Prepared in cooperation with the 

IDAHO DEPARTMENT OF WATER RESOURCES 

WASHINGTON STATE DEPARTMENT OF ECOLOGY

UNIvERSITY OF IDAHO

WASHINGTON STATE UNIvERSITY

Ground-Water Flow Model for the Spokane valley-Rathdrum 

Prairie Aquifer, Spokane County, Washington, and Bonner and 

Kootenai Counties, Idaho

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2007–5044
Cover: Photograph of Spokane Valley, Washington. The U.S. Geological Survey 

streamflow gaging station on the Spokane River at Greenacres, Washington, is to the 

right (upstream) of where Barker Road Bridge crosses the river. (Photograph taken by 

Paul Hsieh, U.S. Geological Survey, 2005.)
Ground-Water Flow Model for the Spokane 

Valley-Rathdrum Prairie Aquifer, Spokane 

County, Washington, and Bonner and 

Kootenai Counties, Idaho

By Paul A. Hsieh, Michael E. Barber, Bryce A. Contor, Md. Akram Hossain, 

Gary S. Johnson, Joseph L. Jones, and Allan H. Wylie

Prepared in cooperation with the

Idaho Department of Water Resources, 

Washington State Department of Ecology, 

University of Idaho, and 

Washington State University

Scientific Investigations Report 2007–5044

U.S. Department of the Interior

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

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2007

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

reproduce any copyrighted materials contained within this report.

Suggested citation:

Hsieh, P.A., Barber, M.E., Contor, B.A., Hossain, Md. A., Johnson, G.S., Jones, J.L., and Wylie, A.H., 2007, 

Ground-water flow model for the Spokane Valley-Rathdrum Prairie Aquifer, Spokane County, Washington, and Bonner 

and Kootenai Counties, Idaho: U.S. Geological Survey Scientific Investigations Report 2007-5044, 78 p.
iii

Contents

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

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

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

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

Previous Investigations........................................................................................................................4

Hydrology of Aquifer......................................................................................................................................5

Geologic Setting....................................................................................................................................5

Areal Extent............................................................................................................................................6

Vertical Extent .......................................................................................................................................8

Hydraulic Properties.............................................................................................................................9

Horizontal Hydraulic Conductivity...........................................................................................10

Vertical Hydraulic Conductivity ...............................................................................................12

Specific Yield..............................................................................................................................12

Inflows to Aquifer................................................................................................................................12

Precipitation................................................................................................................................13

Tributary Basins and Adjacent Uplands.................................................................................16

Lakebed Seepage and Surface Overflow..............................................................................18

Losing Segments of Spokane River........................................................................................20

Return Percolation from Irrigation..........................................................................................20

Effluent from Septic Systems...................................................................................................20

Outflows from Aquifer........................................................................................................................23

Withdrawals from Wells ...........................................................................................................23

Gaining Segments of Spokane River......................................................................................26

Little Spokane River...................................................................................................................26

Subsurface Outflow...................................................................................................................27

Interaction Between Aquifer and Spokane River .........................................................................28

Ground-Water Levels and Movement.............................................................................................32

Ground-Water Flow Model.........................................................................................................................41

Spatial Discretization .........................................................................................................................41

Temporal Discretization.....................................................................................................................41

Boundary Conditions and Implementation of MODFLOW Packages.........................................44

Recharge Package ....................................................................................................................44

Well Package..............................................................................................................................44

Flow and Head Boundary Package ........................................................................................44

River Package.............................................................................................................................44

Streamflow-Routing Package..................................................................................................45

General-Head Boundary Package..........................................................................................47

Hydraulic Conductivity.......................................................................................................................47

Specific Yield.......................................................................................................................................50
iv

Contents—Continued

Model Calibration.........................................................................................................................................52

Nonlinear Least-Squares Regression Method ..............................................................................52

Calibration Data...................................................................................................................................52

Weights for Measured Quantities....................................................................................................53

Estimated Parameters........................................................................................................................54

Assessment of Calibration ................................................................................................................56

Simulated Flows in Aquifer ...............................................................................................................67

Alternative Models .............................................................................................................................68

Model Limitations and Suggestions for Future Work ............................................................................73

Summary and Conclusions.........................................................................................................................73

Acknowledgments.......................................................................................................................................75

References Cited..........................................................................................................................................75

Figures

Figure 1. Map showing location of the Spokane Valley-Rathdrum Prairie aquifer,

Spokane County, Washington, and Bonner and Kootenai Counties, Idaho ......... 3

Figure 2. Map showing locations of wells in and near Spirit and Hoodoo Valleys,

Bonner County, Idaho ..................................................................... 7

Figure 3. Map showing approximate altitude of the base of the Spokane Valley-Rathdrum 

Prairie aquifer, Spokane County, Washington, and Bonner and Kootenai 

Counties, Idaho .............................................................................. 8

Figure 4. Map showing areal extent of clay layer in Hillyard Trough and the Little

Spokane River Arm, Spokane County, Washington .................................... 9

Figure 5. Generalized hydrogeologic section of the Spokane Valley-Rathdrum Prairie

aquifer along hydrogeologic section C-C’ (from Kahle and Bartolino, 2007) ......... 10

Figure 6. Map showing locations of multiple-well aquifer-test sites in the west half of

the Spokane Valley-Rathdrum Prairie aquifer, Washington and Idaho ............... 11

Figure 7. Map showing locations of weather stations, auxiliary vertices, and expanded

triangular network used for interpolation of recharge from precipitation on

permeable surfaces, Spokane Valley-Rathdrum Prairie aquifer, Washington

and Idaho .................................................................................... 14

Figure 8. Graph showing depth to water in well 251 and monthly rate of deep

percolation at the base of the root zone at the well site, Spokane

Valley-Rathdrum Prairie aquifer, Washington and Idaho .............................. 15

Figure 9. Graph showing volumetric rate of recharge from precipitation, Spokane

Valley-Rathdrum Prairie aquifer, Washington and Idaho .............................. 16

Figure 10. Map showing tributary basins that drain to the Spokane Valley-Rathdrum Prairie 

aquifer and to seven lakes that border the aquifer, Washington and Idaho ......... 17
Figures—Continued

Figure 11. Graph showing scaling index used to compute monthly flow from tributary

basins to the Spokane Valley-Rathdrum Prairie aquifer, Washington and Idaho ... 18

Figure 12. Map showing water purveyor service areas and areal distribution of water

purveyor wells, Spokane Valley-Rathdrum Prairie aquifer, 2000–02 .................. 21

Figure 13. Graph showing volumetric rate of return percolation from irrigation and

effluent from septic systems, Spokane Valley-Rathdrum Prairie aquifer,

Washington and Idaho ..................................................................... 22

Figure 14. Map showing sewer hookup density for 2000, Spokane Valley-Rathdrum

Prairie aquifer, Washington and Idaho ................................................... 22

Figure 15. Graph showing withdrawal rates from wells, Spokane Valley-Rathdrum

Prairie aquifer, Washington and Idaho ................................................... 23

Figure 16. Graph showing estimated monthly withdrawal rate for a home outside water

purveyor service areas, Spokane Valley-Rathdrum Prairie aquifer,

Washington and Idaho ..................................................................... 24

Figure 17. Map showing irrigation density for acreages outside water purveyor service

areas and for self-supplied golf courses, Spokane Valley-Rathdrum Prairie

aquifer, Washington and Idaho ............................................................ 25

Figure 18. Map showing locations of gaging stations and streamflow measurement

sites on the Spokane and Little Spokane Rivers, Spokane Valley-Rathdrum

Prairie aquifer, Washington and Idaho ................................................... 26

Figure 19. Graph showing daily streamflow gain on the Little Spokane River between

gaging stations at Dartford (QAD) and near Dartford (QND) and streamflow at

the gaging station at Dartford, Spokane Valley-Rathdrum Prairie aquifer,

Washington and Idaho ..................................................................... 27

Figure 20. Graph showing water levels in well 99 and in Long Lake, Spokane

Valley-Rathdrum Prairie aquifer, Washington and Idaho .............................. 28

Figure 21. Graphs showing streamflow gains or losses between the various gaging

stations and streamflow at the gaging station near Post Falls, Spokane

Valley-Rathdrum Prairie aquifer, Washington and Idaho .............................. 30

Figure 22. Map showing ground-water levels for the Spokane Valley-Rathdrum Prairie

aquifer, Washington and Idaho, September 2004 ....................................... 33

Figure 23. Map showing locations of monitoring wells in the Spokane Valley-Rathdrum

Prairie aquifer, Washington and Idaho ................................................... 34

Figure 24. Hydrograph showing water level in well 60 and stage on the Spokane River at

the gaging station near Otis Orchards, Spokane Valley-Rathdrum Prairie

aquifer, Washington and Idaho ............................................................ 34

Figure 25. Hydrograph showing water level in well 128 and stage on the Spokane River

at the gaging station at Spokane, Spokane Valley-Rathdrum Prairie aquifer,

Washington and Idaho ..................................................................... 36

Figure 26. Hydrograph showing water levels in wells 104 and 107 and in Nine Mile

Reservoir, Spokane Valley-Rathdrum Prairie aquifer, Washington and Idaho ...... 36

Figure 27. Hydrograph showing water levels in wells 92, 209, and 251, Spokane

Valley-Rathdrum Prairie aquifer, Washington and Idaho .............................. 37

Figure 28. Hydrograph showing water levels in wells 234 and 236 and in Lake Pend

Oreille, Spokane Valley-Rathdrum Prairie aquifer, Washington and Idaho ......... 38
vi

Figures—Continued

Figure 29. Hydrograph showing water levels in wells 143 and 159 and in Coeur d’Alene

Lake, Spokane Valley-Rathdrum Prairie aquifer, Washington and Idaho ............ 38

Figure 30. Graph showing water levels in wells 245 and 249, Spokane Valley-Rathdrum 

Prairie aquifer, Washington and Idaho ................................................... 39

Figure 31. Map showing changes in water levels, Spokane Valley-Rathdrum Prairie

aquifer, Washington and Idaho, September 2004 to April 2006 ........................ 40

Figure 32. Map showing active cells in model layer 1, Spokane Valley-Rathdrum 

Prairie aquifer, Washington and Idaho ................................................... 42

Figure 33. Map showing active cells in model layer 3, Spokane Valley-Rathdrum Prairie

aquifer, Washington and Idaho ............................................................ 43

Figure 34. Vertical section A-B-C-D showing how model layers represent the dividing 

of the aquifer from a single hydrogeologic unit into an upper and lower unit

separated by a clay layer, Spokane Valley-Rathdrum Prairie aquifer,

Washington and Idaho ..................................................................... 43

Figure 35. Map showing sections of Spokane River used in the Streamflow-Routing

Package, Spokane Valley-Rathdrum Prairie aquifer, Washington and Idaho ...... 46

Figure 36. Graph showing relation between stage on the Spokane River at Sullivan Road

Bridge (y) and stage at the gaging station near Post Falls (x), Spokane

Valley-Rathdrum Prairie aquifer, Washington and Idaho .............................. 47

Figure 37. Map showing horizontal hydraulic conductivity zones in model layer 1,

Spokane Valley-Rathdrum Prairie aquifer, Washington and Idaho .................. 48

Figure 38. Map showing hydraulic conductivity zones in model layer 2, Spokane

Valley-Rathdrum Prairie aquifer, Washington and Idaho .............................. 49

Figure 39. Map showing horizontal hydraulic conductivity zones in model layer 3,

Spokane Valley-Rathdrum Prairie aquifer, Washington and Idaho .................. 50

Figure 40. Map showing specific yield zones, Spokane Valley-Rathdrum Prairie aquifer,

Washington and Idaho ..................................................................... 51

Figure 41. Graphs showing simulated and measured water levels in selected wells in

various parts of the Spokane-Rathdrum Prairie aquifer, Washington and

Idaho .......................................................................................... 57

Figure 42. Map showing simulated water levels in model layer 1, Spokane

Valley-Rathdrum Prairie aquifer, Washington and Idaho, September 2004 ......... 61

Figure 43. Map showing simulated water levels in model layer 3, Spokane

Valley-Rathdrum Prairie aquifer, Washington and Idaho, September 2004 ......... 62

Figure 44. Graphs showing simulated and measured monthly average streamflow gains

and losses for three Spokane River segments, Spokane Valley-Rathdrum

Prairie aquifer, Washington and Idaho ................................................... 63

Figure 45. Graph showing simulated and measured monthly average streamflow gains

on the Little Spokane River from the gaging stations at Dartford to near

Dartford ....................................................................................... 64

Figure 46. Graphs showing simulated and measured streamflow gains (positive values)

and losses (negative values) on various segments of the Spokane River

during seepage runs of September 13-16, 2004, and August 26-31, 2005 ............ 64
vii

Figures—Continued

Figure 47. Graphs showing simulated and measured streamflow gains (positive values)

and losses (negative values) on three Little Spokane River segments during

seepage runs of September 13-16, 2004, and August 26-31, 2005 ..................... 65

Figure 48. Map showing spatial distribution of weighted water-level residuals in model

layer 1, September 2004, Spokane Valley-Rathdrum Prairie aquifer,

Washington and Idaho ..................................................................... 66

Figure 49. Graph showing weighted water-level residuals and simulated water levels ...... 66

Figure 50. Graph showing weighted flow residuals and simulated streamflow gains

(positive values) or losses (negative values)............................................. 67

Figure 51. Map showing subregions of the Spokane Valley-Rathdrum Prairie aquifer for

water-budget calculations ............................................................... 68

Tables

Table 1. Water levels in wells in and near Spirit and Hoodoo Valleys, 

Bonner County, Idaho ..................................................................... 6

Table 2. Estimated transmission time for precipitation infiltration to travel from base of

root zone to water table, Spokane Valley-Rathdrum Prairie aquifer,

Washington and Idaho ..................................................................... 15

Table 3. Average volumetric rate of recharge and average recharge flux from

precipitation, Spokane Valley-Rathdrum Prairie aquifer, Washington and

Idaho, October 1990 through September 2005 .......................................... 15

Table 4. Drainage area and long-term average inflow to seven lakes that border the

Spokane Valley-Rathdrum Prairie aquifer, Washington and Idaho .................. 19

Table 5. Estimated annual withdrawals by self-supplied industries and withdrawal

rates, Spokane Valley-Rathdrum Prairie aquifer, Washington and Idaho ............ 25

Table 6. Streamflow measurements made on the Spokane River and some tributaries

to determine streamflow gains (positive values) and losses (negative values)

between measurement sites, Spokane Valley-Rathdrum Prairie aquifer,

Washington and Idaho, August 26-31, 2006 ............................................. 29

Table 7. Physical data for wells in water-level monitoring network, Spokane

Valley-Rathdrum Prairie aquifer, Washington and Idaho .............................. 35

Table 8. Model parameters, acceptable intervals, estimated values, and 95-percent

confidence intervals ........................................................................ 55

Table 9. Simulated 10-year average water budget for subregions of the Spokane

Valley-Rathdrum Prairie aquifer ......................................................... 69

Table 10. Esitmated values of parameters for calibrated and alternative models ............ 71

Table 11. Regression statistics for calibrated and alternative models ........................... 72
viii

Conversion Factors and Datums

Conversion Factors

Multiply By To obtain

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

)

cubic foot per day (ft3

/d) 0.02832 cubic meter per day (m3/d)

cubic foot per second (ft3

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

foot (ft) 2.54 centimeter (cm)

foot per day (ft/d) 0.3048 meter per day (m/d)

foot squared per day (ft2

/d) 0.09290 meter squared per day (m2/d)

inch (in.) 2.54 centimeter (cm)

mile (mi) 1.609 kilometer (km)

square mile (mi2

) 2.590 square kilometer (km2)

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

 °F = (1.8 × °C) + 32.

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

 °C = (°F - 32) / 1.8.

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.
Abstract

This report presents a computer model of ground-water 

flow in the Spokane Valley-Rathdrum Prairie (SVRP) aquifer 

in Spokane County, Washington, and Bonner and Kootenai 

Counties, Idaho. The aquifer is the sole source of drinking 

water for more than 500,000 residents in the area. In response 

to the concerns about the impacts of increased ground-water 

withdrawals resulting from recent and projected urban growth, 

a comprehensive study was initiated by the Idaho Department 

of Water Resources, the Washington Department of Ecology, 

and the U.S. Geological Survey to improve the understanding 

of ground-water flow in the aquifer and of the interaction 

between ground water and surface water. The groundwater flow model presented in this report is one component 

of this comprehensive study. The primary purpose of the 

model is to serve as a tool for analyzing aquifer inflows and 

outflows, simulating the effects of future changes in groundwater withdrawals from the aquifer, and evaluating aquifer 

management strategies. The scale of the model and the level of 

detail are intended for analysis of aquifer-wide water-supply 

issues.

The SVRP aquifer model was developed by the Modeling 

Team formed within the comprehensive study. The Modeling 

Team consisted of staff and personnel working under contract 

with the Idaho Department of Water Resources, personnel 

working under contract with the Washington Department of 

Ecology, and staff of the U.S. Geological Survey. To arrive at 

a final model that has the endorsement of all team members, 

decisions on modeling approach, methodology, assumptions, 

and interpretations were reached by consensus.

The ground-water flow model MODFLOW-2000 was 

used to simulate ground-water flow in the SVPR aquifer. 

The finite-difference model grid consists of 172 rows, 256 

columns, and 3 layers. Ground-water flow was simulated 

from September 1990 through September 2005 using 181 

stress periods of 1 month each. The areal extent of the model 

encompasses an area of approximately 326 square miles. 

For the most part, the model extent coincides with the 2005 

revised extent of the Spokane Valley-Rathdrum Prairie aquifer 

as defined in a previous report. However, the model excludes 

Spirit and Hoodoo Valleys because of uncertainties about the 

ground-water flow directions in those valleys and the degree 

of hydraulic connection between the valleys and northern 

Rathdrum Prairie. The SVRP aquifer is considered to be a 

single hydrogeologic unit except in Hillyard Trough and the 

Little Spokane River Arm. In those areas, a continuous clay 

layer divides the aquifer into an upper, unconfined unit and a 

lower, confined unit.

The model includes all known components of inflows to 

and outflows from the aquifer. Inflows to the SVRP aquifer 

include (1) recharge from precipitation, (2) inflows from 

tributary basins and adjacent uplands, (3) subsurface seepage 

and surface overflows from lakes that border the aquifer, 

(4) flow from losing segments of the Spokane River to the 

aquifer, (5) return percolation from irrigation, and (6) effluent 

from septic systems. Outflows from the SVRP aquifer include 

(1) ground-water withdrawals from wells, (2) flow from the 

aquifer to gaining segments of the Spokane River, (3) aquifer 

discharge to the Little Spokane River, and (4) subsurface 

outflow from the lower unit at the western limit of the model 

area near Long Lake. These inflow and outflow components 

are represented in the model by using MODFLOW-2000 

packages.

The parameter-estimation program PEST was used 

to calibrate the SVRP aquifer model. PEST implements a 

nonlinear least-squares regression method to estimate model 

parameters so that the differences between measured and 

simulated quantities are minimized with respect to an optimal 

criterion. Calibration data include 1,573 measurements of 

water levels and 313 measurements of streamflow gains and 

losses along segments of the Spokane and Little Spokane 

Rivers.

Ground-Water Flow Model for the Spokane ValleyRathdrum Prairie Aquifer, Spokane County, Washington, 

and Bonner and Kootenai Counties, Idaho

By Paul A. Hsieh, Michael E. Barber, Bryce A. Contor, Md. Akram Hossain, Gary S. Johnson, Joseph L. Jones, 

and Allan H. Wylie
Model parameters estimated during calibration include 

hydraulic conductivity, specific yield, vertical hydraulic 

conductivity of riverbed sediments, and hydraulic conductance 

of riverbed and lakebed sediments. Simulated water levels and 

streamflow gains and losses generally were in good agreement 

with measured water level and streamflow gains and losses 

throughout most of the aquifer. However, discrepancies 

between measured and simulated quantities do occur in 

local parts of the aquifer. The largest discrepancy between 

measured and simulated water levels occurs in the lower unit 

in northern Hillyard Trough and the Little Spokane River Arm. 

These discrepancies indicate that the lower unit might not be 

accurately represented by the model.

After the model was calibrated, five alternative models 

were evaluated. In each alternative model, one aspect of the 

calibrated model was varied and the alternative model was 

re-calibrated. Results of these alternative model analyses show 

that changes in certain model parameter values can result in 

changes to certain simulated flow components even though the 

overall fit of the alternative model to the measured quantities 

is nearly as good as the calibrated model. This suggests some 

degree of nonuniqueness in the ground-water flow simulated 

by the calibrated model.

The model presented in this report is calibrated using 

significantly more data than are used in previous models. The 

relatively good fit between simulated and measured quantities 

indicates that the overall simulated ground-water flow is a 

reasonable representation of ground-water flow in the SVRP 

aquifer. Nonetheless, the model is subject to limitations. 

In particular, there is insufficient hydrologic information 

to determine ground-water inflow from Spirit and Hoodoo 

Valleys to the SVRP aquifer. In Hillyard Trough and the Little 

Spokane River Arm, ground-water flow in the lower unit is 

not well understood, and simulated water levels do not fit 

measured water levels as well as in other parts of the aquifer. 

There also is significant uncertainty in the simulated seepages 

from Lake Pend Oreille and Coeur d’Alene Lake. Further 

investigations in these parts of the SVRP aquifer could provide 

valuable knowledge that can be used to improve the model in 

the future.

Introduction

The Spokane Valley-Rathdrum Prairie (SVRP) aquifer 

supplies water to more than 500,000 residents in Spokane 

County, Washington, and Bonner and Kootenai Counties, 

Idaho (fig. 1). In 1978, the U.S. Environmental Protection 

Agency designated the aquifer as a “Sole Source Aquifer” 

in response to concerns about aquifer vulnerability to waterquality degradation (Federal Register, 1978). Recent and 

projected urban growth in the aquifer area, which includes 

Spokane, Spokane Valley, and Liberty Lake, Washington, and 

Post Falls and Coeur d’Alene, Idaho, has raised additional 

concerns about the effects of increased ground-water 

withdrawals from the aquifer. To address these concerns, a 

comprehensive hydrologic study was developed in 2004 by 

the Idaho Department of Water Resources, the Washington 

Department of Ecology, and the U.S. Geological Survey 

(USGS) to improve the understanding of ground-water flow 

in the aquifer and of the interaction between ground water 

and surface water. The purpose of the comprehensive study 

is to provide a scientific foundation for management of the 

aquifer and to provide tools that are needed to evaluate aquifer 

management strategies. Development of a ground-water flow 

model of the aquifer is one component of the comprehensive 

study.

The ground-water flow model presented in this report 

was developed by the Modeling Team formed within the 

comprehensive study. The Modeling Team (authors of this 

report) consisted of staff and personnel working under contract 

with the Idaho Department of Water Resources, personnel 

working under contract with the Washington Department of 

Ecology, and staff of the USGS. To arrive at a final model 

that has the endorsement of all team members, decisions 

on modeling approach, methodology, assumptions, and 

interpretations were reached by consensus. The Modeling 

Team operated under the management of the Project Technical 

Leadership Team and received advice and comments from the 

Technical Advisory Committee. In addition to undergoing the 

USGS report review process, the model was reviewed by the 

study’s Peer Review Team.

Purpose and Scope

This report describes the development of a computer 

model to simulate ground-water flow in the SVRP aquifer. 

Steps in model development include: (1) defining the areal 

and vertical extents of the model, (2) defining boundary 

conditions, (3) estimating components and rates of inflows to 

and outflows from the aquifer, and (4) calibrating the model 

by adjusting model parameters (such as hydraulic conductivity 

and specific yield) so the differences between simulated 

and measured quantities (such as water levels and flows) 

are minimized with respect to an objective function. Model 

calibration uses a nonlinear least-squares regression method, 

which enables quantification of parameter uncertainty within 

the context of the regression problem.

The primary purpose of the model is to serve as a tool for 

analyzing SVRP aquifer inflows and outflows, simulating the 

effects of future changes in ground-water withdrawals from 

the aquifer, and evaluating aquifer management strategies. 

 Ground-Water Flow Model for the SVRP Aquifer, Washington and Idaho
Figure 1. Location of the Spokane Valley-Rathdrum Prairie aquifer, Spokane County, Washington, and Bonner and Kootenai 

Counties, Idaho.

wa19-0088_fig01

0 5 10 15 20 KILOMETERS

0 5 10 15 20 MILES

WASHINGTON

IDAHO

MAP AREA

Extent of Spokane Valley-Rathdrum Prairie

 aquifer ground-water flow model

Area of Spokane Valley-Rathdrum Prairie

 aquifer as defined by Kahle and others (2005),

 that is excluded from ground-water flow model

Base modified from U.S. Geological Survey digital data.

City boundaries, 1:24,000, various years (1961-86);

Public land survey, 1:100,000, 1985; Lakes, 1:100,000, 1995;

and rivers, 1:100,000, 1985. North American Datum of 1983 (NAD 83).

EXPLANATION

117°30' 15' 117° 45' 116°30'

48°

 47°

45'

S P O K A N E C O U N T Y

WASHINGTON

IDAHO

Nine Mile Dam

Five

Mile

Prairie

Western Arm

Little Spokane

River Arm

Hillyard Trough

Southern Rathdrum Prairie

Round

Mountain

Northern Rathdrum Prairie

West Channel

Spirit Valley

Hoodoo Valley

Cocolalla

Valley

Ramsey Channel

Chilco Channel

Spokane Valley

Trinity Trough

Upriver Dam Post Falls Dam

P E N D O R E I L L E C O U N T Y

B O N N E R C O U N T Y

K O O T E N A I C O U N T Y

S T E V E N S

C O U N T Y

Little Spoka

ne

River

Spokane River

SPOKANE

COEUR D’ALENE

HAYDEN

ATHOL

POST FALLS

BAYVIEW

Deep Creek

Long

Lake

Coeur d’Alene Lake

Lake

Pend

Oreille

Liberty

Lake

Newman

Lake

Hauser

Lake

Fernan Lake

Hayden Lake

Blanchard

Lake

Spirit

Lake

Twin

Lakes

HangmanCr.

Spokane

River

The scale of the model and the level of detail are intended 

for analysis of aquifer-wide water-supply issues. The model 

presented in this report is not intended for application to 

contaminant-transport issues such as the prediction of 

contaminant traveltimes or flow paths. A contaminanttransport model would require a substantially greater amount 

of hydrogeologic detail for the contamination site.

Although the areal extent of the model differs slightly 

from the areal extent of the SVRP aquifer as defined by Kahle 

and others (2005), this report does not redefine the aquifer 

boundary. Because of the paucity of data for the northern 

extreme of Rathdrum Prairie, the model does not include Spirit 

and Hoodoo Valleys. Ground-water flow directions in Spirit 

and Hoodoo Valleys and the degree of hydraulic connections 

between those valleys and northern Rathdrum Prairie cannot 

be determined with certainty with the presently available 

data. Therefore, during model calibration, the uncertainty in 

ground-water inflow from the valleys is treated by assuming 

different inflow values along the model boundary.

Introduction 
Description of Study Area

The areal extent of the SVRP aquifer model is shown in 

figure 1. The model encompasses an area of approximately 

326 mi2 in eastern Washington and northwestern Idaho. For the 

most part, the model extent coincides with the 2005 revised 

extent of the SVRP aquifer as defined by Kahle and others 

(2005). However, the model excludes Spirit and Hoodoo 

Valleys and three areas where bedrock is close to land surface 

and the aquifer sediments likely are unsaturated. Within the 

model extent, land-surface altitude ranges from about 2,600 ft 

in northern Rathdrum Prairie to about 1,500 ft at the western 

limit of the model near Long Lake. The climate varies from 

subhumid to semiarid and is characterized by warm, dry 

summers and cool, moist winters. Mean annual (1971–2000) 

precipitation is 16.7 in. at the Spokane International Airport, 

Washington; 25.9 in. near Bayview, Idaho; and 28.1 in. at the 

Coeur d’Alene Airport, Idaho (Kahle and others, 2005, p. 6).

The SVRP aquifer is divided into several subregions by 

bedrock outcrops and subsurface bedrock ridges. On the east 

side of the aquifer, a crystalline rock outcrop known as Round 

Mountain and a less prominent bedrock ridge divide the 

aquifer into three channels that connect northern and southern 

Rathdrum Prairie. From west to east, these channels are known 

as West Channel, Ramsey Channel, and Chilco Channel. On 

the west side of the aquifer, two subsurface bedrock ridges 

extend from a basalt highland known as Five Mile Prairie. 

The first ridge extends to the south and the second extends 

to the west. The south-extending ridge, along with Five Mile 

Prairie, divides the aquifer into two arms. The eastern arm is 

known as Hillyard Trough, and the western arm is known as 

Western Arm. At the north end of Hillyard Trough, the aquifer 

continues west in the valley containing the Little Spokane 

River. This part of the aquifer is referred to as the Little 

Spokane River Arm. At the north end of Western Arm, the 

aquifer terminates against the subsurface bedrock ridge that 

extends west from Five Mile Prairie (see Kahle and others, 

2005, p. 18-19). The narrow channel between Western Arm 

and the rest of the aquifer is known as Trinity Trough.

Nine lakes are located along the perimeter of the model 

area. Because the water levels of those lakes are higher than 

the ground-water level in the SVRP aquifer, water seeps from 

the lakebed and recharges the aquifer. The two largest lakes 

are Lake Pend Oreille and Coeur d’Alene Lake. Water levels 

in those lakes are regulated by dams on the respective outlet 

rivers. The outlet of Lake Pend Oreille is the Pend Oreille 

River, which is north of the area shown in the location map 

in figure 1. The outlet of Coeur d’Alene Lake is the Spokane 

River. The seven smaller lakes are Fernan Lake, Hauser Lake, 

Hayden Lake, Liberty Lake, Newman Lake, Twin Lakes, and 

Spirit Lake. These lakes do not have perennial outlet streams. 

However, during wet seasons, if the lake level rises above the 

outlet structure, lake water spills over the outlet structure and 

exits the lake as surface flow. Because of the highly permeable 

nature of the surficial and aquifer material, the surface flow 

soaks into the ground within a short distance of the lake.

The Spokane and Little Spokane Rivers are the major 

surface-water drainages in the model area. The Spokane River 

originates at Coeur d’Alene Lake. Water discharge from that 

lake is regulated by Post Falls Dam. Downstream of the dam, 

the Spokane River is free flowing for about 16 river miles 

until it reaches Upriver Pool near the Centennial Trail Bridge. 

Downstream of Upriver Dam, the river is again free flowing 

for about 4.5 river miles until it enters downtown Spokane. 

In the next 2.5 river miles, the river flows over basaltic rocks 

outside the SVRP aquifer and is regulated by dams in the 

Spokane Falls area. The river re-enters the aquifer at the south 

end of Western Arm and is again free flowing for about 10 

river miles as it meanders northwest to Nine Mile Reservoir. 

Downstream of Nine Mile Dam, the Spokane River is joined 

by the Little Spokane River, which enters the model area at 

the north end of Hillyard Trough and flows toward the west. 

Downstream of the confluence of the two rivers, the Spokane 

River becomes a reservoir known as Long Lake, which is 

regulated by Long Lake Dam.

Previous Investigations

In preparation for developing the ground-water flow 

model presented in this report, Kahle and others (2005) 

compiled geologic and hydrologic information available as 

of June 2005 for the SVRP aquifer. The geologic information 

includes the pre-Tertiary, Tertiary, and Quaternary geology of 

the aquifer and results of previous geophysical investigations. 

The hydrologic information includes the current understanding 

of the hydrogeologic framework, ground-water movement, 

water-budget components, and ground-water/surface-water 

interactions. The compilation also includes descriptions of 

previous ground-water flow models by Bolke and Vaccaro 

(1981), CH2M Hill (1998, 2000), Buchanan (2000), and 

Golder Associates, Inc. (2004). In another report, Kahle and 

Bartolino (2007) refined the hydrogeologic understanding of 

the aquifer. Using drillers’ records and results of available 

geophysical investigations, they developed a contour map 

showing the altitude of the base of the aquifer, mapped the 

extent of a clay layer in Hillyard Trough and in the Little 

Spokane River Arm, and updated the aquifer water budget 

using recently compiled information.

Because previous ground-water flow models of the SVRP 

aquifer are described in detail in the report by Kahle and 

others (2005), the following discussion highlights only the 

similarities and differences among the models. The models 

by Bolke and Vaccaro (1981), CH2M Hill (1998, 2000), and 

Golder Associates, Inc. (2004) encompass the western (mostly 

Washington) part of the aquifer from Post Falls or the IdahoWashington State line to the eastern end of Long Lake. The 

 Ground-Water Flow Model for the SVRP Aquifer, Washington and Idaho
model by Buchanan (2000) encompasses the entire aquifer in 

both States. All four models simulate the interaction between 

the aquifer and the Spokane and Little Spokane Rivers. 

In addition, the model by Golder Associates, Inc. (2004) 

simulates overland flow, river flow, and subsurface flow in the 

unsaturated and saturated zones of the aquifer.

Although evidence available since the late 1990s (see, 

for example, Gruenenfelder, 1997) indicates that an extensive 

clay layer divides the SVRP aquifer into an upper, unconfined 

unit and a lower, confined unit in Hillyard Trough and the 

Little Spokane River Arm, all four previous models treat the 

aquifer effectively as a single, unconfined, hydrogeologic unit. 

The clay layer is not mentioned in the reports by Bolke and 

Vaccaro (1981) and Buchanan (2000). In the reports by CH2M 

Hill (1998, 2000), the clay layer is discussed but the model 

excludes the lower, confined unit. In the report by Golder 

Associates, Inc. (2004), the clay layer is treated as a geologic 

lens within a model layer rather than as a separate model layer. 

Therefore, a single model layer represents both the upper and 

lower units.

The four models are calibrated using different amounts of 

measured data for different time periods. Bolke and Vaccaro 

(1981) presented both a time-averaged simulation and a 

transient simulation for May 1977–April 1978. The model by 

Bolke and Vaccaro (1981) is calibrated using measured water 

levels in 73 wells and ground-water discharge from the SVRP 

aquifer to the Little Spokane River (that is, streamflow gain 

on the river). The calibration is checked by a water-balance 

calculation and by comparing simulated streamflow with 

measured streamflow for three sites on the Spokane River. The 

model by CH2M Hill (1998, 2000) is a steady-state model and 

is calibrated using measured water levels in about 110 wells 

and streamflow gains and losses on the Spokane River during 

September 1994. The calibration is checked by comparing 

simulated and measured water levels and streamflow gains 

and losses for April 1995. The model by Buchanan (2000) 

also is a steady-state model and is calibrated using measured 

water levels in 15 wells during unspecified time periods. The 

calibration data do not include measured streamflow gains 

or losses. The model by Golder Associates, Inc. (2004) is a 

transient model and is calibrated using measured water levels 

in about 20 wells and measured streamflow on the Spokane 

and Little Spokane Rivers during 1994, 1997, and 1999.

An important assumption in the models by Bolke 

and Vaccaro (1981), CH2M Hill (1998, 2000), and Golder 

Associates, Inc. (2004) is the boundary condition at the eastern 

terminus of the model. This boundary condition controls 

ground-water flow from Rathdrum Prairie to the model. The 

model by Bolke and Vaccaro (1981) specifies the hydraulic 

head along the eastern boundary. The calibrated model 

simulates a ground-water inflow of about 400 ft3

/s across 

the boundary. By contrast, the model by CH2M Hill (1998, 

2000) specifies the ground-water inflow across the eastern 

boundary and inflow rate is adjusted during calibration. The 

calibrated model simulates a ground-water inflow of 385 ft3/s. 

The model by Golder Associates, Inc. (2004) specifies a timevarying hydraulic head along the eastern boundary. The report 

by Golder Associates, Inc. (2004, fig. 9.8) does not give the 

simulated flow across the boundary but does indicate that the 

simulated flow across the Washington-Idaho State line, about 

3.5 mi west of the eastern boundary, ranges from about 30 to 

850 ft3/s.

Components of aquifer inflows and outflows are treated 

in somewhat different manners in the four models. In all four 

models, aquifer inflows include recharge from precipitation 

(that is, precipitation minus evapotranspiration), leakage 

from the losing segments of the Spokane River, and varying 

amounts of inflows from tributary basins, adjacent uplands, 

or lakes along the aquifer perimeter. The models by Bolke 

and Vaccaro (1981), CH2M Hill (1998, 2000), and Golder 

Associates, Inc. (2004) also include return percolation from 

irrigation, effluent from septic systems, and inflow across 

the eastern boundary as discussed in the previous paragraph. 

In all four models, ground water discharges to the Little 

Spokane River and to gaining segments of the Spokane 

River. The models by Bolke and Vaccaro (1981), CH2M 

Hill (1998, 2000), and Golder Associates, Inc. (2004) also 

include ground-water withdrawals by water purveyors and by 

industrial, commercial, agricultural, and domestic users. In the 

model by Bolke and Vaccaro (1981), ground-water outflow 

also occurs along a specified-hydraulic head boundary near the 

confluence of the Spokane and Little Spokane Rivers.

Hydrology of Aquifer

The geologic history and the hydrogeologic framework 

of the SVRP aquifer are presented in the reports by Kahle and 

others (2005) and Kahle and Bartolino (2007). The following 

discussion briefly summarizes the geologic setting of the 

study area. The remainder of the discussion focuses on the 

hydrologic information used to develop the ground-water flow 

model. Topics covered include the areal and vertical extent 

of the aquifer, hydraulic properties, inflows and outflows, 

interaction between the aquifer and the Spokane River, and 

ground-water levels and movement.

Geologic Setting

Kahle and Bartolino (2007) described three distinct 

hydrogeologic units in the study area: the SVRP aquifer, the 

Basalt and fine-grained interbeds unit, and the Bedrock unit. 

Together, the Basalt and fine-grained interbeds unit, which 

includes Columbia River basalt and interbedded lacustrine 

deposits of the Latah Formation, and the Bedrock unit, which 

includes Precambrian to Tertiary metamorphic and intrusive 

igneous rocks, laterally bound and underlie the SVRP aquifer.

Hydrology of Aquifer 
The SVRP aquifer consists mostly of sands, gravels, 

cobbles, and boulders primarily deposited by a series of 

catastrophic glacial outburst floods from ancient glacial Lake 

Missoula during the Pleistocene Epoch. Kahle and Bartolino 

(2007) noted that most of the aquifer sediments deposited 

in such a high-energy depositional environment are coarse 

grained. However, they also noted that fine-grained layers of 

clay and silt are scattered throughout the aquifer and likely 

were deposited in large proglacial lakes in the path of the 

Missoula floods. From analysis of drillers’ reports, Kahle and 

Bartolino (2007) found that

“The aquifer generally has a greater percentage 

of finer material near the margins of the valley 

and becomes more coarse and bouldery near 

the center throughout the Rathdrum Prairie and 

Spokane Valley. In the Hillyard Trough, the deposits 

generally are finer grained and the aquifer consists 

of sand with some gravel, silt, and boulders.”

Areal Extent

The areal extent of the SVRP aquifer has been redefined 

several times in the past 30 years. The most recent definition 

is the 2005 revised extent of the SVRP aquifer shown in Kahle 

and others (2005, pl. 2). In most places, the aquifer boundary 

follows the contact between the coarse, highly permeable 

aquifer sediments and the surrounding less permeable bedrock 

and fine-grained material. The 2005 revised extent includes 

Ramsey Channel, Chilco Channel, the south part of Hoodoo 

Valley, and the south part of Cocolalla Valley (fig. 1). These 

four areas lie outside the Sole Source Aquifer as designated by 

the U.S. Environmental Protection Agency in 1978. In revising 

the aquifer boundary, Kahle and others (2005, p. 17) noted that

The 2005 revised extent of the SVRP aquifer extends into 

the west part of Spirit Valley and the south part of Hoodoo 

Valley (fig. 2). Kahle and others (2005, p. 20) stated that

 “In the Hoodoo Valley, historical water-level 

elevations indicated that a water-table divide was 

between Edgemere and Harlem (Walker, 1964). 

Ground water north of the divide moved northward 

toward the Pend Oreille River; ground water south 

of the divide moved southward toward Athol. In 

Spirit Valley, the ground-water divide was near 

Blanchard Lake (Parliman and others, 1980). West 

of the divide, ground water flows northwestward 

toward the Pend Oreille River; east of the divide, 

ground water flows southeastward into the main 

body of the SVRP aquifer.”

An examination of recent water-level data and drillers’ 

reports indicates that the previous characterization is subject 

to uncertainty. During the synoptic water-level measurements 

of September 2004 (Campbell, 2005), water levels in wells 

262 and 263, at the south end of Hoodoo Valley, were several 

feet higher than water levels in wells 260 and 261, which 

are farther to the north (fig. 2, table 1). These water levels 

indicate that ground water flows to the north (away from 

northern Rathdrum Prairie) in almost the entire length of 

Hoodoo Valley. In Spirit Valley, water levels were measured at 

only one well (well 267) during September 2004. However, a 

search of the USGS ground-water database produced several 

water-level measurements for wells in the valley during the 

late summer of 1998 and 1999. Water levels in wells S-1, S-2, 

and 267 (fig. 2, table 1) indicate that the ground water flows 

away from the Rathdrum Prairie in almost the entire length of 

Spirit Valley.

Table 1. Water levels in wells in and near Spirit and Hoodoo Valleys, Bonner 

County, Idaho.

[Well No.: Location of wells is shown in figure 2. Abbreviations: NAVD 88, North American 

Vertical Datum of 1988]

Well 

No. Well name

U.S. Geological 

Survey 

site No.

Date of 

water-level 

measurement

Altitude of 

water level, 

in feet above 

NAVD 88

260 54N 04W 10BBA1 480300116492401 09-16-2004 2,148.52

261 54N 04W 10DCD1 480209116484201 09-16-2004 2,147.45

262 54N 04W 19BCD1 480051116532101 09-16-2004 2,158.17

263 54N 04W 29ABC1 480015116512901 09-16-2004 2,151

264 54N 04W 30BAB1 480021116531201 09-16-2004 2,044.18

266 54N 04W 31BCB1 475849116521601 09-16-2004 2,035.78

S-1 54N 05W 23DBA1 480046116552201 09-15-1999 2,214

267 54N 05W 22ACA1 480101116563601 08-06-1998 2,190

S-2 54N 05W 18AAA1 480207117001401 09-23-1998 2,185

09-20-1999 2,189

“For modeling purposes, it may be 

important to use a more inclusive 

aquifer boundary to better represent 

contributions from adjacent surficial 

deposits that are in hydraulic contact 

with the Sole Source Aquifer.”

For the most part, the extent of the 

model in this report coincides with the 2005 

revised extent. However, the model excludes 

Spirit and Hoodoo Valleys and three areas 

where bedrock is close to land surface and 

the aquifer sediments likely are unsaturated 

(fig. 1). The model extent is not intended to be 

a redefinition of the aquifer. As discussed in 

the following paragraphs, Spirit and Hoodoo 

Valleys are excluded from the model because 

of uncertainties about the ground-water flow 

directions in those valleys and the degree of 

hydraulic connection between the valleys and 

northern Rathdrum Prairie.

 Ground-Water Flow Model for the SVRP Aquifer, Washington and Idaho
Figure 2. Locations of wells in and near Spirit and Hoodoo Valleys, Bonner County, Idaho.

wa19-0088_fig02

0 5 KILOMETERS

0 5 MILES

WASHINGTON

IDAHO

Spirit Lake

Blanchard Lake

Hoodoo Lake

R. 5 W. R. 4 W.

T. 54 N.

T. 53 N.

T. 55 N.

Spirit Valley

Hoodoo Valley

267

266

264

263

262

261

260

S-2

S-1

266

Edgemere

Harlem

Northern Rathdrum Prairie

A

B

BA

Base modified from U.S. Geological Survey digital data.

City boundaries, 1:24,000, various years (1961-86);

Public land survey, 1:100,000, 1985; Lakes, 1:100,000, 1995;

and rivers, 1:100,000, 1985. North American Datum of 1988 (NAD 88).

Area of Spokane Valley-Rathdrum Prairie

 aquifer ground-water flow model

EXPLANATION

Area of Spokane Valley-Rathdrum Prairie

 aquifer as defined by Kahle and others (2005),

 that is excluded from ground-water flow model

Boundary segment of ground-water flow model

Well and No.

The synoptic water-level measurements of September 

2004 (Campbell, 2005) also show a water-level difference of 

about 100 ft over a relatively short distance of about 1 mi from 

the south end of Hoodoo Valley to northern Rathdrum Prairie. 

Water levels in wells 262 and 263 were 2,158.17 and 2,151 ft, 

respectively. Water levels in wells 264 and 266 were 2,044.18 

and 2,035.78 ft, respectively. Drillers’ reports for T. 54 N., 

R. 4 W. and the north half of T. 53 N., R. 4 W. show a similar 

water-level difference from the south end of Hoodoo Valley 

to northern Rathdrum Prairie. The relatively large water-level 

difference over a relatively short distance can indicate the 

presence of a low hydraulic conductivity barrier resulting 

in a poor hydraulic connection between Hoodoo Valley and 

northern Rathdrum Prairie. However, although clay layers 

are noted in some drillers’ reports for the area, conclusive 

evidence of a low hydraulic conductivity barrier is lacking.

Hydrology of Aquifer 
Considered together, available data indicate uncertainty in 

ground-water flow directions in Spirit and Hoodoo Valleys and 

in the degree of hydraulic connection between those valleys 

and northern Rathdrum Prairie. Because of this uncertainty, 

the model in this report terminates at boundary segment A-B 

(fig. 2) and does not extend into Spirit and Hoodoo Valleys. If 

ground-water inflow from these valleys to northern Rathdrum 

Prairie does occur, the inflow is assumed to be negligible. 

During model calibration, the uncertainty in ground-water 

inflow from Spirit and Hoodoo Valleys is treated by assuming 

different inflow values along boundary segment A-B.

In addition to Spirit and Hoodoo Valleys, three areas 

in the 2005 revised extent are excluded from the model 

(fig. 1). The first excluded area is on the east side of northern 

Rathdrum Prairie. Hydrogeologic sections L-L’ and O-O’ in 

Kahle and Bartolino (2007, pl. 2) indicate that bedrock is close 

to land surface in this area and the aquifer sediments likely 

are unsaturated. The second and third excluded areas are two 

narrow zones that border Five Mile Prairie and the subsurface 

bedrock ridge that extends from the prairie toward the south. 

The aquifer sediments in the two narrow zones likely also are 

unsaturated as they lie on the slope of the basalt that outcrops 

to form Five Mile Prairie.

Vertical Extent

The sediments of the SVRP aquifer extend from land 

surface downward to either bedrock or the Latah Formation, 

which is composed predominantly of lacustrine and fluvial 

deposits of siltstone, claystone, and minor sandstone. The 

altitude of the base of the aquifer as determined by Kahle and 

Bartolino (2007) is shown in figure 3. Sediment thickness 

Figure 3. Approximate altitude of the base of the Spokane Valley-Rathdrum Prairie aquifer, Spokane County, Washington, and Bonner 

and Kootenai Counties, Idaho.

0 5 10 15 20 KILOMETERS

0 5 10 15 20 MILES

Little

Spokane R.

Spokane River

Deep

Creek

Coeur d’Alene Lake

Lake

Pend

Oreille

Hangman

Creek

117°30'

48°

47°

45'

117° 116°30'

1600,600

1,600

1,600

1,600

1,600

1,600

1,600

1,600

1,600

1,600

1,400

1,400 1,400

1,400

1,800

1800,800

1800,800

1,800

1,800

1,800

1,800

1,800

1,800

1,800

1,800

1,800 2,000 2000

2,000

2,000

2,000

2,000

2,000

2000,000

2000,000

2,200

2,200

2,200

2,200

2,200 2,200

2,200

2200,200

2,400

Spokane R.

Long

Lake

Liberty

Lake

Newman

Lake

Hauser

Lake

Fernan Lake

Hayden Lake

Spirit

Lake

Twin

Lakes

EXPLANATION

1,200

Extent of Spokane Valley-Rathdrum Prairie 

aquifer ground-water flow model 

Altitude of base of Spokane Valley–

 Rathdrum Prairie aquifer—

 Contour interval is 200 feet. Datum

 is North American Vertical Datum

 of 1988

Area underlain by shallow bedrock

   Ground-Water Flow Model for the SVRP Aquifer, Washington and Idaho
(from land surface to the base of the aquifer) is largest in 

the central part of the aquifer and thins to zero at the aquifer 

perimeter. The maximum sediment thickness is about 800 ft 

in northern Rathdrum Prairie, 500 ft near the WashingtonIdaho State line, and 700 ft in Hillyard Trough. However, the 

saturated thickness (from the water table to the base of the 

aquifer) can be substantially less than the sediment thickness. 

For example, in northern Rathdrum Prairie, the depth from 

the land surface to the water table can exceed 500 ft, and the 

maximum saturated thickness is between 200 and 300 ft. In 

the area underlain by shallow bedrock (fig. 3) the saturated 

zone likely is a thin veneer overlying the bedrock or is entirely 

absent.

The SVRP aquifer is considered to be a single 

hydrogeologic unit except in Hillyard Trough and the Little 

Spokane River Arm. In those areas, a continuous clay layer 

divides the aquifer into an upper unconfined unit and a lower 

confined unit. Kahle and Bartolino (2007) characterize this 

clay layer as a “fine-grained layer.” However, the term “clay 

layer” is used in this report to emphasize the low-permeability 

character of the layer. The areal extent of the clay layer as 

mapped by Kahle and Bartolino (2007) is shown in figure 4. 

The vertical extent of the clay layer is shown in hydrogeologic 

section C-C’ in figure 5. According to Kahle and Bartolino 

(2007), the average thickness of the clay layer is 215 ft in 

Hillyard Trough and 130 ft in the Little Spokane River Arm. 

In Hillyard Trough, the altitude of the top of the clay layer is 

between 1,660 and 1,720 ft. In the Little Spokane River Arm, 

the altitude of the top of the clay layer is between 1,500 and 

1,700 ft.

Hydraulic Properties

The SVRP aquifer is a highly productive aquifer. Wells 

in the aquifer yield as much as several thousand gallons per 

minute with relatively little drawdown. Many wells penetrate 

only the upper part (less than 100 ft) of the aquifer’s saturated 

thickness. Kahle and others (2005, p. 19-20) noted that 

hydraulic conductivity of the aquifer sediments is at the upper 

end of values measured in the natural environment. However, 

the aquifer also contains local zones of less permeable, finegrained sedimentary materials.

Figure 4. Areal extent of clay layer in Hillyard Trough and the Little Spokane River Arm, 

Spokane County, Washington.

0 5 KILOMETERS

0 5 MILES

117°30' 117°20'

47°45'

47°40'

Spokane River

Little Spokane

River

Long

Lake

Deep

Creek

Spokane

River

C C’

C’

C

Area of Spokane Valley-

 Rathdrum Prairie aquifer 

 ground-water flow model

EXPLANATION

Extent of clay layer in 

 Hillyard Trough and

 Little Spokane River Arm

Line of hydrogeologic section (see fig. 5)

Base modified from U.S. Geological Survey digital data.

City boundaries, 1:24,000, various years (1961-86);

Public land survey, 1:100,000, 1985; Lakes, 1:100,000, 1995;

and rivers, 1:100,000, 1985. North American Datum of 1983 (NAD 83).

Hydrology of Aquifer 
Figure 5. Generalized hydrogeologic section of the Spokane Valley-Rathdrum Prairie 

aquifer along hydrogeologic section C-C’ (from Kahle and Bartolino, 2007).

Horizontal Hydraulic Conductivity

Multiple-well aquifer tests, specific-capacity data, and 

computer model analyses have been used in previous studies 

to estimate the horizontal hydraulic conductivity (Kh) of 

the SVRP aquifer. Aquifer tests and specific capacity data 

are more numerous in more populated regions of the SVRP 

aquifer—from the city of Spokane, Washington, on the west 

to Post Falls and Coeur d’Alene, Idaho, on the east. There 

is significantly less information on hydraulic properties in 

Rathdrum Prairie north of Post Falls and Coeur d’Alene.

Data for nine multiple-well aquifer tests conducted in the 

west half of the SVRP aquifer (fig. 6) are given by CH2M Hill 

(1998, 2000). These data appear to be the only multiple-well 

aquifer-test data readily available from published sources. 

Estimated Kh

 values given in the reports range from 500 to 

6,200 ft/d (CH2M Hill, 1998, p. 2-22 and tables 2-7; CH2M 

Hill, 2000, table E-2). However, analyses of the tests are 

complicated by the fact that the pumped and observation wells 

do not penetrate the entire thickness of the aquifer.

Specific capacity is the ratio of pumping rate to 

drawdown in a well after a given pumping duration. However, 

there is no commonly accepted pumping duration and it can 

be highly variable from one test to the next. Consequently, Kh

values estimated from specific-capacity data are not as reliable 

as values estimated from multiple-well aquifer-test data.

19-0088_Figure 05

1,000

1,600

1,400

1,200

1,800

2,400

2,200

2,000

2,600

?

?

?

?

? ?

? ?

?

?

?

Hillyard Trough

26N/42E-27D0426N/42E-23E01

26N/43E-19P01

26N/43E-19R01

26N/43E-20N01

26N/43E-20J02

26N/43E-21J0126N/43E-22M01

26N/43E-22J01

FEET

500

450

400

300

350

650

600

550

METERS

750

700

C FEETC'

W E Spokane River SECTION D-D'

1,000

1,600

1,400

1,200

1,800

2,400

2,200

2,000

2,600

NORTH AMERICAN VERTICAL DATUM OF 1988 (NAVD 88)

Base altitude inferred from

Newcomb and others, 1953

EXPLANATION

PROJECT WELL AND NO.

? APPROXIMATE HYDROGEOLOGIC CONTACT–Dashed and queried where least certain

WATER TABLE –Approximate position in Spokane Valley-

 Rathdrum Prairie aquifer (modified from Campbell, 2005)

Bedrock unit and undifferentiated deposits

HYDROGEOLOGIC UNITS

Spokane Valley–Rathdrum Prairie;

 blue denotes fine-grained layers within aquifer

Basalt and fine-grained interbeds unit;

 orange denotes basalt, green denotes fine-grained interbed

26N/46E-30D01

VERTICAL EXAGGERATION X 10

0 2 4 MILES

0 2 4 6 KILOMETERS

10 Ground-Water Flow Model for the SVRP Aquifer, Washington and Idaho
Sagstad (1977), Bolke and Vaccaro (1981), and CH2M 

Hill (1998) estimated Kh from single-well specific capacity 

data. Bolke and Vaccaro (1981, p. 18) indicated a decrease in 

Kh values in a westerly direction from the Post Falls area. They 

stated:

“The decrease in values in the down-valley direction 

is indicative of the change in valley-fill material, 

which, in general grades from coarse to fine in a 

westerly direction.”

The estimated Kh

 values given by Bolke and Vaccaro (1981) 

average about 6,000 ft/d in the Post Falls area, about 4,300 ft/d 

in Spokane Valley, about 2,600 ft/d near Spokane, and about 

860 ft/d in Hillyard Trough and the Little Spokane River Arm.

CH2M Hill (1998, p. 2-24 and tables 2-8) indicated that 

Kh values estimated from analysis of 31 specific capacity tests 

range from 100 to 6,200 ft/d for the west half of the SVRP 

aquifer. The central 50 percent of the values range from 400 to 

3,000 ft/d. However, CH2M Hill (1998) also noted that

“For many wells, including those closest to the 

State line, the computed transmissivities [horizontal 

hydraulic conductivity times saturated thickness] 

appear to underestimate the likely aquifer 

transmissivity because the wells pump at low rates 

or are shallow (for example, they penetrate only 

a very small fraction of the aquifer’s saturated 

thickness).”

Figure 6. Locations of multiple-well aquifer-test sites in the west half of the Spokane Valley-Rathdrum Prairie aquifer, Washington and 

Idaho.

wa19-0088_fig06

0 5 10 15 20 KILOMETERS

0 5 10 15 20 MILES

Little

Spokane River

Spokane River

Deep

Creek

Coeur d’Alene Lake

Lake

Pend

Oreille

Hangman

Creek

117°30'

48°

47°

45'

117° 116°30'

Liberty

Lake

Newman

Lake Hauser

Lake

Fernan Lake

Hayden Lake

Spirit

Lake

Twin

Lakes

Long

Lake

Spokane R. 1,200-2,100

1,100-2,500

500-3,500

2,500

4,200-6,200

2,000-4,200

950

3,000

2,500

 Site of multiple-well aquifer 

 test —Number is range of 

 estimated hydraulic conductivity 

 values, in feet per day (from CH2M Hill, 

 1998, 2000)

EXPLANATION

Extent of Spokane Valley-Rathdrum 

 Prairie aquifer ground-water 

 flow model

Base modified from U.S. Geological Survey digital data.

City boundaries, 1:24,000, various years (1961-86);

Public land survey, 1:100,000, 1985; Lakes, 1:100,000, 1995;

and rivers, 1:100,000, 1985. North American Datum of 1983 (NAD 83).

1,100-2,500

Hydrology of Aquifer 11
In a hydrogeologic study of southern Rathdrum Prairie, 

Sagstad (1977, p. 39-40) analyzed specific-capacity data 

for 20 wells in the Post Falls area and 4 wells in the Coeur 

d’Alene area. The estimated Kh values given by Sagstad (1977) 

range from 250 to 2,100 ft/d in the Post Falls area and from 

240 to 900 ft/d in the Coeur d’Alene area. Sagstad noted that

“Transmissive characteristics in the Post Falls area 

generally are higher than in the Coeur d’Alene area. 

Well logs show that greater percentages of coarse 

gravels, pebbles, and sands are present in the Post 

Falls area than in the Coeur d’Alene area.”

This characterization is consistent with a steeper water-table 

gradient in the Coeur d’Alene area than in the Post Falls area, 

as shown by the ground-water level map of Campbell (2005).

Horizontal hydraulic conductivity values used in previous 

computer models generally were higher than values estimated 

from multiple-well aquifer tests and specific-capacity data. 

In the model by Bolke and Vaccaro (1981, p. 20), Kh

 values 

ranged from about 1,000 to 11,000 ft/d. These values result 

from an upward adjustment, by a factor of 1.9, of the initial 

values estimated from specific-capacity data. In the final 

model of CH2M Hill (2000, fig. J-4), Kh values ranged 

from 2,000 ft/d in northern Hillyard Trough to 7,000 ft/d at 

the Idaho-Washington State line. In the model by Golder 

Associates, Inc. (2004, fig. 6.8), Kh values ranged from about 

260 ft/d in northern Hillyard Trough to about 57,000 ft/d at the 

Idaho-Washington State line.

The model by Buchanan (2000) is the only previous 

model that encompasses the entire SVRP aquifer. In that 

model, on the east side of the aquifer, a zone that has a Kh

value of 11,000 ft/d extends from Lake Pend Oreille toward 

the west through northern Rathdrum Prairie and then toward 

the south through West Channel into southern Rathdrum 

Prairie. Lower Kh values (220 ft/d or less) are assigned to 

areas near the aquifer perimeter and in side valleys. This 

characterization is consistent with the steeper water-table 

gradients in side valleys in which Hauser, Hayden, Newman, 

and Spirit Lakes are located (see water-level map of Campbell, 

2005). On the west side of the aquifer, Kh

 values are similar to 

those in the CH2M Hill (2000) model.

Considered together, available data indicate that Kh values 

in the central part of the SVRP aquifer range from about 1,000 

ft/d to several tens of thousands of feet per day. In Hillyard 

Trough and in the vicinity of Coeur d’Alene, Kh values appear 

to be near the low end of the range. Near the aquifer perimeter 

and in side valleys, Kh values might be a few hundred feet per 

day or less.

Vertical Hydraulic Conductivity

Few field-measured vertical hydraulic conductivity (Kv) 

data were available for the SVRP aquifer and for the clay layer 

that separates the upper and lower aquifer units in Hillyard 

Trough and the Little Spokane River Arm. In the models by 

CH2M Hill (1998, 2000) and Golder Associates, Inc. (2004), 

the ratio of Kh

 to Kv in the aquifer was assumed to be 10:1 and 

3:1, respectively. The report by Golder Associates, Inc. (2003, 

p. 5-24) refers to an aquifer test conducted near the Colbert 

Landfill (about 10 mi north of Spokane) where a clay layer 

separates an upper aquifer from a lower aquifer. Both the clay 

layer in Hillyard Trough and the clay layer near the Colbert 

Landfill are believed to have been deposited within a glacial 

lake environment. Golder Associates, Inc. (2003, p. 5-24) 

noted that

“During pump tests at wells near the Colbert 

Landfill (Landau Associates, 1991) no response 

in the upper sands and gravels was observed 

during pumping from the lower sands and gravels, 

indicating that the glacial lake sediments act as a 

vertical hydraulic barrier between the upper and 

lower sand and gravel units in this area.”

However, it is uncertain if this characterization also applies to 

the clay layer in Hillyard Trough and the Little Spokane River 

Arm.

Specific Yield

Few field-measured specific yield (SY) data were available 

for the SVRP aquifer. In the model by Bolke and Vaccaro 

(1981), SY values initially were estimated from published 

tables that relate SY to grain size and subsequently adjusted 

during model calibration. Values of SY in the calibrated 

model ranged from about 0.1 to 0.2. In the model by Golder 

Associates, Inc. (2004, fig. 6-8), a similar procedure yielded 

SY values that ranged from 0.125 to 0.3. The models by CH2M 

Hill (1998, 2000) and Buchanan (2000) do not consider SY

because those models assume steady-state flow conditions.

Inflows to Aquifer

Inflows to the SVRP aquifer include (1) recharge from 

precipitation, (2) inflows from tributary basins and adjacent 

uplands, (3) subsurface seepage and surface overflows from 

lakes that border the aquifer, (4) flow from losing segments 

of the Spokane River to the aquifer, (5) return percolation 

from irrigation, and (6) effluent from septic systems. For 

the ground-water flow model in this report, monthly inflows 

were estimated for October 1990 through September 2005. 

Areally distributed inflow components, such as recharge from 

precipitation, are computed on a raster grid with a cell size 

of 1,320 by 1,320 ft. To facilitate preparation of model input 

data, the raster grid is aligned with the finite-difference grid 

used in the model.

12  Ground-Water Flow Model for the SVRP Aquifer, Washington and Idaho
Precipitation

Recharge from precipitation refers to that part of 

precipitation that infiltrates into the subsurface and percolates 

downward to reach the water table. Precipitation can enter the 

subsurface by falling on a permeable surface and infiltrating 

into the ground or falling on an impermeable (paved) surface 

and running off to a recharge (“dry”) well, an infiltration 

basin, or an adjacent permeable surface. In both cases, part 

of the precipitation is consumed by evapotranspiration, either 

on land surface or as the water percolates through the root 

zone (typically, the top several feet of subsurface). The water 

that percolates below the root zone is referred to as deep 

percolation. The assumption is that evapotranspiration does 

not occur below the root zone, and water that becomes deep 

percolation eventually reaches the water table.

If precipitation falls on a permeable surface within the 

SVRP aquifer, the entire amount of precipitation is assumed 

to enter the ground and there is no overland runoff. This 

assumption is reasonable for the SVRP aquifer because the 

aquifer material is highly permeable. The monthly rate of 

deep percolation resulting from precipitation on a permeable 

surface is denoted by DP and is expressed in units of length 

over time—for example, inches per month.

To estimate DP, the FAO Penman-Monteith method 

developed by Allen and others (1998) was used to estimate 

evapotranspiration. Bartolino (2007) used the method to 

estimate daily evapotranspiration for 1990–2005 at six weather 

stations in the vicinity of the model area (fig. 7). Daily deep 

percolation at each weather station was determined from 

a daily soil-moisture balance calculation (Bartolino, 2007, 

p. 11, eq. 19). The daily deep percolation values then were 

aggregated over each month to determine the DP value for each 

weather station.

The areal distribution of DP is estimated by linearly 

interpolating the DP values for the six weather stations. 

To perform the interpolation, an initial triangular network 

is constructed using the six weather stations as vertices. 

However, this initial network does not encompass the full 

extent of the model. Therefore, three auxiliary vertices are 

added to expand the network (fig. 7). The DP at an auxiliary 

vertex is set equal to the DP at the closest weather station. For 

any point within the expanded network, DP is interpolated 

linearly from the DP values for the three vertices of the triangle 

containing the point.

If precipitation falls on an impermeable surface and 

then runs off to a recharge well, an infiltration basin, or an 

adjacent permeable surface, the assumption is that 15 percent 

of the runoff is consumed by evapotranspiration and the 

remaining 85 percent becomes deep percolation. Because 

infiltration is focused into the recharge well, infiltration basin, 

or along the edges of the adjacent permeable surface, loss to 

evapotranspiration probably is relatively low. The monthly 

rate of deep percolation resulting from precipitation on an 

impermeable surface is denoted by DI.

Two data sets are used to estimate the areal distribution 

of DI. The first contains values for the amount of impermeable 

surface in the model area, and the second contains values 

for precipitation throughout the model area. The amount of 

impermeable surface is estimated from aerial photographs 

and National Land-Cover Data (http://erg.usgs.gov/isb/pubs/

factsheets/fs10800.html) for four periods: 1990–95, 1996–99, 

2000–02, and 2002–05. Within each period, the amount of 

impermeable surface is assumed to remain constant in time. 

Precipitation throughout the model area is obtained from 

PRISM-derived data. PRISM is an acronym for “Parameterelevation Regressions on Independent Slopes Model” and was 

developed by the Oregon Climate Service of Oregon State 

University (http://www.ocs.orst.edu/prism/products/). PRISM 

uses point data, a digital elevation model, and other spatial 

data sets to generate gridded estimates of several spatial and 

temporal climatic parameters, including precipitation.

The areal distribution of DI

 is calculated on a raster grid 

with a cell size of 1,320 by 1,320 ft. For each cell in the grid, 

DI is calculated as

DI = P × f

I

 × 0.85, (1)

where

P is the monthly precipitation rate, and

f

I is the fraction of the cell’s surface area that is 

impermeable.

Runoff from impermeable surfaces in certain parts of Spokane 

and Coeur d’Alene where the runoff is routed into a sewer 

overflow system that discharges to the Spokane River was 

excluded from the calculation.

Combining DP and DI

, the monthly rate of deep 

percolation (D) at a cell is calculated as

D = (1 – f

I

) DP + DI . (2)

Note that for any given month, D is the downward flux at 

the base of the root zone. This downward infiltration must 

travel through the unsaturated zone to reach the water table. 

According to unsaturated flow theory, the transmission time of 

an infiltration front to a given depth depends on the prevailing 

moisture conditions in the unsaturated zone. In this study, a 

simple approximation is adopted in which the traveltime to the 

water table is linearly related to the depth of the water table.

To estimate the transmission time to the water table, 

water levels recorded at well 251 are compared to the monthly 

rates of deep percolation (D) for the well site (fig. 8). Well 251 

is selected for analysis because the water level in the well is 

Hydrology of Aquifer 13
about 400 ft below land surface and the well has a long period 

of record. For each water year during which the hydrograph 

peak and trough are not masked by the long-term trend, the 

point of steepest water-level rise, assumed to be approximately 

halfway between the trough and the peak, is indicated by a 

triangle on the graph. The time of steepest water-level rise 

is assumed to correspond to the time of greatest recharge at 

the water table. The steepest water level rise typically occurs 

around May (fig. 8). However, deep percolation at the base of 

the root zone is greatest around the preceding December or 

early January. This observation indicates a transmission time 

of about 5 months is needed for precipitation infiltration to 

reach the water table at a depth of about 400 ft. The estimated 

transmission time to reach the water table at different depths, 

based on linearly prorating the transmission time to the depth 

of 400 ft, is shown in table 2.

wa19-0088_fig07

0 5 10 15 20 KILOMETERS

0 5 10 15 20 MILES

Spokane River

117°30'

48°

47°

45'

48°

15'

117° 116°30'

Little

Spoka

ne River

Deep Creek

Coeur d’Alene Lake

Lake

Pend

Oreille

Liberty

Lake

Newman

Lake

Hauser

Lake

Fernan Lake

Hayden

Lake

Spirit

Lake

Twin

Lakes

Long

Lake

Spokane

R.

Priest River Experiment

Station, Idaho

Newport,

Washington

Sandpoint Experiment

Station, Idaho

Bayview Model

Basin, Idaho

Coeur d'Alene, Idaho

 Spokane

Inernational

Airport, Washington

Extent of Spokane Valley-Rathdrum Prairie

 aquifer ground-water flow model

Expanded triangular network

Weather station

Auxiliary vertex

EXPLANATION

Pend Oreille River

Base modified from U.S. Geological Survey digital data.

City boundaries, 1:24,000, various years (1961-86);

Public land survey, 1:100,000, 1985; Lakes, 1:100,000, 1995;

and rivers, 1:100,000, 1985. North American Datum of 1983 (NAD 83).

Figure 7. Locations of weather stations, auxiliary vertices, and expanded triangular network used for interpolation of 

recharge from precipitation on permeable surfaces, Spokane Valley-Rathdrum Prairie aquifer, Washington and Idaho.

14  Ground-Water Flow Model for the SVRP Aquifer, Washington and Idaho
Figure 8. Depth to water in well 251 and monthly rate of deep percolation at the base of the root zone at the well site, 

Spokane Valley-Rathdrum Prairie aquifer, Washington and Idaho.

wa19-0088_fig08

DEPTH TO WATER IN WELL 251,

IN FEET BELOW LAND SURFACE

1990 1995 2000 2005

425

415

405

395

385

DEEP PERCOLATION AT BASE OF ROOT ZONE,

IN INCHES PER MONTH

0

2

4

6

8

Well 251

Deep

percolation

Table 2. Estimated transmission time for precipitation infiltration 

to travel from base of root zone to water table, Spokane ValleyRathdrum Prairie aquifer, Washington and Idaho.

Depth from land surface 

to water table 

(feet)

Transmission time 

(months)

0 - 40 0

40 - 120 1

120 - 200 2

200 - 280 3

280 - 360 4

360 - 440 5

The monthly volumetric rate of recharge (in cubic feet 

per second) from precipitation to the (1) Idaho side of the 

SVRP aquifer model, (2) Washington side of the model, and 

(3) entire model is shown in figure 9. The volumetric rate of 

recharge from precipitation is lower on the Washington side 

than on the Idaho side. Also, the recharge peaks earlier on the 

Washington side than on the Idaho side, because the water 

table generally is at a shallower depth on the Washington side 

than on the Idaho side.

The average volumetric rate of recharge and the average 

recharge flux from October 1990 through September 2005 is 

shown in table 3. For the entire model, the average volumetric 

rate of recharge from precipitation is 228 ft3

/s. The average 

volumetric rate of recharge on the Idaho side of the model is 

about twice that on the Washington side. The average recharge 

flux is calculated as the average volumetric rate divided by 

the surface area. Although precipitation generally is higher on 

the Idaho side of the model than on the Washington side, the 

recharge flux is about the same on both sides. This is because 

a higher percentage of precipitation enters the subsurface 

through recharge wells and infiltration basins on the 

Washington side than on the Idaho side. Infiltration through 

recharge wells and infiltration basins generally is subject to 

less evapotranspiration loss than infiltration through permeable 

surfaces.

Table 3. Average volumetric rate of recharge and average 

recharge flux from precipitation, Spokane Valley-Rathdrum Prairie 

aquifer, Washington and Idaho, October 1990 through September 

2005.

[Volumetric rate of recharge: PS, recharge from precipitation on 

permeable surface; IS, Recharge from precipitation runoff from impermeable 

surface to recharge well, infiltration basin, or adjacent permeable surface; 

Abbreviations: ft3/s, cubic foot per second; mi2, square mile; in/yr, inch per 

year; ft/d, foot per day]

Region

Volumetric rate of recharge 

(ft3

/s)

Surface 

area 

(mi2

)

Recharge flux 

PS IS Total (in/yr) (ft/d)

Idaho side 133 22 155 211 10.0 0.0023

Washington side 43 30 73 115 8.6 .0020

Entire model 176 52 228 326 9.5 .0022

Hydrology of Aquifer 15
The regression equations in StreamStats are developed 

by relating the mean annual discharge values for long-term 

gaging stations to various physical and climatic characteristics 

(basin characteristics) of the upstream drainage basin. For 

a gaged site, the mean annual discharge is the average of 

all annual discharges in the data record or during a specific 

period of years. For an ungaged site, the estimated mean 

annual discharge is a long-term average for a time period 

that is comparable to the length of record used to develop the 

regression equations. Typically, the data record spans tens of 

years.

To apply StreamStats, 72 tributary basin outlet points 

are selected along the model boundary (fig. 10). For each 

outlet point, the upstream drainage basin is delineated. The 

combination of all delineated basins encompasses most of 

the surrounding uplands that drain into the SVRP aquifer. 

Small gaps between adjacent basins are not included. For each 

delineated basin, StreamStats is used to estimate the mean 

annual discharge at the outlet point. Because bedrock occurs 

either at the basin surface or under a thin layer of soil, the 

assumption is that minimal subsurface discharge occurs at 

the outlet. Therefore, stream discharge accounts for nearly all 

discharge from the basin. However, as the stream crosses from 

the tributary basin onto the aquifer, all stream water quickly 

soaks into the ground because of the highly permeable nature 

of the aquifer material.

The 72 tributary basins range in area from 0.3 to 24 mi2

. 

The estimated mean annual discharge ranges from 0.0037 to 

15 ft3

/s, and the sum of the mean annual discharges of all 72 

Figure 9. Volumetric rate of recharge from precipitation, Spokane Valley-Rathdrum Prairie aquifer, Washington and Idaho.

wa19-0081_fig09

0

200

400

600

800

VOLUMETRIC RATE OF RECHARGE FROM PRECIPITATION,

1990 1995 2000 2005

IN CUBIC FEET PER SECOND

Entire SVRP aquifer model

Idaho side

Washington

side

Tributary Basins and Adjacent Uplands

The SVRP aquifer receives flow from higher altitude 

regions immediately adjacent to the aquifer. These regions are 

referred to as tributary basins and adjacent uplands or simply 

as tributary basins. A tributary basin might drain directly 

into the aquifer or drain into a lake that, in turn, recharges 

the aquifer. Tributary basins that drain directly to the aquifer 

are shown in figure 10. Recharge from these basins to the 

aquifer is estimated in this section of the report. Tributary 

basins that drain into seven of the nine lakes that border the 

aquifer are shown in figure 10—Lake Pend Oreille and Coeur 

d’Alene Lake are not included. Recharge from the lakes to the 

aquifer is estimated in section, “Lakebed Seepage and Surface 

Overflows.”

Flow from tributary basins to the SVRP aquifer is 

estimated using regional regression equations developed by 

Hortness and Berenbrock (2001). These regression equations 

are developed for Idaho and parts of adjacent States to 

estimate the mean annual discharge at ungaged sites on 

streams that are unaffected by regulations and (or) diversions. 

The methodology uses the USGS StreamStats web application 

(Ries and others, 2004) and the ArcGIS-ArcHydro application 

developed by Environmental Systems Research Institute, Inc. 

Both tools make use of the same techniques and underlying 

data sets. In the following discussion, this methodology is 

referred to as StreamStats. The estimation of tributary basin 

discharge to the aquifer was performed by Jon Hortness and is 

included in the report by Kahle and Bartolino (2007).

16  Ground-Water Flow Model for the SVRP Aquifer, Washington and Idaho
tributary basins is 112 ft3

/s. StreamStats also provides error 

statistics on the estimated discharge values. Typically, the 

67-percent confidence interval ranges from 0.4 to 1.6 times the 

estimated value. An assessment of the StreamStats estimates 

is included in the section, “Lakebed Seepage and Surface 

Overflow.”

The mean annual discharge estimated by StreamStats is 

the long-term average flow from a tributary basin to the SVRP 

aquifer. For a particular month, the flow from the tributary 

basin is higher or lower than the long-term average value. To 

Figure 10. Tributary basins that drain to the Spokane Valley-Rathdrum Prairie aquifer and to seven lakes that border the aquifer, 

Washington and Idaho.

wa19-0081_fig10

0 5 10 15 20 KILOMETERS

0 5 10 15 20 MILES

Little

Spokane R.

Spokane River

Deep

Creek

Coeur d’Alene Lake

Lake

Pend

Oreille

Long

Lake

Hangman

Creek

O O'

117°30'

48°

47°

45'

117° 116°30'

O'

O

Spokane R.

EXPLANATION

 Tributary basin that drains to Spokane Valley-Rathdrum Prairie aquifer 

 Tributary basin that drains to lake

 Extent of Spokane Valley-Rathdrum Prairie aquifer 

 ground-water flow model

 Outlet point of tributary basin that drains to Spokane 

 Valley-Rathdrum Prairie aquifer

 Line of basin-yield outflow calculation by Pluhowski 

 and Thomas (1968)

Base modified from U.S. Geological Survey digital data.

City boundaries, 1:24,000, various years (1961-86);

Public land survey, 1:100,000, 1985; Lakes, 1:100,000, 1995;

and rivers, 1:100,000, 1985. North American Datum of 1983 (NAD 83).

allow for temporal (monthly) variability in flow, a scaling 

index is used to convert the long-term average flow into the 

flow for a particular month. In this study, monthly flow on 

the Little Spokane River at the gaging station at Dartford, 

Washington, is assumed to be an appropriate scaling index. 

Because the tributary basins that drain to the Little Spokane 

River are in close proximity to the tributary basins that drain 

to the aquifer, these two sets of basins likely share similar 

physical and climatic characteristics.

Hydrology of Aquifer 17
The monthly flow from a tributary basin to the SVRP 

aquifer is estimated as

Q Q

L

L

Q

m a

m

a

m

= × ,

where 

is the flow from the tributary basin for a 

given month, 

Q is the long-term average flow (mean an a nual

discharge) estimated by StreamStats for 

the tributary basin, 

is the corresponding flow on the Little 

Spokane

Lm

 River at the gaging station 

at Dartford for the given month, and 

is the long-term average flow on the Little 

Sp

La

okane River at the gaging station at

Dartford. 

(3)

The scaling index (Lm / La) for each month from October 

1990 to September 2005 is shown in figure 11. The long-term 

average flow at the gaging station at Dartford is computed 

using discharge data for 1960–2000.

Lakebed Seepage and Surface Overflow

The SVRP aquifer is recharged by lakes that border the 

aquifer. Of the nine lakes along the perimeter of the model 

area, only the two largest lakes (Lake Pend Oreille and Coeur 

d’Alene Lake) have perennial outlet streams. For the seven 

smaller lakes, outflow occurs as subsurface seepage through 

the lakebed and occasional surface overflow when the lake 

level rises above the outlet structure. Because of the highly 

permeable nature of the aquifer material, the surface overflow 

soaks into the ground within a short distance of the lake. 

Therefore, the combined outflow from subsurface lakebed 

seepage and surface overflow is the amount of recharge from 

the seven smaller lakes to the aquifer.

In principle, the outflow from a lake can be estimated by 

the following water-balance equation:

OL

 = I

L

 + PL – EL – SL,	 (4)

where

OL is rate of outflow from the lake,

I

L is the rate of inflow to the lake from the surrounding 

tributary basins,

PL is the rate of direct precipitation on the lake surface,

EL is the rate of evaporation from the lake, and

SL is the rate of change in storage in the lake.

Figure 11. Scaling index used to compute monthly flow from tributary basins to the Spokane Valley-Rathdrum Prairie 

aquifer, Washington and Idaho.

0

2

4

6

1990 1995 2000 2005

SCALING INDEX

18 Ground-Water Flow Model for the SVRP Aquifer, Washington and Idaho
Murray (2007) evaluated the terms on the right-hand side of 

equation 4 for Hayden Lake on a monthly basis from 1998 

through 2005. She noted that PL – EL – SL typically is a small 

percentage of I

L

. Therefore, monthly outflow from the lake 

can be approximated reasonably by the monthly inflow from 

surrounding tributary basins. This approximation is assumed 

to be applicable to the seven smaller lakes along the perimeter 

of the model area (that is, excluding Lake Pend Oreille and 

Coeur d’Alene Lake).

Murray (2007) estimated inflows to the seven lakes using 

the same procedure as that used to estimate flow from tributary 

basins to the SVRP aquifer. For each lake, outlet points are 

placed along the lake perimeter. For each outlet point, the 

upstream tributary basin is delineated (fig. 10) and StreamStats 

is used to estimate the mean annual discharge. The sum of the 

mean annual discharges for the surrounding tributary basins 

is the long-term average inflow to the lake (table 4). Finally, 

the monthly inflow to the lake is estimated by multiplying the 

long-term average inflow by the scaling index (Lm / La

) in the 

same manner as that used to estimate monthly flow from a 

tributary basin to the aquifer (eq. 3). Assuming monthly inflow 

to the lake equals monthly outflow from the lake, the longterm average inflow to the lake times the scaling index is the 

monthly flow from the lake to the aquifer.

An assessment of the StreamStats methodology can be 

made by comparing the StreamStats estimates with basin 

water yields calculated by previous investigators. Pluhowski 

and Thomas (1968) estimated the water yield of the Rathdrum 

Prairie Basin, defined as all areas upstream of a northnorthwest to south-southeast line drawn at the gaging station 

on the Spokane River near Otis Orchards (see line O-O’ in 

fig. 10). For this basin, they estimated a water yield of 530 

ft3/s. When expressed in terms of the flow components used 

in this study, this basin water yield represents the sum of (1) 

recharge from precipitation to that part of the SVRP aquifer 

upstream of line O-O’, (2) inflows from all tributary basins to 

that part of the aquifer upstream of line O-O’, and (3) inflows 

to the aquifer from Fernan, Hauser, Hayden, Newman, Spirit, 

and Twin Lakes. The second and third items are estimated 

by StreamStats. For the period of study (1990–2005), the 

average values of the three items are 165, 86, and 294 ft3

/s, 

respectively, which sum to 545 ft3/s. Although this sum is 

somewhat higher than Pluhowski and Thomas’s (1968) basin 

water yield of 530 ft3/s, especially because Pluhowski and 

Thomas’s Rathdrum Prairie Basin includes Spirit Valley, 

which is excluded from the model extent in this study, the 

comparison does indicate that the StreamStats estimates used 

in this study are consistent with basin water yields calculated 

by previous investigators.

For Lake Pend Oreille and Coeur d’Alene Lake, a large 

part of the lake inflow is discharged to the outlet stream. By 

comparison, lakebed seepage is a relatively small quantity. 

Therefore, estimating lakebed seepage by a water-balance 

calculation for the lake might lead to results that are highly 

uncertain. Nonetheless, previous investigators have made 

water-balance calculations for Coeur d’Alene Lake. Seepage 

from Coeur d’Alene Lake often is estimated in combination 

with seepage from the segment of the Spokane River upstream 

of the gaging station near Post Falls. In a study of groundwater inflow to the Rathdrum Prairie, Anderson (1951, p. 

20-21) stated that

“Some ground water is believed to be derived 

from Coeur d’Alene Lake (and the Spokane River 

between the lake and Post Falls). Approximately 

three-fourths of the inflow to the lake is gaged and 

a comparison of estimated total inflow with total 

surface outflow plus evaporation indicates a seepage 

loss of about 300 second-feet to ground water.”

However, given that the mean annual flow measured at the 

gaging station near Post Falls for 1913–2001 is about 6,200 

ft3

/s (Kahle and others, 2005, p. 41), the calculated seepage 

loss of 300 ft3

/s would be within the error in the discharge 

measurements and is therefore highly uncertain.

Sagstad (1977) applied Darcy’s Law to estimate recharge 

to the SVRP aquifer from Coeur d’Alene Lake and from the 

segment of the Spokane River upstream of the gaging station 

near Post Falls. The Darcy’s Law calculation was performed 

for three sections, with flow across section C-C’ (Sagstad 

1977, fig. 13 and table 7) approximately representing seepage 

from Coeur d’Alene Lake. Using a hydraulic conductivity 

of 535 ft/d, a saturated thickness of 150 ft, and a watertable gradient of 0.00303, the flow across section C-C’ was 

calculated to be 37 ft3/s.

Table 4. Drainage area and long-term average inflow to seven 

lakes that border the Spokane Valley-Rathdrum Prairie aquifer, 

Washington and Idaho.

[Abbreviations: mi2, square mile; ft3/s, cubic foot per second]

Lake Drainage area 

(mi2

)

Long-term 

average inflow 

(ft3

/s)

Fernan Lake 18.8 12.9

Hauser Lake 18.3 17.4

Hayden Lake 51.6 62.0

Liberty Lake 11.3 4.9

Newman Lake 24.8 20.2

Spirit Lake 32.8 48.4

Twin Lakes 26.9 35.1

Hydrology of Aquifer 19
For Lake Pend Oreille, Pluhowski and Thomas (1968) 

estimated lakebed seepage by a water-balance calculation 

for the east (mostly Idaho) part of the SVRP aquifer. Their 

seepage estimate for Lake Pend Oreille is 50 ft3/s. However, 

they noted that because of uncertainties in various flow 

components, “the actual contribution to the aquifer from Pend 

Oreille Lake may be as much as 200 cfs.”

Losing Segments of Spokane River

Losing segments of the Spokane River occur where the 

stream level is higher than the hydraulic head in the aquifer 

directly under the streambed. Along a losing segment, water 

seeps from the stream and recharges the aquifer. Consequently, 

there is less streamflow at the downstream end of a losing 

segment than at the upstream end of the segment. The amount 

of flow from the Spokane River to the SVRP aquifer is 

discussed in the section, “Interaction between Aquifer and 

Spokane River.”

Return Percolation from Irrigation

Return percolation from irrigation refers to that 

part of applied irrigation water that is not consumed by 

evapotranspiration but instead percolates downward past the 

root zone and eventually reaches the water table. Irrigation 

includes landscape irrigation (such as lawn watering), 

agricultural irrigation, and golf course irrigation. For the 

period of study (1990–2005), nearly all irrigation water 

applied within the model area is derived from ground-water 

pumpage. Therefore, return percolation from irrigation 

actually is water that came from the aquifer.

To estimate water use for landscape irrigation, monthly 

ground-water withdrawals by water purveyors and by domestic 

users outside water purveyor service areas is divided into an 

indoor-use component and an outdoor-use component. For 

each year, during January, February, March, November, and 

December, the outdoor-use component is assumed to be zero. 

Therefore, ground-water withdrawal during those 5 months is 

entirely for indoor use. For April through October, the indooruse component is assumed to be the average withdrawal 

during the aforementioned 5 months with no outdoor use. The 

outdoor-use component is any withdrawal in excess of the 

indoor-use component. The entire outdoor-use component is 

assumed to be for landscape irrigation. Based on studies by 

Oad and others (1997) and Dukes and others (2005), landscape 

irrigation efficiency is estimated to be 60 percent. Therefore, 

40 percent of the outdoor-use component percolates back to 

the aquifer.

In a water purveyor service area, ground water is pumped 

from supply wells and distributed to users in the service area. 

Water purveyor service areas are delineated for four periods: 

1990–95, 1996–99, 2000–02, and 2003–05. Within each 

period, service areas are assumed to remain unchanged. Water 

purveyor service areas during 2000–02 are shown in figure 12. 

For each service area, return percolation is computed from 

pumpage records for supply wells in the entire service area 

(see section, “Withdrawals from Wells”) and then distributed 

uniformly over the service area. For the City of Spokane, 

however, the service area southwest of the city (fig. 12) is 

excluded from the return percolation calculation as that area 

is relatively undeveloped and landscape irrigation is expected 

to be minimal. Outside water purveyor service areas, groundwater withdrawals are estimated on a cell-by-cell basis on a 

raster grid (see section, “Withdrawals from Wells”). Return 

percolation is applied to the same cell from which ground 

water is withdrawn.

For agricultural irrigation outside water purveyor 

service areas and for self-supplied golf courses, groundwater withdrawals are estimated from crop acreage, irrigation 

demand, and an assumed irrigation efficiency of 60 percent. 

Therefore, 40 percent of the pumped water is assumed to 

percolate back to the aquifer.

The monthly rate of return percolation from all types 

of irrigation for the entire model area is shown in figure 13. 

From October 1990 through September 2005, the average rate 

of return percolation from irrigation is 54 ft3

/s for the entire 

model area.

Effluent from Septic Systems

For a water user who discharges to a septic system, 95 

percent of the indoor use is assumed to become effluent from 

septic systems that percolates back to the aquifer. For a water 

user who discharges to a sewer system, the assumption is that 

none of the indoor use returns to the aquifer. To determine 

the areal distribution of effluent from septic systems, a raster 

of sewer hookup density is constructed for each year from 

1990 to 2005 using spatial data of city limits, sewer district 

boundaries, and density of sewer hook-ups within each sewer 

district. The sewer hookup density raster for 2000 is shown in 

figure 14. For each cell in the raster, the sewer hookup density 

is the fraction of homes in the cell that are connected to a 

public sewer system. If the sewer hookup density is zero, the 

cell is not in a sewer district and all homes in the cell discharge 

to septic systems. In this case, 95 percent of the indoor water 

use within the cell is returned to the aquifer. If the sewer 

hookup density is 1, then the cell is within a sewer district 

20 Ground-Water Flow Model for the SVRP Aquifer, Washington and Idaho
and all homes in the cell discharge to a sewer system. In this 

case, none of the indoor water use within the cell is returned 

to the aquifer. If the sewer hookup density is greater than zero 

but less than 1, then either the cell is partially within a sewer 

district or the cell is entirely within a sewer district but not all 

homes in the cell discharge to the sewer system. In this case, 

effluent from the septic system is 0.95 (1 – ds) times the indoor 

water use in the cell, where ds is the sewer hookup density.

The monthly rate of effluent from septic systems for 

the entire model area is shown in figure 13. The gradually 

declining rate is caused by expansion of sewer districts. From 

October 1990 through September 2005, the average rate of 

effluent from septic systems is 23 ft3/s for the entire model 

area.

Figure 12. Water purveyor service areas and areal distribution of water purveyor wells, Spokane Valley-Rathdrum Prairie aquifer, 

2000–02.

wa19-0088_fig15

0 5 10 15 20 KILOMETERS

0 5 10 15 20 MILES

Little Spokane R.

Spokane River

Deep

Creek

Coeur d’Alene Lake

Lake

Pend

Oreille

Liberty

Lake

Newman

Lake

Long

Lake

Hauser

Lake

Fernan Lake

Hayden Lake

Spirit

Lake

Twin

Lakes

Hangman

Creek

EXPLANATION

Water purveyor service areas, 2000–02

Water purveyor service area excluded from

return percolation calculation

Extent of Spokane Valley-Rathdrum Prairie

aquifer ground-water flow model

Water purveyor wells

48°

47°

45'

117°30' 117° 116°30'

Spokane R.

Base modified from U.S. Geological Survey digital data.

City boundaries, 1:24,000, various years (1961-86);

Public land survey, 1:100,000, 1985; Lakes, 1:100,000, 1995;

and rivers, 1:100,000, 1985. North American Datum of 1983 (NAD 83).

Hydrology of Aquifer  21
wa19-0088_fig13

0

100

200

1990 1995 2000 2005

VOLUMETRIC FLOW RATE,

IN CUBIC FEET PER SECOND

Return

percolation

from irrigation

Effluent

from

septic

systems

250

150

50

Figure 13. Volumetric rate of return percolation from irrigation and effluent from septic systems, Spokane ValleyRathdrum Prairie aquifer, Washington and Idaho.

Figure 14. Sewer hookup density for 2000, Spokane Valley-Rathdrum Prairie aquifer, Washington and Idaho.

0 5 10 15 20 KILOMETERS

0 5 10 15 20 MILES

Little Spokane R.

Spokane River

Deep

Creek

Coeur d’Alene Lake

Lake

Pend

Oreille

Liberty

Lake

Newman

Lake

Long

Lake

Hauser

Lake

Fernan Lake

Hayden Lake

Spirit

Lake

Twin

Lakes

Hangman

Creek

 EXPLANATION

Sewer hookup density for 2000

 1.0

 0.75

 0.25

 0.1

 0

117°30'

48°

47°

45'

117° 116°30'

Spokane R.

Base modified from U.S. Geological Survey digital data.

City boundaries, 1:24,000, various years (1961-86);

Public land survey, 1:100,000, 1985; Lakes, 1:100,000, 1995;

and rivers, 1:100,000, 1985. North American Datum of 1983 (NAD 83).

22  Ground-Water Flow Model for the SVRP Aquifer, Washington and Idaho
Outflows from Aquifer

Outflows from the SVRP aquifer include (1) groundwater withdrawals from wells, (2) ground-water discharge 

from the aquifer to gaining segments of the Spokane River, (3) 

ground-water discharge from the aquifer to the Little Spokane 

River, and (4) subsurface outflow at the western limit of the 

model area near Long Lake. For the ground-water flow model 

in this report, monthly outflows were estimated for October 

1990 through September 2005. Areally distributed outflow 

components were computed on a raster grid with a cell size of 

1,320 by 1,320 ft that was aligned with the finite-difference 

grid used in the model.

Withdrawals from Wells

Withdrawals of ground water from the SVRP aquifer 

were estimated for four categories: (1) withdrawals by water 

purveyors, (2) withdrawals by domestic users outside water 

purveyor service areas, (3) withdrawals for agricultural 

irrigation outside water purveyor service areas and by selfsupplied golf courses, and (4) withdrawals by self-supplied 

industries. The combined monthly withdrawal rate for all four 

categories is shown in figure 15. From October 1990 through 

September 2005, the average combined withdrawal rate is 

317 ft3/s. Individual withdrawal rates for each category are 

discussed in the following paragraphs.

Data on withdrawals by water purveyors were obtained 

from 21 water purveyors in the model area for 1990–2005. 

This work was performed in conjunction with ongoing USGS 

water-use data collection (Molly Maupin, U.S. Geological 

Survey, written commun., 2006). The areal distribution of 

159 water purveyor wells is shown in figure 12. Monthly 

withdrawals were obtained for 125 of the 159 wells, and 

annual withdrawals were obtained for the other 34 wells. For 

wells with annual withdrawal data, monthly withdrawals are 

estimated by distributing the annual withdrawal to each month 

on the basis of the monthly pumping pattern of other wells 

operated by the same water purveyor or by another water 

purveyor serving a similar community. The estimated monthly 

withdrawal rate by all water purveyors is shown in figure 15. 

From October 1990 through September 2005, the average 

withdrawal rate for this category is 205 ft3

/s.

Figure 15. Withdrawal rates from wells, Spokane Valley-Rathdrum Prairie aquifer, Washington and Idaho.

0

200

400

600

800

1990 1995 2000 2005

WITHDRAWAL RATE, IN CUBIC FEET PER SECOND

Withdrawals by water purveyors

Withdrawals by domestic users outside purveyor service areas

Withdrawals for agricultural irrigation outside purveyor service areas and by self-supplied golf courses

Withdrawals by self-supplied industries

Total withdrawal

EXPLANATION

Hydrology of Aquifer  23
Data on withdrawals by domestic users outside 

water purveyor service areas were not available. Monthly 

withdrawals were assumed to be similar to the average 

monthly withdrawals for a water purveyor-supplied home in 

Spokane. The estimated monthly withdrawal rate for a home 

outside water purveyor service areas is shown in figure 16. A 

study by the City of Spokane estimated the rate of indoor use 

for a home is 25.4 ft3/d (L. Brewer, oral commun., 2006). This 

indoor use rate is assumed to apply year round for a home 

outside water purveyor service areas. From April through 

October, an outdoor-use rate (for landscape irrigation) is added 

to the indoor-use rate. The outdoor-use rate is based on the 

outdoor-use pattern for Spokane.

To estimate the areal distribution of withdrawals by 

domestic users outside water purveyor service areas, the 

density of homes outside those areas was estimated on a 

raster grid. The number of homes in each cell was estimated 

from land-cover data and aerial photographs. The monthly 

withdrawal rate in each cell was computed as the number of 

homes in the cell times the monthly withdrawal rate of a home 

(fig. 16). The estimated monthly withdrawal rate for all homes 

outside water purveyor service areas is shown in figure 15. 

From October 1990 through September 2005, the average 

withdrawal rate for this category is 28 ft3/s.

Withdrawals for agricultural irrigation outside water 

purveyor service areas and by self-supplied golf courses were 

estimated from irrigation acreages and irrigation demand. 

Nearly all withdrawals in this category were on the Idaho 

side of the SVRP aquifer. Irrigation acreages were estimated 

from Idaho water rights data and by inspection of aerial 

photographs. Irrigation acreages are shown on a raster grid 

of irrigation density in figure 17. For each cell in the raster, 

the irrigation density is the percentage of the cell’s area that 

is irrigated. If the irrigation density is 1, all of the cell’s area 

is irrigated. Conversely, if the irrigation is zero, no irrigation 

occurs in the cell.

To estimate irrigation demand, a single crop mix is 

calculated from data published by the National Agricultural 

Statistics Service (2003, 2004, 2005) and information obtained 

from the Jacklin Seed Company (G. Jacklin, oral commun., 

2006) on grass-seed acreage. Average evapotranspiration 

rates for each crop are obtained from Allen and Brockway 

(1983) for Coeur d’Alene. The evapotranspiration rate for 

grass seed is based on the evapotranspiration rate for pasture 

but is adjusted to reflect a shorter irrigation season. For 

golf courses, the evapotranspiration rate for alfalfa is used. 

Monthly precipitation was obtained from PRISM-derived data 

downloaded from Oregon State University (http://www.ocs.

orst.edu/prism/products/). Assuming that 75 percent of the 

monthly precipitation was effective in meeting crop needs, the 

monthly irrigation demand for a cell is calculated as:

R = A × dr

 × max(ET - 0.75P, 0), (5)

where

R is the monthly irrigation demand,

A is the area of the cell,

dr is the irrigation density of the cell,

ET is the monthly evapotranspiration, and

P is the monthly precipitation.

Assuming an irrigation efficiency of 60 percent, the monthly 

withdrawal for irrigation at a cell is calculated as R divided by 

0.6. Therefore, 40 percent of the irrigation water percolates 

back to the SVRP aquifer. The estimated monthly withdrawal 

rate for agricultural irrigation outside water purveyor service 

areas and for self-supplied golf courses is shown in figure 15. 

From October 1990 through September 2005, the average 

withdrawal rate for this category is 51 ft3/s.

For withdrawals by self-supplied industries, only users 

that withdraw more than 500 acre-ft/yr (0.7 ft3

/s) were 

explicitly included in the model. On the Washington side of 

the SVRP aquifer, seven wells that meet this criterion were 

identified from a review of withdrawal data reported by CH2M 

Hill (1998) and Golder Associates, Inc. (2004). On the Idaho 

side, two wells were identified. Withdrawals for both wells 

were estimated from the Idaho water rights database (http://

www.idwr.idaho.gov/gisdata/new%20data%20download/

water_rights.htm). Typically, the actual withdrawal is 

somewhat less than the full water right. Therefore the actual 

withdrawal was assumed to be five-sevenths of the full water 

right. The estimated annual withdrawals from these nine 

wells and the withdrawal rates, assuming constant year-round 

pumping, are given in table 5. The total estimated withdrawal 

rate is about 34 ft3

/s. This rate is assumed to remain constant 

from October 1990 to September 2005 (fig. 15).

Figure 16. Estimated monthly withdrawal rate for a 

home outside water purveyor service areas, Spokane 

Valley-Rathdrum Prairie aquifer, Washington and Idaho.

0

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

20

10

40

30

60

Indoor use

 Outdoor use

WITHDRAWAL RATE,

IN CUBIC FEET PER DAY

70

50

24  Ground-Water Flow Model for the SVRP Aquifer, Washington and Idaho
Figure 17. Irrigation density for acreages outside water purveyor service areas and for self-supplied golf courses, Spokane ValleyRathdrum Prairie aquifer, Washington and Idaho.

wa19-0088_fig17

0 5 10 15 20 KILOMETERS

0 5 10 15 20 MILES

Little Spokane River

Spokane River

Deep

Creek

Coeur d’Alene Lake

Lake

Pend

Oreille

Hangman

Creek

117°30'

48°

47°

45'

117° 116°30'

Liberty

Lake

Newman

Lake

Hauser

Lake

Fernan Lake

Hayden Lake

Spirit

Lake

Twin

Lakes

Long

Lake

Spokane R.

EXPLANATION

Irrigation density

 0.8 to1.0

 0.6 to 0.8

 0.4 to 0.6

 0.2 to 0.4

 0.01 to 0.2

0

Base modified from U.S. Geological Survey digital data.

City boundaries, 1:24,000, various years (1961-86);

Public land survey, 1:100,000, 1985; Lakes, 1:100,000, 1995;

and rivers, 1:100,000, 1985. North American Datum of 1983 (NAD 83).

Table 5. Estimated annual withdrawals by self-supplied industries and withdrawal rates, Spokane ValleyRathdrum Prairie aquifer, Washington and Idaho.

[Abbreviations: acre-ft, acre-foot; ft3/s, cubic foot per second]

Well identification State Annual withdrawal 

(acre-ft)

Withdrawal rate 

(ft3

/s)

Diamond National Corporation Idaho 517 0.71

Riley Creek Lumber Company Idaho 627 .87

Kaiser Mead N Plant Washington 2,812 3.88

Kaiser Mead S Plant Washington 565 .78

Kaiser Trentwood Section 2 Washington 4,098 5.66

Kaiser Trentwood Section 10 Washington 10,538 14.55

Kaiser Trentwood Section 11 Washington 398 .55

Honeywell-Johnson Matthey Electric Washington 517 .71

Hydrology of Aquifer  25
Gaining Segments of Spokane River

Gaining segments of the Spokane River occur where the 

stream level is lower than the hydraulic head in the aquifer 

directly under the streambed. Along a gaining segment, 

ground water discharges from the aquifer and flows to 

the stream. Consequently, there is more streamflow at the 

downstream end of a gaining segment than at the upstream 

end of the segment. The amount of discharge from the SVRP 

aquifer to the Spokane River is discussed in the section, 

“Interaction Between Aquifer and Spokane River.”

Little Spokane River

The SVRP aquifer discharges to the Little Spokane 

River along the valley north of Five Mile Prairie. This part 

of the aquifer is known as the Little Spokane River Arm. 

Ground-water discharge to the river can be estimated from 

the streamflow gain between two gaging stations on the Little 

Spokane River (fig. 18). In the following discussion, QAD

refers to streamflow at the gaging station at Dartford, which is 

about 1 mi upstream of where the Little Spokane River enters 

the aquifer, and QND refers to streamflow at the gaging station 

near Dartford, which is about 3 mi upstream of where the 

Little Spokane River flows into the Spokane River (fig. 18). 

Figure 18. Locations of gaging stations and streamflow measurement sites on the Spokane and Little Spokane Rivers, Spokane ValleyRathdrum Prairie aquifer, Washington and Idaho.

0 5 10 15 20 KILOMETERS

0 5 10 15 20 MILES

Little Spokane River

Spokane

River

Deep Cr.

Coeur d’Alene Lake

Lake

Pend

Oreille

Hangman

Cr.

117°30'

48°

47°

45'

117° 116°30'

Liberty

Lake

Newman

Lake

Hauser

Lake

Fernan Lake

Hayden Lake

Spirit

Lake

Twin

Lakes

Long

Lake

Spokane R.

EXPLANATION

Extent of Spokane Valley-Rathdrum

 Prairie aquifer ground-water flow model

Streamflow measurement site

Gaging station

Base modified from U.S. Geological Survey digital data.

City boundaries, 1:24,000, various years (1961-86);

Public land survey, 1:100,000, 1985; Lakes, 1:100,000, 1995;

and rivers, 1:100,000, 1985. North American Datum of 1983 (NAD 83).

Spokane R. below

Nine Mile Dam

Spokane R. at

Centennial Trail Bridge

Spokane R.

Sullivan Rd

Little Spokane R.

near mouth

Little Spokane R.

at Waikiki Rd.

Little Spokane R.

near Dartford

Little Spokane R.

at Dartford

Spokane R. near

Otis Orchards

Spokane R.

at Greenacres

Spokane R.

at Flora Rd.

Spokane R.

at Spokane Spokane R. below

Greene St. Bridge

Spokane R. below

Upriver Dam

Spokane R. below

T.J. Meenach Bridge

Spokane 

wastewatertreatment 

plant

Hangman Cr.

at Spokane

Spokane R.

near Post Falls

Spokane R. near

Coeur d’Alene Lake Deep Cr.near

confluence

Spokane R. at

Rifle Club Rd.

26  Ground-Water Flow Model for the SVRP Aquifer, Washington and Idaho
The streamflow gain between the two gaging stations is QND

– QAD. The assumption is that this gain represents groundwater discharge from the aquifer.

Daily values of QND – QAD and QAD are shown in 

figure 19. The data record is from October 1997 through 

September 2005. The average streamflow gain for the entire 

data set is 248 ft3/s. There does not appear to be a strong 

trend of increasing or decreasing streamflow gain with QAD. 

However, when QAD is greater than 500 ft3/s, the scatter in 

the plotted points becomes much larger. The scatter likely is 

caused by inherent streamflow measurement error, estimated 

as 5 percent of the measured streamflow value (Sauer 

and Meyer, 1992). At higher streamflows, the computed 

streamflow gain can fall within the streamflow measurement 

error and, thus, become highly uncertain.

Streamflow measurements at a site near the mouth of 

the Little Spokane River during September 2004 and August 

2005, indicate a streamflow loss of 16 to 31 ft3

/s from the 

gaging station near Dartford to the streamflow measurement 

site near the mouth of the river. These results are unexpected 

in a ground-water discharge area of the SVRP aquifer and 

could be caused by inaccuracies in streamflow measurements. 

Because the streamflow loss is small, the interaction between 

the aquifer and the Little Spokane River downstream of the 

gaging station near Dartford might be justifiably characterized 

as minimal.

Subsurface Outflow

At the west end of the Little Spokane River Arm, 

the model terminates at the eastern end of Long Lake (the 

reservoir on the Spokane River behind Long Lake Dam). In 

this area, the aquifer is divided into an upper unit and a lower 

unit by a clay layer. The upper unit is in direct hydraulic 

contact with Long Lake. Ground-water discharge from the 

upper unit to Long Lake probably is minimal because most 

of the outflow likely has entered the Little Spokane River. 

Hydrologic conditions in the lower unit are not well known. 

Drillers’ reports indicate that the lower unit remains confined 

under Long Lake for at least 1 or 2 mi beyond the model 

boundary, but the clay layer eventually pinches out, thus 

allowing outflow from the lower unit to Long Lake (John 

Covert, Washington State Department of Ecology, written 

commun., 2005). This interpretation is supported by an 

apparent correlation between the water level in well 99, which 

is completed in the lower unit, and the lake level of Long Lake 

(fig. 20). Declines in the water level in the well during January 

and March 2005 coincide with declines in the lake level. 

However, the rate of outflow from the lower unit is not known.

Figure 19. Daily streamflow gain on the Little Spokane River between gaging stations 

at Dartford (QAD) and near Dartford (QND) and streamflow at the gaging station at 

Dartford, Spokane Valley-Rathdrum Prairie aquifer, Washington and Idaho.

0

100

200

300

400

0 400 800 1,200 1,600 2,000

QAD , IN CUBIC FEET PER SECOND

QND– QAD , IN CUBIC FEET PER SECOND

Hydrology of Aquifer  27
Interaction Between Aquifer and Spokane River

For nearly its entire length within the study area, the 

Spokane River interacts dynamically with the SVRP aquifer. 

Kahle and others (2005) summarized previous investigations 

of this river-aquifer interaction. MacInnis and others (2004, p. 

15) characterized segments of the Spokane River as gaining, 

losing, transitional (varying between gaining and losing 

depending on the magnitude of the river flow), or minimal. 

These characterizations were based on nearly simultaneous 

streamflow measurements (seepage run) made on the Spokane 

River during September 13-16, 2004 (Kahle and others, 

2005, p. 44, table 15), and estimated low-flow values based 

on historical data and computer modeling. Therefore, these 

characteristics describe the Spokane River during low-flow 

conditions, which occur in late summer.

During 2005 and 2006, additional streamflow 

measurements were made on the Spokane River to refine the 

understanding of the river-aquifer interaction (table 6, fig. 18). 

The USGS conducted seepage runs along the Spokane River 

during August 26-31, 2005, and on August 8, 2006. The 

seepage run during August 26-31, 2005, encompassed nearly 

the entire length of the Spokane River in the study area (from 

just downstream of Coeur d’Alene Lake to below Nine Mile 

Dam). The seepage run on August 8, 2006, focused on the 

river segment from the Centennial Trail Bridge to the site 

below Greene Street Bridge (fig. 18). In addition, Gregory and 

Covert (2005) measured stream-water temperature along the 

Spokane River to detect ground-water discharge to the river. 

They noted that

“In late summer/early autumn conditions, the 

Spokane River discharges to the SVRP aquifer 

upstream from Sullivan Road. Downstream from 

this point, the river receives aquifer water.”

The seepage run during August 26-31, 2005, indicates 

that the Spokane River lost 606 ft3/s from the most 

upstream measurement site near Coeur d’Alene Lake to the 

measurement site at Flora Road (table 6). From the Flora 

Road to the Centennial Trail Bridge site, data indicate a net 

gain of 360 ft3/s. However, Gregory and Covert (2005) noted 

that the gaining segment actually occurs downstream of the 

Sullivan Road site (fig. 18). From the Centennial Trail Bridge 

site to the site below Greene Street Bridge, data indicate a 

net gain in streamflow of 233 ft3/s. However, the seepage run 

on August 8, 2006, indicates that this segment of the river 

actually comprises of a losing segment from the Centennial 

Trail Bridge site to the site downstream of Upriver Dam and 

a gaining segment from the site downstream of Upriver Dam 

to the site downstream of Greene Street Bridge. The river lost 

112 ft3

/s from the site downstream of Greene Street Bridge 

to the gaging station at Spokane. From this gaging station 

to the site downstream of Nine Mile Dam, after subtracting 

inflows from Hangman Creek and effluent from the Spokane 

wastewater-treatment plant, the net gain from ground-water 

discharge to the river is about 267 ft3/s. Overall, from the 

most upstream site near Coeur d’Alene Lake to the most 

downstream site below Nine Mile Dam, the Spokane River 

gained a net amount of 142 ft3

/s from exchange with the 

aquifer based on the August 26-31, 2005 seepage run.

Figure 20. Water levels in well 99 and in Long Lake, Spokane Valley-Rathdrum 

Prairie aquifer, Washington and Idaho.

wa19-0088_fig20

1,538

1,542

1,546

1,550

1,554

1,558

J M M J S N J M M J S N J M M

2004 2005 2006

ALTITUDE OF WATER LEVEL IN 

WELL 99, IN FEET

ALTITUDE OF WATER LEVEL IN 

LONG LAKE, IN FEET

Long Lake

Well 99

1,524

1,528

Recently

pumped 1,532

1,536

1,540

1,544

28 Ground-Water Flow Model for the SVRP Aquifer, Washington and Idaho
For times other than late summer, the characteristics of 

streamflow gains and losses on the Spokane River are less 

well known. Daily streamflow data are available since 1999 

for four gaging stations on the Spokane River in the model 

area (fig. 18). Streamflow gains or losses between the various 

gaging stations and streamflow at the gaging station near Post 

Falls (QPF) are shown in figure 21. Subtracting the streamflow 

at an upstream gaging stream from the streamflow at a 

downstream gaging station gives the streamflow gain between 

the two stations if the result is positive and the streamflow loss 

if the result is negative. Although figure 21 shows significant 

scatter among the plotted points, a linear relation is fitted to 

that part of the data where QPF is less than or equal to 10,000 

ft3/s. The fitted line provides an overall description of how 

the streamflow gains or losses vary with QPF. However, when 

QPF is greater than 10,000 ft3/s, the scatter is so large that it is 

difficult to discern a trend.

The fitted lines in figures 21A and 21B indicate that the 

river segments from the gaging station at Post Falls to both 

the gaging stations near Otis Orchards and at Greenacres are 

net losing segments, and the magnitude of streamflow loss 

increases with increasing QPF. Conversely, the fitted lines in 

figures 21C and 21D indicate that the river segments from both 

gaging stations near Otis Orchards and at Greenacres to the 

gaging station at Spokane are net gaining segments, and the 

magnitude of the streamflow gain decreases with increasing

QPF. Lastly, the fitted line in figure 21E indicates that the river 

segment from gaging stations near Post Falls to at Spokane is a 

net gaining segment when QPF is less than about 7,000 ft3

/s but 

is a net losing segment when QPF is greater than about 7,000 

ft3/s. The river segment between the gaging stations near Otis 

Orchards and at Greenacres is not considered because the two 

gaging stations are close to each other and the computed gains 

or losses generally fall within the streamflow measurement 

errors except for very low streamflows.

Table 6. Streamflow measurements made on the Spokane River and some tributaries to determine streamflow 

gains (positive values) and losses (negative values) between measurement sites, Spokane Valley-Rathdrum Prairie 

aquifer, Washington and Idaho, August 26-31, 2005, and August 8, 2006.

[Location of sites or gaging station shown in figure 18. Abbreviations: ft3/s, cubic foot per second; --, not computed; —, not measured]

Measurement site or gaging station

August 26-31, 2005 August 8, 2006

Discharge 

(ft3

/s)

Gain or loss 

(ft3/s)

Discharge

(ft3/s)

Gain or loss 

(ft3/s)

Spokane River near Coeur d’Alene Lake 738 -- — --

Spokane River near Post Falls 447 -291 — --

Spokane River near Otis Orchards 256 -191 — --

Spokane River at Greenacres 146 -110 — --

Spokane River at Flora Road 132 -14 — --

Spokane River at Centennial Trail Bridge 492 360 579 --

Spokane River below Upriver Dam — -- 525 -54

Spokane River below Greene Street Bridge 725 233 869 344

Spokane River at Spokane 613 -112 767 -102

Hangman Creek at Spokane 1.5 -- — --

Spokane River below T.J. Meenach Bridge 703 88.5 — --

Spokane Wastewater Treatment Plant discharge1 56 -- — --

Spokane River at Rifle Club Road 797 38 — --

Deep Creek near confluence 0 -- — --

Spokane River below Nine Mile Dam 938 141 — --

1

Spokane Wastewater Treatment Plant discharge value provided by John Convert, Washington State Department of Ecology, written. 

commun., 2005.

Hydrology of Aquifer  29
wa19-0088_fig21AB

–400

–800

0

400

800

0 10,000 20,000 30,000

QPF , IN CUBIC FEET PER SECOND

QOO– QPF , IN CUBIC FEET PER SECOND

QOO– QPF = –0.04076 x QPF – 150.8

A. Daily streamflow gain (positive value) or loss (negative value) on the Spokane River 

 between gaging stations near Post Falls (QPF) and near Otis Orchards (QOO) and 

 streamflow at gaging station near Post Falls.

–400

–800

0

400

800

0 10,000 20,000 30,000

QPF , IN CUBIC FEET PER SECOND

QGA– QPF , IN CUBIC FEET PER SECOND

QGA– QPF = –0.01908 x QPF – 227.5

B. Daily streamflow gain (positive value) or loss (negative value) on the Spokane River 

 between gaging stations near Post Falls (QPF) and at Greenacres (QGA) and streamflow at 

 gaging station near Post Falls.

Figure 21. Streamflow gains or losses between the various gaging stations and 

streamflow at the gaging station near Post Falls, Spokane Valley-Rathdrum Prairie 

aquifer, Washington and Idaho.

30 Ground-Water Flow Model for the SVRP Aquifer, Washington and Idaho
Figure 21.—Continued.

wa19-0088_fig21CD

–500

–1,000

0

500

1,000

1,500

0 10,000 20,000 30,000

QPF , IN CUBIC FEET PER SECOND

QSP– QOO , IN CUBIC FEET PER SECOND

QSP– QOO = –0.01349 x QPF + 519.8

C. Daily streamflow gain (positive value) or loss (negative value) on the Spokane River 

 between gaging stations near Otis Orchards (QOO) and at Spokane (QSP) and streamflow 

 at gaging station near Post Falls (QPF ).

–500

–1,000

0

500

1,000

1,500

0 10,000 20,000 30,000

QPF , IN CUBIC FEET PER SECOND

QSP– QGA , IN CUBIC FEET PER SECOND

QSP– QGA = –0.03517 x QPF + 596.5

D. Daily streamflow gain (positive value) or loss (negative value) on the Spokane River 

 between gaging stations at Greenacres (QGA) and at Spokane (QSP) and streamflow at 

 gaging station near Post Falls (QPF ).

Hydrology of Aquifer  31
Ground-Water Levels and Movement

Campbell (2005) constructed a map of ground-water 

levels in the SVRP aquifer on the basis of water levels 

measured in 268 wells during September 2004. An updated 

version of the ground-water level map that takes into account 

more recent information, such as updated altitudes of well 

sites and the base of the aquifer, is shown in figure 22. 

Notable features of the water-level map include the relatively 

flat hydraulic gradient in southern Rathdrum Prairie and the 

steeper gradients along the aquifer margins and in side valleys. 

On the Washington side of the aquifer, the hydraulic gradient 

tends to increase from east to west. In Hillyard Trough and 

the Little Spokane River Arm, water levels are shown for the 

upper unit. Too few data are available to construct a map of 

hydraulic head in the lower unit.

The generalized direction of ground-water flow is shown 

in figure 22. Ground water flows from northern Rathdrum 

Prairie through West Channel and Ramsey (Middle) Channel 

into southern Rathdrum Prairie. Flow through Chilco Channel 

probably is small because of the small saturated thickness in 

the channel. From southern Rathdrum Prairie, ground water 

flows toward the west through Spokane Valley to the Spokane 

area, where the flow bifurcates. Part of the flow moves through 

Trinity Trough into the Western Arm of the aquifer. And part 

moves to the north into Hillyard Tough and finally to the west 

into the Little Spokane River Arm.

During the summer of 2004, a network of 56 wells 

was established to measure ground-water levels in the study 

area (fig. 23, table 7). Measurements were made manually 

about once a month. Pressure transducers were installed in 

8 of the 56 wells to automatically record water levels once 

an hour. Additional water-level data for certain wells in the 

network also are available from previous studies. Water levels 

measured during 2000 and 2001 by Caldwell and Bowers 

(2003) are available for 11 wells in the vicinity of the Spokane 

River. Historical water-level data dating back to 1993 and 

earlier are available for four wells in the network.

Data from the well network indicate that ground-water 

level fluctuations are characteristic of different parts of the 

aquifer. For example, in Spokane Valley, ground-water levels 

are strongly controlled by stage on the Spokane River. The 

water level in well 60 and the stage on the Spokane River 

at the gaging station near Otis Orchards from June 2004 to 

December 2005 are shown in figure 24. To facilitate visual 

comparison, the well hydrograph is plotted with respect to the 

vertical axis on the left, and the stage on the Spokane River is 

plotted with respect to the vertical axis on the right. Individual 

peaks in the river stage during the high-flow period from 

December 2004 to June 2005 correspond closely to individual 

peaks in the well hydrograph.

Figure 21.—Continued.

wa19-0088_fig21E

–500

–1,000

0

500

1,000

1,500

0 10,000 20,000 30,000

QPF , IN CUBIC FEET PER SECOND

QSP– QPF , IN CUBIC FEET PER SECOND

QSP– QPF = –0.05426 x QPF + 369.1

E. Daily streamflow gain (positive value) or loss (negative value) on the Spokane River 

 between gaging stations near Post Falls (QPF) and at Spokane (QSP) and streamflow at 

 gaging station near Post Falls.

32  Ground-Water Flow Model for the SVRP Aquifer, Washington and Idaho
wa19-0088_fig22

0 5 10 15 20 KILOMETERS

0 5 10 15 20 MILES

Little

Spokane R.

Spokane River

Deep

Creek

Coeur d’Alene Lake

Lake

Pend

Oreille

Liberty

Lake

Newman

Lake

Long

Lake

Hauser

Lake

Fernan Lake

Hayden Lake

Spirit

Lake

Twin

Lakes

Hangman

Creek

48°

47°

45'

117°30' 117° 116°30'

EXPLANATION

Altitude of water level, in feet, of the Spokane Valley- 

 Rathdrum Prairie aquifer, September 2004—

 Datum is North American Vertical Datum of 1988

2,220 to 2,300

2,140 to 2,220

2,060 to 2,140

1,980 to 2,060

1,900 to 1,980

1900

1820

1980

2060

2060

Spokane R.

1400

1,820 to 1,900

1,740 to 1,820

1,660 to 1740

1,580 to 1,660

1,500 to 1,580

Water-level contour—Shows altitude of water level, September 2004. 

 Contour interval is 20 feet. Datum is North American 

 Vertical Datum of 1988

Generalized direction of ground-water flow

Site of water-level measurement

Base modified from U.S. Geological Survey digital data.

City boundaries, 1:24,000, various years (1961-86);

Public land survey, 1:100,000, 1985; Lakes, 1:100,000, 1995;

and rivers, 1:100,000, 1985. North American Datum of 1983 (NAD 83).

Figure 22. Ground-water levels for the Spokane Valley-Rathdrum Prairie aquifer, Washington and Idaho, September 2004 (revised after 

Campbell, 2005).

Hydrology of Aquifer  33
Figure 23. Locations of monitoring wells in the Spokane Valley-Rathdrum Prairie aquifer, Washington and Idaho.

wa19-0088_fig23

Base modified from U.S. Geological Survey digital data.

City boundaries, 1:24,000, various years (1961-86);

Public land survey, 1:100,000, 1985; Lakes, 1:100,000, 1995;

and rivers, 1:100,000, 1985. North American Datum of 1983 (NAD 83). 0 5 10 15 20 KILOMETERS

0 5 10 15 20 MILES

Little Spokane River

Spokane River

Deep Cr.

Coeur d’Alene Lake

Lake

Pend

Oreille

Hangman

Cr. 117°30'

48°

47°

45'

117° 116°30'

Liberty

Lake

Newman

Lake

Hauser

Lake

Fernan Lake

Hayden Lake

Spirit

Lake

Twin

Lakes

Long

Lake

Spokane R.

Montoring well and No.

Streamflow gaging station

 92

EXPLANATION

Extent of Spokane Valley-Rathdrum Prairie

 aquifer ground-water flow model

225

223

267

252

251

249

246

245

236

234

219

217

213

171 167

209

197 202 99

107

104

115

134

2

6 8

128

13

16

58

44

77

79

81

93 94

92

87 71

84, 86

91

60

24

137

138

157, 158

Little Spokane River

near Dartford

Little Spokane River

at Dartford

139

97

178

159

150 143 140

Spokane River

at Spokane

Spokane River

at Greenacres

Spokane River

near Otis Orchards

Spokane River

near Post Falls

248

Hangman Creek

at Spokane

Figure 24. Water level in well 60 and stage on the Spokane River at the gaging station near Otis 

Orchards, Spokane Valley-Rathdrum Prairie aquifer, Washington and Idaho.

1,928

1,932

1,936

1,940

1,944 2,020

2,004

2,008

2,012

2,016

J J A S O N D J F M A M J J A S O N D

2004 2005

WATER LEVEL IN WELL 60, IN FEET

SPOKANE RIVER STAGE, IN FEET

Well 60

Spokane River

near Otis Orchards

34  Ground-Water Flow Model for the SVRP Aquifer, Washington and Idaho
Table 7. Physical data for wells in water-level monitoring network, Spokane Valley-Rathdrum Prairie aquifer, Washington and Idaho.

[Abbreviations: USGS, U.S. Geological Survey; NAD 83, North American Datum of 1983; NAVD 88, North American Vertical Datum of 1988; ft, foot]

Well No. USGS Site No. USGS Site Name Longitude

(NAD 83)

Latitude

(NAD 83)

Land surface altitude

(ft, NAVD 88)

Hole depth

(ft)

Aquifer

unit

2 473928117275001 25N 42E 14CDD1 473928 1172757 1,801.50 136 Gravel

6 474045117255201 25N 43E 07CBB1 474045 1172552 1,904 258 Gravel

8 474054117232401 25N 43E 08ADD1 474054 1172324 1,948.42 101 Gravel

13 474037117193303 25N 43E 11DAA3 474037 1171933 1,952.16 120 Gravel

16 473958117185801 25N 43E 13BCD1 473956 1171913 1,943.34 85 Gravel

24 473912117205103 25N 43E 22ADA3 473912 1172051 1,932 108 Gravel

44 474026117115301 25N 44E 11DDAC1 474026 1171153 1,965.90 69 Gravel

58 473839117131101 25N 44E 22DDD1 473834 1171307 2,082.59 250 Gravel

60 473852117115301 25N 44E 23DAA1 473858 1171157 2,046.42 210 Gravel

71 474140117060401 25N 45E 03BDDA1 474140 1170604 2,056.37 117 Gravel

77 474156117091601 25N 45E 05BBBC1 474155 1170916 2,049.36 128 Gravel

79 474109117091701 25N 45E 07AAAA4 474109 1170917 2,025.36 100 Gravel

81 474038117091201 25N 45E 08CBBC1 474039 1170913 2,021.23 97 Gravel

84 474106117060501 25N 45E 10BAAA1 474106 1170605 2,034.34 72 Gravel

86 474107117060502 25N 45E 10BAAA3 474106 1170605 2,034.20 140 Gravel

87 474033117062501 25N 45E 10CBDA1 474034 1170625 2,041.70 97 Gravel

91 473925117053201 25N 45E 15DDCC1 473925 1170540 2,076.87 155 Gravel

92 474011117072901 25N 45E 16BAB1 474011 1170737 2,059.70 Unknown Gravel

93 474016117085601 25N 45E 17BBAA5 474016 1170856 2,035.60 113 Gravel

94 473928117083001 25N 45E 17DCCB1 473928 1170830 2,047.74 287 Gravel

97 474049117023501 25N 46E 07BCAD1 474049 1170235 2,178.13 248 Gravel

99 474629117305101 26N 42E 04CCDB1 474627 1173055 1,729 321 Gravel

104 474427117312101 26N 42E 20ABAC1 474435 1173132 1,627.86 60 Gravel

107 474434117303201 26N 42E 21BAA1 474436 1173032 1,668.48 101 Gravel

115 474604117242901 26N 43E 08BCC1 474606 1172429 1,788.03 465 Gravel

128 474303117215901 26N 43E 27CCB1 474304 1172159 2,034 225 Gravel

134 474216117244501 26N 43E 31DAA1 474216 1172445 2,063.14 223 Gravel

137 474349117025201 26N 45E 24DDDA1 474349 1170252 2,107.97 178 Gravel

138 474304117025201 26N 45E 25DDAA1 474304 1170252 2,084.17 146 Gravel

139 474226117024801 26N 46E 31CBBC1 474225 1170248 2,094.02 151 Gravel

140 474226116444101 50N 03W 06DAA1 474224 1164448 2,220.91 185 Gravel

143 474229116474501 50N 04W 02ACC1 474229 1164745 2,224 440 Gravel

150 474242116540301 50N 05W 01ACBB1 474242 1165413 2,197.43 243 Gravel

157 474134117002201 50N 05W 07DABC1 474134 1170022 2,073.32 79 Gravel

158 474134117002202 50N 05W 07DABC2 474135 1170022 2,073.54 46 Gravel

159 474147116544001 50N 05W 12BCAD1 474147 1165440 2,130.58 200 Gravel

167 474720116532101 51N 04W 06CCDD1 474720 1165321 2,240.47 310 Gravel

171 474643116461901 51N 04W 12DBD1 474643 1164621 2,308.78 248 Gravel

178 474445116521601 51N 04W 20CCCC1 474447 1165221 2,249.18 298 Gravel

197 474450117003301 51N 05W 19DCAC1 474450 1170033 2,126.80 178 Gravel

202 474440116545801 51N 05W 26AAAA1 474440 1165506 2,232.79 268 Gravel

209 474329116582801 51N 05W 33CBA1 474320 1165832 2,145.42 Unknown Gravel

213 475211116443801 52N 03W 07ADDA1 475211 1164438 2,313.98 147 Gravel

217 475322116522201 52N 04W 06AAAA1 475322 1165222 2,411.75 445 Gravel

219 475202116484301 52N 04W 10DAB1 475202 1164843 2,311.71 340 Gravel

223 475002116521101 52N 04W 20CCB1 475002 1165208 2,270.56 500 Multiple

225 475019116494701 52N 04W 21DAA1 475019 1164947 2,284.41 348 Gravel

234 475705116371201 53N 02W 07DBD1 475705 1163712 2,441.61 442 Gravel

236 475736116341701 53N 02W 09AAC1 475737 1163421 2,294.83 351 Gravel

245 475510116391201 53N 03W 24CCCD1 475509 1163912 2,457.37 340 Gravel

246 475400116404201 53N 03W 34ADAA1 475400 1164042 2,460.25 237 Gravel

248 475656116520401 53N 04W 08CCDA1 475656 1165204 2,548.30 503 Gravel

249 475622116470101 53N 04W 13CBBD1 475622 1164701 2,505.01 506 Gravel

251 475439116503401 53N 04W 28CAB1 475438 1165038 2,432.26 449 Gravel

252 480128116375301 54N 02W 18CBB1 480128 1163753 2,320.43 72 Gravel

267 480101116563601 54N 05W 22ACA1 480101 1165639 2,317.17 175 Gravel

Hydrology of Aquifer  35
In Hillyard Trough, ground-water level fluctuations are 

less dynamic than those in Spokane Valley. The water level 

in well 128 and the stage on the Spokane River at the gaging 

station at Spokane are shown in figure 25. The rise and fall 

of the well hydrograph are more gradual than the rise and fall 

of the river stage because wells in Hillyard Trough are farther 

from the Spokane River than wells in Spokane Valley.

In the north part of Western Arm, ground-water levels 

are strongly controlled by the level of Nine Mile Reservoir. 

Water levels in wells 104 and 107 and the level of Nine 

Mile Reservoir are shown in figure 26. Wells 104 and 107 

are located, respectively, on the west and east sides of Nine 

Mile Reservoir. The rise and fall of the well hydrographs are 

closely correlated with the rise and fall of the reservoir. During 

periods when the level of the reservoir is constant, the water 

level in well 107 is nearly identical to level of the reservoir, 

but the water level in well 104 is about 0.5 ft above the level of 

the reservoir. This indicates that ground water discharges from 

the SVRP aquifer to Nine Mile Reservoir.

In northern Rathdrum Prairie, ground-water level 

fluctuations are substantially different from those in Spokane 

Valley. Long-term water levels in wells 92, 209, and 251 for 

1990–2005 are shown in figure 27. Well 92 is in Spokane 

Valley. The hydrograph of well 92 is similar to the hydrograph 

of well 60 (fig. 24)—the water level exhibits multiple peaks 

that correlate to peaks in the stage on the Spokane River 

during winter and spring. By contrast, well 251 is in northern 

Rathdrum Prairie. The hydrograph of well 251 is controlled 

primarily by recharge from precipitation. Although most of 

the precipitation infiltrates into the land surface during winter 

and spring, the long transmission time from land surface to 

the water table at depths of 400–500 ft caused the hydrograph 

to peak in July and August. Well 209 is in the transition zone 

between southern Rathdrum Prairie and Spokane Valley. 

The hydrograph of well 209 exhibits characteristics that are 

intermediate between those of wells 92 and 251.

Figure 26. Water levels in wells 104 and 107 and in Nine Mile Reservoir, Spokane 

Valley-Rathdrum Prairie aquifer, Washington and Idaho.

Figure 25. Water level in well 128 and stage on the Spokane River at the gaging station at 

Spokane, Spokane Valley-Rathdrum Prairie aquifer, Washington and Idaho.

1,842

1,846

1,850

1,718

1,722

1,726

J J A S O N D J F M A M J J A S O N D

2004 2005

ALTITUDE OF WATER LEVEL 

IN WELL 128, IN FEET

SPOKANE RIVER STAGE,

IN FEET

Well 128

Spokane River at Spokane

36  Ground-Water Flow Model for the SVRP Aquifer, Washington and Idaho
The falling limbs of the hydrograph of well 251 are more 

gradual then the falling limbs of the hydrograph of well 92 

(fig. 27). This may be explained by the fact that ground water 

in northern Rathdrum Prairie must discharge through West 

Channel and Ramsey Channel, which can constrict groundwater flow. During several years of higher-than-average 

recharge, such as from 1995 to 1997, ground-water levels in 

northern Rathdrum Prairie can exhibit an increasing trend over 

several years. Conversely, during several years of lower-thanaverage recharge, such as from 2000 to 2005, ground-water 

levels in northern Rathdrum Prairie can exhibit a declining 

trend over several years.

In the vicinity of Lake Pend Oreille, ground-water 

levels in the SVRP aquifer are strongly controlled by the lake 

level. The hydrographs of wells 234 and 236 and the level 

of Lake Pend Oreille are shown in figure 28. The lake level 

is maintained at about 2,066 ft during the summer and at a 

lower level during the rest of the year. This lake level signal is 

exhibited by the hydrographs of wells 234 and 236. The water 

levels shown in figure 28 also indicate that flow is from the 

lake toward wells 234 and 236.

In the vicinity of Coeur d’Alene Lake and Post Falls, 

ground-water levels do not show a clear relation to the level in 

the lake (and the arm of the Spokane River above Post Falls). 

The hydrographs of wells 143 and 159 and the level of Coeur 

d’Alene Lake are shown in figure 29. Note that the lake level 

is about 100 ft higher than the water levels in the wells. From 

June to mid-September, the lake level is maintained at about 

2,132 ft. From mid-September to December, the gates at Post 

Falls Dam are opened incrementally to lower the lake to its 

natural autumn level. From December to June, the dam gates 

usually are left open to allow the lake level to rise and fall in 

response to inflows from storms and snowmelt. The water 

levels in wells 143 and 159 do not follow the lake level. In 

well 143, the water level peaks in November when the lake 

level is declining. In well 159, the water level peaks in July 

when the lake level is high. Then, for the next 1 to 2 months, 

the water level declines when the lake level is constant. These 

fluctuations in ground-water levels are not explained readily 

by a simple conceptualization of lake-aquifer interaction 

in which ground-water levels near the lake are expected to 

rise in response to greater subsurface leakage from the lake 

during times of high lake levels, and fall in response to less 

subsurface leakage from the lake during times of low lake 

levels.

Water levels in wells along the perimeter of the SVRP 

aquifer generally are not indicative of hydrologic conditions 

in the main part of the aquifer. Along the perimeter of the 

aquifer, bedrock is close to land surface, the saturated zone 

is thin, hydraulic conductivity tends to be low, and groundwater levels can be tens to more than 100 ft higher than 

in the main part of the aquifer. In addition, ground-water levels 

along the perimeter can rise and decline abruptly in response 

to episodic inflows from surrounding uplands. The effects 

of these inflows typically are damped out in the main part 

of the aquifer. The hydrograph of well 245, which is located 

near the east boundary of northern Rathdrum Prairie, and the 

hydrograph of well 249, which is located in the central part 

of northern Rathdrum Prairie, are shown in figure 30. 

Figure 27. Water levels in wells 92, 209, and 251, Spokane Valley-Rathdrum Prairie 

aquifer, Washington and Idaho.

wa19-0088_fig27

1990 1995 2000 2005

ALTITUDE OF WATER LEVEL IN WELL 251, IN FEETALTITUDE OF WATER LEVEL IN WELL 209, IN FEET

ALTITUDE WATER LEVEL IN WELL 92, IN FEET

Well 92

Well 209

Well 251

2,020

2,030

2,040

2,050

2,010

2,000

1,995

2,005

2,015

2,025

1,985

1,975

1,970

1,980

1,990

2,000

1,960

1,950

Hydrology of Aquifer  37
Figure 28. Water levels in wells 234 and 236 and in Lake Pend Oreille, Spokane Valley-Rathdrum Prairie aquifer, 

Washington and Idaho.

Figure 29. Water levels in wells 143 and 159 and in Coeur d’Alene Lake, Spokane ValleyRathdrum Prairie aquifer, Washington and Idaho.

wa19-0088_fig28

2,050

2,054

2,058

2,062

2,066

2,070

1990 1995 2000 2005

ALTITUDE OF WATER LEVEL, INFEET

Lake Pend Oreille

Well 236

Well 234

2,024

2,016

2,008

2,130

2,122

2,114

J F M A M J J A S O N D

2004

J F M A M J J A S O N D

2005

ALTITUDE OF WATER LEVEL 

IN WELL, IN FEET

ALTITUDE OF WATER LEVEL 

IN LAKE, IN FEET

Coeur d’Alene Lake

Well 159

Well 143

2,004

2,028

2,020

2,012

2,110

2,118

2,126

2,134

38 Ground-Water Flow Model for the SVRP Aquifer, Washington and Idaho
The water level in well 245 is about 150 ft higher than the 

water level in well 249. During the period of record shown in 

figure 30, the water level is relatively stable in the central part 

of northern Rathdrum Prairie. However, the water level near 

the east boundary indicates a substantial rise and fall during 

the winter and spring in response to storm and snowmelt 

runoff.

Along the losing segment of the Spokane River from the 

gaging stations near Post Falls to at Greenacres, the riverbed 

is above the regional water table and water leaks out of the 

river to recharge the aquifer. Caldwell and Bowers (2003, p. 

18-19 and fig. 7) noted that a narrow ground-water mound 

can develop beneath the river as a result of vertical infiltration 

from the river. A well located close to the riverbank might 

penetrate the ground-water mound and encounter a water 

level that is substantially higher than the water level that 

predominates in regions away from the river. Wells 84, 157, 

and 158 in the well network appear to fit that description. 

For example, from June 2004 to December 2005, the average 

water level in well 84 is 1,990 ft. For the same period, the 

water levels in wells 71 and 87, both about 3,500 ft from well 

84 and the Spokane River (fig. 23), are, respectively, 1,963 

and 1,961 ft. The water levels in wells 71 and 87 are consistent 

with the regional trend of the water table, but the water level in 

well 84 likely characterizes the local ground-water condition 

immediately beneath the Spokane River.

During April 2006, synoptic water-level measurements 

were made in the same 268 wells used for the September 

Figure 30. Water levels in wells 245 and 249, Spokane Valley-Rathdrum Prairie aquifer, 

Washington and Idaho.

2004 synoptic water-level measurements (Campbell, 2005). 

The two water-level data sets provide an opportunity to 

compare spring and autumn water levels throughout the SVRP 

aquifer. The change in water level from September 2004 to 

April 2006 is shown in figure 31. At each well, the change is 

computed as the water level in April 2006 minus the water 

level in September 2004. A positive change indicates a rise in 

water level from September 2004 to April 2006. A negative 

change indicates a decline in water level from September 

2004 to April 2006. Excluded from figure 31 are wells that 

were pumped prior to or during either synoptic measurement 

session.

The water-level changes shown in figure 31 display many 

of the water-level characteristics discussed in the preceding 

paragraphs. For the most part, water levels rose in the aquifer 

in response to recharge from precipitation and river infiltration 

during the winter of 2005-06 and the spring of 2006. However, 

exceptions occur in the vicinity of Lake Pend Oreille and in 

the vicinity of Coeur d’Alene Lake. The level of Lake Pend 

Oreille is lower in spring than in autumn. Figure 31 shows 

that ground-water levels in the vicinity of Lake Pend Oreille 

declined from September 2004 to April 2006. The magnitude 

of the water-level decline is largest close to the lake and 

decreases with distance away from the lake. The areal extent 

of the decline indicates that the lake level could influence 

ground-water levels over a distance from the lake to about 

halfway between Lake Pend Oreille and Spirit Lake.

In the vicinity of Coeur d’Alene Lake, ground-water 

levels also declined from September 2004 to April 2006. 

2,170

2,174

2,178

2,182

2,186 2,040

2,024

2,028

2,032

2,036

J J A S O N D J F M A M J J A S O N D

2004 2005

ALTITUDE OF WATER LEVEL IN 

WELL 245, IN FEET

ALTITUDE OF WATER LEVEL IN 

Well 249

WELL 249, IN FEET

Well 245

Hydrology of Aquifer  39
September 2004 is the end of the summer period when Coeur 

d’Alene Lake was maintained at about 2,132 ft. During 

April 2006, the lake level rose from about 2,129 to 2,132 ft. 

However, as discussed previously, the ground-water levels 

in the vicinity of Coeur d’Alene Lake do not show a clear 

relation to the lake level. Therefore, it is difficult to provide a 

clear explanation for the decline in ground-water levels in the 

vicinity of the lake from September 2004 to April 2006.

Along the margins of Rathdrum Prairie, water levels rose 

by tens of feet. By contrast, in the central part of Rathdrum 

Prairie, the water level rose by a few feet. The data support 

the concept that water levels along the aquifer margins are 

strongly controlled by inflows from adjacent uplands and 

tributary basins. The large fluctuations in water levels along 

the aquifer margins are damped out in the central part of the 

aquifer.

Figure 31. Changes in water levels, Spokane Valley-Rathdrum Prairie aquifer, Washington and Idaho, September 2004 to April 2006.

wa19-0088_fig31

0 5 10 15 20 KILOMETERS

0 5 10 15 20 MILES

Little

Spokane R.

Spokane River

Deep

Creek

Coeur d’Alene Lake

Lake

Pend

Oreille

Hangman

Creek

117°30'

48°

47°

45'

117° 116°30'

Spokane R.

Long

Lake

Liberty

Lake

Newman

Lake

Hauser

Lake

Fernan Lake

Hayden Lake

Spirit

Lake

Twin

Lakes

EXPLANATION

Water-level change from

September 2004 to April 2006, in feet

–10 to –5

 –5 to 0

 0 to 5

 5 to 10

 10 to 20

 20 to 50

Extent of Spokane Valley-Rathdrum 

 Prairie aquifer ground-water 

 flow model

Base modified from U.S. Geological Survey digital data.

City boundaries, 1:24,000, various years (1961-86);

Public land survey, 1:100,000, 1985; Lakes, 1:100,000, 1995;

and rivers, 1:100,000, 1985. North American Datum of 1983 (NAD 83).

40 Ground-Water Flow Model for the SVRP Aquifer, Washington and Idaho
The water-level rise in Spokane Valley is substantially 

larger than the water-level rise in Rathdrum Prairie (excluding 

the wells along the prairie margins). This is due to the 

presence of the Spokane River, which is a major source of 

recharge to Spokane Valley. In addition, the greater depths 

to the water table in Rathdrum Prairie require a longer time 

for precipitation infiltration to reach the water table. Thus, 

water-level rises in Rathdrum Prairie generally lags behind the 

water-level rise in Spokane Valley.

Ground-Water Flow Model

The ground-water flow model known as MODFLOW2000 (Harbaugh and others, 2000) is used to simulate 

ground-water flow in the SVRP aquifer. MODFLOW-2000 

is a computer program that numerically solves the threedimensional ground-water flow equation for a porous medium 

by using the finite-difference method. The modular design 

of MODFLOW-2000 uses packages to represent various 

components of the ground-water flow system, such as 

recharge, withdrawal from wells, and interactions between the 

aquifer and surface-water bodies.

Spatial Discretization

A model grid of 172 rows, 256 columns, and 3 layers is 

used to represent the SVRP aquifer. In the horizontal direction, 

each cell has a dimension of 1,320 by 1,320 ft. The areal 

extent of the grid is larger than the model area. The larger 

grid is intended to accommodate possible enlargements of the 

model area in future studies. Cells outside the model area are 

designated as inactive in MODFLOW-2000. The active cells in 

model layers 1 and 3 and shown in figures 32 and 33.

Vertically, the SVRP aquifer is represented by either 

one or three model layers. Except in Hillyard Trough and 

Little Spokane River Arm, the aquifer is represented by 

the top model layer (layer 1); cells in model layers 2 and 

3 are inactive. In Hillyard Trough and the Little Spokane 

River Arm, the aquifer is represented by three model layers. 

Vertical section A-B-C-D in figure 34 shows how the model 

layers represent the dividing of the aquifer from a single 

hydrogeologic unit into an upper and a lower unit separated 

by a clay layer. From A to B, the aquifer exists as a single 

hydrogeologic unit and is represented solely by model 

layer 1. Model layers 2 and 3 have zero thicknesses and are 

designated as inactive. From B to C, the aquifer exists as 

a single hydrogeologic unit but is represented by all three 

model layers. Model layer 2 has a uniform thickness of 1 ft, 

but model layer 3 gradually thickens from B to C. Point C 

marks the location where the aquifer divides into an upper 

and a lower unit. From C to D, model layer 1 represents the 

upper unit, model layer 2 represents the clay layer, and model 

layer 3 represents the lower unit. The altitude at the top of the 

clay layer is set at 1,700 ft in Hillyard Trough and decreases 

to 1,500 ft in the Little Spokane River Arm. The thickness 

of the clay layer is assumed to be 150 ft in both Hillyard 

Trough and the Little Spokane River Arm. In the input data for 

MODFLOW-2000, model layer 1 is specified as an unconfined 

layer. Model layers 2 and 3 are specified as confined layers.

Temporal Discretization

Ground-water flow in the SVRP aquifer is simulated for 

September 1990 through September 2005. The simulation 

period is divided into 181 stress periods of 1 month each. 

Monthly stress periods are needed to simulate the dynamic 

interaction between the aquifer and the Spokane River as 

shown by the hydrograph in figure 24. Each stress period 

consists of one time step. Using one time step per stress period 

was determined to be adequate because trial simulations using 

several time steps per stress period yielded essentially the 

same results.

Because the distribution of heads in the aquifer is 

unknown at the beginning of the simulation period, the initial 

heads are computed assuming a steady-state ground-water 

flow system during the first stress period (September 1990). 

The heads simulated during the first stress period serves as 

the initial conditions for the rest of the transient simulation. 

In reality, flow in the aquifer is not at steady state during 

September 1990, and the simulated heads for that month 

do not accurately represent the actual heads in the aquifer. 

To eliminate the effects of this error, the first 5 years of the 

simulation is considered a start-up period. No attempt is 

made to fit simulated quantities to measured quantities during 

the start-up period. Instead, hydrologic data from October 

1995 to September 2005 are used for model calibration. The 

calibrated model is expected to produce simulated conditions 

at the end of the start-up period (October 1995) that are close 

to the corresponding actual conditions in the aquifer, because 

the error caused by the assumed initial conditions would have 

dissipated.

Ground-Water Flow Model  41
Figure 32. Active cells in model layer 1, Spokane Valley-Rathdrum Prairie aquifer, Washington and Idaho.

wa19-0088_Figure 32

0 5 10 15 20 KILOMETERS

15 10 5 0 20 MILES

tti L

R enakopS el iver

Sp

okane River

Deep

Cre ke

Coeur d’Alene Lake

Lake

Pend

Oreille

LibertyLake

NewmanLake

LongLake

HauserLake

Fernan Lake

Hayden Lake

SpiritLake

TwinLakes

HangmanCr.

Active cellActive cell that receives flow  from a tributary basinActive cell used in River PackageActive cell that receives flow from a lake, excluding Lake Pend  Oreille and Coeur d’Alene Lake

EXPLANATION 48°

47°45'

117°30' 117° 116°30'

Spokane River

42  Ground-Water Flow Model for the SVRP Aquifer, Washington and Idaho
Figure 33. Active cells in model layer 3, Spokane Valley-Rathdrum Prairie aquifer, 

Washington and Idaho.

Figure 34. Vertical section A-B-C-D showing how model layers represent the dividing of the 

aquifer from a single hydrogeologic unit into an upper and lower unit separated by a clay layer, 

Spokane Valley-Rathdrum Prairie aquifer, Washington and Idaho. wa19-0088_fig33

0 5 KILOMETERS

0 5 MILES

117°30' 117°20'

47°45'

47°40'

Spokane River

Little Spokane

River

Long

Lake

Deep

Creek

Spokane

River

A

B

C

D

Active cell in model layer 3, 

not under clay layer

Active cell in model layer 3, 

under clay layer

Active cell in model layer 3 where 

General-Head Boundary 

Package is applied 

Inactive cell in model layer 3 

Line of cross section (see fig. 34)

EXPLANATION

A D

1,200

Vertical exaggeration x12

1,400

1,600

1,800

2,000

2,200

Model layer 1

Water table

Model layer 3

Model layer 2

D C

B A

ALTITUDE, IN FEET

1,320 ft

Ground-Water Flow Model  43
Boundary Conditions and Implementation of 

MODFLOW Packages

A no-flow boundary is specified on the bottom of the 

model domain. Along the sides of the model domain, the 

no-flow boundary is specified except for locations where the 

aquifer receives inflow from tributary basins and lakes and 

where ground water exits the lower unit at the west end of the 

Little Spokane River Arm. Boundary conditions other than the 

no-flow boundary are implemented in the model using various 

MODFLOW packages. These packages represent inflow and 

outflow components discussed previously in the sections, 

“Inflows to Aquifer” and “Outflows from Aquifer.”

Recharge Package

The Recharge Package is used to simulate recharge from 

precipitation. Recharge is applied to the top model layer. For 

each active cell and for each stress period (that is, for each 

month), the recharge is the precipitation infiltration flux (in 

units of feet per day) that enters the ground-water table. This 

recharge is derived from (1) precipitation that infiltrates into 

permeable land surfaces and (2) precipitation runoff from 

impermeable land surfaces to recharge wells, infiltration 

basins, or adjacent permeable surfaces.

Well Package

The Well Package is used to simulate withdrawals from 

wells, return percolation from irrigation, and effluent from 

septic systems. For each active cell and for each stress period, 

a positive well flow value (in units of cubic feet per day) 

indicates a volumetric recharge rate, and a negative well flow 

value indicates a volumetric withdrawal rate. In the model, 

the well flow value for each cell is the net result of well 

withdrawals, return percolation from irrigation, and effluent 

from septic systems. For example, within a water purveyor 

service area, a cell that contains a water-supply well has a 

large negative well flow value, which is dominated by the well 

withdrawal rate. By contrast, a cell that does not contain a 

water-supply well has a small positive well flow value, which 

represents return percolation from landscape irrigation.

Flow and Head Boundary Package

The Flow and Head Boundary Package (Leake and Lilly, 

1997) is used to simulate flows to the SVRP aquifer from 

tributary basins and from all lakes except Lake Pend Oreille 

and Coeur d’Alene Lake. For each tributary basin that drains 

to the aquifer (fig. 10), flow from the basin is assigned as 

specified flow to the active cell that contains or is closest to 

the outflow point. An exception to this procedure is made 

along the east margin of northern Rathdrum Prairie. In that 

area, the saturated zone is a thin layer overlying bedrock. If 

flow from the tributary basin is applied to a single cell, a very 

large buildup in head would result. To avoid this situation, 

flow is distributed uniformly among active cells along the 

model boundary. The cells that receive flow from tributary 

basins are shown in figure 32. For the lakes, flow to the aquifer 

is distributed uniformly among active model cells along the 

lakeshore (fig. 32).

River Package

The River Package is used to simulate the Little Spokane 

River, Lake Pend Oreille, and Coeur d’Alene Lake. In the 

River Package, a river reach refers to the section of a river with 

a model cell. For a river reach, the volumetric flow rate across 

the riverbed to the underlying model cell is computed as

Q C h h

Q

rb rb r a

rb

= − ( ),

where 

is the flow rate across the riverbed, 

is the conductance of the riverbed, 

is the river

C

h

rb

r stage, and 

h is the hydraulic head in the cell underlyin a g 

the riverbed, if the bottom of the 

riverbed is below the water table in 

the cell, or the altitude of the bottom 

of the riverbed, if the bottom of the 

riverbed is above the water table in 

the cell. 

(6)

The conductance of the riverbed is given by

C K wL

m

K

rb

v

v

= ,

where 

is the vertical hydraulic conductivity of 

the riverbed sediment, 

w is the width of the river reach,

is the length of the river reach, and 

is the thickness

L

m of the riverbed sediment. 

(7)

However, Kv

, w, L, and m are not individually specified in the 

River Package. Instead, conductance of the riverbed, Crb, is 

specified. Model cells used in the River Package are shown in 

figure 32.

To apply the River Package to the Little Spokane River, 

the following procedure is used to determine the river stage, 

hr. For each stress period, the average stage (over the stress 

44  Ground-Water Flow Model for the SVRP Aquifer, Washington and Idaho
period) is computed at the gaging stations on the Little 

Spokane River at Dartford and near Dartford from daily 

recorded values. Because no gaging stations are located on the 

Little Spokane River downstream of the gaging station near 

Dartford, the stage at the mouth of the river (where the river 

joins the Spokane River) is estimated from the Nine Mile Falls 

USGS 7.5-minute topographic quadrangle to be 1,544 ft. For 

each river reach between the gaging stations at Dartford and 

near Dartford, the stage is estimated by assuming a uniform 

gradient in river level between the two gaging stations. A 

similar procedure is used to estimate the stage of a river reach 

between the gaging station near Dartford and the mouth of the 

river.

For each reach of the Little Spokane River in the model, 

the altitude of the bottom of the riverbed is assumed to be 10 

ft below the river stage. This assumption is inconsequential 

to the simulation because the simulated head in the cell 

underlying the river reach is always higher than the bottom 

of the riverbed. Therefore, ha

 in equation 6 is always the 

hydraulic head in the cell. A single riverbed conductance 

is assigned to all reaches of the Little Spokane River. This 

conductance value, denoted as C-LSR, is estimated by model 

calibration.

Because equation 6 also can be used to describe lakebed 

seepage, the River Package is used to simulate subsurface 

seepage from Lake Pend Oreille and Coeur d’Alene Lake. For 

this usage, the terms river and riverbed in the definitions of 

variables in equation 6 are replaced, respectively, by lake and 

lakebed. The River Package is implemented at model cells 

underlying the near-shore region of the lake. For each stress 

period, the lake stage is specified as the average of the daily 

recorded lake levels over the stress period. Based on lake 

bathymetry data, the altitude of the bottom of the lakebed in 

the near-shore region is set to 1,860 ft for Lake Pend Oreille 

and 2,090 ft for Coeur d’Alene Lake. As in the case of the 

Little Spokane River, a single lakebed conductance is assigned 

to each lake. These two conductance values, denoted as C-PO 

for Lake Pend Oreille and C-CDA for Coeur d’Alene Lake, 

are estimated by model calibration.

Streamflow-Routing Package

The Streamflow-Routing Package (Prudic and others, 

2004) is used to simulate the interaction between the Spokane 

River and the SVRP aquifer. In the Streamflow Routing 

Package, equations used to compute flow between the stream 

and the aquifer were the same as equations 6 and 7 in the 

River Package. However, the Streamflow-Routing Package 

is more powerful than the River Package because it provides 

various methods for routing water through a stream network.

The Streamflow-Routing Package requires data on 

stream-channel geometry. In the SVRP aquifer model, the 

channel of the Spokane River is approximated as having a 

rectangular cross section. Channel width and the altitude of 

the top of the streambed are estimated at 16 control points 

along the river from field measurements and data from 

previous studies (Seitz and Jones, 1981; Annear and others, 

2001) and then linearly interpolated between control points. 

The thickness of the streambed sediments (m) is assumed 

to be 1 ft. This assumption is not critical to the simulation 

because the streambed conductance depends on Kv/m, where 

Kv is the vertical hydraulic conductivity of the streambed 

sediments. Thus, the actual variation in m can be included in 

the variability of Kv.

To represent Kv of the streambed sediment, the Spokane 

River in the model area is divided into 11 sections (fig. 35). 

Within each section, Kv is assumed to be uniform. Selection 

of the 11 river sections is based partially on factors that 

might affect Kv. For example, the character of the streambed 

sediment in a free-flowing part of the river might be different 

from the character of the streambed sediment where the river 

is a reservoir behind a dam. In addition, Caldwell and Bowers 

(2003) noted that the transport of fine-grained material with 

the leaking water from the Spokane River might decrease Kv

of the streambed sediments along a losing section of the river. 

The Kv values of the 11 river sections are denoted by KVSR-1 

through KVSR-11 and are estimated by model calibration.

Although the Streamflow-Routing Package is capable 

of simulating stream depth as a function of stream discharge 

using either Manning’s equation or a rating curve, this 

capability is not used in the SVRP aquifer model because 

of the presence of dams, spillways, and reservoirs on the 

Spokane River. Instead, for every time step, stream stage is 

specified for every stream reach in the model. In this usage, 

the Streamflow-Routing Package functions in a manner similar 

to the River Package with the added capability of calculating a 

stream water budget for each stream reach.

Data used to determine stream stages include: (1) stage 

measurements at the four gaging stations on the Spokane 

River, (2) stage measurements for 1999 and 2000 for seven 

bridges across the Spokane River (Reanette Boese, Spokane 

County Utilities, written commun., 2006), and (3) levels of 

Coeur d’Alene Lake, Long Lake, and Nine Mile Reservoir. 

The altitude of Upriver Reservoir is assumed to be constant 

at 1,914 ft. The given data provide stream stages at 15 control 

points along the Spokane River. Between control points, 

stream stage is estimated by linear interpolation. If the stage 

data at a control point do not span the entire simulation period 

(1990–2005), a mathematical relation is developed between 

the available stage measurements at the control point and 

the corresponding stage at the gaging station near Post Falls 

(which has stage data for the entire simulation period). The 

relation between the stage at the Sullivan Road Bridge (where 

stage data are available for 1999 and 2000) and the stage at 

the gaging station near Post Falls is shown in figure 36. The 

equation shown in figure 36 is used to estimate the stage at 

Sullivan Road Bridge for periods with no stage data.

Ground-Water Flow Model  45
Figure 35. Sections of Spokane River used in the Streamflow-Routing Package, Spokane Valley-Rathdrum Prairie aquifer, Washington and Idaho.

wa19-0088_fig35

0 5 10 15 20 KILOMETERS

15 10 5 0 20 MILES

tti L

R enakopS el iver

Sp

okane R.

Deep

Cre ke

Coeur d’Alene Lake

Lake

Pend

Oreille

LibertyLake

NewmanLake

LongLake

HauserLake

Fernan Lake

Hayden Lake

SpiritLake

TwinLakes

HangmanCr.

KVSR-1

KVSR-2

KVSR-3

KVSR-4 KVSR-5

KVSR-6

KVSR-7

KVSR-8

KVSR-9

KVSR-10

KVSR-11

EXPLANATION

Extent of Spokane Valley- Rathdrum Prairie ground-water flow modelActive cells that interact with Spokane RiverSpokane River section and parameter name

48°

47°45'

117°30' 117° 116°30'

Spokane River

KVSR-7

46  Ground-Water Flow Model for the SVRP Aquifer, Washington and Idaho
General-Head Boundary Package

The General-Head Boundary Package is used to simulate 

ground-water outflow from the lower unit at the west end of 

the Little Spokane River Arm. Available field data indicate 

that the lower unit extends beyond the model boundary, and 

ground water flows in the unit for an unknown distance before 

eventually discharging into Long Lake. The General-Head 

Boundary Package is used as an approximate representation of 

this conceptualization and is applied to four active cells at the 

west edge of model layer 3 (fig. 33). Ground-water outflow 

from each cell is computed as

Q C h h

C

h

b b b a

b

b

= − ( ),

where 

is the boundary conductance, 

is the water level in Long Lake, and 

h is the hydraulic head in a the cell. 

(8)

For each stress period, hb is the average of the daily recorded 

levels of Long Lake over the stress period. A single boundary 

conductance, denoted as C-OUT, is assigned to all four cells, 

and the conductance is estimated by model calibration.

Hydraulic Conductivity

To represent the spatial distribution of Kh in the SVRP 

aquifer, active cells in model layer 1 are grouped into 22 

zones, denoted by HK1-1 through HK1-22, as shown in 

figure 37. Within each zone, Kh is uniform. Zones HK1-1 

through HK1-11 represent the central part of the aquifer 

in Rathdrum Prairie, Spokane Valley, and the upper unit in 

Hillyard Trough and the Little Spokane River Arm. Zones 

HK1-12 and HK1-13 represent Trinity Trough and Western 

Arm, respectively. Zones HK1-14 and HK1-15 represent the 

areas in the vicinity of Hayden and Coeur d’Alene. Zones 

HK1-16 through HK1-22 represent side valleys and regions 

of shallow bedrock along the aquifer margins. Zone HK1-22 

is included as a separate zone to represent a small side valley 

north of Coeur d’Alene. This arrangement provides greater 

flexibility during model calibration to simulate water levels 

in well 140, which is located in the side valley. The Kh

 values 

for zones HK1-1 through HK1-22 are estimated by model 

calibration. For all active cells in model layer 1, Kv is set to 

3,000 ft/d. This Kv value is intended to allow water to flow 

freely from model layer 1 into model layer 3 in the region 

south of the clay layer in Hillyard Trough.

Active cells in model layer 2 are grouped into two zones 

as indicated in figure 38. One zone represents the clay layer 

in Hillyard Trough and the Little Spokane River Arm. Kh and 

Kv values of these cells are set to 10-8 ft/d. The other zone 

provides hydraulic connection between model layers 1 and 3 

in the region south of the clay layer. These are the 1-ft thick 

cells shown along section B-C in figure 34. Kh and Kv values 

of these cells are set to 3,000 ft/d.

Active cells in model layer 3 are grouped into two 

zones as shown in figure 39. Zone HK3-1 consists of cells in 

Hillyard Trough. Zone HK3-2 consists of cells in the Little 

Spokane River Arm. Kh

 values of both zones are estimated by 

model calibration. As in the case of model layer 1, all active 

cells in model layer 3 are assigned a Kv value of 3,000 ft/d to 

establish a direct hydraulic connection between model layers 1 

and 3 in the region south of the clay layer in Hillyard Trough.

Figure 36. Relation between stage on the Spokane 

River at Sullivan Road Bridge (y) and stage at the 

gaging station near Post Falls (x), Spokane ValleyRathdrum Prairie aquifer, Washington and Idaho.

wa19-0088_fig36 STAGE ON SPOKANE RIVER AT SULLIVAN ROAD BRIDGE, IN FEET

2,055 2,065

STAGE ON SPOKANE RIVER AT GAGING STATION 

NEAR POST FALLS, IN FEET

2,075

1,932

1,940

1,948

y = 2.55 (x – 2,050.88)0.8 + 1,924

1,956

Ground-Water Flow Model  47
Figure 37. Horizontal hydraulic conductivity zones in model layer 1, Spokane Valley-Rathdrum Prairie aquifer, Washington and Idaho.

wa19-0088_fig37

0 5 10 15 20 KILOMETERS

0 5 10 15 20 MILES

Little

Spokane River

Spokane River

Deep

Creek

Coeur d’Alene Lake

Lake

Pend

Oreille

Liberty

Lake

Newman

Lake

Long

Lake Hauser

Lake

Fernan Lake

Hayden Lake

Spirit

Lake

Twin

Lakes

Hangman

Creek

117°30'

48°

47°

45'

117° 116°30'

HK1-11

HK1-10

HK1-9

HK1-8

HK1-6

HK1-5

HK1-4

HK1-3

HK1-2 HK1-1

HK1-19

HK1-12

HK1-15 HK1-22

HK1-14

HK1-17

HK1-18

HK1-16

HK1-7

HK1-13

HK1-20

HK1-21

48 Ground-Water Flow Model for the SVRP Aquifer, Washington and Idaho
Figure 38. Hydraulic conductivity zones in model layer 2, Spokane Valley-Rathdrum Prairie 

aquifer, Washington and Idaho.

wa19-0088_fig38

0 5 KILOMETERS

0 5 MILES

117°30' 117°20'

47°45'

47°40'

Spokane River

Little Spokane

River

Long

Lake

Deep

Creek

Spokane

River

EXPLANATION

Area of model layer 2 where horizontal and 

 vertical hydraulic conductivity are 3,000 feet per day

Area of model layer 2 where horizontal and 

 vertical hydraulic conductivity are 10–8 feet per day

Area of model layer 2 where cells are inactive

Ground-Water Flow Model  49
wa19-0088_fig39

0 5 KILOMETERS

0 5 MILES

117°30' 117°20'

47°45'

47°40'

Spokane River

Little Spokane

River

Long

Lake

Deep

Creek

Spokane

River

HK3-2

HK3-1

Model layer 3, inactive cells

Figure 39. Horizontal hydraulic conductivity zones in model layer 3, Spokane Valley-Rathdrum 

Prairie aquifer, Washington and Idaho.

Specific Yield

The spatial distribution of SY is represented by three 

zones as shown in figure 40. Because SY values are expected 

to fall within a relatively limited range (0.1 – 0.3), fewer 

zones are used to represent the spatial distribution of SY

than to represent the spatial distribution of Kh. Zone SY-1 

represents, approximately, that part of the SVRP aquifer 

where water-level fluctuations are controlled primarily by 

recharge from precipitation. By contrast, zone SY-2 represents, 

approximately, that part of the aquifer where water-level 

fluctuations are controlled primarily by the Spokane River. 

Zone SY-3 represents regions of shallow bedrock along the 

margins of Rathdrum Prairie where water-level fluctuations 

are controlled by both recharge from precipitation and inflows 

from tributary basins and adjacent uplands. The three SY

 values 

are estimated by model calibration.

50 Ground-Water Flow Model for the SVRP Aquifer, Washington and Idaho
Figure 40. Specific yield zones, Spokane Valley-Rathdrum Prairie aquifer, Washington and Idaho.

wa19-0088_fig40

0 5 10 15 20 KILOMETERS

0 5 10 15 20 MILES

Little Spokane River

Spokane River

Deep

Creek

Coeur d’Alene Lake

Lake

Pend

Oreille

Liberty

Lake

Newman

Lake Long

Lake

Hauser

Lake

Fernan Lake

Hayden Lake

Spirit

Lake

Twin

Lakes

Hangman

Creek

117°30'

48°

47°

45'

117° 116°30'

SY-1

SY-1

SY-2

SY-3

SY-3

Spokane R.

Ground-Water Flow Model  51
Model Calibration

Model calibration is the adjustment of model parameters 

(such as hydraulic conductivity and specific yield) so that the 

differences between simulated and measured quantities (such 

as water levels and flows) are minimized with respect to an 

objection function. This section of the report describes the 

nonlinear least-squares regression method used for calibration, 

the calibration data, and the calibration results. The calibration 

is assessed by examining how well the simulated quantities 

fit the measured quantities. Model assumptions are examined 

by comparing the calibrated model with several alternative 

models.

Nonlinear Least-Squares Regression Method

The parameter estimation program PEST version 10 

(Doherty, 2004) is used to calibrate the SVRP aquifer model. 

PEST implements a nonlinear least-squares regression method 

to estimate model parameters by minimizing the sum of 

squared weighted residuals:

Φ = ( ) =

∑ w r

N

w

i i

i

N

i

2

1

,

where 

is the number of measurements, 

is the weight for the measured quantity, 

and 

is the 

th

t

i

r i i

h th residual, defined as the 

measured quantity minus th

i

e 

corresponding simulated quantity. th i

(9)

The sum of squared weighted residuals, Φ, also is known 

as the objective function. PEST uses the Gauss-MarquardtLevenberg method to minimize Φ. Details of this method are 

given in the PEST user’s manual (Doherty, 2004).

The weight, wi

, reflects the importance of the i

th measured 

quantity on the regression. A measurement with a large wi

asserts a large influence on the regression and, therefore, 

the estimated parameter values. Conversely, a measurement 

with a small wi asserts a small influence on the regression 

and estimated parameter values. Note that the notation for 

wi follows the convention used in the PEST manual. Other 

authors (for example, Hill, 1998) express equation 9 as

Φ = =

∑ωi i

i

N

r 2

1

, (10)

and define ωi as the weight. Therefore, wi as used in this report 

is equivalent to the square root of ωi

 in equation 10.

Calibration Data

The SVRP aquifer model is calibrated using both waterlevel and flow measurements. Water-level measurements 

include:

Synoptic water-level measurements for 268 wells during 

September 2004,

Monthly water-level measurements for 55 monitoring 

wells from the summer of 2004 through September 2005,

Water-level measurements by Caldwell and Bowers 

(2003) for 11 wells in the vicinity of the Spokane River 

during 2000 and 2001, and

Historical water-level data for four wells in the monitoring 

network (wells 92, 209, 236, and 251) from October 1995 

to September 2005.

A total of 1,573 water-level measurements are used in the 

model calibration. Water-level measurements for several 

wells are excluded as calibration data. A well is excluded if it 

meets one of the following criteria: (a) the well is completed 

in bedrock or in both bedrock and in aquifer sediments, (b) 

the well encounters the narrow ground-water mound beneath 

the losing segment of the Spokane River upstream of the 

gaging station at Greenacres, and (c) the well is located along 

the aquifer boundary where water levels differ by more than 

several tens of feet from those of nearby wells.

Flow measurements consist of streamflow gains and 

losses along segments of the Spokane and Little Spokane 

Rivers. Flow measurements include:

Monthly average streamflow gains and losses from 

October 1995 to July 1999 on the segment of the Spokane 

River from the gaging stations near Post Falls to at 

Spokane,

Monthly average streamflow gains and losses from 

August 1999 to September 2005 on the segment of the 

Spokane River from the gaging stations near Post Falls to 

at Greenacres,

Monthly average streamflow gains and losses from 

August 1999 to September 2005 on the segment of the 

Spokane River from the gaging stations at Greenacres to 

at Spokane,

Monthly average streamflow gains from October 1997 

to September 2005 on the segment of the Little Spokane 

River from the gaging stations at Dartford to near 

Dartford,

Synoptic streamflow gains and losses on the Spokane 

and Little Spokane Rivers during the seepage run during 

September 13–16, 2004, and

1.

2.

3.

4.

1.

2.

3.

4.

5.

52  Ground-Water Flow Model for the SVRP Aquifer, Washington and Idaho
Synoptic streamflow gains and losses on the Spokane 

and Little Spokane Rivers during the seepage run during 

August 26–31, 2005.

A total of 313 flow measurements are used in the model 

calibration.

Weights for Measured Quantities

A standard approach to determining the weight for a 

measured quantity is to calculate wi as the inverse of the 

standard deviation of the error associated with the i

th measured 

quantity (Hill, 1998). To apply this approach, the following 

sources of error are evaluated for water-level measurements:

Error in the altitude of land surface datum at the well 

site. For all wells used in this study, the land surface 

datum errors are given in the column under the heading 

“Accuracy of land-surface altitude” in table 1 of the 

water-level map by Campbell (2005). These errors range 

from 0.01 to 20 ft. The altitudes of several well sites have 

been re-surveyed since the publication of the water-level 

map.

Error in the measurement method. For the September 

2004 synoptic water-level data listed in table 1 of the 

water-level map by Campbell (2005), the measurement 

method is given in the column under the heading “Method 

of water-level measurement.” Six measurement method 

codes are listed: A (airline), C (calibrated airline), G 

(pressure gage), S (steel tape), T (electric tape), and V 

(calibrated electric tape). For the weight calculation, the 

error is assumed to be 5 ft for A, C, and G, and 0.02 ft 

for S, T, and V. For all other water-level measurements 

used in the model calibration, the measurement method 

is assumed to be S, T, or V, and the error is assumed to be 

0.02 ft.

Error from local disturbance, such as pumping. This error 

is inferred from a water-level status code that is recorded 

with each water-level measurement. If the code is P (the 

well is being pumped at the time of measurement), the 

error is assumed to be 10 ft. If the code is R (the well 

was recently pumped prior to the time of measurement), 

the error is assumed to be 2 ft. If the code is S (a nearby 

well is being pumped at the time of measurement) or T 

(a nearby well was recently pumped prior to the time of 

measurement), the error is assumed to be 1 ft. If no code 

is recorded, the error is assumed to be zero.

Following Hill’s (1998, p. 46) suggestion, each of the 

above error values is interpreted as a 95-percent confidence 

interval. For example, a land-surface datum error of 10 ft is 

6.

1.

2.

3.

interpreted to mean that the probability is 95 percent that the 

actual land-surface datum is within ±10 ft of the measured 

datum. If the error is assumed to be a normally distributed 

random variable, then a 10-ft error is equal to 1.96 times the 

standard deviation. Therefore, the standard deviation is 5.1 ft. 

To account for multiple sources of errors, the variance (square 

of standard deviation) for each error source is computed, 

the variances are summed, and the square root of the sum is 

calculated. This gives the standard deviation of the total error. 

The weight then is computed as the inverse of this standard 

deviation.

A similar procedure (Hill, 1998, p. 46-47) is used to 

determine the weights of the flow measurements (streamflow 

gains and losses). Because the streamflow gain or loss on 

a river segment is determined from the difference between 

streamflows at the upstream and downstream ends of the river 

segment, the variances of the errors of the two streamflow 

values are summed. Based on the study by Sauer and Meyer 

(1992), the error of a streamflow measurement is estimated 

to be 5 percent of the measured value. To illustrate the weight 

calculation, suppose the streamflow at the upstream gaging 

station is 1,000 ft3/s and the streamflow at the downstream 

gaging station is 800 ft3/s, giving a streamflow loss of 200 

ft3/s. For the streamflow at the upstream gaging station, the 

error is 50 ft3/s. Interpreting this error to equal 1.96 times the 

standard deviation gives a standard deviation of 25.5 ft3/s, or a 

variance of 651 (ft3/s)2. By the same reasoning, the error of the 

streamflow at the downstream gaging station is 40 ft3/s, giving 

a standard deviation of 20.4 ft3/s and a variance of 416 (ft3/s)2. 

The variance of the error of the streamflow loss (200 ft3/s) 

is the sum of the variances of the upstream and downstream 

streamflows, or 1,067 (ft3/s)2. The standard deviation is 32.7 

ft3/s, and the weight is the inverse of the standard deviation.

Initial calibration runs using weights determined by 

the previously described procedure indicated that the sum of 

squares of weighted residuals is heavily dominated by the 

water-level measurements, and the fit to flow measurements 

is poor. This results from the fact that (1) the number of 

water-level measurements is five times the number of 

flow measurements, and (2) relative errors in water-level 

measurements generally are much smaller than relative 

errors in flow measurements. To create a more even balance 

between the weighted water-level residuals and the weighted 

flow residuals, the weights for water-level measurements 

are reduced by adding 5 ft to the standard deviations of the 

water-level errors. This adjustment improves the fit to flow 

measurements without substantially degrading the fit to waterlevel measurements.

Model Calibration  53
Estimated Parameters

The calibration set-up as previously described involves 42 

parameters. These parameters are listed in the first column of 

table 8. During the initial calibration phase, it was determined 

that calibration data were insensitive to HK1-21 (the 

horizontal hydraulic conductivity of the side valley in which 

Newman Lake is located) and KVSR-11 (the vertical hydraulic 

conductivity of the streambed sediments in the segment of the 

Spokane River in the Little Spokane River Arm). In addition, 

initial calibration runs yielded relatively high values of COUT (the boundary conductance for outflow from the lower 

unit to Long Lake), but the estimated C-OUT was nonunique 

and varied from one calibration run to another depending on 

starting parameter values. In addition, the estimated value of 

HK3-2 (horizontal hydraulic conductivity of the lower unit in 

the Little Spokane Arm) was unreasonably high. Therefore, 

the four previously mentioned parameters are not estimated by 

calibration but are assigned as follows. HK1-21 is set to 150 

ft/d. This Kh

 value results in a simulated water level of about 

2,100 ft in the vicinity of Newman Lake. This water level is 

about 30 ft below the level of Newman Lake. KVSR-11 is set 

to 0.1 ft/d. This Kv value limits the interaction between the 

aquifer and the segment of the Spokane River in the Little 

Spokane River Arm. The limited interaction is reasonable 

because most of the ground-water discharge in the Little 

Spokane River Arm would enter the Little Spokane River. 

C-OUT is set to a relatively high value of 106 ft2/d. Increasing 

or decreasing this value by one order of magnitude has little 

or no effect on the simulation results. However, setting a high 

value for C-OUT is nearly equivalent to setting the head at the 

outflow boundary close to the level of Long Lake. HK3-2 is 

set to 2,000 ft/d. This value based on Kh values estimated for 

the Hillyard Trough area (see fig. 6).

The PEST program requires specifying an acceptable 

interval for each estimated parameter. The lower and 

upper limits defining this interval are given in the third and 

fourth columns of table 8. For hydraulic conductivity and 

conductance, the acceptable interval is set fairly wide, with 

the expectation that the estimated value would fall within the 

acceptable interval. For specific yield, the acceptable interval 

is set from 0.1 to 0.3. PEST always yields an estimated value 

that is within the parameter’s acceptable interval (inclusive of 

the upper and lower limits).

Estimated values obtained from the calibration are given 

in the fifth column of table 8. A value in bold indicates that 

the estimated value is at either the upper or lower limit of 

the acceptable interval. In the central part of the aquifer in 

Rathdrum Prairie and in Spokane Valley, estimated Kh values 

ranged from 6,170 to 22,100 ft/d. In Hillyard Trough, the 

Little Spokane River Arm, and Western Arm, estimated Kh

values ranged from 1,980 to 3,110 ft/d. In the Coeur d’Alene 

area, the estimated Kh value is 1,290 ft/d. For side valleys and 

regions of shallow bedrock along the margins of the aquifer, 

estimated Kh values ranged from 5 to 140 ft/d. These estimated

Kh values generally are consistent with Kh values estimated in 

previous studies (see the discussion in the section, “Hydraulic 

Properties.”).

Estimated SY values are 0.1 for SY-1, 0.19 for SY-2, and 

0.21 for SY-3. The estimated SY-1 value is at the lower limit 

of the acceptable range. The implication of this calibration 

result is explored using alternative model C in the section, 

“Alternative Models.”

Estimated Kv

 values of streambed sediments indicate that 

these parameters are related to gaining and losing segments 

of the Spokane River. Along losing segments of the Spokane 

River, estimated Kv values of streambed sediments are less 

than 1 ft/d (parameters KVSR-1 to KVSR-4, KVSR-6, and 

KVSR-8). Along gaining segments of the Spokane River, 

estimated Kv values of streambed sediments are greater than 1 

ft/d (parameters KVSR-5, KVSR-7, KVSR-9, and KVSR-10). 

These results support the suggestion by Caldwell and Bowers 

(2003) that the transport of fine-grained material with the 

leaking water from the Spokane River might decrease Kv of the 

streambed sediments along a losing segment of the river.

The sixth and seventh columns of table 8 give the linear, 

95-percent confidence interval for the estimated parameter 

values. These confidence intervals should be interpreted 

with caution. The confidence intervals are approximate and 

are computed under the assumption that the model is linear 

with respect to the parameters in the vicinity of the estimated 

values. If this linearity assumption is not valid, then the 

confidence intervals are inaccurate.

54  Ground-Water Flow Model for the SVRP Aquifer, Washington and Idaho
Table 8. Model parameters, acceptable intervals, estimated values, and 95-percent confidence intervals.

[Value in italic indicates parameter is specified and not estimated; value in bold indicates that estimated value is at either upper or lower limit of acceptable 

interval. Abbreviations: ft/d, foot per day; --, dimensionless; ft2/d, foot squared per day; n.a., not assigned; n.c., not calculated]

Parameter Units

Acceptable interval

Estimated value

95-percent confidence interval

Lower limit Upper limit Lower limit Upper limit

HK1-1 ft/d 100 50,000 13,000 6,440 26,400

HK1-2 ft/d 100 50,000 6,170 4,300 8,860

HK1-3 ft/d 100 50,000 17,100 13,500 21,700

HK1-4 ft/d 100 50,000 12,100 10,800 13,500

HK1-5 ft/d 100 50,000 22,100 20,200 24,300

HK1-6 ft/d 100 50,000 19,100 17,800 20,400

HK1-7 ft/d 100 50,000 7,470 6,820 8,170

HK1-8 ft/d 100 50,000 9,500 8,040 11,200

HK1-9 ft/d 1 5,000 2,630 2,400 2,870

HK1-10 ft/d 1 5,000 2,180 2,020 2,360

HK1-11 ft/d 1 5,000 1,980 1,710 2,300

HK1-12 ft/d 1 5,000 608 485 762

HK1-13 ft/d 1 5,000 3,110 2,470 3,920

HK1-14 ft/d 1 5,000 90 82 98

HK1-15 ft/d 1 5,000 1,290 755 2,190

HK1-16 ft/d 1 5,000 55 53 56

HK1-17 ft/d 1 5,000 5 4 7

HK1-18 ft/d 1 5,000 78 74 82

HK1-19 ft/d 1 5,000 95 93 97

HK1-20 ft/d 1 5,000 64 55 76

HK1-21 ft/d n.a. n.a. 150 n.c. n.c.

HK1-22 ft/d 1 5,000 140 131 150

HK3-1 ft/d 1 5,000 207 155 276

HK3-2 ft/d n.a. n.a. 2,000 n.c. n.c.

SY-1 -- .1 .3 .1 .08 .13

SY-2 -- .1 .3 .19 .16 .21

SY-3 -- .1 .3 .21 .18 .23

KVSR-1 ft/d .01 10 .054 .047 .062

KVSR-2 ft/d .01 10 .25 .23 .27

KVSR-3 ft/d .01 10 .27 .24 .29

KVSR-4 ft/d .01 10 .14 .10 .20

KVSR-5 ft/d .01 10 9.4 7.3 12.2

KVSR-6 ft/d .01 10 .01 .005 1.8

KVSR-7 ft/d .01 10 10 5.6 18

KVSR-8 ft/d .01 10 .3 .20 .45

KVSR-9 ft/d .01 10 10 1.70 59

KVSR-10 ft/d .01 10 10 .63 159

KVSR-11 ft/d n.a. n.a. .1 n.c. n.c.

C-LSR ft2

/d 10-10 1010 40,600 36,100 45,700

C-PO ft2

/d 10-10 1010 241,000 102,000 572,000

C-CDA ft2

/d 10-10 1010 77,800 40,000 151,000

C-OUT ft2

/d n.a. n.a. 106 n.c. n.c.

Model Calibration  55
Assessment of Calibration

The results of the calibration can be assessed by 

comparing simulated and measured quantities and by 

examining the weighted residuals. Simulated and measured 

water levels in selected wells in various parts of the SVRP 

aquifer are shown in figure 41. Except as noted, the same scale 

is used for all vertical axes for ease of comparison of waterlevel fluctuations in different wells.

For the upgradient end of the aquifer near Lake Pend 

Oreille, simulated and measured water levels in well 236 

are in close agreement (fig. 41A). For northern Rathdrum 

Prairie, simulated and measured water levels in well 251 

(fig. 41B) and well 249 (fig. 41C) are compared. In well 251, 

the simulated rise in water level during 1996 and 1997 is not 

as large as the measured rise. In both wells 249 and 251, the 

simulated fluctuations during 2004 and 2005 are somewhat 

larger in magnitude than the measured fluctuations. These 

discrepancies likely are due to two simplifying assumptions 

used in calculating recharge from precipitation: (1) use of 

a triangular network to linearly interpolate recharge from 

precipitation (fig. 7), and (2) use of a linear relation between 

depth of water table and transmission time for precipitation 

infiltration to reach water table (table 2).

For southern Rathdrum Prairie, simulated and measured 

water levels in well 225 (fig. 41D) and well 178 (fig. 41E) 

are compared. In both wells, the simulated water levels are 

close to the measured water levels, but the character of the 

simulated fluctuations during 2004–05 does not match the 

character of the measured fluctuations. The same discrepancy 

is shown in the simulated and measured water levels in well 

143 (fig. 41F), which is near Coeur d’Alene Lake. These 

discrepancies indicate that the temporal distribution of 

recharge to southern Rathdrum Prairie (including the Coeur 

d’Alene area) might not be represented accurately in the 

model for 2004–05.

Simulated and measured water levels in well 246 near the 

east edge of Rathdrum Prairie and in well 140 in a side valley 

between Coeur d’Alene and Hayden are shown in figures 41G

and 41H, respectively. Note that the vertical axes in both 

figures span 120 ft. Although these wells have few long-term 

historical data for calibration, the simulation indicates that 

water levels along the margins of Rathdrum Prairie can rise 

and decline by substantial amounts.

Simulated and measured water levels in wells in various 

parts of Spokane Valley are shown in figures 41I to 41M. 

Overall, the seasonal rise and decline of simulated water levels 

reproduce the measured water levels. However, simulated 

fluctuations on short time scales do not always match the 

measured fluctuations. For example, each year during 

September, the measured water level in well 92 (fig. 41K) 

rises in response to the rise in Spokane River stage as the gates 

at the Post Falls Dam are opened. However, the simulated 

water level during September does not always follow the 

rises in the measured water level. This discrepancy might be 

a consequence of assuming a rectangular cross-section for 

the channel of the Spokane River. Under this assumption, the 

streambed area through which seepage occurs is independent 

of river stage. In reality, under low-flow conditions typical 

of later summer, the river might occupy only a portion of the 

streambed. As the river stage rises in September, the river 

might occupy a larger area of streambed. Because the model 

does not simulate this increase in wetted streambed area, the 

simulated exchange between the aquifer and the Spokane 

River might be inaccurate during early autumn.

Simulated and measured water levels in well 6 in Trinity 

Trough are shown in figure 41N. The magnitude of the 

simulated water-level fluctuation is substantially smaller than 

the magnitude of the measured fluctuation. During 2004–05, 

the water level in well 6 rose and declined by about 15 ft. 

This is nearly double the magnitude of the fluctuations in 

other wells in Spokane Valley. Well 6 is located in an area of 

steep hydraulic gradient as ground water is channeled through 

Trinity Trough into Western Arm (fig. 22). The discrepancy 

between simulated and measured water levels in well 6 might 

indicate that a finer model grid (with smaller model cells) is 

needed to more accurately represent Trinity Trough.

Simulated and measured water levels in well 128 in 

Hillyard Trough and in well 107 in Western Arm are shown 

in figures 41O and 41P, respectively. For both wells, the 

simulated water levels are in fairly good agreement with the 

measured water levels.

Simulated and measured water levels in the lower unit 

are shown in figures 41Q and 41R. Well 115 (fig. 41Q) is 

at the north end of Hillyard Trough. The drawdowns during 

2004–05 are a result of pumping of a nearby production well. 

However, the simulated drawdowns were substantially larger 

than the measured drawdowns, indicating that the estimated 

value of KH3-1 might be too low. Well 99 (fig. 41R) is located 

on the west side of the Little Spokane River Arm. In this 

well, simulated water levels were about 15 ft higher than the 

measured water levels. Considered together, the relatively 

poor fits to measured water levels in wells 115 and 99 indicate 

that the lower unit might not be represented accurately by the 

model.

 Simulated water levels in model layer 1 during 

September 2004 are shown in figure 42. The contours 

of simulated water levels shown in figure 42 are in good 

agreement with the contours of measured water levels shown 

in figure 22. Simulated water levels in model layer 3 during 

September 2004 are shown in figure 43. Because water-level 

measurements are available for only two wells in the lower 

unit, too few data are available to construct a map of measured 

water levels in the lower unit.

56  Ground-Water Flow Model for the SVRP Aquifer, Washington and Idaho
Figure 41. Simulated and measured water levels in selected wells in various parts of the SpokaneRathdrum Prairie aquifer, Washington and Idaho.

wa19-0088_fig41ABCDE

2,040

2,050

2,060

2,070

A. Well 236

Measured

Simulated (start-up)

Simulated (calibrated)

1990 1995 2000 2005

2,010

2,020

2,030

2,040

2,050

B. Well 251

2,010

2,020

2,030

2,040

2,050

ALTITUDE OF WATER LEVEL, IN FEET

C. Well 249

2,010

2,020

2,030

2,040

2,050

D. Well 225

1,990

2,000

2,010

2,020

2,030

E. Well 178

Model Calibration  57
Figure 41.—Continued.

wa19-0088_fig41FGHI

2,200

2,160

2,240

 2,280

G. Well 246

2,080

2,040

2,120

2,160

H. Well 140

1,975

1,985

1,995

2,005

2,015

I. Well 209

1990 1995 2000 2005

2,010

2,020

2,030

2,040

2,050

ALTITUDE OF WATER LEVEL, IN FEET

F. Well 143

Measured

Simulated (start-up)

Simulated (calibrated)

58 Ground-Water Flow Model for the SVRP Aquifer, Washington and Idaho
wa19-0088_fig41JKLMN

1,940

1,950

1,960

1,970

1,980

K. Well 92

1,920

1,930

1,940

1,950

ALTITUDE OF WATER LEVEL, IN FEET

L. Well 60

1,870

1,880

1,890

1,900

M. Well 16

1,750

1,760

1,770

1,780

N. Well 6

1990 1995 2000 2005

1,960

1,970

1,980

1,990

2,000

J. Well 139

Measured

Simulated (start-up)

Simulated (calibrated)

Figure 41.—Continued.

Model Calibration  59
wa19-0088_fig41OPQR

1,590

1,600

1,610

1,620

P. Well 107

1,830

1,840

1,850

1,860

O. Well 128

Measured

Simulated (start-up)

Simulated (calibrated)

1,675

1,685

1,695

1,705

1,715

ALTITUDE OF WATER LEVEL, IN FEET

Q. Well 115

1,540

1,550

1,560

1,570

1,580

R. Well 99

1990 1995 2000 2005

Figure 41.—Continued.

60 Ground-Water Flow Model for the SVRP Aquifer, Washington and Idaho
Figure 42. Simulated water levels in model layer 1, Spokane Valley-Rathdrum Prairie aquifer, Washington and Idaho, September 2004.

wa19-0088_fig42

0 5 10 15 20 KILOMETERS

0 5 10 15 20 MILES

Little

Spokane R.

Spokane River

Deep

Creek

Coeur d’Alene Lake

Lake

Pend

Oreille

Liberty

Lake

Newman

Lake

Long

Lake

Hauser

Lake

Fernan Lake

Hayden Lake

Spirit

Lake

Twin

Lakes

Hangman

Creek

48°

47°

45'

117°30' 117° 116°30' 1900

1820

1980

2060

1660

Spokane R.

EXPLANATION

Altitude of simulated water level, in feet, in model layer 1,

 September 2004—Datum is North American Vertical 

 Datum of 1988

2,220 to 2,300

2,140 to 2,220

2,060 to 2,140

1,980 to 2,060

1,900 to 1,980

1400

1,820 to 1,900

1,740 to 1,820

1,660 to 1740

1,580 to 1,660

1,500 to 1,580

Simulated water-level contour—Shows altitude of water level,

 September 2004. Contour interval, 20 feet. Datum is North 

 American Vertical Datum of 1988

Base modified from U.S. Geological Survey digital data.

City boundaries, 1:24,000, various years (1961-86);

Public land survey, 1:100,000, 1985; Lakes, 1:100,000, 1995;

and rivers, 1:100,000, 1985. North American Datum of 1983 (NAD 83).

Model Calibration  61
Simulated and measured monthly average streamflow 

gains and losses for the Spokane River segments from the 

gaging stations near Post Falls to at Spokane, near Post Falls 

to at Greenacres, and at Greenacres to at Spokane are shown 

in figure 44. The length of the vertical line extending above 

or below a plotted point indicates the measurement error, 

calculated as 1.96 times the standard deviation used in the 

calculation of measurement weights. During times of high 

streamflow (winter and spring), measurement errors can 

be very large. The simulated streamflow gains and losses 

generally agree with the measured gains and losses during late 

summer, when streamflow is low and the measurement error is 

small.

For the Spokane River segment from the gaging stations 

near Post Falls to at Greenacres (fig. 44B), data show a 

decrease in the magnitude of streamflow loss during the first 

halves of 2003 and 2004 when the river stage is high. This 

is an unexpected phenomenon because the altitude of the 

bottom of the riverbed from the gaging stations near Post Falls 

to at Greenacres is always above the water table. Therefore, 

the magnitude of streamflow loss is expected to increase (as 

indicated by the simulated quantity) during winter and spring 

when the river stage is high. If the measured streamflow at the 

gaging station at Greenacres is inaccurate (too low) during the 

first halves of 2003 and 2004, the error might account for the 

poorer fit during the same two periods for the segment from 

gaging stations at Greenacres to at Spokane (fig. 44C).

The simulated and measured monthly average streamflow 

gains on the Little Spokane River from the gaging stations at 

Dartford to near Dartford are in overall agreement as shown 

in figure 45. The simulated streamflow gain represents the 

regional discharge from the SVRP aquifer and is remarkably 

constant. The measured streamflow gain exhibits fluctuations 

during winter and spring. Nonetheless, the simulated gains are 

within the error bounds of the measured gains.

Simulated and measured streamflow gains and losses on 

various Spokane River segments during the seepage runs of 

September 13-16, 2004, and August 26-31, 2005, are shown 

in figure 46. The measurements made during the seepage runs 

might be characterized as nearly instantaneous. By contrast, 

the simulated streamflow gains and losses represent quantities 

averaged over a stress period (1 month). Nonetheless, the 

simulated and measured quantities are in relatively good 

agreement.

wa19-0088_fig43

0 5 KILOMETERS

0 5 MILES

117°30' 117°20'

47°45'

47°40'

Spokane River

Little Spokane

River

Long

Lake

Deep

Creek

Spokane

R.

EXPLANATION

1400

1740

1660

1580

1820

1,820 to 1,900

1,740 to 1,820

1,660 to 1740

1,580 to 1,660

1,500 to 1,580

Simulated water-level contour—Shows

 altitude of water level, September 2004.

 Contour interval, 20 feet. Datum is North 

 American Vertical Datum of 1988

Base modified from U.S. Geological Survey digital data.

City boundaries, 1:24,000, various years (1961-86);

Public land survey, 1:100,000, 1985; Lakes, 1:100,000, 1995;

and rivers, 1:100,000, 1985. North American Datum of 1983 (NAD 83).

Altitude of simulated water level, 

 in feet, in model layer 3, 

 September 2004—Datum is 

 North American Vertical 

 Datum of 1988

Boundary of Spokane Valley-Rathdrum 

 Prairie ground-water flow model

Southern limit of clay layer

Figure 43. Simulated water levels in model layer 3, Spokane Valley-Rathdrum Prairie aquifer, Washington and Idaho, September 

2004.

62  Ground-Water Flow Model for the SVRP Aquifer, Washington and Idaho
Figure 44. Simulated and measured monthly average streamflow gains and losses for three Spokane River segments, 

Spokane Valley-Rathdrum Prairie aquifer, Washington and Idaho.

wa19-0088_fig44ABC

0

1,000

500

–500

–1,000

–1,500

1,500

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999

Measured

Simulated (start-up)

Simulated (calibrated)

0

1,000

500

–500

–1,000

–1,500

1,500

STREAMFLOW GAIN OR LOSS, IN CUBIC FEET PER SECOND

1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

Measured

Simulated (calibrated)

1,000

500

0

–500

–1,000

–1,500

1,500

1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

Measured

Simulated (calibrated)

C. Simulated and measured streamflow gains (positive values) or losses (negative values) on the Spokane

 River segment from gaging stations at Greenacres to at Spokane. 

A. Simulated and measured streamflow gains (positive values) or losses (negative values) on the Spokane

 River segment from gaging stations near Post Falls to at Spokane. 

B. Simulated and measured streamflow gains (positive values) or losses (negative values) on the Spokane 

 River segment from gaging stations near Post Falls to at Greenacres. 

Model Calibration  63
Figure 45. Simulated and measured monthly average streamflow gains on the Little Spokane River from the gaging 

stations at Dartford to near Dartford.

Figure 46. Simulated and measured streamflow gains (positive values) and losses (negative values) on various 

segments of the Spokane River during seepage runs of September 13-16, 2004, and August 26-31, 2005.

wa19-0088_fig45

100

200

300

400

0

STREAMFLOW GAIN,

IN CUBIC FEET PER SECOND

1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

Measured

Simulated (calibrated)

200

0

–200

600

400

–200

–400

STREAMFLOW GAIN OR LOSS,

IN CUBIC FEET PER SECOND

200

400

0

near Coeur d’Alene L.

to near Post Falls

Near Post Falls to

near Otis Orchards

Near Otis Orchards to

Greenacres

Greenacres to

Flora Road

Flora Rd. to

Centennial Trail Br.

Centennial Trail Br.

to below Greene St.

below Greene St. to

gage at Spokane

gage at Spokane to below 

T.J. Meenach Br.

below T.J. Meenach Br.

to Rifle Club Rd.

Rifle Club Rd. to

below Nine Mile Dam

Measured Simulated

September 13-16, 2004

August 26-31, 2005

64  Ground-Water Flow Model for the SVRP Aquifer, Washington and Idaho
Simulated and measured streamflow gains and losses 

on three Little Spokane River segments during the seepage 

runs of September 13-16, 2004, and August 26-31, 2005, are 

shown in figure 47. The simulated streamflow gains and losses 

are close to the measured streamflow gains and losses except 

for the river segment from the gaging station near Dartford 

to the streamflow measurement site near the mouth of the 

river. For this segment, the data indicate a small streamflow 

loss. As discussed previously, this Little Spokane River is not 

expected to lose streamflow in the Little Spokane River Arm 

of the SVRP aquifer because this is an area of ground-water 

discharge. The calibrated model simulates essentially no 

interaction between the aquifer and this segment of the Little 

Spokane River.

The spatial distribution of weighted water-level residuals 

in model layer 1 during September 2004 is shown in figure 48. 

Ideally, weighted residuals should be distributed randomly 

throughout the model area. In figure 48, however, the positive 

weighted residuals tend to cluster locally with other positive 

weighted residuals, and the negative weighted residuals tend 

to cluster locally with other negative weighted residuals. The 

clustering likely is caused by the representation of aquifer 

properties by zones of uniform values. This simplification 

might be considered a source of model error in the sense that 

such error can be reduced by implementing a more complex 

distribution of aquifer properties in the model. Nonetheless, 

zonation is widely accepted as a useful approach for model 

calibration. Although the spatial distribution of weighted 

residuals shown in figure 48 cannot be characterized as an 

ideal distribution, the distribution does not show signs of gross 

model errors over large parts of the aquifer.

The weighted water-level residuals and simulated water 

levels are shown in figure 49. The plotted points show a fairly 

random distribution of weighted residuals above and below 

zero for all simulated water levels. This feature indicates that 

the simulated water levels fit measured water levels as well in 

an upgradient region (such as northern Rathdrum Prairie) as 

in a downgradient region (such as Western Arm). Similarly, 

the plotted points for weighted flow residuals and simulated 

streamflow gains and losses (fig. 50) show a fairly random 

distribution of points above and below zero for all simulated 

quantities. This feature indicates that the simulated flows fit 

the measured flows as well in a gaining river segment as in 

a losing river segment. The vertical band of points shown in 

figure 50 corresponds to weighted flow residuals for the Little 

Spokane River. The cluster pattern results from the fact that 

simulated streamflow gains on the Little Spokane River are 

relatively constant over time whereas the measured streamflow 

gains show fluctuations during the winter and spring (fig. 45).

Figure 47. Simulated and measured streamflow 

gains (positive values) and losses (negative 

values) on three Little Spokane River segments 

during seepage runs of September 13-16, 2004, 

and August 26-31, 2005.

200

0

–200

200

0

–200

STREAMFLOW GAIN OR LOSS,

IN CUBIC FEET PER SECOND

at Dartford to

Waikiki Rd.

Waikiki Rd. to

near Dartford

near Dartford to

near mouth of river

Measured Simulated

September 13-16, 2004

August 26-31, 2005

Model Calibration  65
Figure 48. Spatial distribution of weighted water-level residuals in model layer 1, September 2004, Spokane Valley-Rathdrum Prairie 

aquifer, Washington and Idaho.

Figure 49. Weighted water-level residuals and simulated water levels.

wa19-0088_fig48

0 5 10 15 20 KILOMETERS

0 5 10 15 20 MILES

Little

Spokane R.

Spokane River

Deep

Creek

Coeur d’Alene Lake

Lake

Pend

Oreille

Hangman

Creek

117°30'

48°

47°

45'

117° 116°30'

Spokane R.

Long

Lake

Liberty

Lake

Newman

Lake Hauser

Lake

Fernan Lake

Hayden Lake

Spirit

Lake

Twin

Lakes

 Weighted water-level residuals in 

 model layer 1, September 2004

 –6.2 to –2

 –2 to –1

 –1 to –0.25

 –0.25 to 0

EXPLANATION

Extent of Spokane Valley-Rathdrum

 Prairie aquifer ground-water 

 flow model

0 to 0.25

0.25 to 1

1 to 2

2 to 3.4

Base modified from U.S. Geological Survey digital data.

City boundaries, 1:24,000, various years (1961-86);

Public land survey, 1:100,000, 1985; Lakes, 1:100,000, 1995;

and rivers, 1:100,000, 1985. North American Datum of 1983 (NAD 83).

0

2

–2

–4

–6

4

6

WEIGHTED WATER-LEVEL RESIDUAL

1,500 1,600 1,700 1,800 1,900

ALTITUDE OF SIMULATED WATER LEVEL, IN FEET

2,000 2,100 2,200 2,300

66  Ground-Water Flow Model for the SVRP Aquifer, Washington and Idaho
Simulated Flows in Aquifer

To examine simulated ground-water movement, the 

SVRP aquifer is divided into eight subregions, seven of which 

are shown in figure 51. The eighth subregion is the lower 

unit, which underlies Hillyard Trough and the Little Spokane 

River Arm. For each subregion, simulated inflows, outflows, 

and change in storage are averaged over the model calibration 

period from October 1995 to September 2005 and are given 

under the Calibrated Model column in table 9. Additional 

columns in table 9 contain water-budget components for 

alternative models discussed in the next section of this report. 

Note that quantities averaged over the 10-year calibration 

period have slightly different values than the corresponding 

15-year averaged quantities given in the sections, “Inflows to 

Aquifer” and “Outflows from Aquifer”.

For each subregion, inflow components include recharge 

from precipitation, flow from tributary basins, flow from lakes 

and losing segments of rivers, and flow from the upstream 

subregion. Outflow components include net water use (well 

withdrawals minus return percolation from irrigation and 

effluent from septic systems), flow to gaining segments of 

rivers, and flow to the downstream subregion.

Many of the flow components in the subregional water 

budget are specified in the model. These components are 

shown in italic and include recharge from precipitation, net 

water use, and flow to the aquifer from tributary basins and 

from all lakes except Lake Pend Oreille and Coeur d’Alene 

Lake. In addition, the model is calibrated to fit measured 

flows between the aquifer and the Spokane and Little Spokane 

Rivers. Therefore, the flow components that are neither 

specified nor constrained by calibration data are: (1) inflow to 

the aquifer from Lake Pend Oreille, (2) inflow to the aquifer 

from Coeur d’Alene Lake, and (3) outflow from the aquifer 

(lower unit) to Long Lake. The calibrated model gives a 10-

year average flow of 67 ft3/s from Lake Pend Oreille to the 

aquifer, 138 ft3/s from Coeur d’Alene Lake to the aquifer, and 

27 ft3/s from the lower unit to Long Lake.

On the east side of the aquifer, flow from a subregion 

to the adjacent downgradient subregion increases along the 

direction of ground-water flow. The simulated 10-year average 

flow from northern Rathdrum Prairie to southern Rathdrum 

Prairie is 286 ft3/s, of which 72 percent flows through West 

Channel, 26 percent flows through Ramsey Channel, and 2 

percent flows through Chilco Channel (table 9, fig. 51). The 

simulated 10-year average flow from southern Rathdrum 

Prairie to eastern Spokane Valley is 823 ft3

/s. The boundary 

between eastern Spokane Valley and the Spokane area 

coincides with Sullivan Road, which crosses the Spokane 

River near the point where the river changes from a losing 

stream to a gaining stream. The simulated 10-year average 

flow from eastern Spokane Valley to the Spokane area is 1,280 

ft3/s.

From the Spokane area, the simulated 10-year average 

outflows are 623 ft3/s to the Spokane River, 288 ft3/s to 

Hillyard Trough, and 264 ft3

/s through Trinity Trough to 

Western Arm. From Hillyard Trough, the simulated 10-year 

average outflows are 254 ft3/s to the upper unit in Little 

Spokane River Arm (and eventually to the Little Spokane 

River) and 27 ft3/s to the lower unit (and eventually to Long 

Lake).

Figure 50. Weighted flow residuals and simulated streamflow gains (positive values) or losses (negative 

values).

wa19-0088_fig50

0

–4

–8

4

8

10

WEIGHTED FLOW RESIDUAL

–200 400 600 800 1,000

SIMULATED STREAMFLOW GAIN OR LOSS, IN CUBIC FEET PER SECOND

–600 –400 0 200

Spokane River

Little Spokane River

Model Calibration  67
Figure 51. Subregions of the Spokane Valley-Rathdrum Prairie aquifer for water-budget calculations.

wa19-0088_fig51

0 5 10 15 20 KILOMETERS

0 5 10 15 20 MILES

Little Spokane River

Spokane River

Deep

Creek

Coeur d’Alene Lake

Lake

Pend

Oreille

Liberty

Lake

Newman

Lake

Northern Rathdrum Prairie

Southern

Rathdrum Prairie

Eastern Spokane Valley

Spokane Area

Little Spokane

River Arm

Western

Arm

Hillyard

Trough

Long

Lake

Hauser

Lake

Fernan Lake

Hayden Lake

Spirit

Lake

Twin

Lakes

Hangman

Creek

117°30'

48°

47°

45'

117° 116°30'

Spokane R.

Alternative Models

To examine the assumptions in the SVRP aquifer model, 

five alternative models are analyzed. In each alternative 

model, one aspect of the calibrated model is modified and 

the alternative model is recalibrated. Estimated values of 

parameters and regression statistics from the calibrated and 

alternative models are given in tables 10 and 11.

In alternative model A, the value of HK1-15 (Kh of the 

zone next to Coeur d’Alene Lake) is set to 500 ft/d (table 9). 

This value is similar to the hydraulic conductivity value 

used in Sagstad’s (1977) Darcy’s Law calculation of seepage 

from Coeur d’Alene Lake (see section, “Lakebed Seepage 

and Surface Overflow”). In the calibrated model, the value 

of HK1-15 is 1,290 ft/d and the simulated 10-year average 

seepage from Coeur d’Alene Lake is 138 ft3/s. Using a lower 

68 Ground-Water Flow Model for the SVRP Aquifer, Washington and Idaho
Table 9. Simulated 10-year average water budget for subregions of the Spokane Valley-Rathdrum Prairie aquifer.

[Values are in cubic foot per second. Value in italic indicates flow component is specified; value in bold indicates flow component is not specified]

Calibrated

model

Alternative model

A B C D E

northern 

Rathdrum

Prairie

Inflow

recharge from precipitation 77 77 77 77 77 77

tributary basins 63 63 63 63 108 63

Lake Pend Oreille 67 107 65 64 95 75

Spirit Lake 55 55 55 55 55 55

Twin Lakes 40 40 40 40 40 40

Outflow

net water use -6 -6 -6 -6 -6 -6

West Channel -206 -235 -204 -204 -259 -212

Ramsey Channel -75 -87 -75 -75 -96 -78

Chilco Channel -5 -5 -5 -5 -5 -5

Decrease in Storage -9 -9 -9 -9 -9 -9

southern

Rathdrum

Prairie

Inflow

West Channel 206 235 204 204 259 212

Ramsey Channel 75 87 75 75 96 78

Chilco Channel 5 5 5 5 5 5

recharge from precipitation 74 74 74 74 74 74

tributary basins 20 20 20 20 20 20

Hayden Lake 70 70 70 70 70 70

Fernan Lake 15 15 15 15 15 15

Coeur d’Alene Lake 138 29 175 132 104 137

Spokane River 274 287 276 272 265 273

Outflow net water use -57 -57 -57 -57 -57 -57

eastern Spokane Valley -823 -769 -861 -813 -855 -830

Decrease in Storage 4 4 4 4 5 4

eastern 

Spokane

Valley

Inflow

southern Rathdrum Prairie 823 769 861 813 855 830

recharge from precipitation 30 30 30 30 30 30

tributary basins 16 16 16 16 16 16

Hauser Lake 20 20 20 20 20 20

Newman Lake 23 23 23 23 23 23

Liberty Lake 6 6 6 6 6 6

Spokane River 377 378 378 377 377 378

Outflow net water use -16 -16 -16 -16 -16 -16

Spokane area -1,280 -1,227 -1,319 -1,270 -1,312 -1,288

Decrease in Storage 2 2 2 2 2 2

value of HK1-15 in alternative model A effectively limits the 

seepage from Coeur d’Alene Lake. Compared to the calibrated 

model, alternative model A has higher Kh values in northern 

Rathdrum Prairie (HK1-1, HK1-2, and HK1-3) and a lower 

conductance value for the Coeur d’Alene lakebed sediments 

(C-CDA) (table 10). The simulated 10-year average seepage 

from Lake Pend Oreille to the aquifer increases to 107 ft3/s 

(table 9), and the simulated 10-year average seepage from 

Coeur d’Alene Lake to the aquifer decreases to 29 ft3/s, which 

is close to Sagstad’s (1977) estimated of 37 ft3/s. However, as 

shown in table 11, the sum of squared weighted residuals from 

alternative model A is only about 1 percent higher than that 

from the calibrated model. This indicates that the simulated 

quantities from alternative model A fit the measured quantities 

nearly as well as the simulated quantities from the calibrated 

model.

Model Calibration  69
Calibrated 

model

Alternative model

A B C D E

Spokane 

area

Inflow

eastern Spokane Valley 1,280 1,227 1,319 1,270 1,312 1,288

recharge from precipitation 30 30 30 30 30 30

tributary basins 4 4 4 4 4 4

Outflow

net water use -133 -133 -133 -133 -133 -133

Spokane River -623 -616 -620 -618 -625 -628

Hillyard Trough -293 -293 -349 -293 -293 -293

Western Arm -264 -218 -249 -260 -295 -268

Decrease in Storage 0 0 0 0 0 0

Western 

Arm

Inflow

Spokane area 264 218 249 260 295 268

recharge from precipitation 7 7 7 7 7 7

tributary basins 16 16 16 16 16 16

Outflow net water use -3 -3 -3 -3 -3 -3

Spokane River -283 -238 -269 -280 -314 -287

Decrease in Storage 0 0 0 0 0 0

Hillyard 

Trough

Inflow

Spokane area 293 293 349 293 -293 293

recharge from precipitation 14 14 14 14 14 14

tributary basins 3 3 3 3 3 3

Outflow

net water use -28 -28 -28 -28 -28 -28

Little Spokane River Arm -254 -254 -254 -254 -254 -254

Lower Unit -27 -27 -84 -27 -27 -27

Decrease in Storage 0 0 0 0 0 0

Little 

Spokane

River Arm

Inflow

Hillyard Trough 254 254 254 254 254 254

recharge from precipitation 5 5 5 5 5 5

tributary basins 6 6 6 6 6 6

Outflow

net water use 0 0 0 0 0 0

Little Spokane River -257 -257 -257 -257 -257 -257

Spokane River/Long Lake -8 -8 -8 -8 -8 -8

Decrease in Storage 0 0 0 0 0 0

Lower 

Unit

Inflow Hillyard Trough 27 27 84 27 27 27

Outflow net water use 0 0 0 0 0 0

Long Lake -27 -27 -84 -27 -27 -27

Decrease in Storage 0 0 0 0 0 0

Table 9. Simulated 10-year average water budget for subregions of the Spokane Valley-Rathdrum Prairie aquifer—

Continued.

[Values are in cubic foot per second. Value in italic indicates flow component is specified; value in bold indicates flow component is not 

specified]

70 Ground-Water Flow Model for the SVRP Aquifer, Washington and Idaho
Table 10. Esitmated values of parameters for calibrated and alternative models.

[ft/d; foot per day; --, dimensionless; ft2/d, foot squared per day; value in italic indicates that parameter is specified and not estimated]

Parameter Units Calibrated 

model

Alternative model

A B C D E

HK1-1 ft/d 13,000 17,700 12,600 12,600 18,400 14,300

HK1-2 ft/d 6,170 8,340 6,100 6,050 7,660 6,610

HK1-3 ft/d 17,100 21,000 16,600 17,200 23,200 17,800

HK1-4 ft/d 12,100 11,900 12,600 11,800 13,200 12,200

HK1-5 ft/d 22,100 20,700 23,100 21,900 22,700 22,400

HK1-6 ft/d 19,100 18,300 19,700 19,000 19,600 19,100

HK1-7 ft/d 7,470 7,020 7,830 7,400 7,720 7,520

HK1-8 ft/d 9,500 8,480 10,400 9,410 10,200 9,610

HK1-9 ft/d 2,630 2,690 2,650 2,630 2,590 2,620

HK1-10 ft/d 2,180 2,170 2,170 2,180 2,190 2,190

HK1-11 ft/d 1,980 1,990 1,990 1,990 1,990 1,990

HK1-12 ft/d 608 501 571 599 681 617

HK1-13 ft/d 3,110 2,600 2,960 3,070 3,440 3,160

HK1-14 ft/d 90 89 90 85 90 90

HK1-15 ft/d 1,290 500 1,580 1,240 1,000 1,280

HK1-16 ft/d 55 55 55 54 54 55

HK1-17 ft/d 5 5 5 5 5 5

HK1-18 ft/d 78 78 78 78 78 78

HK1-19 ft/d 95 95 95 95 94 95

HK1-20 ft/d 64 64 65 65 64 64

HK1-21 ft/d 150 150 150 150 150 150

HK1-22 ft/d 140 145 140 141 142 140

HK3-1 ft/d 207 208 619 207 206 206

HK3-2 ft/d 2,000 2,000 7,030 2,000 2,000 2,000

SY-1 -- .10 .10 .10 .081 .10 .10

SY-2 -- .19 .19 .19 .19 .19 .18

SY-3 -- .21 .21 .21 .21 .21 .21

KVSR-1 ft/d .054 .057 .055 .054 .052 .054

KVSR-2 ft/d .25 .25 .25 .25 .25 .25

KVSR-3 ft/d .27 .27 .27 .27 .27 .27

KVSR-4 ft/d .14 .14 .14 .14 .14 .14

KVSR-5 ft/d 9.4 10 8.7 9.1 9.1 9.4

KVSR-6 ft/d .01 .01 .01 .01 .01 .01

KVSR-7 ft/d 10 10 10 10 10 10

KVSR-8 ft/d .30 .26 .29 .30 .33 .31

KVSR-9 ft/d 10 10 10 10 10 10

KVSR-10 ft/d 10 10 10 10 10 10

KVSR-11 ft/d .1 .1 .1 .1 .1 .1

C-LSR ft2

/d 40,600 40,800 40,700 40,700 40,500 40,600

C-PO ft2

/d 241,000 388,000 233,000 235,000 330,000 267,000

C-CDA ft2

/d 77,800 16,100 98,600 74,693 59,000 40,600

C-OUT ft2

/d 106 106 106 106 106 106

Model Calibration  71
In alternative model B, the value of HK3-2 (Kh

 values 

for the lower unit in the Little Spokane River Arm) is not 

specified but is estimated. The calibration yields an HK3-2 

value of 7,030 ft/d (table 10). The simulated 10-year average 

flow from the lower unit to Long Lake increases to 83 ft3

/d 

(table 9). The sum of squared weighted residuals decreases to 

less than that of the calibrated model, indicating an improved 

fit to measured water levels in wells 99 and 115. However, 

this improved fit is attained at the cost of allowing HK3-2 to 

take on a value substantially higher than the upper limit of the 

acceptable range for this parameter. The occurrence of this 

trade-off is an additional indication that the model might not 

accurately represent the lower unit.

In alternative model C, the lower limit of the acceptable 

range for SY-1 is decreased from 0.1 to 0. In this case, the 

calibration yields an SY-1 value of 0.08 (table 10). However, 

the estimated values and the regression statistics from 

alternative model C are very close to those from the calibrated 

model (table 9 and 11). This indicates that setting a lower limit 

of 0.1 for SY-1 results in just as good a fit as allowing SY-1 to 

take on an estimated value less than 0.1.

In alternative model D, an inflow of 40 ft3

/s is applied to 

the model boundary near Spirit and Hoodoo Valleys (boundary 

segment A-B in fig. 2). In the calibrated model, there is no 

inflow across this boundary segment to the aquifer. Compared 

to the calibrated model, alternative model D has higher Kh

values in northern Rathdrum Prairie (HK1-1, HK1-2, and 

HK1-3) (table 10). The simulated 10-year average seepages 

are 95 ft3/s from Lake Pend Oreille and 104 ft3/s from Coeur 

d’Alene Lake (table 9). As in alternative model A, the sum of 

squared weighted residuals from alternative model D is just 

slightly higher than that from the calibrated model (table 11). 

This indicates that amount of inflow from the model boundary 

near Spirit and Hoodoo Valleys cannot be determined reliably 

from calibration of the SVRP aquifer model with the present 

calibration data.

Alternative model E is a hybrid calibration that consists 

of a steady-state model in additional to the transient model. 

The steady-state model simulates average conditions for water 

year 2005 (October 2004 through September 2005). Simulated 

water levels and flows from the steady-state model are fitted to 

averages of water levels and flows measured during water year 

2005. The purpose of simultaneously calibrating a transient 

and a steady-state model is to examine the use of the SVRP 

aquifer model for steady-state simulations.

In general, a steady-state model can be used to simulate 

average conditions over a period (for example, one year) if (1) 

heads throughout the aquifer at the beginning of the period are 

similar to the heads at the end of the period and (2) the model 

is linear or nearly linear with respect to head. Because the 

SVRP aquifer is a water-table aquifer, nonlinearity can occur 

along the aquifer margins, where saturated thickness is small. 

To avoid such nonlinearity, measured water levels along the 

aquifer margins are excluded from the steady-state calibration 

data, with the understanding that the steady-state model might 

not accurately simulate water levels along the aquifer margins.

The sum of squared weighted residuals from 

alternative model E is higher than that from the calibrated 

model (table 11), because the hybrid calibration involves 

additional residuals. However, the estimated parameter 

values in alternative model E are in close agreement with the 

corresponding values in the calibrated model (table 10). This 

indicates that a steady-state model can be used to simulate 

average conditions during water year 2005 with reasonable 

accuracy (except for water levels along the aquifer margins). 

The applicability of the steady-state model is due to the fact 

that heads throughout the aquifer at the beginning of the 

water year (October 2004) are similar to heads at the end of 

the water year (September 2005). By contrast, applying the 

steady-state model to simulate average conditions for water 

year 1998 might lead to inaccurate results because water levels 

in Rathdrum Prairie rose about 15 ft from the beginning of the 

water year to the end of the water year (fig. 27).

Table 11. Regression statistics for calibrated and alternative models.

Regression statistic Calibrated 

model

Alternative model

A B C D E

Sum of 

squared 

weighted

residuals

All 1,500 1,512 1,472 1,497 1,514 1,520

Water levels, model layer 1 590 601 590 589 598 602

Water levels, model layer 3 74 74 47 74 74 77

Spokane River gains and losses 682 682 681 679 688 685

Little Spokane River gains and losses 155 155 154 155 154 156

Standard error of weighted residuals 0.901 0.905 0.893 0.900 0.905 0.896

72  Ground-Water Flow Model for the SVRP Aquifer, Washington and Idaho
Model Limitations and Suggestions for 

Future Work

Although the SVRP aquifer model presented in this 

report provides a relatively good fit between simulated and 

measured quantities, indicating that the overall simulated 

ground-water flow is a reasonable representation of groundwater flow in the SVRP aquifer, the model is subject to 

limitations. These limitations, discussed in the following 

paragraphs, should be taken into consideration when using the 

model.

 In general, the scale of the model and the level of 

detail are intended for analysis of aquifer-wide water-supply 

issues. Although the model might be useful for providing 

boundary conditions for smaller scale investigations, the 

model lacks sufficient details for direct application to smallscale investigations such as the analysis of capture zone for 

an individual well. Additionally, the model is not intended 

for application to contaminant-transport issues such as 

the prediction of contaminant traveltimes or flow paths. A 

contaminant-transport model would require a substantially 

greater amount of hydrogeologic detail for the contamination 

site.

There is insufficient hydrologic information to determine 

ground-water inflow from Spirit and Hoodoo Valleys to the 

SVRP aquifer. The calibrated model in this report assumes no 

inflow from Spirit and Hoodoo Valleys. However, alternative 

model D indicates that the amount of inflow from the Spirit 

and Hoodoo Valleys cannot be determined reliably from model 

calibration with the present calibration data. Monitoring water 

levels in and near Spirit and Hoodoo Valleys could provide 

important data to better evaluate inflows from those valleys to 

the SVRP aquifer.

In Hillyard Trough and the Little Spokane River Arm, 

ground-water flow in the lower unit is not well understood. 

Water levels in the lower unit are monitored in only two 

wells, and the horizontal and vertical extents of the clay layer 

separating the upper and lower units are not well known. 

The model assumes that the clay layer completely isolates 

hydraulic contact between the lower and upper units, but this 

assumption requires critical evaluation. Monitoring water 

levels and conducting aquifer tests in both the upper and lower 

units might lead to better understanding of the hydrogeology 

in Hillyard Trough and Little Spokane River Arm.

There is significant uncertainty in the simulated seepages 

from Lake Pend Oreille and Coeur d’Alene Lake. In addition, 

the water-level fluctuations in wells near Coeur d’Alene Lake 

are not well understood. A detailed study of ground-water flow 

in the Coeur d’Alene area, including aquifer tests to estimate 

hydraulic conductivity in the vicinity of Coeur d’Alene Lake, 

might provide data that can be used to constrain the simulated 

seepage from Coeur d’Alene Lake.

Summary and Conclusions

The ground-water flow model presented in this report 

is a component of a comprehensive study initiated by the 

Idaho Department of Water Resources, the Washington 

Department of Ecology, and the U.S. Geological Survey 

to improve the understanding of ground-water flow in the 

Spokane Valley-Rathdrum Prairie (SVRP) aquifer and of 

the interaction between ground water and surface water. The 

model was developed by the Modeling Team formed within 

the comprehensive study. The Modeling Team consisted of 

staff and personnel working under contract with the Idaho 

Department of Water Resources, personnel working under 

contract with the Washington Department of Ecology, and 

staff of the U.S. Geological Survey. To arrive at a final model 

that has the endorsement of all team members, decisions 

on modeling approach, methodology, assumptions, and 

interpretations were reached by consensus. The primary 

purpose of the model is to serve as a tool for analyzing 

aquifer inflows and outflows, simulating the effects of future 

changes in ground-water withdrawals from the aquifer, and 

for evaluating aquifer management scenarios. The scale of 

the model and the level of detail are intended for analysis of 

aquifer-wide water-supply issues.

The SVRP aquifer model encompasses an area of 

approximately 326 square miles. For the most part, the model 

extent coincides with the 2005 revised extent of the SVRP 

aquifer as defined in previous reports. However, the model 

excludes Spirit and Hoodoo Valleys because of uncertainties 

about the ground-water flow directions in those valleys and 

the degree of hydraulic connection between the valleys and 

northern Rathdrum Prairie. In addition, the model excludes 

three areas, one in northern Rathdrum Prairie and two in the 

vicinity of Five Mile Prairie, because the aquifer sediments in 

those areas likely are unsaturated.

The SVRP aquifer is considered to be a single 

hydrogeologic unit except in Hillyard Trough and the Little 

Spokane River Arm. In those areas, a continuous clay layer 

divides the aquifer into an upper, unconfined unit and a 

lower, confined unit. Both the upper and lower units extend 

from Hillyard Trough toward the west through the Little 

Spokane River Arm. The model terminates at the east end of 

Long Lake, a reservoir on the Spokane River. At this model 

boundary, the upper unit is in direct hydraulic connection with 

Long Lake. Available field data indicate that the clay layer and 

lower unit extend beyond the model boundary for an unknown 

distance. However, the confining clay layer eventually pinches 

out, allowing ground water in the lower unit to discharge into 

Long Lake.

The SVRP aquifer model includes all known components 

of inflows to and outflows from the aquifer. Inflows to 

the SVRP aquifer include (1) recharge from precipitation, 

(2) inflows from tributary basins and adjacent uplands, 

Summary and Conclusions  73
(3) subsurface seepage and surface overflows from lakes 

that border the aquifer, (4) flow from losing segments of the 

Spokane River to the aquifer, (5) return percolation from 

irrigation, and (6) effluent from septic systems. Outflows from 

the SVRP aquifer include (1) ground-water withdrawals from 

wells, (2) flow from the aquifer to gaining segments of the 

Spokane River, (3) aquifer discharge to the Little Spokane 

River, and (4) subsurface outflow from the lower unit at the 

western limit of the model area near Long Lake.

The ground-water flow model MODFLOW-2000 is used 

to simulate ground-water flow in the SVRP aquifer. The finitedifference model grid consists of 172 rows, 256 columns, and 

3 layers. Ground-water flow is simulated for September 1990 

through September 2005 using 181 stress periods of 1 month 

each. Aquifer heads at the start of the simulation period are 

unknown, and the ground-water flow system was assumed 

to be at a steady state during the first stress period. Because 

aquifer heads simulated during this stress period serve as the 

initial conditions for the rest of the transient simulation, the 

first 5 years of the simulation period is considered to be a 

start-up period. No attempt is made to fit simulated quantities 

to measured quantities during the start-up period. Instead, 

the model is calibrated using water-level and flow data for 

October 1995 to September 2005.

Boundary conditions representing inflow and outflow 

components are implemented using packages in MODFLOW2000. The Recharge Package is used to simulate recharge 

from precipitation. The Well Package is used to simulate 

withdrawals from wells, return percolation from irrigation, and 

effluent from septic systems. The Flow and Head Boundary 

Package is used to simulate flows to the aquifer from tributary 

basins and from all lakes except Lake Pend Oreille and Coeur 

d’Alene Lake. The River Package is used to simulate the Little 

Spokane River, Lake Pend Oreille, and Coeur d’Alene Lake. 

The Streamflow-Routing Package is used to simulate the 

interaction between the Spokane River and the aquifer. The 

General-Head Boundary Package is used to simulate groundwater outflow from the lower unit at the west end of the 

Little Spokane River Arm. The spatial distribution of aquifer 

properties such as hydraulic conductivity and specific yield 

are represented by dividing the aquifer into zones. Within each 

zone, the aquifer property is assumed to be uniform.

The parameter estimation program PEST is used to 

calibrate the SVRP aquifer model. PEST implements a 

nonlinear least-squares regression method to estimate model 

parameters by minimizing the sum of squared weighted 

residuals. Calibration data include 1,573 measurements 

of water levels and 313 measurements of streamflow 

gains and losses along segments of the Spokane and Little 

Spokane Rivers. Weights for the measurements initially are 

determined from errors associated with the measurements and 

subsequently adjusted to balance the influence of water-level 

measurements and flow measurements on the regression.

A total of 38 model parameters are estimated during 

calibration. These parameters include 22 values of horizontal 

hydraulic conductivity, 3 values of specific yield, 10 values 

of vertical hydraulic conductivity of riverbed sediments of the 

Spokane River, and 3 values of hydraulic conductances for 

riverbed sediments in the Little Spokane River and for lakebed 

sediments in Lake Pend Oreille and Coeur d’Alene Lake. 

An additional four model parameters are not estimated by 

calibration because the calibration data are insensitive to these 

parameters. Instead, the parameters are assigned reasonable 

values.

Model calibration gives the following results. In the 

central part of the aquifer in Rathdrum Prairie and in Spokane 

Valley, estimated horizontal hydraulic conductivity (Kh

) values 

range from about 6,200 to 22,000 feet per day. In Hillyard 

Trough, the Little Spokane River Arm, and Western Arm, 

estimated Kh values range from about 2,000 to 3,000 feet per 

day. In the Coeur d’Alene area, the estimated Kh

 value is 1,290 

feet per day. For side valleys and regions of shallow bedrock 

along the margins of the aquifer, the estimated Kh values range 

from 4 to 137 feet per day. Estimated specific yield values 

range from 0.10 to 0.21. For the Spokane River bed sediments, 

estimated values of vertical hydraulic conductivity range from 

0.01 to 10 feet per day.

In general, the simulated water levels and flows 

(streamflow gains and losses) are in good agreement with 

the measured water levels and flows throughout most of the 

aquifer. The greatest discrepancies between measured and 

simulated quantities occur in the two wells (wells 99 and 115) 

completed in the lower unit. These discrepancies indicate 

that the lower unit might not be represented accurately by the 

model. For southern Rathdrum Prairie (including the Coeur 

d’Alene area), simulated water-level fluctuations during 2004–

05 do not agree with the observed measured fluctuation. This 

discrepancy indicates that the temporal distribution of recharge 

to southern Rathdrum Prairie might not be represented 

accurately in the model for 2004–05.

The calibrated model gives a 10-year average flow of 

67 cubic feet per second (ft3

/s) from Lake Pend Oreille to the 

aquifer, 138 ft3

/s from Coeur d’Alene Lake to the aquifer, 

and 27 ft3/s from the lower unit to Long Lake. To examine 

the assumptions in the SVRP aquifer model, five alternative 

models are analyzed. In each alternative model, one aspect of 

the calibrated model is modified and the alternative model is 

recalibrated. Results of these alternative model analyses show 

that changes in certain model parameter values can result in 

changes to certain simulated flow components even though the 

overall fit of the alternative model to the measured quantities 

is nearly as good as the calibrated model. This suggests some 

degree of nonuniqueness in the ground-water flow simulated 

by the calibrated model.

74  Ground-Water Flow Model for the SVRP Aquifer, Washington and Idaho
The SVRP model presented in this report is calibrated 

using significantly more data than are used in previous models. 

The relatively good fit between simulated and measured 

quantities indicates that the overall simulated ground-water 

flow is a reasonable representation of ground-water flow in 

the SVRP aquifer. Nonetheless, the model has limitations. 

In particular, there is insufficient hydrologic information 

to determine ground-water inflow from Spirit and Hoodoo 

Valleys to the SVRP aquifer. In Hillyard Trough and the Little 

Spokane River Arm, ground-water flow in the lower unit is 

not well understood, and simulated water levels do not fit 

measured water levels as well as in other parts of the aquifer. 

There also is significant uncertainty in the simulated seepages 

from Lake Pend Oreille and Coeur d’Alene Lake. Further 

investigations in these parts of the SVRP aquifer could provide 

valuable knowledge that can be used to improve the model in 

the future.

Acknowledgments

The authors thank the following individuals who 

provided data that were essential for development of the 

ground-water flow model in this report. Molly Maupin of 

the USGS collected pumping records from water purveyors 

for 1990 through 2005. Jon Hortness of the USGS provided 

estimates of inflows to the SVRP aquifer from tributary 

basins and adjacent uplands. Reanette Boese of the Spokane 

County Utilities provided Spokane River stage measurements 

for 1999–2000. Richard Martindale of the Idaho Panhandle 

Health District provided information on sewer districts and 

septic systems in Idaho. Lloyd Brewer of the City of Spokane 

provided water-use data for Spokane. The staff of the Post 

Falls, Idaho, and Spokane, Washington, USGS field offices 

provided support for field data collection. Reviews by Guy 

Gregory and John Covert of the Washington State Department 

of Ecology, Joel Massmann of Keta Waters, John Lindgren of 

the Idaho Department of Water Resources, Gary Stevens of the 

Idaho Department of Environmental Quality, Lloyd Brewer of 

the City of Spokane, and Devin Galloway and Richard Yager 

of the USGS are gratefully acknowledged.

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78 Ground-Water Flow Model for the SVRP Aquifer, Washington and Idaho
Manuscript approved for publication, April 9, 2007

Prepared by the USGS Publishing Network, 

Bill Gibbs 

David Jones

Cathy Martin

Linda Rogers

Bobbie Jo Richey

For more information concerning the research in this report, contact the 

Washington Water Science Center Director, 

U.S. Geological Survey, 934 Broadway — Suite 300

Tacoma, Washington 98402

http://wa.water.usgs.gov
Hsieh and others SIR 2007–5044 Ground-Water Flow Model for the Spokane Valley-Rathdrum Prairie Aquifer, 

Spokane County, Washington, and Bonner and Kootenai Counties, Idaho