 12413210,

South Fork Coeur d’Alene River at Elizabeth Park, near Kellogg, Idaho, water

years 1999-2004 .............................................................................. 9

Figure 5. Graphs showing mean annual estimated dissolved and total trace-metal

loads and mean streamflow at 10 streamflow-gaging stations in the

Spokane River basin, Idaho and Washington, water years 1999–2004 ............... 15

Figure 6. Graphs showing mean annual flow-weighted concentrations of trace metals

and mean streamflow at 10 streamflow-gaging stations in the Spokane River

basin, Idaho and Washington, water years 1999–2004 ................................. 22

Figure 7. Graphs showing estimated long-term mean annual dissolved and total

trace-metal loads and mean streamflow at four streamflow-gaging stations

in the Spokane River basin, Idaho ......................................................... 27
iv

Figure 8. Graphs showing mean annual flow-weighted concentrations of dissolved

and total cadmium, lead, and zinc and mean annual streamflow at four

streamflow-gaging stations in the Spokane River basin, Idaho........................ 30

Figure 9. Graphs showing comparison of short- and long-term trace-metal load 

estimates at four streamflow-gaging stations in the Spokane River 

basin, Idaho ................................................................................. 32

Figure 10. Graphs showing estimated flow-weighted concentration of dissolved and

total cadmium and mean annual streamflow at South Fork Coeur d’Alene

River at Pinehurst, water years 1991–2004 and North Fork Coeur d’Alene

River at Enaville, Idaho, water years 1992–2004 ....................................... 34

Tables

Table 1. Streamflow-gaging stations in the Spokane River basin, Idaho and

Washington, where streamflow and water-quality samples were collected

to estimate flow-weighted concentrations and cadmium, lead, and zinc

loads, water years 1991–2004 ............................................................ 5

Table 2. Regression coefficients and coefficients of determination (R 2) used to

estimate dissolved and total cadmium, lead, and zinc loads at 10

streamflow-gaging stations in the Spokane River basin, Idaho and

Washington, water years 1999–2004 ...................................................... 10

Table 3. Estimated mean annual trace-metal loads for 10 streamflow-gaging stations

in the Spokane River basin, Washington and Idaho, water years 1999–2004. ...... 13

Table 4. Comparison of trace-metal load estimates for water years 1999–2001 (this

study) with estimates by Clark (2002) for nine streamflow-gaging stations in

the Spokane River basin, Idaho and Washington ....................................... 17

Table 5. Estimated mean annual flow-weighted concentrations of trace metals for 10

streamflow-gaging stations in the Spokane River basin, Idaho and

Washington, water years 1999–2004 ...................................................... 20

Table 6. Regression coefficients and coefficients of determination (R 2) for load

models used to estimate dissolved and total cadmium, lead, and zinc loads

at four streamflow-gaging stations in the Spokane River basin, Idaho ............... 24

Table 7. Estimated long-term mean annual trace-metal loads and flow-weighted

concentrations of cadmium, lead, and zinc at four streamflow-gaging

stations in the Spokane River basin, Idaho ............................................. 25

Table 8. Comparison of short- and long-term trace-metal load estimates for four

streamflow-gaging stations in the Spokane River basin, Idaho........................ 31

Table 9. Comparison of trend analysis results for trace-metal loads and

flow-weighted concentrations based on the Mann-Kendall test and

LOADEST regression analysis for four streamflow-gaging stations in the

Spokane River basin, Idaho, water years 1991–2004.................................... 34

Figures—Continued
Conversion Factors and Datums

Conversion Factors

Multiply By To obtain

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

cubic meter per second (m3/s) 70.07 acre-foot per day 

kilogram (kg) 2.205 pound avoirdupois 

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

kilogram per year (kg/y) 2.205 pound avoirdupois per year

square mile (mi2) 259.0 hectare

square mile (mi2) 2.590 square kilometer

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

°F=(1.8×°C)+32.

Datums

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

(NAVD 88).

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

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

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Abstract

Streamflow and trace-metal concentration data collected 

at 10 locations in the Spokane River basin of northern Idaho 

and eastern Washington during 1999-2004 were used as 

input for the U.S. Geological Survey software, LOADEST, to 

estimate annual loads and mean flow-weighted concentrations 

of total and dissolved cadmium, lead, and zinc.

Cadmium composed less than 1 percent of the total metal 

load at all stations; lead constituted from 6 to 42 percent of the 

total load at stations upstream from Coeur d’Alene Lake and 

from 2 to 4 percent at stations downstream of the lake. Zinc 

composed more than 90 percent of the total metal load at 6 of 

the 10 stations examined in this study.

Trace-metal loads were lowest at the station on Pine 

Creek below Amy Gulch, where the mean annual total 

cadmium load for 1999–2004 was 39 kilograms per year 

(kg/yr), the mean estimated total lead load was about 1,700 

kg/yr, and the mean annual total zinc load was 14,000 kg/yr. 

The trace-metal loads at stations on North Fork Coeur d’Alene 

River at Enaville, Ninemile Creek, and Canyon Creek also 

were relatively low.

Trace-metal loads were highest at the station at Coeur 

d’Alene River near Harrison. The mean annual total cadmium 

load was 3,400 kg/yr, the mean total lead load was 240,000 

kg/yr, and the mean total zinc load was 510,000 kg/yr for 

1999–2004. Trace-metal loads at the station at South Fork 

Coeur d’Alene River near Pinehurst and the three stations on 

the Spokane River downstream of Coeur d’Alene Lake also 

were relatively high. Differences in metal loads, particularly 

lead, between stations upstream and downstream of Coeur 

d’Alene Lake likely are due to trapping and retention of metals 

in lakebed sediments.

LOADEST software was used to estimate loads for water 

years 1999–2001 for many of the same sites discussed in 

this report. Overall, results from this study and those from a 

previous study are in good agreement. Observed differences 

between the two studies are attributable to streamflow 

differences in the two regression models, 1999–2001 and 

1999-2004.

Flow-weighted concentrations (FWCs) calculated from 

the estimated loads for 1999–2004 were examined to aid 

interpretation of metal load estimates, which were influenced 

by large spatial and temporal variations in streamflow. FWCs 

of total cadmium ranged from 0.04 micrograms per liter(μg/L) 

at Enaville to 14 μg/L at Ninemile Creek. Total lead FWCs 

were lowest at Long Lake (1.3 μg/L) and highest at Ninemile 

Creek (120 μg/L). Elevated total lead FWCs at Harrison 

confirmed that the high total lead loads at this station were 

not simply due to higher streamflow. Conversely, relatively 

low total lead loads combined with high total lead FWCs at 

Ninemile and Canyon Creeks reflected low streamflow but 

high concentrations of total lead. Very low total lead FWCs 

(1.3 to 2.7 μg/L) at the stations downstream of Coeur d’Alene 

Lake are a result both of deposition of lead-laden sediments 

in the lake and dilution by additional streamflow. Total zinc 

FWCs also demonstrated the effect of streamflow on load 

calculations, and highlighted source areas for zinc in the basin. 

Total zinc FWCs at Canyon and Ninemile Creeks, 1,600 μg/L 

and 2,200 μg/L, respectively, were by far the highest in the 

basin but contributed among the lowest total zinc loads due to 

their relatively low streamflow. Total zinc FWCs ranged from 

38 to 67 μg/L at stations downstream of Coeur d’Alene Lake, 

but total zinc load estimates at these stations were relatively 

high because of high mean streamflow compared to other 

stations in the basin.

Long-term regression models for 1991 to 2003 or 2004 

were developed and annual trace-metal loads and FWCs 

were estimated for Pinehurst, Enaville, Harrison, and Post 

Falls to better understand the variability of metal loading 

with time. Long-term load estimates are similar to the results 

for 1999-2004 in terms of spatial distribution of metal loads 

throughout the basin.

LOADEST results for 1991-2004 indicated that 

statistically significant downward temporal trends for 

dissolved and total cadmium, dissolved zinc, and total lead 

were occurring at Pinehurst, Enaville, Harrison, and Post 

Falls. Additionally, data for Enaville and Post Falls showed 

significant downward trends for dissolved lead and total zinc 

loads; Harrison total zinc loads also decreased with time. The 

Annual Trace-Metal Load Estimates and Flow-Weighted 

Concentrations of Cadmium, Lead, and Zinc in the 

Spokane River Basin, Idaho and Washington, 1999-2004

By Mary M. Donato
Mann-Kendall trend test results agreed with the LOADEST 

trend results in most cases, but gave contradictory results for 

total zinc at Pinehurst and at Post Falls.

Long- and short-term load and flow-weighted 

concentration estimates yielded valuable information about 

metal storage and transport processes, and demonstrated 

that water quality data are a great aid in understanding these 

processes.

Introduction

Since the late 1800s, mining and ore-processing activities 

in the South Fork Coeur d’Alene River basin have altered the 

water quality, aquatic biological, and hydrologic conditions 

in the 6,680-mi2 Spokane River basin of northern Idaho 

and eastern Washington (fig. 1). Historical ore-processing 

activities resulted in large quantities of metal-rich tailings 

that were placed in and along streams (Long, 1998). The 

tailings have produced, and continue to produce, trace-metalcontaminated water (Hartz, 1993; Woods, 2001a; Woods, 

2001b) and extensive deposits of trace-metal-contaminated 

sediment throughout the South Fork Coeur d’Alene River 

basin, the channel and flood plain of the main-stem Coeur 

d’Alene River (Spruill, 1993; Fousek, 1996; Bookstrom 

and others, 2001), and the lakebed of Coeur d’Alene Lake 

(Horowitz and others, 1995; Woods and Beckwith, 1997). 

Annual snowmelt runoff, frequent rain-on-snow events, and 

occasional floods continue to transport and redistribute tracemetal-contaminated sediments throughout the Coeur d’Alene 

River basin, into the Spokane River of eastern Washington 

(Maret and Skinner, 2000; Grosbois and others, 2001), and as 

far downstream as the Columbia River (Bortleson and others, 

1994). The National Sediment Inventory (U.S. Environmental 

Protection Agency [USEPA], 1997) identified the Coeur 

d’Alene River and Lake as “areas of probable concern for 

sediment contamination,” the most severe contamination 

category in their assessment.

In 1998, USEPA initiated a Remedial Investigation/

Feasibility Study (RI/FS) of the Spokane River basin under 

the authority of the Comprehensive Environmental Response, 

Compensation, and Liability Act (CERCLA) of 1980, which 

requires USEPA to evaluate contaminant release, fate, and 

transport. The Remedial Investigation (RI) phase involves 

data collection to characterize site conditions, development 

of conceptual models, determination of the nature and extent 

of trace-element contamination, and risk assessment for 

human health and the environment. The RI phase is followed 

by the Feasibility Study (FS) phase where remedial action 

alternatives are developed and evaluated.

Considerable effort is underway to mitigate the adverse 

environmental effects of past mining in the basin, primarily in 

the South Fork Coeur d’Alene River valley and its tributaries 

(Beckwith, 1998). The USEPA is directing cleanup of a 

Superfund site surrounding the defunct Bunker Hill Mine 

and ore-processing complex in Kellogg, Idaho (figs. 1 and 2). 

The State of Idaho, other Federal agencies, and the mining 

industry also are conducting site-specific sediment-removal, 

reclamation, and stream-channel rehabilitation projects. 

The USEPA is evaluating environmental contamination and 

remediation options in mining-affected areas outside the 

Superfund site in areas of the lower Coeur d’Alene River, its 

flood plain and adjacent wetlands, and the lakebed of Coeur 

d’Alene Lake (Beckwith, 1998). In addition, the USEPA and 

the Idaho Department of Environmental Quality currently are 

under court order to develop Total Maximum Daily Loads 

for a number of water bodies that do not support one or more 

designated uses in the Spokane River basin because of tracemetal contamination.

In support of these activities, streamflow and tracemetal chemistry data collected by the U.S. Geological Survey 

(USGS) during several previous and ongoing scientific studies 

at 10 USGS streamflow-gaging stations in the Spokane River 

basin were compiled and analyzed (table 1; figs. 1, 2). The 

data then were used to estimate annual trace-metal loads at the 

10 stations for water years (WY) 1999–2003 or 1999–2004. 

Trace-metal loads for WY 1991–2003 or 1991–2004 also were 

estimated at four of the stations. The purpose of this report 

is to present and describe the results of the load estimations 

of dissolved and total cadmium (Cd), lead (Pb), and zinc 

(Zn) in numerous stream reaches in the Spokane River basin, 

Idaho and Washington, and to discuss inferences about metal 

transport and storage processes indicated by these results. The 

estimates in this study also were compared to earlier estimates 

of trace metal loads in the Spokane River basin as reported by 

Clark (2003).

   Annual Trace-Metal Load Estimates and Flow-Weighted Concentrations, Spokane River Basin, ID and WA, 1999-2004
ID_3084_fig01

ID

AHO

WASH INGTON

EXPLANATION

South Fork Coeur d’Alene River basin (see fig. 2)Bunker Hill Box, Operable Units 1 and 2Streamflow-gaging station and number (see table 1) 5

48

47

118 117 116

115

Base compiled from U.S. Geological SurveyState base maps 1:1,000,000

00

10

10

30 MILES

30 KILOMETERS

20

20

COEUR D'ALENE

INDIAN

RESERVATION

Pine

Creek

Hangman

Creek

River

North

Fork

Coeur

d'Alene

River

St Joe

River

Fork

Fork

North

St

Maries

River

Spokane

River

Columbia

River

Spokane

Little

River

Spokane

South

Coeur

d'Alene

Lake

Long

Lake

Franklin D

Roosevelt

Lake

Kellogg

Harrison

Enaville

Cataldo

Pinehurst

Post

Falls

St Maries

Coeur

d'Alene

Plummer

Spokane

Santa

Smelterville

Calder

Wallace

Mullan

BONNER

KOOTENAI

BENEWAH

LATAH

SHOSHONE

ADAMS WHITMAN

LINCOLN

SPOKANE

STEVENS

WASHINGTON

IDAHO

IDAHO

MONTANA 9

7

3

6

5

10

8

Figure 1. Locations of streamflow-gaging stations and Bunker Hill Box, including Operable Units 1 and 2, in the Spokane and Coeur d’Alene River basins, Idaho and Washington.

Introduction 
ID_3084_fig02

Cataldo

Enaville

Pinehurst

Kellogg Smelterville

Osburn

Wallace

Murray

Prichard

Mullan

Gulch

Government

Moon

Gulch

Prichard

Creek

Canyon

Creek

Pine Creek

Ninemile

Creek

Placer

Creek

Big Creek

South

Fork

Coeur

d'Alene River

North

Fork

Coeur

d'Alene

River

1

2

3

4

5

116

6

 15' 116 07' 30" 116 00' 115 52' 30" 115 45'

47 37' 30"

47 30'

SHOSHONE

COUNTY

00

5

5

10 MILES

10 KILOMETERS 6

EXPLANATION

Bunker Hill Box, Operable Units 1 and 2, and Bunker Hill Superfund siteStreamflow-gaging station and number (see table 1)

Figure 2. Locations of streamflow-gaging stations on and near the South Fork Coeur d’Alene River, Idaho.

   Annual Trace-Metal Load Estimates and Flow-Weighted Concentrations, Spokane River Basin, ID and WA, 1999-2004
Table 1. Streamflow-gaging stations in the Spokane River basin, Idaho and Washington, where streamflow and water-quality samples 

were collected to estimate flow-weighted concentrations and cadmium, lead, and zinc loads, water years 1991–2004.

[Gaging station locations are shown in figures 1 and 2. Type of streamflow record: C, continuous; S. simulated. Number of samples used in estimation: some 

samples did not include all constituents. Abbreviations: USGS, U.S. Geological Survey; ID, Idaho; WA, Washington]

Gaging 

station 

No.

USGS 

identification 

No.

USGS gaging station name

Abbreviated 

gaging station 

name

Type of 

streamflow 

record

Water years for which 

loads were simulated

Number of 

samples used in 

estimation

1 12413125 Canyon Creek above mouth at Wallace, ID Canyon Creek C 1999–2003 41–42

2 12413130 Ninemile Creek above mouth at Wallace, ID Ninemile Creek C 1999–2003 40–43

3 12413210 South Fork Coeur d’Alene River at 

Elizabeth Park, near Kellogg, ID

Elizabeth Park C 1999–2004 27

4 12413445 Pine Creek below Amy Gulch near Pinehurst, IDAmy Gulch C 1999–2003 37–43

5 12413470 South Fork Coeur d’Alene River near Pinehurst, IDPinehurst C 1999–2004; 1991–2004 77; 104–120

6 12413000 North Fork Coeur d’Alene River at 

Enaville, ID

Enaville C 1999–2004; 1991–2004 69–70; 86–111

7 12413860 Coeur d’Alene River near Harrison, ID Harrison S 1999–2004; 1991–2004 45–46; 61–142

8 12417598 Spokane River at Lake Outlet at 

Coeur d’Alene, ID

Outlet C 2003–2004 15

9 12419000 Spokane River near Post Falls, ID Post Falls C 1999–2003; 1991–2003 60–61; 78–120

10 12433000 Spokane River at Long Lake, WA Long Lake C 1999–2003 29

Description of Study Area

The occurrence and transport of trace metals in the 

Spokane River basin are controlled primarily by the metal 

source and input rate, the tendency of metals to adhere to 

sediment, and the transport of water and sediment through 

the basin. Primary sources of trace metals to the South Fork 

Coeur d’Alene, Coeur d’Alene, and Spokane Rivers include 

headwater streams, tributary inflows, ground-water inflow, 

overland runoff from flood plains, and erosion of streambank 

and streambed materials. Once trace metals are mobilized in 

an aquatic system, they can be redistributed in streams and 

rivers, especially during high streamflow when transport is at 

a maximum.

Figure 3 is a schematic diagram showing relative 

locations and hydrologic relationships among the 10 sites. 

Stations 1 and 2, referred to as Canyon Creek and Ninemile 

Creek, are on tributaries to the South Fork Coeur d’Alene 

River. Station 3, referred to as Elizabeth Park, is on the 

South Fork Coeur d’Alene River at Elizabeth Park, Idaho. 

These three sites are upstream of the area designated by the 

USEPA as Operable Unit 2 (OU2). Station 4, referred to as 

Amy Gulch, is on Pine Creek, a tributary to the South Fork 

Coeur d’Alene River whose mouth is in OU2; however, the 

monitoring station is upstream of the OU2 boundary. Station 

5, referred to as Pinehurst, near Pinehurst, Idaho, is on the 

South Fork Coeur d’Alene River downstream of the mouth 

of Pine Creek and in OU2. Station 6, referred to as Enaville, 

at Enaville, Idaho, is on the North Fork Coeur d’Alene River, 

which drains a part of the basin where minimal mining activity 

has taken place. Station 7, referred to as Harrison, near 

Harrison, Idaho, is on the Coeur d’Alene River downstream 

of the confluence of the North Fork and South Fork Coeur 

d’Alene Rivers. Because it represents the accumulated flow 

from the entire Coeur d’Alene River basin the Harrison site is 

important for monitoring metals carried into Coeur d’Alene 

Lake. Stations 8, 9, and 10 (referred to as Outlet, Post Falls, 

and Long Lake, respectively) are downstream of Coeur 

d’Alene Lake, on the mainstem Spokane River (figs. 1, 2).

Description of Study Area 
ID_3084_fig03

2

1

3

4

5

6

7

8

9

10

Coeur d'Alene Lake

Long Lake

Bunker Hill Box

Operable Units 1 and 2

Spokane River Spokane River

South Fork Coeur

 d'Alene River

 South Fork Coeur

 d'Alene River

Coeur d'Alene River

North Fork 

Ninemile Creek

Canyon Creek

Coeur

 d'Alene River

3 Streamflow-gaging station 

 and number

Pine Creek

EXPLANTION

Figure 3. Relative locations and hydrologic relationships among 10 streamflow-gaging stations in the 

Spokane River basin, Idaho and Washington. Gaging station names are shown in table 1.

 Annual Trace-Metal Load Estimates and Flow-Weighted Concentrations, Spokane River Basin, ID and WA, 1999-2004
Method For Estimating Trace-Metal 

Loads

Annual loads of dissolved and total cadmium (DCd and 

TCd), dissolved and total lead (DPb and TPb) and dissolved 

and total zinc (DZn and TZn) were estimated using the USGS 

software LOADEST, which uses instantaneous streamflow 

data and constituent concentrations to calibrate a regression 

model that describes constituent loads in terms of various 

functions of streamflow and time (Runkel and others, 2004). 

The software then uses the regression model to estimate loads 

over a user-specified interval for which daily mean streamflow 

data are provided. Model output includes statistical data to 

enable the user to evaluate the quality of the model. Model 

output also includes upper and lower 95-percent confidence 

interval (CI) for the entire estimation period to provide 

an understanding of the precision of the estimates. In this 

study, separate regression models were calibrated for each 

constituent for each of the 10 sampling sites. Daily loads were 

calculated and summed to obtain annual loads.

The software performs calibration procedures and makes 

load estimates using four statistical estimation methods: 

Adjusted Maximum Likelihood Estimation (AMLE), 

Maximum Likelihood Estimation (MLE), Linear Attribution 

Method (LAM), and Least Absolute Deviation (LAD). The 

user chooses the most appropriate method for the data being 

analyzed. AMLE and MLE are suitable when the model 

calibration errors (residuals) are normally distributed; AMLE 

is the more appropriate method of the two when the calibration 

data set contains censored data (for example, data that are 

reported as below or above some threshold). LAM and LAD 

are useful when the residuals are not normally distributed. 

The AMLE estimation method was selected because in many 

cases the calibration data sets included censored data. The 

initial model calibration residuals for each constituent were 

tested for normality by plotting the natural log of the residuals 

for the AMLE model against their Z-scores, both given in the 

LOADEST output file. These plots yielded generally straight 

lines, indicating that the residuals were normally distributed.

LOADEST software allows the user to choose 

between selecting the general form of the regression from 

among several predefined models and letting the software 

automatically choose the best model, based on the Akaike 

Information Criterion (Akaike, 1981). The selection criterion 

is designed to achieve a good balance between using as many 

predictor variables as possible to explain the variance in load 

while minimizing the standard error of the resulting estimates. 

For this study, the software was allowed to choose the best 

model.

Output regression equations take the following general 

form:

ln(L) = a + b(ln Q) + c(ln Q2) + d[sin(2•T)]

+ e[cos(2•T)] + fT + gT2 (1)

where

L is constituent load, in kg/d;

Q is streamflow, in ft3

/s;

T is time in decimal years from 

beginning of calibration period; 

and

a, b, c, d, e, f, and g are regression coefficients.

Some regression equations in this study did not include 

all of the above terms, depending on the particular model 

chosen by the software. A complete discussion of the theory 

and principles behind calibration and estimation methods used 

by the LOADEST software is given by Runkel and others 

(2004).

Model Input

Streamflow

Streamflow at 9 of the 10 gaging stations in the sampling 

network was measured using standard USGS methods 

(Buchanan and Somers, 1968, 1969; Carter and Davidian, 

1968), using a continuous record of water stage calibrated to 

periodic measurements. Station 7, however, is situated within 

the backwater created by Coeur d’Alene Lake and therefore 

does not have a valid stage-streamflow relation. To estimate 

streamflow at this station, the USGS streamflow model FourPt 

was used (Delong and others, 1997). The model uses channel 

geometry and water-stage data at upstream and downstream 

gaging stations in the stream reach being modeled. The model 

was calibrated using streamflow measurements over a wide 

range of streamflows and lake stage and was used to compute 

a daily mean streamflow for station 7 for WYs 1991–2004. 

Streamflow at station 8 was not available for WY 2003–04; 

therefore, streamflow for station 9 was used for 2003–04.

Model Input 
Trace-Metal Concentrations

Short-term regression models were calibrated at 10 

sites using trace-metal concentration data collected during 

1999–2004. Long-term regression models also were calibrated 

for four of the sites using concentration data collected during 

1991–2004. Trace-metal load estimates were compared for 

1991–2004 based on two different regression models. This 

aspect of the study is discussed in the section “Estimated 

Trace-Metal Loads, 1999-2004”

The trace-metal concentration data used in this report 

were collected as part of a number of studies by the USGS. 

These studies included the Northern Rockies Intermontane 

Basins study of the National Water Quality Assessment 

program (NROK NAWQA) and the Remedial Investigation/

Feasibility Study (RI/FS) of the Spokane River basin (URSGreiner, 2001; Woods, 2001b; Clark and others, 2004). 

Because these studies had somewhat different objectives, the 

number of trace metal samples varied widely among the 10 

sites (table 1). Nevertheless, all trace-metal concentration data 

were collected by USGS personnel and analyzed in USGS 

laboratory facilities using consistent methods and quality 

control measures.

The sampling approach for the NAWQA and RI/FS 

studies was to allocate samples over the full range of the 

station hydrograph to develop a robust relation between tracemetal concentration and streamflow. Generally, samples were 

collected on a fixed-interval frequency, and additional samples 

were collected during low or high streamflow. Additional 

samples were collected as part of the RI/FS study during 

significant streamflow events such as rain-on-snow, spring 

snowmelt runoff, and thunderstorms to characterize tracemetal transport during those times.

Samples at all stations were collected using nonmetallic 

samplers and cross-sectional, depth-integrated sampling 

procedures (Edwards and Glysson, 1988). Samples were 

composited and subsampled using a polyethylene churn or 

Teflon cone-splitting device. Samples for whole-water 

recoverable (total) analyses were withdrawn directly from the 

splitting device. Samples for dissolved (smaller than 0.45-μm 

diameter) analyses were withdrawn directly from either the 

churn splitter or a subsample of the cone splitter and passed 

through a pre-rinsed, 0.45-μm pore size, disposable Gelman 

capsule filter. All trace-metal samples were preserved with 

2 mL of Ultrex nitric acid. Samples were shipped in plastic 

coolers to the USGS National Water Quality Laboratory 

(NWQL) in Denver, Colo. About 10 percent of the samples 

were submitted as blanks, field spikes, and duplicates for 

quality assurance purposes as described by Friedman and 

Erdmann (1982) and Mueller and others (1997).

Samples at all stations were analyzed at the NWQL using 

similar analytical techniques. At the NWQL, samples were 

analyzed for total and dissolved concentrations of selected 

trace metals. Samples for total analysis were digested by 

heating with dilute hydrochloric acid and were filtered prior 

to analysis. Trace-metal concentrations were determined by 

atomic absorption spectrometry in conjunction with a graphite 

furnace and inductively coupled plasma-mass spectrometry 

(Fishman, 1993). Quality assurance/quality control procedures 

used at the NWQL were documented by Pritt and Raese 

(1995).

Trace-metal concentrations generally vary in relation 

to streamflow. For example, in samples collected during 

WYs 1999-2004 from the South Fork Coeur d’Alene River 

at Elizabeth Park, concentrations of TCd and TZn generally 

decreased and TPb increased with increasing streamflow 

(fig. 4). This relation between streamflow and concentration is 

typical of many mining-affected streams in the region (Clark, 

2002).

Some samples having Cd concentrations near the 

minimum analytical detection limit were reported to have 

DCd concentration slightly higher than the TCd concentration, 

a situation that probably is an artifact of the analytical 

methods at low concentration. In most cases, the difference 

between DCd and TCd was less than 10 percent of the 

total concentration; therefore, the Cd in these samples was 

considered to be entirely dissolved.

Regression Statistics

Regression coefficients and coefficients of determination 

(R2) for the best-fit regression models for loads of dissolved 

and total Cd, Pb, and Zn for the 10 studied sites are presented 

in table 2. Coefficients of determination tended to be 

somewhat inflated because of the form of the regression 

equation. Because load is a function of flow (equation 1) a 

strong relation (high R2) is expected, unless large variations 

are in concentration. Nevertheless, the relatively high R2

values indicate that, with few exceptions, the models for 

all constituents successfully simulated the variability in 

constituent loads at most sites.

Table 2 also shows p-values for coefficients of terms 

representing time as a variable in the regression equation 

(coefficient “f”). These coefficients can indicate trends in 

the load estimations. Because the 5-year interval for these 

estimates was considered insufficient to yield reliable trend 

results, they are not discussed here. However, trends for 14-

year load estimates are discussed later in the report.

 Annual Trace-Metal Load Estimates and Flow-Weighted Concentrations, Spokane River Basin, ID and WA, 1999-2004
ID_3084_fig04

TOTAL CADMIUM

TOTAL LEAD

TOTAL ZINC

STREAMFLOW, IN CUBIC FEET PER SECOND

TRACE-METAL CONCENTRATION, IN MICROGRAMS PER LITER

200

400

600

800

1,000

1,200

1,400

1,600

0

200

400

600

0

2

4

6

8

10

0

0 500 1,000 1,500 2,000 2,500

Figure 4. Relation between trace-metal concentrations and streamflow at 

U.S. Geological Survey streamflow-gaging station 12413210, South Fork Coeur 

d’Alene River at Elizabeth Park, near Kellogg, Idaho, water years 1999-2004.

Regression Statistics   
Gaging 

station No.

Gaging station 

name

Regression coefficient

R 2 

(percent) a b c d e f p-value 

of f g p-value of g

Dissolved cadmium (DCd)

1 Canyon Creek 0.6008 0.6817 -0.0712 -0.1205 0.3826 -0.0688 0.028 – – 87.1

2 Ninemile Creek -.5595 .7737 – -.1230 .2380 -.0759 .009 – – 92.6

3 Elizabeth Park 1.0269 .5743 -.0637 .0154 .0336 -.0569 <.001 0.0236 0.0080 96.8

4 Amy Gulch -2.3365 .9049 – .0337 .1811 – – – – 98.3

5 Pinehurst 1.9199 .5719 -.0785 .0344 -.2288 -.0655 <.001 – – 94.2

6 Enaville -2.3963 1.0208 – – – – – – – 93.8

7 Harrison 1.8232 .8408 -.1047 -.2305 -.0058 -.0649 .031 .0496 .0308 96.3

8 Outlet .2334 1.1621 – – – -.1219 .136 – – 98.3

9 Post Falls .5099 1.1001 -.0375 .1454 .2112 – – – – 97.9

10 Long Lake .1045 1.6900 – .4741 .6026 -.2017 .023 – – 84.5

Total cadmium (TCd)

1 Canyon Creek 0.5374 0.7350 – -0.1634 0.3522 -0.0707 0.045 – – 86.9

2 Ninemile Creek -.4732 .8761 – -.2349 .1934 -.0521 .107 – – 92.6

3 Elizabeth Park 1.0808 .7157 -.0380 .0015 .1456 -.0627 .0018 0.0240 0.0410 95.5

4 Amy Gulch -2.3283 1.0044 .0138 -.0965 .1151 .1977 <.001 -.1089 .002 97.6

5 Pinehurst 1.9876 .8189 – .2659 -.1850 – – – – 74.3

6 Enaville -2.7061 1.2194 .1678 – – – – – – 95.4

7 Harrison 2.0707 1.0881 – -.3835 .0169 – – – – 91.8

8 Outlet .0350 1.1977 – – – -.1230 .040 – – 99.2

9 Post Falls .7253 1.1318 – .1774 .1474 -.0542 .052 – – 97.7

10 Long Lake .2041 1.0004 – .1834 .9167 -.0511 .131 – – 95.6

Dissolved lead (DPb)

1 Canyon Creek 0.9883 0.9365 – – – -0.1679 <0.001 – – 96.3

2 Ninemile Creek .0933 .9595 -0.095 – – -.1014 <.001 – – 97.8

3 Elizabeth Park .5524 .6525 – -0.0734 -0.3841 -.0925 – – – 95.9

4 Amy Gulch -2.3582 1.2459 .0747 – – – – – – 95.5

5 Pinehurst 1.4363 .7143 .0519 -.2162 .2869 -.0999 <.001 – – 93.7

6 Enaville -1.3537 1.1351 – – – .1263 – – – 86.3

7 Harrison 3.2026 1.2784 – – – – – – – 88.0

8 Outlet .1119 1.1162 – -.4152 -.2898 -.4273 .060 – – 92.0

9 Post Falls .5045 1.3779 .1794 – – .2774 .009 – – 84.7

10 Long Lake .9372 2.2135 – – – -.1964 .054 – – 75.7

Table 2. Regression coefficients and coefficients of determination (R 2) used to estimate dissolved and total cadmium, lead, and zinc 

loads at 10 streamflow-gaging stations in the Spokane River basin, Idaho and Washington, water years 1999–2004.

[Gaging station locations are shown in figures 1 and 2. Regression equation is ln(L) = a + b(ln Q) + c(ln Q2) + d[sin(2•T)] + e[cos(2•T)] + fT + gT2, where L is 

constituent load; Q is streamflow; T is time in decimal years from beginning of calibration period; a, b, c, d, e, f, and g are regression coefficients. Symbols: –, 

explanatory variable not used in regression; <, less than]

10 Annual Trace-Metal Load Estimates and Flow-Weighted Concentrations, Spokane River Basin, ID and WA, 1999-2004
Gaging 

station No.

Gaging station 

name

Regression coefficient

R 2 

(percent) a b c d e f p-value 

of f g p-value of g

Total lead (TPb)

1 Canyon Creek 1.6834 1.567 0.3162 – – -0.2964 <0.001 – – 93.2

2 Ninemile Creek .7112 1.5924 .2343 – – -.1343 .001 – – 96.8

3 Elizabeth Park 1.6364 1.7137 .3457 – – – – – – 88.7

4 Amy Gulch -2.3381 1.8454 .4037 – – – – – – 93.6

5 Pinehurst 3.1628 1.8606 .4051 0.5561 -0.0612 -.1068 .0317 – – 90.0

6 Enaville -.4893 1.9066 .3115 – – – – – – 91.7

7 Harrison 4.4844 1.5341 .3381 – – – – – – 91.6

8 Outlet 1.9154 1.1945 – – – – – – – 89.5

9 Post Falls 2.2820 1.4423 .1874 .2101 -.4583 – – – – 95.1

10 Long Lake 2.4961 2.2704 – – – – – – – 92.7

Dissolved zinc (DZn)

1 Canyon Creek 5.7756 0.7615 -0.1145 -0.0403 0.5307 -0.0364 0.264 -0.057 0.069 86.3

2 Ninemile Creek 4.5379 .82 – -.2289 .3347 -.0859 .021 – – 88.9

3 Elizabeth Park 6.0871 .6176 -.0896 .0677 .0472 -.0595 <.001 .0227 .0230 96.5

4 Amy Gulch 3.3121 .868 – -.0204 .3607 – – – – 97.2

5 Pinehurst 6.8909 .5413 -.0431 .0484 -.1875 -.0652 <.001 – – 94.8

6 Enaville 2.9204 1.2150 -.0919 – – – – – – 93.6

7 Harrison 7.0699 .7142 -.0750 -.1846 .1819 -.0644 .004 – – 97.0

8 Outlet 5.6784 1.0963 – .2530 -.1315 – – – – 99.5

9 Post Falls 6.2192 1.0794 – .0487 .3295 -.0544 <.001 – – 99.5

10 Long Lake 5.7269 1.1329 – -.007 .9901 -.076 .010 – – 97.3

Total zinc (TZn)

1 Canyon Creek 5.4991 0.7975 – -0.2311 0.4613 – – – – 86.3

2 Ninemile Creek 4.5792 .9167 – -.3632 .2502 -0.0663 0.098 – – 89.7

3 Elizabeth Park 6.0733 .7258 -.0409 .0644 .1397 -.0523 .00918 0.0202 0.0901 95.6

4 Amy Gulch 3.4019 .961 – -.1631 .2743 – – – – 96.5

5 Pinehurst 6.8546 .6726 .0597 .1023 -.2355 -.0505 .078 – – 80.7

6 Enaville 2.9576 1.4643 – – – – – – – 93.5

7 Harrison 7.0231 .9963 .0466 -.4025 .0801 – – – – 93.1

8 Outlet 5.7451 1.0763 – .1994 -.1721 -.1200 .013 – – 99.6

9 Post Falls 6.2629 1.1028 – .0365 .2954 -.0232 .102 – – 99.1

10 Long Lake 6.2721 1.4376 -.2892 -.0054 .8105 -.0724 .028 -.073 .012 97.7

Table 2. Regression coefficients and coefficients of determination (R 2) used to estimate dissolved and total cadmium, lead, and zinc 

loads at 10 streamflow-gaging stations in the Spokane River basin, Idaho and Washington, water years 1999–2004.—Continued

[Gaging station locations are shown in figures 1 and 2. Regression equation is ln(L) = a + b(ln Q) + c(ln Q2) + d[sin(2•T)] + e[cos(2•T)] + fT + gT2, where L is 

constituent load; Q is streamflow; T is time in decimal years from beginning of calibration period; a, b, c, d, e, f, and g are regression coefficients. Symbols: –, 

explanatory variable not used in regression; <, less than]

Regression Statistics 11
Estimated Trace-Metal Loads, 

1999-2004

Estimated annual trace-metal loads of dissolved and total 

Cd, Pb, and Zn for 1999-2004 (referred to as “short-term” 

loads) for each site are presented in table 3. The overall mean 

total metal loads (the sum of TCd, TPb, and TZn) and the 

mean percentage of each metal of the sum are presented in 

table 3. Graphs showing mean streamflow and mean dissolved 

and total Cd, Pb, and Zn loads for each site for 1999–2004 are 

shown in figure 5.

Cadmium Loads

Estimated mean annual TCd loads were relatively low at 

Amy Gulch (39 kg/yr), Enaville (58 kg/yr), Ninemile Creek 

(200 kg/yr), and Canyon Creek (510 kg/yr). Conversely, 

estimated mean annual TCd loads were relatively high at 

Harrison (3,400 kg/yr), Pinehurst (2,600 kg/yr), Post Falls 

(1,500 kg/yr), and Elizabeth Park (1,200 kg/yr). The lowest 

single annual TCd load, 18 kg, was estimated at Amy Gulch 

in 2001. Results for 2001, the lowest-flow year, marked the 

lowest TCd loads at every station in the basin.

The maximum annual TCd load, 5,100 kg, was at 

Harrison in 1999. The same year also marked maximum loads 

at Pinehurst (3,400 kg), Post Falls (2,100 kg), and Elizabeth 

Park (1,800 kg) (table 3). TCd loads for the entire estimation 

period were high at these four sites.

Dissolved Cd typically ranged from about 70 to 100 

percent of the TCd load (fig. 5). In general, the DCd/TCd 

ratio was higher at stations where ground water was a 

volumetrically important component of the streamflow, for 

example, on tributary streams farther upstream in the basin or 

at stations at the outlet of Coeur d’Alene Lake or Long Lake. 

DCd also is higher at most stations during low-flow years 

such as 2001, because of the greater contribution of ground 

water to streamflow. Relatively high DCd/TCd ratios (about 

0.9 and greater) were estimated at stations 1 through 4. The 

lowest DCd/TCd was estimated at Harrison, where less than 

70 percent of the TCd was in the dissolved state.

In a few instances, DCd loads exceeded TCd loads 

because DCd concentrations were reported to be slightly 

higher than TCd concentrations when Cd levels were at or 

near the minimum analytical detection limit. Again, these 

situations indicate that essentially the entire Cd load is in the 

dissolved state.

Lead Loads

Stations 4 (Amy Gulch), 2 (Ninemile Creek), and 6 

(Enaville) had relatively low mean annual estimated TPb 

loads: 1,700, 2,100, and 3,600 kg, respectively. The lowest 

single annual TPb load, 64 kg, was at station 4 in 2001. 

Minimum TPb loads for 1999-2004 occurred mainly in 2001 

at most stations, although 2003 also was characterized by low 

TPb loads.

The highest overall mean TPb loads in the basin were 

at Harrison (station 7). Mean annual TPb loads at Harrison 

exceeded those at all other stations by a factor of 6 or more. 

The maximum annual TPb load in the basin, about 590,000 

kg, was at Harrison for 2002. Water year 2002 also marked 

peak TPb loads at seven other stations in the basin, which may 

reflect flushing of accumulated Pb-laden sediment that was 

stored in the stream channels during the previous year.

The DPb/TPb ratio varied in time and space and reflected 

the relative contribution of ground water (high in DPb) to 

total streamflow and the presence (low DPb/TPb) or absence 

(high DPb/TPb) of Pb-carrying suspended particulate and 

(or) colloidal material. DPb/TPb ranged from less than 10 

percent to nearly 30 percent of the TPb load. The highest 

DPb/TPb loads were at stations 8, 9, and 10 (downstream of 

Coeur d’Alene Lake) and at stations 1 and 2, all stations where 

ground water contribution to streamflow is high and suspended 

sediment tends to be low under normal flow conditions.

Zinc Loads

Zinc is the primary constituent of metal loads in the 

Spokane River basin, composing more than 90 percent of the 

total metal load at 6 of the 10 stations. Relatively low mean 

annual TZn loads occurred at Amy Gulch (14,000 kg) and at 

Enaville (17,000 kg). The low loads at these two stations likely 

reflect the low availability of zinc in the source areas upstream 

of these stations.

Mean annual loads of TZn at Harrison were about 

510,000 kg/yr and far exceeded those at any other station. The 

single highest annual TZn load in the basin (760,000 kg) was 

at Harrison in 1999. High TZn loads also were measured at 

Pinehurst and Post Falls, with mean annual loads greater than 

350,000 kg. Mean annual loads of about 265,000 and 280,000 

kg of TZn were estimated at the stations at Coeur d’Alene 

Lake outlet and Long Lake, respectively.

12 Annual Trace-Metal Load Estimates and Flow-Weighted Concentrations, Spokane River Basin, ID and WA, 1999-2004
Water year

Gaging station number and name

1

Canyon 

Creek

2

Ninemile 

Creek

3

Elizabeth 

Park

4

Amy 

Gulch

5

Pinehurst

6

Enaville

7

Harrison

8

Outlet

9

Post 

Falls

10

Long 

Lake

Mean daily streamflow (m3/s)

1999 1.7 0.5 12 5.6 19 64 86 – 210 260

2000 1.5 .5 10 4.6 17 57 77 – 190 240

2001 .9 .2 4.3 1.6 7 22 30 – 76 110

2002 1.7 .7 12 5.7 19 70 93 – 220 250

2003 1.2 .3 7.2 3.3 11 39 52 130 130 160

2004 – – 7.2 – 11 41 54 130 – –

 Mean 1.4 0.4 8.8 4.1 14 49 65 130 165 200

Dissolved cadmium (DCd) load

1999 640 240 1,600 54 3,000 58 3,400 – 1,400 1,000

2000 580 220 1,300 47 2,600 52 2,800 – 1,300 860

2001 350 100 700 16 1,300 19 1,100 – 460 –

2002 500 210 1,100 54 2,300 64 2,600 – 1,400 600

2003 400 130 880 34 1,700 36 1,600 960 890 370

2004 – – 920 – 1,600 37 1,600 820 – –

 Mean 490 180 1,100 41 2,100 44 2,200 890 1,100 710

Total cadmium (TCd) load

1999 660 250 1,800 30 3,400 73 5,100 – 2,100 980

2000 600 230 1,500 41 3,000 71 4,400 – 1,900 900

2001 360 110 690 18 1,400 19 1,400 – 590 –

2002 530 250 1,200 70 3,200 108 4,700 – 1,900 830

2003 410 140 930 35 2,200 43 2,400 1,100 1,000 510

2004 – – 970 – 2,200 37 2,100 970 – –

 Mean 510 200 1,200 39 2,600 58 3,400 1,000 1,500 810

Dissolved lead (DPb) load

1999 1,400 480 1,300 98 2,700 150 23,000 – 1,700 4,400

2000 1,000 410 1,000 77 2,100 150 20,000 – 2,200 3,500

2001 530 170 540 20 1,000 60 6,400 – 710 –

2002 840 390 950 120 2,000 260 27,000 – 5,200 2,800

2003 490 210 620 55 1,200 150 13,000 1,400 2,500 760

2004 – – 580 – 1,100 180 13,000 940 – –

 Mean 850 330 830 74 1,700 160 17,000 1,200 2,500 2,900

Total lead (TPb) load

1999 10,000 2,600 23,000 910 64,000 3,200 250,000 – 20,000 11,000

2000 5,000 2,000 16,000 1,000 46,000 4,700 320,000 – 18,000 11,000

2001 1,900 390 2,500 64 5,100 440 37,000 – 4,300 –

2002 6,300 4,900 32,000 5,400 95,000 10,000 590,000 – 27,000 13,000

2003 1,400 540 6,000 1,000 17,000 2,100 170,000 6,500 7,600 4,200

2004 – – 5,200 – 7,000 1,000 81,000 6,300 – –

 Mean 4,900 2,100 14,000 1,700 39,000 3,600 240,000 6,400 15,000 9,800

Table 3. Estimated mean annual trace-metal loads for 10 streamflow-gaging stations in the Spokane River basin, Washington and 

Idaho, water years 1999–2004.

[Gaging station locations are shown in figures 1 and 2. All values in kilograms unless otherwise noted. Abbreviation: m3/s, cubic meter per second. –, no 

estimate]

Estimated Trace-Metal Loads, 1999-2004 13
Water year

Gaging station number and name

1

Canyon 

Creek

2

Ninemile 

Creek

3

Elizabeth 

Park

4

Amy 

Gulch

5

Pinehurst

6

Enaville

7

Harrison

8

Outlet

9

Post 

Falls

10

Long 

Lake

Dissolved zinc (DZn) load

1999 83,000 41,000 250,000 16,000 430,000 13,000 580,000 – 470,000 280,000

2000 91,000 37,000 200,000 14,000 380,000 11,000 500,000 – 420,000 250,000

2001 54,000 16,000 100,000 4,800 200,000 3,800 220,000 – 130,000 –

2002 78,000 34,000 170,000 15,000 330,000 14,000 440,000 – 410,000 220,000

2003 54,000 20,000 140,000 10,000 250,000 7,600 300,000 250,000 240,000 130,000

2004 – – 142,000 – 240,000 8,000 290,000 230,000 – –

 Mean 72,000 30,000 167,000 12,000 310,000 9,600 310,000 240,000 330,000 220,000

Total zinc (TZn) load

1999 81,000 41,000 260,000 18,000 490,000 22,000 760,000 – 480,000 330,000

2000 78,000 38,000 210,000 16,000 430,000 20,000 660,000 – 440,000 350,000

2001 48,000 16,000 100,000 5,000 220,000 5,300 220,000 – 140,000 –

2002 79,000 38,000 190,000 18,000 410,000 28,000 690,000 – 460,000 300,000

2003 65,000 21,000 140,000 12,000 290,000 12,000 370,000 290,000 280,000 150,000

2004 – – 140,000 – 260,000 12,000 330,000 240,000 – –

 Mean 70,000 31,000 170,000 14,000 350,000 17,000 510,000 265,000 360,000 280,000

Total metals

1999 92,000 44,000 280,000 19,000 560,000 25,000 1,000,000 – 500,000 340,000

2000 84,000 40,000 230,000 17,000 480,000 25,000 980,000 – 460,000 360,000

2001 50,000 17,000 100,000 5,100 230,000 5,800 260,000 – 100,000 –

2002 86,000 43,000 220,000 23,000 510,000 38,000 1,300,000 – 490,000 310,000

2003 67,000 22,000 150,000 13,000 310,000 14,000 540,000 300,000 290,000 160,000

2004 – – 150,000 – 270,000 13,000 410,000 250,000 – –

 Mean 76,000 33,000 190,000 15,000 390,000 20,000 750,000 275,000 370,000 293,000

Mean percentage of total metals (by weight)

Cadmium 0.67 0.59 0.63 0.25 0.65 0.29 0.45 0.39 0.40 0.27

Lead 6.6 6.3 7.4 11 10 18 32 2.4 4.0 3.4

Zinc 93 93 92 89 89 82 68 97 96 96

Table 3. Estimated mean annual trace-metal loads for 10 streamflow-gaging stations in the Spokane River basin, Washington and 

Idaho, water years 1999–2004.—Continued

[Gaging station locations are shown in figures 1 and 2. All values in kilograms unless otherwise noted. Abbreviation: m3/s, cubic meter per second. –, no 

estimate]

14 Annual Trace-Metal Load Estimates and Flow-Weighted Concentrations, Spokane River Basin, ID and WA, 1999-2004
ID_3084_fig05

 Dissolved

 Total

Mean streamflow

0.1

1.0

10

100

1,000

10,000

0.1

1.0

10

100

1,000

10,000

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

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

Canyon Ck

Ninemile Ck

Elizabeth Park

Amy Gulch

Pinehurst

Enaville

Harrison

Outlet

Post Falls

Long Lake

MEAN ANNUAL TRACE-METAL LOAD, IN KILOGRAMS

CADMIUM

LEAD

ZINC

MEAN STREAMFLOW, IN CUBIC METERS PER SECOND

Figure 5. Mean annual estimated dissolved and total trace-metal 

loads and mean streamflow at 10 streamflow-gaging stations in the 

Spokane River basin, Idaho and Washington, water years 1999–2004. 

Estimated Trace-Metal Loads, 1999-2004 15
The DZn/TZn ratio typically was greater than 0.8 at most 

stations, indicating that zinc occurs primarily in the dissolved 

state. Although TZn loads were relatively low in 2001 due 

to low streamflow, the ratio DZn/TZn was higher in 2001 

compared to other years resulting from high concentrations of 

DZn in ground water and less dilution of ground water inflow 

from snowmelt runoff.

Spatial Patterns in Metal Loads and Proportions

Mean total metal load (the sum of TCd, TPb, and TZn) 

for 1999-2004 ranged from about 15,000 kg/y at station 4 to 

about 750,000 kg/y at station 7 (table 3). The percentage of Cd 

of the total metal load at all stations was less than 1 percent. 

The percentage of Pb of the total metal load increased from 

about 6 percent at station 2 to about 32 percent downstream at 

station 7. Downstream of Coeur d’Alene Lake the percentage 

of Pb dropped to 2 to 4 percent at stations 8 through 10. Zn 

accounted for the highest proportion of the total metal load 

at all stations. In contrast to Pb, Zn regularly decreased from 

about 93 percent at stations 1 through 3 to about 68 percent at 

station 7. At stations 8 through 10, the proportion of Zn was 

the highest in the basin, about 96 percent.

Notable systematic differences between total metal loads 

at stations in different parts of the basin are partly due to 

the direct relation between streamflow and load. Generally, 

the further downstream the station’s location in the Coeur 

d’Alene River basin, the higher the total metal load, because 

streamflow increases downstream. (Enaville is an exception; 

although it has relatively high mean streamflow, loads are 

relatively low because of low concentrations). Total metal 

loads at the three stations downstream of Coeur d’Alene Lake 

(stations 8, 9, and 10) are much less than metal loads at station 

7. Only about 4,200 kg/y of metals enter Coeur d’Alene Lake 

from the St. Joe River via mean annual streamflow of about 

2,900 ft3/s (Clark, 2002).

The decrease in metal loads at gaging stations 

downstream of Coeur d’Alene Lake likely is due to the 

well-documented process of retention of metals, particularly 

Pb, in lakebed sediments (Horowitz and others, 1995; Clark, 

2002). This study did not consider metals in the St. Joe 

River, but because Pb is primarily carried by particulate 

matter, a substantial amount of Pb is likely being deposited 

in the lakebed sediments. The relative proportions, as well 

as quantities, of metals upstream and downstream of the lake 

support this inference. The relative proportions of Cd, Pb, and 

Zn change notably upstream and downstream of the lake. For 

example, the percentage of TPb of the total metal load ranges 

from 6 percent at station 2 to 32 percent at station 7. However, 

TPb is only 2-4 percent of the total metal load at stations 8 

through 10.

Conversely, Zn, which generally occurs in the dissolved 

state, accounts for a much higher proportion of the total metal 

load at stations 8 – 10 than at station 7. Indeed, the DZn/TZn 

load ratio rises sharply from 0.68 at stations 7 to 0.97 at station 

8. These data suggest that metals are affected by different 

storage and transport processes, and that water-quality data aid 

significantly in understanding these processes.

Comparison with Previous Load Estimates

Clark (2002) used LOADEST software to estimate loads 

for WY 1999-2001 at many of the same sites studied in this 

report. The input calibration data used by Clark were identical 

to the calibration data used in this study for 1999-2001, but 

these data also included data for 2002-04. Comparing Clark’s 

results with those from this study is another opportunity to 

compare load estimates for the same time interval based on 

two different regressions.

Metal load estimates from this study for nine sites 

during 1999–2001 (produced by regressing data from 1999-

2004) were compared with Clark’s (2002) estimates for 

the same interval (table 4). Overall, there was fairly good 

agreement between the two sets of results. However, notable 

discrepancies, including 2001 TPb results, were identified in 

estimates for Enaville. TPb annual load estimate from this 

study was higher than Clark’s estimate by a factor of almost 2. 

Smaller differences in TPb estimates for other years occurred 

at Harrison, Amy Gulch, and Elizabeth Park; most TPb 

discrepancies are negative, that is, estimates from this study 

were greater than Clark’s estimates. One important difference 

between the two time intervals is that very low streamflow in 

2001 strongly influenced Clark’s results because that year’s 

data constituted a greater proportion of the total available data, 

whereas, 2001 data were a smaller proportion of the available 

data for this study. Because the software uses the relation 

between flow and concentration to perform the regressions, 

including the subsequent 3 years’ TPb and streamflow data 

probably resulted in a regression line with a steeper slope, 

causing estimated loads to be somewhat higher. In addition, 

this difference was probably more noticeable with Pb than 

with Cd or Zn because of the effect of streamflow on sediment 

transport and consequently, Pb load.

16  Annual Trace-Metal Load Estimates and Flow-Weighted Concentrations, Spokane River Basin, ID and WA, 1999-2004
Water 

year

Canyon Creek Ninemile Creek Elizabeth Park Amy Gulch Pinehurst

This study Clark (2002) This study Clark (2002) This study Clark (2002) This study Clark (2002) This study Clark (2002)

Annual dissolved cadmium (DCd) load

1999 640 590 240 220 1,600 1,400 54 59 3,000 2,300

2000 580 590 220 220 1,300 1,300 47 50 2,600 2,600

2001 350 380 100 120 700 680 16 18 1,300 1,300

Annual total cadmium (TCd) load

1999 660 640 250 240 1,800 1,700 30 64 3,400 3,600

2000 600 640 230 230 1,500 1,500 41 50 3,000 2,900

2001 360 390 110 120 690 680 18 15 1,400 1,300

Annual dissolved lead (DPb) load

1999 1,400 1,300 480 450 1,300 1,600 98 91 2,700 2,600

2000 1,000 1,000 410 380 1,000 1,100 77 73 2,100 1,900

2001 530 500 170 160 540 450 20 20 1,000 950

Annual total lead (TPb) load

1999 10,000 11,000 2,600 2,400 23,000 26,000 910 590 64,000 60,000

2000 5,000 5,400 2,000 2,100 16,000 15,000 1,000 500 46,000 34,000

2001 1,900 2,100 390 500 2,500 1,500 64 64 5,100 3,900

Annual dissolved zinc (DZn) load

1999 83,000 86,000 41,000 36,000 250,000 200,000 16,000 16,000 430,000 420,000

2000 91,000 86,000 37,000 35,000 200,000 190,000 14,000 13,000 380,000 380,000

2001 54,000 54,000 16,000 19,000 100,000 100,000 4,800 4,500 200,000 200,000

Annual total zinc (TZn) load

1999 81,000 86,000 41,000 38,000 260,000 230,000 18,000 15,000 490,000 500,000

2000 78,000 86,000 38,000 38,000 210,000 210,000 16,000 14,000 430,000 420,000

2001 48,000 54,000 16,000 19,000 100,000 100,000 5,000 5,000 220,000 200,000

Table 4. Comparison of trace-metal load estimates for water years 1999–2001 (this study) with estimates by Clark (2002) for nine 

streamflow-gaging stations in the Spokane River basin, Idaho and Washington.

[All values rounded, in kilograms. Abbreviation: na, not available]

Estimated Trace-Metal Loads, 1999-2004 17
Water 

year

Enaville Harrison Post Falls Long Lake

This study Clark (2002) This study Clark (2002) This study Clark (2002) This study Clark (2002)

Annual dissolved cadmium (DCd) load

1999 58 73 3,400 3,500 1,400 1,800 1,000 950

2000 52 64 2,800 2,800 1,300 1,500 860 950

2001 19 25 1,100 1,000 460 450 na na

Annual total cadmium (TCd) load

1999 73 100 5,100 4,500 2,100 2,300 980 950

2000 71 95 4,400 3,500 1,900 1,900 900 860

2001 19 28 1,400 1,300 590 540 na na

Annual dissolved lead (DPb) load

1999 150 180 23,000 17,000 1,700 2,800 4,400 4,500

2000 150 170 20,000 15,000 2,200 2,700 3,500 1,900

2001 60 50 6,400 5,000 710 590 na na

Annual total lead (TPb) load

1999 3,200 2,900 250,000 210,000 20,000 21,000 11,000 11,000

2000 4,700 4,500 320,000 240,000 18,000 19,000 11,000 10,000

2001 440 160 37,000 40,000 4,300 4,000 na na

Annual dissolved zinc (DZn) load

1999 13,000 10,000 580,000 540,000 470,000 450,000 280,000 270,000

2000 11,000 9,100 500,000 450,000 420,000 400,000 250,000 260,000

2001 3,800 3,100 220,000 210,000 130,000 130,000 na na

Annual total zinc (TZn) load

1999 22,000 22,000 760,000 680,000 480,000 450,000 330,000 340,000

2000 20,000 20,000 660,000 540,000 440,000 440,000 350,000 330,000

2001 5,300 5,300 220,000 220,000 140,000 150,000 na na

Table 4. Comparison of trace-metal load estimates for water years 1999–2001 (this study) with estimates by Clark 

(2002) for nine streamflow-gaging stations in the Spokane River basin, Idaho and Washington.—Continued

[All values rounded, in kilograms. Abbreviation: na, not available]

18  Annual Trace-Metal Load Estimates and Flow-Weighted Concentrations, Spokane River Basin, ID and WA, 1999-2004
Flow-Weighted Concentrations of 

Trace Metals, 1999-2004

Constituent loads are calculated as the product of 

streamflow and concentration. Because load calculation results 

are strongly governed by variations in streamflow, a calculated 

load may not be the best measure by which to analyze metal 

transport and behavior. Interpretation of calculated annual 

loads is improved by examining mean annual Flow-Weighted 

Concentrations (FWCs) of trace metals. A FWC is an estimate 

of the mean actual concentration in a total volume of water 

flowing past a site in a specific period, such as a month or 

year.

FWCs permit a better understanding of metal transport 

in the context of variable streamflow conditions and relative 

location in the basin. For example, table 5 shows mean daily 

streamflow from 1999 to 2004 ranged from less than 1 m3/s 

at Ninemile Creek to greater than 200 m3/s at Long Lake. 

Mean annual streamflow at Amy Gulch varied by more than 

a factor of 3 during this period. By examining FWCs, a better 

understanding of the true differences in metal transport can be 

gained.

Mean annual FWCs for total and dissolved Cd, Pb, and 

Zn are presented in table 5; the overall mean FWCs for the 

10 sites are presented in figure 6. Mean annual FWCs were 

calculated by the following method. Daily load estimates for 

each constituent, in kilograms, were summed to obtain annual 

total loads for each year. Annual sums were divided by the 

total streamflow, in cubic meters, for that year. Appropriate 

conversion factors were applied to obtain mean annual FWCs 

in micrograms per liter.

Total Cd FWCs in the Coeur d’Alene basin ranged from 

14 to 0.04 μg/L. Concentrations were highest at Ninemile 

Creek and Canyon Creek, where estimated loads were 

relatively low due to low mean streamflow. TCd FWCs were 

lowest at Enaville; TCd loads also were low at this station, 

indicating overall low metal contribution from the North 

Fork Coeur d’Alene River. Low TCd FWCs were measured 

at stations downstream from Coeur d’Alene Lake, even 

though TCd loads at these sites were relatively high. This 

demonstrates that although concentrations may be low because 

of high streamflow, the overall transport of trace metals can be 

high.

Total Pb FWCs were highest at Ninemile Creek, Canyon 

Creek, and Harrison. Elevated TPb FWCs at Harrison 

confirmed that the high TPb loads at this station were not 

simply due to higher streamflow; downstream dilution did 

not compensate for additional sources of metal. Conversely, 

relatively low TPb loads together with high TPb FWCs at 

Ninemile and Canyon Creeks reflect low streamflow but high 

concentrations of TPb.

Total zinc FWCs also demonstrated the profound effect 

of streamflow on load calculations, and indicated source areas 

for zinc in the basin. Total Zn FWCs at Ninemile and Canyon 

Creeks are by far the highest in the basin but contribute among 

the lowest TZn loads due to their relatively low streamflow. 

Conversely, stations downstream from Coeur d’Alene Lake 

exhibit among the lowest TZn FWCs, but TZn load estimates 

are high because of their high mean streamflow relative to 

other stations in the basin.

Effects of dilution were recognized by examining TZn 

loads together with FWCs at Pinehurst and Harrison. Mean 

annual flows at Pinehurst generally were about 20 percent 

of those at Harrison. Although mean annual TZn loads at 

Pinehurst were slightly less than loads at Harrison, the mean 

annual FWCs of TZn at Pinehurst were more than 3 times 

greater than FWCs at Harrison, due to downstream dilution of 

metals by inflow of the North Fork Coeur d’Alene River just 

downstream of Pinehurst (fig. 6).

Total metal loads and FWCs at Enaville are among the 

lowest in the basin. This likely reflects an upstream source 

area low in metals in addition to the effects of dilution by 

relatively high streamflow at this site. Overall, these data 

confirm the low metal contribution of this part of the basin to 

the whole.

Flow-Weighted Concentrations of Trace Metals, 1999-2004 19
Water 

year

Gaging station number and name

1

Canyon 

Creek

2

Ninemile 

Creek

3

Elizabeth 

Park

4

Amy 

Gulch

5

Pinehurst

6

Enaville

7

Harrison

8

Outlet

9

Post 

Falls

10

Long 

Lake

Mean annual streamflow (m3/s)

1999 1.7 0.5 12 5.6 19 64 86 – 210 260

2000 1.5 .5 10 4.6 17 57 77 – 190 240

2001 .9 .2 4.3 1.6 7 22 30 – 76 110

2002 1.7 .7 12 5.7 19 70 93 – 220 250

2003 1.2 .3 7.2 3.3 11 39 52 130 130 160

2004 – – 7.2 – 11 41 54 130 – –

Mean 1.4 0.5 8.8 4.1 14 49 65 130 165 200

Dissolved cadmium (DCd) FWC

1999 12 14 4.2 0.31 4.8 0.03 1.2 – 0.21 0.13

2000 13 14 3.9 .32 5.0 .03 1.2 – .21 .11

2001 12 15 5.0 .33 6.4 .03 1.2 – .19 –

2002 9.1 9.8 2.9 .30 3.7 .03 .87 – .21 .08

2003 11 12 3.8 .33 4.9 .03 .97 0.23 .21 .07

2004 – – 4.1 – 4.6 .03 .92 .20 – –

Mean 11 13 4.0 0.32 4.9 0.03 1.1 0.21 0.21 0.10

Total cadmium (TCd) FWC

1999 12 15 4.9 0.17 5.5 0.04 1.9 – 0.31 0.12

2000 13 15 4.5 .28 5.8 .04 1.8 – .30 .12

2001 13 15 5.1 .36 6.6 .03 1.4 – .25 –

2002 9.6 11 3.3 .39 5.2 .05 1.6 – .27 .10

2003 11 13 4.1 .34 6.3 .03 1.5 0.27 .25 .10

2004 – – 4.3 – 6.2 .03 1.2 .23 – –

Mean 12 14 4.4 0.31 5.9 0.04 1.6 0.25 0.28 0.11

Dissolved lead (DPb) FWC

1999 25 28 3.4 0.56 4.4 0.07 8.4 – 0.25 0.53

2000 22 26 3.2 .52 4.1 .08 8.4 – .35 .46

2001 19 24 4.0 .41 5.0 .09 6.7 – .29 –

2002 15 18 2.5 .65 3.3 .12 9.0 – .74 .35

2003 13 20 2.7 .53 3.3 .12 7.7 0.33 .61 .15

2004 – – 2.6 – 3.1 .14 7.4 .23 – –

Mean 19 23 3.1 0.53 3.9 0.10 7.9 0.28 0.45 0.37

Table 5. Estimated mean annual flow-weighted concentrations of trace metals for 10 streamflow-gaging stations in the Spokane River 

basin, Idaho and Washington, water years 1999–2004.

[Gaging station locations are shown in figures 1 and 2. All values are in micrograms per liter unless otherwise noted. Abbreviations: FWC, flow-weighted 

concentration; m3/s, cubic meter per second. Symbols: –, no estimate]

20 Annual Trace-Metal Load Estimates and Flow-Weighted Concentrations, Spokane River Basin, ID and WA, 1999-2004
Water 

year

Gaging station number and name

1

Canyon 

Creek

2

Ninemile 

Creek

3

Elizabeth 

Park

4

Amy 

Gulch

5

Pinehurst

6

Enaville

7

Harrison

8

Outlet

9

Post 

Falls

10

Long 

Lake

Total lead (TPb) FWC 

1999 190 150 61 5.2 100 1.6 91 – 2.9 1.3

2000 110 130 48 7.0 87 2.6 130 – 2.9 1.4

2001 67 55 18 1.3 25 .65 39 – 1.8 –

2002 110 230 84 30 160 4.5 200 – 3.9 1.7

2003 38 51 26 9.8 48 1.7 100 1.5 1.8 .8

2004 – – 23 – 20 .77 48 1.5 – –

Mean 100 120 43 11 73 2.0 100 1.5 2.7 1.3

Dissolved zinc (DZn)

1999 1,600 2,400 660 89 700 6.4 210 – 69 34

2000 2,000 2,300 620 94 720 6.2 200 – 68 33

2001 1,900 2,300 760 96 980 5.6 230 – 55 –

2002 1,400 1,600 400 85 550 6.2 150 – 58 27

2003 1,500 1,900 600 100 710 6.1 180 61 58 26

2004 – – 630 – 680 6.2 170 57 – –

Mean 1,700 2,100 610 93 720 6.1 190 59 62 30

Total zinc (TZn)

1999 1,500 2,400 700 100 800 11 280 – 71 40

2000 1,700 2,400 650 110 820 11 270 – 71 46

2001 1,700 2,200 740 100 1,000 7.8 230 – 60 –

2002 1,400 1,800 490 100 670 13 230 – 67 38

2003 1,800 2,000 610 110 810 10 230 68 67 29

2004 – – 640 – 750 9.0 200 57 – –

Mean 1,600 2,200 640 100 810 10 240 63 67 38

Table 5. Estimated mean annual flow-weighted concentrations of trace metals for 10 streamflow-gaging stations in the Spokane River 

basin, Idaho and Washington, water years 1999–2004.—Continued

[Gaging station locations are shown in figures 1 and 2. All values are in micrograms per liter unless otherwise noted. Abbreviations: FWC, flow-weighted 

concentration; m3/s, cubic meter per second. Symbols: –, no estimate]

Flow-Weighted Concentrations of Trace Metals, 1999-2004 21
ID_3084_fig06

MEAN ANNUAL TRACE METAL FLOW-WEIGHTED CONCENTRATION, IN MICROGRAMS PER LITER

CADMIUM

LEAD

ZINC

0.10

1

10

100

1,000

10,000

Canyon Ck

Ninemile Ck

Elizabeth Park

Amy Gulch

Pinehurst

Enaville

Harrison

Outlet

Post Falls

Long Lake

0.01

0.10

1

10

100

1,000

0.10

1

10

100

1,000

0.10

1

10

100

1,000

10,000

0.01

0.10

1

10

100

1,000

0.10

1

10

100

1,000

 Dissolved

 Total

MEAN STREAMFLOW, IN CUBIC METERS PER SECOND

Mean streamflow

Figure 6. Mean annual flow-weighted concentrations of trace metals and 

mean streamflow at 10 streamflow-gaging stations in the Spokane River 

basin, Idaho and Washington, water years 1999–2004.

22 Annual Trace-Metal Load Estimates and Flow-Weighted Concentrations, Spokane River Basin, ID and WA, 1999-2004
Estimated Trace-Metal Loads and 

Flow-Weighted Concentrations, 

1991-2004

Regression models were developed and annual tracemetal loads and FWCs were estimated for Enaville (1992-

2004), Pinehurst, Harrison, (1991-2004) and Post Falls (1991-

2003) to understand the variability of metal loading with 

time. Because these load models and estimates cover a longer 

interval than the 1999-2004 study, they are described as “longterm.” Developing long-term regression models for these four 

sites also enabled a comparison of the load estimates produced 

for 1991-2004 with those for 1999-2004 and an examination 

of the robustness of the two differently calibrated models, 

specifically for the latter period. The metal concentration data 

for 1999–2004 were the same as those used for the short-term 

load simulation. The regression coefficients and coefficients 

of determination (R2) of the long-term models are presented 

in table 6. Long-term annual load estimates and FWCs are 

summarized in table 7. Graphs of annual estimated loads at all 

sites for all years are shown in figure 6. The graphs in figure 7

summarize the long-term mean annual metal loads for the 

four stations. The mean annual flow-weighted concentrations 

calculated from the long-term loads are shown in figure 8.

Long-term load estimates are similar to the results 

for 1999-2004 in terms of mean annual metal loads. For 

example, the largest mean loads for the estimation period were 

measured at Harrison and Pinehurst. Generally, the highest 

estimated loads were in 1996 and 1997 at all four stations, 

primarily because of extremely high flows. High loads also 

were measured in 1991. The lowest loads prior to 1999 

generally were in lower-flow years, 1994 and 1998. Although 

low estimated loads in 1994 and 1998 probably reflect low 

streamflow, estimated loads in 1998 also may reflect a lack 

of sufficient time for metals to accumulate upstream after 

unusually high flows in 1996 and 1997 had scoured the 

channels.

Comparison with 1999-2004 Estimates

Long-term trace-metal load estimates for 1999-2004 

generally compare well with the estimates produced by the 

short-term model for the overlapping time interval, but the 

similarity of the estimates varies from station to station, from 

year to year, and from metal to metal (table 8, fig. 9). The 

most consistent results for a single station were obtained from 

the gaging station at Pinehurst, where the long-term results for 

all metals are within 10 percent of the short-term estimates, 

except the TCd estimate for 2004. The estimated TZn load at 

Pinehurst is within 1 percent of the short-term estimates for 

all six years. Large differences in TCd load estimates were 

computed at Enaville in 1999 and in TPb estimates at Enaville, 

Harrison, and Post Falls for a number of years.

Because the load regression is based on the log-linear 

relation between flow and concentration, if the calibration 

data files represent a different range of flow or concentration 

values, the resulting regression may yield different load 

estimates due to the differences in regression line slopes. 

This is exemplified by the data for Pinehurst. The shortterm load estimates for DPb are 17 to 54 percent lower than 

the long-term estimates. Comparison of the two calibration 

files shows the likely reason: the mean concentrations for 

most metals were in good agreement, except that the mean 

DPb concentration of the short-term calibration data was 

about 50 percent lower than that of the long-term data. This 

demonstrates the importance of assuring that input data 

adequately represent conditions during the time interval for 

which loads are estimated. Results should not be extrapolated 

beyond the time interval or flow regime represented by the 

input data.

Because the calibration data used in the long-term 

regressions and the short-term regressions were the same 

for 1999–2004, differences in the estimated loads for that 

period result from the differences in the regression equations 

produced by the model in the two cases. For example, the 

short- and long-term estimates for 2001 were similar at all 

four stations; the differences never exceeded 10 percent 

for any metal. This may be related to low streamflow (and 

consequently, load estimates) in 2001, the lowest in the 

14-year estimation interval, a year that was common to both 

short- and long-term regressions. Conversely, the highest-flow 

years in the 14-year interval were 1996 and 1997, years that 

were not included in the short-term model regressions. The 

two models probably treat low-flow conditions similarly, but 

provide different results for high-flow conditions because 

calibration data for the long-term model provided data for lowand high-flow years, whereas data for the short-term model 

did not.

The value of long-term versus short-term load estimates 

ultimately depends on data available, questions being 

addressed, and purposes for which the results will be used. In 

this study, longer estimation intervals gave a more complete 

account of metal loading in the basin at the four long-term 

stations. Nevertheless, the most vital criterion to achieve the 

best results using LOADEST is that the calibration data be 

representative of the conditions, both hydrologic and temporal, 

during the simulated interval.

Estimated Trace-Metal Loads and Flow-Weighted Concentrations, 1991-2004 23
Table 6. Regression coefficients and coefficients of determination (R 2

) for load models used to estimate dissolved and total cadmium, 

lead, and zinc loads at four streamflow-gaging stations in the Spokane River basin, Idaho.

[Gaging station locations are shown in figures 1 and 2. Long-term records are for the periods: Enaville (1992-2004), Pinehurst, Harrison (1991-2004), and Post 

Falls (1991-2003). Regression equation is ln(L) = a + b(ln Q) + c(ln Q2) + d[sin(2•T)] + e[cos(2•T)] + fT + gT2, where L is constituent load; Q is streamflow; 

T is time in decimal years from beginning of calibration period; a, b, c, d, e, f, and g are regression coefficients. Symbols: –, explanatory variable not used in 

regression; <, less than]

Gaging 

station No.

Gaging station 

name

Regression coefficient

R2 

(percent) a b c d e f p-value 

of f g p-value of g

Dissolved cadmium (DCd)

5 Pinehurst 2.1468 .6013 -0.0736 0.0798 0.2101 -0.0474 <0.001 -0.0029 0.04 93.4

6 Enaville -1.8831 .9942 .0274 -.0116 .2051 -.2843 <.001 .0293 .001 96.1

7 Harrison 1.9382 .8366 -.1057 .0908 .1527 -.0810 <.001 – – 96.1

9 Post Falls 1.1276 .9752 – .0184 .2246 -.1639 <.001 – – 92.8

Total cadmium (TCd)

5 Pinehurst 2.1242 0.7909 – -0.2745 -0.0834 -0.0465 <0.001 – – 78.7

6 Enaville -1.8090 1.2197 .1347 -.0131 -.0983 -.2970 <.001 0.0253 <0.001 96.7

7 Harrison 2.0933 1.0082 – -.3460 -.2173 -.0635 <.001 – – 84.9

9 Post Falls 1.1790 .9300 – -.3064 -.2743 -.1188 <.001 – – 91.8

Dissolved lead (DPb)

5 Pinehurst 1.6491 0.8307 – 0.0812 -0.3614 -0.0361 0.076 – – 75.1

6 Enaville -1.3706 1.1582 0.0967 .0073 .1954 -.1412 .003 0.0376 <0.001 87.8

7 Harrison 3.1052 1.2741 – – – – – – – 86.5

9 Post Falls .5203 .8471 -.0112 .3731 .5585 -.1728 <.001 .0447 <.001 73.8

Total lead (TPb)

5 Pinehurst 3.4695 1.7819 0.4036 -0.2484 -0.3025 -0.0621 <0.001 -0.0099 0.073 90.0

6 Enaville .1051 1.6317 .3643 – – -.1820 <.001 – – 87.6

7 Harrison 4.5146 1.5535 .2601 -.1865 -.2269 -.0511 <.001 – – 89.6

9 Post Falls 2.5816 .9318 .0323 -.5161 .1712 -.1419 <.001 .0203 <.001 82.6

Dissolved zinc (DZn)

5 Pinehurst 7.1708 0.5584 -0.0380 0.0513 0.1592 -0.0564 <0.001 -0.0041 0.004 92.8

6 Enaville 2.8899 1.2202 -.0541 .0984 .1196 -.0713 <.001 .0176 <.001 93.2

7 Harrison 7.0855 .7306 -.0690 .2148 .078 -.0915 <.001 – – 97.1

9 Post Falls 6.3186 .9838 – -.2365 .347 -.0509 <.001 – – 98.9

Total zinc (TZn)

5 Pinehurst 7.0444 0.6686 0.0533 -0.2184 0.0185 -0.0500 <0.001 – – 82.9

6 Enaville 3.1654 1.2801 .0890 – – -.1199 <.001 – – 88.0

7 Harrison 7.0945 .9485 – -.2767 -.2635 -.0300 .004 – – 82.2

9 Post Falls 6.3402 .9369 – -.0723 -.4163 -.0388 <.001 – – 93.5

24 Annual Trace-Metal Load Estimates and Flow-Weighted Concentrations, Spokane River Basin, ID and WA, 1999-2004
Water 

year

Mean 

streamflow 

(m3/s)

Mean annual trace-metal loads (kg) Mean annual flow-weighted concentrations (mg/L)

Cadmium Lead Zinc Cadmium Lead Zinc

Dissolved 

(DCd) 

Total 

(TCd) 

Dissolved 

(DPb) 

Total 

(TPb) 

Dissolved 

(DZn) 

Total 

(TZn) 

Dissolved 

(DCd) 

Total 

(TCd) 

Dissolved 

(DPb) 

Total 

(TPb) 

Dissolved 

(DZn) 

Total 

(TZn) 

Pinehurst

1991 20 4,200 5,800 4,200 94,000 640,000 790,000 6.4 8.9 6.5 150 990 1,200

1992 9 2,600 2,900 2,000 9,900 400,000 440,000 9.2 10 7.1 35 1,500 1,600

1993 12 2,700 3,100 2,700 32,000 430,000 470,000 7.0 8.2 7.0 83 1,100 1,200

1994 8 2,200 2,300 1,700 11,000 350,000 350,000 8.7 9.2 7.0 42 1,400 1,400

1995 15 3,300 3,800 2,700 46,000 510,000 530,000 7.0 8.1 5.8 98 1,100 1,100

1996 23 3,900 5,400 3,600 670,000 610,000 710,000 5.3 7.2 4.8 900 820 960

1997 24 3,500 4,600 4,000 240,000 550,000 630,000 4.7 6.0 5.3 310 720 840

1998 12 2,600 2,800 2,200 21,000 400,000 390,000 6.6 7.1 5.7 53 1,000 1,000

1999 19 3,000 3,600 3,200 66,000 460,000 500,000 4.9 5.9 5.2 110 740 810

2000 17 2,700 3,100 2,600 48,000 400,000 430,000 5.1 5.9 5.0 91 760 820

2001 7 1,300 1,400 1,200 5,700 210,000 220,000 6.4 6.6 5.9 27 1,000 1,100

2002 19 2,300 3,000 2,900 95,000 340,000 410,000 3.8 4.9 4.7 150 560 680

2003 11 1,700 2,000 1,700 16,000 250,000 290,000 4.9 5.7 4.7 44 700 810

2004 11 1,600 1,900 1,700 6,600 220,000 270,000 4.5 5.4 4.7 19 640 760

Enaville

1991 – – – – – – – – – – – – –

1992 29 1,500 740 1,500 2,000 19,000 18,000 1.6 0.80 1.6 2.2 21 20

1993 43 1,200 770 1,600 6,100 24,000 31,000 .86 .56 1.2 4.5 17 23

1994 26 390 230 470 1,600 10,211 14,000 .46 .28 .56 1.9 12 16

1995 57 460 450 740 13,000 19,000 36,000 .26 .25 .41 7.4 10 20

1996 84 410 630 830 73,000 24,000 61,000 .16 .23 .31 27 8.8 23

1997 78 260 340 600 20,000 19,000 48,000 .11 .14 .25 8.0 7.8 20

1998 42 91 78 170 1,500 8,400 14,000 .07 .06 .13 1.2 6.3 11

1999 64 100 110 250 3,700 12,000 24,000 .05 .06 .12 1.8 6.2 12

2000 57 71 86 200 4,300 10,000 20,000 .04 .05 .11 2.4 5.6 11

2001 22 23 20 65 400 3,500 4,800 .03 .03 .10 .58 5.1 7.0

2002 70 64 94 300 6,300 13,000 22,000 .03 .04 .13 2.8 5.8 9.9

2003 39 31 37 140 1,200 6,800 8,500 .02 .03 .11 .95 5.4 6.8

2004 41 32 33 170 500 7,900 7,000 .02 .03 .13 .38 6.1 5.3

Table 7. Estimated long-term mean annual trace-metal loads and flow-weighted concentrations of cadmium, lead, and zinc at four 

streamflow-gaging stations in the Spokane River basin, Idaho.

[Long-term records are for the periods: Enaville (1992-2004), Pinehurst, Harrison (1991-2004), and Post Falls (1991-2003). Abbreviations: m3/s, cubic meter 

per second; mg/L, microgram per liter. –, no estimate]

Estimated Trace-Metal Loads and Flow-Weighted Concentrations, 1991-2004 25
Water 

year

Mean 

streamflow 

(m3/s)

Mean annual trace-metal loads (kg) Mean annual flow-weighted concentrations (mg/L)

Cadmium Lead Zinc Cadmium Lead Zinc

Dissolved 

(DCd) 

Total 

(TCd) 

Dissolved 

(DPb) 

Total 

(TPb) 

Dissolved 

(DZn) 

Total 

(TZn) 

Dissolved 

(DCd) 

Total 

(TCd) 

Dissolved 

(DPb) 

Total 

(TPb) 

Dissolved 

(DZn) 

Total 

(TZn) 

Harrison

1991 96 7,300 10,000 26,000 730,000 1,400,000 1,100,000 2.4 3.4 8.5 240 460 380

1992 42 3,700 4,200 8,900 71,000 750,000 510,000 1.2 1.4 2.9 23 250 170

1993 64 4,000 5,000 16,000 220,000 760,000 620,000 1.3 1.6 5.4 73 250 210

1994 38 2,700 3,000 8,300 67,000 540,000 400,000 .89 1.0 2.7 22 180 130

1995 75 4,300 6,000 19,000 330,000 810,000 770,000 1.4 2.0 6.3 110 270 260

1996 120 5,500 10,000 35,000 2,600,000 1,000,000 1,300,000 1.8 3.3 12 850 330 430

1997 100 4,200 6,800 31,000 690,000 770,000 940,000 1.4 2.2 10 230 250 310

1998 57 2,900 3,600 13,000 99,000 520,000 530,000 .94 1.2 4.3 33 170 180

1999 86 3,400 5,000 23,000 240,000 600,000 740,000 1.1 1.6 7.4 80 200 250

2000 77 2,900 4,300 20,000 260,000 510,000 660,000 .95 1.4 6.6 86 170 220

2001 30 1,200 1,400 6,300 36,000 210,000 240,000 .38 .47 2.1 12 71 80

2002 93 2,600 4,300 26,000 410,000 440,000 700,000 .85 1.4 8.7 140 150 230

2003 52 1,600 2,400 12,000 110,000 290,000 410,000 .53 .79 4.1 37 95 140

2004 54 1,600 2,200 12,000 66,000 280,000 410,000 .53 .73 4.1 22 91 130

Post Falls

1991 230 7,700 6,100 27,000 57,000 720,000 690,000 1.1 0.85 3.7 7.9 100 96

1992 98 2,900 2,600 7,000 17,000 320,000 320,000 .94 .83 2.3 5.5 100 100

1993 150 3,800 3,400 6,000 22,000 420,000 420,000 .79 .70 1.3 4.7 89 87

1994 84 1,800 1,700 2,200 9,000 240,000 240,000 .68 .65 .82 3.4 89 92

1995 180 3,300 3,200 2,700 14,000 500,000 510,000 .58 .57 .48 2.5 90 90

1996 290 4,300 4,200 2,700 18,000 740,000 740,000 .47 .46 .30 2.0 81 81

1997 290 3,700 3,900 2,500 20,000 670,000 670,000 .41 .42 .27 2.1 73 73

1998 140 1,600 1,800 1,200 8,200 320,000 340,000 .35 .39 .27 1.8 71 75

1999 210 2,000 2,300 1,700 12,000 450,000 470,000 .29 .34 .25 1.8 66 70

2000 190 1,600 1,900 1,700 11,000 400,000 430,000 .25 .31 .27 1.7 65 69

2001 76 510 660 870 4,600 140,000 160,000 .21 .27 .36 1.9 57 64

2002 220 1,300 1,600 2,800 14,000 390,000 420,000 .18 .23 .40 2.0 56 60

2003 130 660 940 2,300 8,300 250,000 280,000 .16 .22 .55 2.0 59 66

2004 – – – – – – – – – – – – –

Table 7. Estimated long-term mean annual trace-metal loads and flow-weighted concentrations of cadmium, lead, and zinc at four 

streamflow-gaging stations in the Spokane River basin, Idaho.—Continued

[Long-term records are for the periods: Enaville (1992-2004), Pinehurst, Harrison (1991-2004), and Post Falls (1991-2003). Abbreviations: m3/s, cubic meter 

per second; mg/L, microgram per liter. –, no estimate]

26  Annual Trace-Metal Load Estimates and Flow-Weighted Concentrations, Spokane River Basin, ID and WA, 1999-2004
ID_3084_fig07A

ENAVILLE

PINEHURST

HARRISON

POST FALLS

MEAN ANNUAL TRACE-METAL LOAD, IN KILOGRAMS

A. CADMIUM

10

100

1,000

10,000

1

10

100

1,000

10,000

10

100

1,000

10,000

100,000

1

10

100

1,000

10,000

1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004

1

10

100

1,000

10,000

1

10

100

1,000

10,000

10

100

1,000

10,000

100,000

1

10

100

1,000

10,000

1

 Dissolved

 Total

MEAN STREAMFLOW, IN CUBIC METERS PER SECOND

Mean streamflow

Figure 7. Estimated long-term mean annual dissolved and total trace-metal loads and mean 

streamflow at four streamflow-gaging stations in the Spokane River basin, Idaho. Long-term models 

used records for the periods: Enaville (1992-2004), Pinehurst, Harrison (1991-2004), and Post Falls 

(1991-2003).

Estimated Trace-Metal Loads and Flow-Weighted Concentrations, 1991-2004 27
ID_3084_fig07B

ENAVILLE

PINEHURST

HARRISON

POST FALLS

MEAN ANNUAL TRACE-METAL LOAD, IN KILOGRAMS

B. LEAD

10

100

1,000

10,000

100,000

1

10

100

1,000

10,000

100,000

1,000,000

1

10

100

1,000

10,000

100,000

1,000,000

10,000,000

1

10

100

1,000

10,000

100,000

10

100

1,000

10,000

100,000

1

10

100

1,000

10,000

100,000

1,000,000

1

10

100

1,000

10,000

100,000

1,000,000

10,000,000

1

10

100

1,000

10,000

100,000

1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004

MEAN STREAMFLOW, IN CUBIC METERS PER SECOND

 Dissolved

 Total

Mean streamflow

Figure 7.—Continued.

28  Annual Trace-Metal Load Estimates and Flow-Weighted Concentrations, Spokane River Basin, ID and WA, 1999-2004
ID_3084_fig07C

ENAVILLE

PINEHURST

HARRISON

POST FALLS

MEAN ANNUAL TRACE-METAL LOAD, IN KILOGRAMS

C. ZINC

100

1,000

10,000

100,000

1

10

100

1,000

10,000

100,000

1,000,000

10,000,000

1

10

100

1,000

10,000

100,000

1,000,000

1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004

1

10

100

1,000

10,000

100,000

1,000,000

10

100

1,000

10,000

100,000

1

10

100

1,000

10,000

100,000

1,000,000

10,000,000

1

10

100

1,000

10,000

100,000

1,000,000

1

10

100

1,000

10,000

100,000

1,000,000

10

MEAN STREAMFLOW, IN CUBIC METERS PER SECOND

 Dissolved

 Total

Mean streamflow

Figure 7.—Continued.

Estimated Trace-Metal Loads and Flow-Weighted Concentrations, 1991-2004 29
ID_3084_fig08

0.1

1.0

10.0

100.0

1,000.0

0.1

1.0

10.0

100.0

1,000.0

1.0

10.0

100.0

1,000.0

10,000.0

0.1

1.0

10.0

100.0

1,000.0

0.1

1.0

10.0

100.0

1,000.0

1.0

10.0

100.0

1,000.0

10,000.0

Pinehurst Enaville Harrison Post Falls

MEAN ANNUAL FLOW-WEIGHTED CONCENTRATION, IN MICROGRAMS PER LITER

CADMIUM

LEAD

ZINC

MEAN ANNUAL STREAMFLOW, IN CUBIC METERS PER SECOND

Total

Dissolved

Mean annual streamflow

Figure 8. Mean annual flow-weighted concentrations of dissolved and total 

cadmium, lead, and zinc and mean annual streamflow at four streamflowgaging stations in the Spokane River basin, Idaho. Long-term models used 

records for the periods: Enaville (1992-2004), Pinehurst, Harrison (1991-2004), 

and Post Falls (1991-2003).

30 Annual Trace-Metal Load Estimates and Flow-Weighted Concentrations, Spokane River Basin, ID and WA, 1999-2004
Table 8. Comparison of short- and long-term trace-metal load estimates for four streamflow-gaging stations in the Spokane River 

basin, Idaho.

[All values are in kilograms. Short-term records are for 1999-2004. Long-term records are for the periods: Enaville (1992-2004), Pinehurst, Harrison (1991-

2004), and Post Falls (1991-2003). Symbols: –, no estimate]

Water 

year

Trace-metal loads

Pinehurst Enaville Harrison Post Falls

Short-term Long-term Short-term Long-term Short-term Long-term Short-term Long-term

Annual dissolved cadmium (DCd) load

1999 3,000 3,000 58 100 3,400 3,400 1,400 2,000

2000 2,600 2,700 52 71 2,800 2,900 1,300 1,600

2001 1,300 1,300 19 23 1,100 1,200 460 510

2002 2,300 2,300 64 64 2,600 2,600 1,400 1,300

2003 1,700 1,700 36 31 1,600 1,600 890 660

2004 1,600 1,600 37 32 1,600 1,600 – –

Annual total cadmium (TCd) load

1999 3,400 3,600 73 110 5,100 5,000 2,100 2,300

2000 3,000 3,100 71 86 4,400 4,300 1,900 1,900

2001 1,400 1,400 19 20 1,400 1,400 590 660

2002 3,200 3,000 108 94 4,700 4,300 1,900 1,600

2003 2,200 2,000 43 37 2,400 2,400 1,000 940

2004 2,200 1,900 37 33 2,100 2,200 – –

Annual dissolved lead (DPb) load

1999 2,700 3,200 150 250 23,000 23,000 1,700 1,700

2000 2,100 2,600 150 200 20,000 20,000 2,200 1,700

2001 1,000 1,200 60 65 6,400 6,300 710 870

2002 2,000 2,900 260 300 27,000 26,000 5,200 2,800

2003 1,200 1,700 150 140 13,000 12,000 2,500 2,300

2004 1,100 1,700 180 170 13,000 12,000 – –

Annual total lead (TPb) load

1999 64,000 66,000 3,200 3,700 250,000 240,000 20,000 12,000

2000 46,000 48,000 4,700 4,300 320,000 260,000 18,000 11,000

2001 5,100 5,700 440 400 37,000 36,000 4,300 4,600

2002 95,000 95,000 10,000 6,300 590,000 410,000 27,000 14,000

2003 17,000 16,000 2,100 1,200 170,000 110,000 7,600 8,300

2004 7,000 6,600 1,000 500 81,000 66,000 – –

Annual dissolved zinc (DZn) load

1999 430,000 460,000 13,000 12,000 580,000 600,000 470,000 450,000

2000 380,000 400,000 11,000 10,000 50,000 510,000 420,000 400,000

2001 200,000 210,000 3,800 3,500 220,000 210,000 130,000 140,000

2002 330,000 340,000 14,000 13,000 440,000 440,000 410,000 390,000

2003 250,000 250,000 7,600 6,800 300,000 290,000 240,000 250,000

2004 240,000 220,000 8,000 7,900 290,000 280,000 – –

Annual total zinc (TZn) load

1999 490,000 500,000 22,000 24,000 760,000 740,000 480,000 470,000

2000 430,000 430,000 20,000 20,000 660,000 660,000 440,000 430,000

2001 220,000 220,000 5,300 4,800 220,000 240,000 140,000 160,000

2002 410,000 410,000 28,000 22,000 690,000 700,000 460,000 420,000

2003 290,000 290,000 12,000 8,500 370,000 410,000 280,000 280,000

2004 260,000 270,000 12,000 7,000 330,000 410,000 – –

Estimated Trace-Metal Loads and Flow-Weighted Concentrations, 1991-2004  31
ID_3084_fig09 DIFFERENCE BETWEEN SHORT- AND LONG--TERM TRACE-METAL LOAD ESTIMATES, IN PERCENTAGE OF SHORT-TERM VALUE

ENAVILLE

PINEHURST HARRISON

POST FALLS

-20

0

20

40

60

-60

-40

-20

0

20

40

60

1999 2000 2001 2002 2003 2004

1999 2000 2001 2002 2003 2004

-10

0

10

20

-20

0

20

40

60

80

100 Total Cadmium

Total Lead

Total Zinc

Figure 9. Comparison of short- and long-term trace-metal load estimates at four streamflow-gaging 

stations in the Spokane River basin, Idaho. Short-term models used records from 1999-2004. Long-term 

models are based on records for the periods: Enaville (1992-2004), Pinehurst, Harrison (1991-2004), and 

Post Falls (1991-2003). Load estimates were not made at Post Falls for 2004.

32 Annual Trace-Metal Load Estimates and Flow-Weighted Concentrations, Spokane River Basin, ID and WA, 1999-2004
Temporal Trends in Long-Term Metal Load 

Estimates

To gain a better understanding of metal loading through 

time, the long-term annual trace metal load estimates were 

evaluated for possible temporal trends. LOADEST results 

include information on trends in the form of a statistically 

significant coefficient for a “time” term (labeled “f” in tables 2

and 6) in the regression equation. If present, a negative 

coefficient signifies an overall downward trend during 

the estimation period; a positive term indicates an upward 

trend. More complex changes with time are indicated by the 

presence of a statistically significant “time-squared” (t2) 

term in addition to a time term. In this study, a p-value of less 

than or equal to 0.05 (greater than or equal to the 95 percent 

confidence level) was considered statistically significant. 

According to the LOADEST results, all four long-term sites 

showed significant downward trends for DCd, TCd, DZn, and 

TPb. In addition, Enaville and Post Falls showed significant 

downward trends for DPb and TZn loads; Harrison TZn loads 

also decreased significantly with time. No gaging stations 

showed an increasing trend.

Metal loads and FWCs at Enaville generally decreased 

with time from 1992-2004, but some explanation is needed 

for the unusually smooth descending curve displayed by the 

annual mean FWCs of DCd (fig. 10), in which estimates 

ranged from 1.6 to 0.02 μg/L DCd. Measured concentrations 

of DCd were low at Enaville. Of the 86 concentration 

measurements used in the calibration file, 68 were censored 

data, for example, below the minimum detection limit 

(MDL). The 18 measured concentrations above the MDL 

ranged from 0.02 to 1.0 μg/L. Analytical precision improved 

during the estimation period and the MDLs changed from 

1 to 0.04 μg/L in 2001. The LOADEST software is able to 

accommodate varying detection limits and applies statistical 

methods to enable use of censored data in the regression. The 

regular shape of the time-series graph of DCd FWC likely is 

an expression of combined effects of the high proportion of 

censored data for this constituent, modeling process itself, 

and actual decreases in FWC with time. A similar pattern is 

exhibited by DCd at Post Falls, another station with abundant 

censored calibration data. An example from Pinehurst of TCd 

FWCs calculated from data with no censored data among 77 

samples is given for comparison (fig. 10).

A simple but useful independent test for trends in 

time-series data is the non-parametric Mann-Kendall trend 

test. Generally, this test determines whether values tend to 

monotonically increase or decrease with time. Because this 

test is non-parametric, the data need not be assumed to have 

a normal distribution. This is a valid test as long as no serial 

correlation is present in the data (Helsel and Hirsch, 1995). 

Because annual data were analyzed, there was no sensitivity to 

seasonal changes. The relatively long 13- or 14-year interval 

helps minimize the effects of extreme years.

The results of applying the Mann-Kendall trend test to 

estimated annual load and FWC data are compared with the 

trend information from the long-term LOADEST regression 

results in table 9.

Results of the two types of trend analysis agree in 

most cases. For example, both methods indicate that all four 

long-term sites had statistically significant (with 95 percent 

confidence) decreasing trends for Cd loads and FWCs for 

the estimation period, with a few cases of disagreement; for 

instance, the methods gave contradictory results for TZn at 

Pinehurst and at Post Falls.

It should be noted that the Mann-Kendall test results 

simply show whether an overall monotonic change exists for 

the entire 13- or 14-year period. Therefore, trends for shorter 

intervals in this period are not precluded. For example, based 

on the Mann-Kendall test, FWCs of DPb show a significant 

increasing trend for 1999-2003 at Post Falls. The statistical 

results must be viewed in the context of other information 

known about the data and about the system as a whole.

Estimated Trace-Metal Loads and Flow-Weighted Concentrations, 1991-2004  33
ID_3084_fig10

 Dissolved

 Total

 Mean annual streamflow

0.01

0.10

1.00

10.00

1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004

10.0

100.0

1.0

10.0

1991

1

10

100

FLOW-WEIGHTED CADMIUM CONCENTRATION, 

IN MICROGRAMS PER LITER

MEAN ANNUAL STREAMFLOW, IN CUBIC METERS PER SECOND

ENAVILLE

PINEHURST

Figure 10. Estimated flow-weighted concentration of dissolved and total cadmium and 

mean annual streamflow at South Fork Coeur d’Alene River at Pinehurst, water years 

1991–2004 and North Fork Coeur d’Alene River at Enaville, Idaho, water years 1992–2004.

Table 9. Comparison of trend analysis results for trace-metal loads and flow-weighted concentrations based on the Mann-Kendall test 

and LOADEST regression analysis for four streamflow-gaging stations in the Spokane River basin, Idaho, water years 1991–2004.

[Abbreviations: Y, statistically significant downward trend at greater than 95-percent confidence level; N, no significant trend. –, comparison not made]

Trace metal

Trace-metal loads

Pinehurst Enaville Harrison Post Falls

Mann-Kendall LOADEST Mann-Kendall LOADEST Mann-Kendall LOADEST Mann-Kendall LOADEST

Dissolved cadmium (DCd) Y Y Y Y Y Y Y Y

Total cadmium (TCd) Y Y Y Y Y Y Y Y

Dissolved lead (DPb) Y1 N Y Y N N Y Y

Total lead (TPb) N Y Y1 Y N Y Y Y

Dissolved zinc (DZn) Y Y Y Y Y Y Y1 Y

Total zinc (TZn) Y N Y Y Y1 Y N Y

Flow-weighted concentrations

Dissolved cadmium (DCd) Y – Y – Y – Y –

Total cadmium (TCd) Y – Y – Y – Y –

Dissolved lead (DPb) Y – Y – N – Y –

Total lead (TPb) N – Y – N – Y –

Dissolved zinc (DZn) Y – Y – Y – Y –

Total zinc (TZn) Y – Y – Y1 – Y –

1

Result was statistically significant at the 90- to 95-percent confidence level.

34 Annual Trace-Metal Load Estimates and Flow-Weighted Concentrations, Spokane River Basin, ID and WA, 1999-2004
Summary

Streamflow and trace-metal chemistry data collected 

at 10 U.S. Geological Survey streamflow-gaging stations 

in the Spokane River basin were used as input for the U.S. 

Geological Survey software, LOADEST, to estimate annual 

loads and mean flow-weighted concentrations of total and 

dissolved cadmium, lead, and zinc for 1999 to 2004. These 

estimates yielded valuable information about processes of 

metal storage and transport, and demonstrated that water 

quality data are a great aid in understanding these processes.

Cadmium composed less than 1 percent of the total metal 

load at all stations; mean annual total cadmium loads for 

1999-2004 ranged from 39 kilograms at Amy Gulch to 3,400 

kilograms at Harrison. Overall, mean annual total cadmium 

loads were lowest at the stations at Amy Gulch, Enaville, 

Ninemile Creek, and Canyon Creek, and highest at Harrison, 

Pinehurst, Post Falls, and Elizabeth Park.

Dissolved cadmium typically ranged from about 70 

percent to about 100 percent of the total cadmium load. 

The ratio of dissolved to total cadmium was higher at 

stations where ground water was a volumetrically important 

component of the streamflow. Ratio of dissolved cadmium 

to total cadmium also was higher at most stations during 

low-flow years such as 2001, because of the greater relative 

contribution of ground water to streamflow. Dissolved 

cadmium/total cadmium ratios about 0.9 and greater were 

estimated at stations upstream from Pinehurst. The lowest 

dissolved cadmium/total cadmium ratio was estimated at 

Harrison.

Total lead constituted from 6 to 42 percent of the total 

load at stations upstream of Coeur d’Alene Lake and from 

2 to 4 percent at stations downstream of the lake. Gaging 

stations at Amy Gulch, Ninemile Creek, and Enaville yielded 

the lowest mean annual estimated total lead loads in the basin. 

By far the highest overall mean total lead load in the basin 

was estimated at Harrison. Maximum total lead loads at eight 

stations in the basin were estimated for 2002, which may 

reflect flushing of accumulated lead-laden sediment stored in 

the stream channels during 2001, a low-flow year.

The ratio of dissolved lead to total lead varied in time and 

space and reflected the relative contribution of ground water to 

total streamflow and the variation in the amount of suspended 

particulate and (or) colloidal material. Dissolved lead ranged 

from less than 10 percent to nearly 30 percent of the total lead 

load.

Zinc composed more than 90 percent of the total metal 

load at 8 of the 10 gaging stations examined in this study. 

Mean annual total zinc loads for 1999-2004 ranged from 

510,000 kilograms at Harrison to 14,000 kilograms at Amy 

Gulch. Low annual total zinc loads at Amy Gulch and at 

Enaville likely reflect the low availability of metals in the 

source areas upstream of these stations. Estimated mean 

annual total zinc loads at Harrison far exceeded those at any 

other station. The three highest estimated annual total zinc 

loads were at Harrison, Post Falls, and Pinehurst. Total zinc 

loads also were high at Outlet, Post Falls, and Long Lake.

Dissolved zinc/total zinc ratios typically were greater 

than 0.8 at most gaging stations. Although estimated total 

zinc loads were relatively low in 2001 due to the effect of 

streamflow on the load calculation, the ratio of dissolved zinc 

to total zinc load was higher in 2001 than in other years, owing 

to less dilution and a higher proportion of ground water input 

to streamflow.

Notable systematic differences between total metal 

loads at stations in different parts of the basin are partly due 

to the direct relation between streamflow and load. Above 

Coeur d’Alene Lake, gaging stations with higher streamflow 

generally yielded higher loads. However, the total metal loads 

at the three stations downstream of Coeur d’Alene Lake, 

where mean streamflow is 2-3 times higher than at Harrison, 

are relatively small. Based on the dissolved metal data and the 

relative proportions of metals, this observation was interpreted 

as lead trapped by sediments in Coeur d’Alene Lake and the 

transmission of cadmium and zinc.

Flow-weighted concentrations (FWCs) calculated 

from the estimated loads were examined to enhance the 

interpretation of metal load estimates, which were influenced 

by large spatial and temporal differences in streamflow. Total 

cadmium FWCs ranged from 14 to 0.04 μg/L and were highest 

at Ninemile and Canyon Creeks and lowest at Enaville. 

Both total cadmium loads and mean streamflow were low at 

Enaville, indicating overall low metal contribution from the 

North Fork Coeur d’Alene River. The combination of low 

total cadmium FWCs and high total cadmium loads at gaging 

stations downstream of Coeur d’Alene Lake was attributed to 

the effects of downstream dilution with higher streamflow.

Total lead FWCs were highest at Ninemile Creek, 

Canyon Creek, and Harrison. The elevated total lead FWCs 

at Harrison confirmed that the high total lead loads at this 

station were not simply due to higher streamflow. Conversely, 

relatively low total lead loads combined with high total lead 

FWCs at Ninemile and Canyon Creeks reflect relatively low 

streamflow but high concentrations of total lead.

Total zinc FWCs also demonstrate the strong effect of 

streamflow on load calculations, and confirm source areas for 

zinc in the basin. Total zinc FWCs at Ninemile and Canyon 

Creeks are by far the highest in the basin but contribute 

among the lowest total zinc loads due to their relatively 

low streamflow. Conversely, stations downstream of Coeur 

d’Alene Lake exhibit among the lowest total zinc FWCs, but 

total zinc load estimates are high because of their high mean 

streamflow relative to other stations in the basin.

Summary  35
An earlier version of the LOADEST software was used 

in a previous study to estimate loads for WY1999-2001 for 

many of the same sites included in this report. Overall, there 

was good agreement between results of this study and the 

previous study. However, notable discrepancies are attributable 

to important differences in streamflow for 1999-2001 versus 

1999-2004. Very low streamflow in 2001 strongly influenced 

Clark’s results because that year’s data constituted a greater 

proportion of total available data, whereas, 2001 data were 

a smaller proportion of the available data for this study. 

Because the software uses the relation between streamflow 

and concentration to calculate the regressions, estimated loads 

during the overlapping years were somewhat different for the 

two data sets.

Long-term regression models for 1991 to 2003 or 2004 

were developed and annual trace-metal loads and FWCs 

were estimated for Enaville, Pinehurst, Harrison, and Post 

Falls to better understand the variability of metal loading 

with time. Long-term load estimates were compared with 

those for 1999-2004 to examine the robustness of the two 

differently calibrated models. Long-term estimates mirror 

the results for 1999-2004 in terms of the mean annual metal 

loads. The largest mean loads for the estimation period were 

measured at Harrison and Pinehurst. The largest estimated 

loads were in 1996 and 1997 (both very high streamflow 

years) at all four stations; high loads also were measured 

in 1991. The lowest loads prior to 1999 were in 1994 and 

1998. Although low estimated loads in 1994 probably reflect 

low streamflow, estimated loads in 1998 may have reflected 

both low streamflow and lack of sufficient time for metals to 

accumulate upstream after unusually high flows had scoured 

the channels during 1996 and 1997.

The long- and short-term estimates for these four sites 

for 1999-2004, the interval common to both estimates, 

were similar, but with notable differences. Because the 

LOADEST software uses the relation between streamflow and 

concentration to perform the regressions, and the calibration 

data files for 1999-2004 represented a smaller data set than 

the 1991-2004 data set, different regression models resulted. 

Differing results demonstrate the importance of assuring that 

models not be extrapolated beyond the time interval and range 

of streamflow represented by the input data used to calibrate 

them.

LOADEST results suggested that statistically significant 

downward trends during 1991-2004 were at Enaville, 

Pinehurst, Harrison, and Post Falls for dissolved cadmium, 

total cadmium, total lead, and dissolved zinc. Additionally, 

data from Enaville and Post Falls showed significant 

downward trends for dissolved lead and total zinc loads; 

Harrison total zinc loads also diminished with time. The 

Mann-Kendall trend test also was applied to the load data and 

the results agreed with the LOADEST trend results in most 

cases, but gave contradictory results for total zinc at Pinehurst 

and Post Falls. The Mann-Kendall test indicated significant 

downward trends in FWCs for all constituents except 

dissolved lead and total lead at Harrison and for total lead at 

Pinehurst.

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38  Annual Trace-Metal Load Estimates and Flow-Weighted Concentrations, Spokane River Basin, ID and WA, 1999-2004

This page left intentionally blank.
Manuscript approved for publication, July 25, 2006

Prepared by the USGS Publishing Network, 

Publishing Service Center, Tacoma, Washington

Bob Crist

Debra Grillo

Virginia Renslow

Bobbie Jo Richey

For more information concerning the research in this report, contact the 

Idaho Water Science Center Director, 

230 Collins Road

Boise, Idaho 83702-4520

http://id.water.usgs.gov
SIR 2006–5188 Donato

Annual Trace-Metal Load Estimates and Flow-Weighted Concentrations of Cadmium, Lead, and Zinc in the 

Spokane River Basin, Idaho and Washington, 1999-2004