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Streamflow Trends in the 

Spokane River and Tributaries, 

Spokane Valley/Rathdrum Prairie, 

Idaho and Washington

By Jon E. Hortness and John J. Covert

Prepared in cooperation with the

Idaho Department of Water Resources and the

Washington State Department of Ecology

Scientific Investigations Report 2005-5005

U.S. Department of the Interior

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

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2005

For sale by U.S. Geological Survey, Information Services

Box 25286, Denver Federal Center

Denver, CO 80225

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

World Wide Web: http://www.usgs.gov/

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

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

reproduce any copyrighted materials contained within this report.

Suggested citation:

Hortness, J.E., and Covert, J.J., 2005, Streamflow trends in the Spokane River and tributaries, Spokane Valley/

Rathdrum Prairie, Idaho and Washington: U.S. Geological Survey Scientific Investigations Report 2005-5005, 17 p. 
iii

Contents

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

Introduction ................................................................................................ 1

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

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

Available Data ............................................................................................. 2

Trend Analysis ............................................................................................. 6

Spokane River near Post Falls, Idaho (12419000) ................................................ 6

Spokane River at Spokane, Washington (12422500) .......................................... 10

Hangman Creek at Spokane, Washington (12424000) .......................................... 12

Little Spokane River at Dartford, Washington (12431000) .................................... 12

Ground-Water / Surface-Water Exchanges ......................................................... 14

Spokane River ....................................................................................... 14

Little Spokane River ................................................................................. 16

Summary ................................................................................................... 16

References Cited .......................................................................................... 17
iv

Figures

Figure 1. Map showing location of study area, general extent of basin-fill aquifers and 

Spokane Valley/Rathdrum Prairie sole-source aquifer, Idaho and Washington ... 3

Figure 2. Map showing location of gaging stations in the Spokane Valley/Rathdrum 

Prairie, Idaho and Washington ......................................................... 4

Figure 3. Graph showing period of records for gaging stations located within the 

extent of the Spokane Valley/Rathdrum Prairie Aquifer, Idaho and Washington ... 5

Figure 4. Graphs showing trend analysis of August and September monthly mean 

streamflows for the Spokane River near Post Falls, Idaho, 1968 –2002 ............ 7

Figure 5. Graph showing monthly mean stage of Coeur d’Alene Lake, Idaho, 

August and September, 1968–2002 ...................................................... 7

Figure 6. Graph showing annual 7-day low streamflows for the Spokane River near 

Post Falls, Idaho, 1914–2002 ............................................................ 8

Figure 7. Graph showing monthly mean streamflows for the Spokane Valley Farms 

Canal at Post Falls, Idaho, June, July, and August, 1911–1966 ..................... 9

Figure 8. Graph showing trend analysis of annual 7-day low streamflows for the 

Spokane River near Post Falls, Idaho, 1968 –2002 .................................... 9

Figure 9. Graphs showing trend analysis of August and September monthly mean 

streamflows for the Spokane River at Spokane, Washington, 1968–2002 ......... 10

Figure 10. Graph showing annual 7-day low streamflows for the Spokane River at 

Spokane, Washington, 1891–2002 ...................................................... 11

Figure 11. Graph showing trend analysis of annual 7-day low streamflows for the 

Spokane River at Spokane, Washington, 1968 –2002 ................................. 11

Figure 12. Graphs showing trend analysis of September and October monthly mean 

streamflows for the Little Spokane River at Dartford, Washington, 

1930–32, 1947–2002 ........................................................................ 12

Figure 13. Graph showing trend analysis of annual 7-day low streamflows for the 

Little Spokane River at Dartford, Washington, 1930–32 and 1947 –2002 ............ 13

Figure 14. Graphs showing trend analysis of differences in September, October, and 

November monthly mean streamflow between the gaging stations at 

Spokane River at Post Falls, Idaho, and Spokane River at Spokane, 

Washington, 1968–2002 .................................................................. 15

Tables

Table 1. Summary of existing continuous streamflow records for gaging stations located within 

the extent of the Spokane Valley/Rathdrum Prairie aquifer .............................. 5
v

Conversion Factors, Datums, and Acronymns

Conversion Factors

Multiply By To obtain

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

cubic foot (ft3) 0.02832 cubic meter

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

foot (ft) 0.3048 meter

foot per second (ft/s) 0.3048 meter per second

gallon per minute 3.785 liter per minute

inch (in.) 2.54 centimeter

inch per year (in/yr) 25.4 millimter per year

mile (mi) 1.609 kilomter per hour

square mile (mi2) 2.590 square kilometer

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

 °F=(1.8×°C)+32.

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 National Geodetic Vertical Datum of 1929 

(NGVD 29).

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

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

Abbreviations

Abbreviation Meaning

IDWR Idaho Department of Water Resources

USGS U.S. Geological Survey

WADOE Washington State Department of Ecology
Abstract

A clear understanding of the aquifer and river dynamics 

within the Spokane Valley/Rathdrum Prairie is essential in 

making proper management decisions concerning groundwater and surface-water appropriations. Management of the 

Spokane Valley/Rathdrum Prairie aquifer is complicated 

because of interstate, multi-jurisdictional responsibilities, and 

by the interaction between ground water and surface water.

Kendall’s tau trend analyses were completed on 

monthly mean (July through December) and annual 7-day 

low streamflow data for the period 1968–2002 from gaging 

stations located within the Spokane Valley/Rathdrum Prairie. 

The analyses detected trends of decreasing monthly mean 

streamflow at the following gaging stations: Spokane River 

near Post Falls, Idaho (August and September); Spokane River 

at Spokane, Washington (September); and Little Spokane 

River at Dartford, Washington (September and October); and 

decreasing annual 7-day low streamflows at the following 

gaging stations: Spokane River near Post Falls, Idaho and 

Spokane River at Spokane, Washington. Limited analyses of 

lake-level, precipitation, tributary inflow, temperature, and 

water-use data provided little insight as to the reason for the 

decreasing trends in streamflow.

A net gain in streamflow occurs between the gaging 

stations Spokane River near Post Falls, Idaho and Spokane 

River at Spokane, Washington. Significant streamflow 

losses occur between the gaging stations Spokane River 

near Post Falls, Idaho and Spokane River at Greenacres, 

Washington; most, if not all, of the gains occur downstream 

from the Greenacres gaging station. Trends of decreasing net 

streamflow gains in the Spokane River between the near Post 

Falls and at Spokane gaging stations were detected for the 

months of September, October, and November.

Introduction

The Spokane Valley/Rathdrum Prairie (SVRP) aquifer 

is the sole source of drinking water for more than 400,000 

residents in Spokane County, Washington, and Kootenai 

County, Idaho. The area includes the rapidly growing cities of 

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

Coeur d’Alene and Post Falls, Idaho. Recent and projected 

urban, suburban, industrial, and commercial growth has raised 

concerns about potential future impacts on water availability 

and water quality in the SVRP aquifer and the Spokane River 

and its tributaries. Water-resource concerns include growing 

demands on ground water, low streamflow in reaches of the 

Spokane River, a decrease in streamflows over time, and 

water-quality problems associated with changing land-use 

activities. Water-resource demands are increasing at a time 

when aquifer and river dynamics are not well understood. 

This understanding is essential in making proper management 

decisions concerning ground-water and surface-water 

appropriations in the SVRP area.

Management of the SVRP aquifer is complicated because 

of the interstate, multi-jurisdictional responsibilities for the 

aquifer. The states of Washington and Idaho have primary 

responsibility for water allocation and water quality. However, 

local governments are increasingly being called upon to 

consider water supply and quality implications in land-use 

planning. Aquifer management also is complicated by the 

interaction between ground water and surface water. Groundwater exchanges with surface water influence surface-water 

flow rates and water quality, and vice versa.

Streamflow Trends in the Spokane River and Tributaries, 

Spokane Valley/Rathdrum Prairie, Idaho and Washington

By Jon E. Hortness and John J. Covert
The Idaho and Washington Districts of the U.S. 

Geological Survey (USGS), in cooperation with the 

Washington Department of Ecology (WADOE) and the Idaho 

Department of Water Resources (IDWR), have begun work 

on several projects to assist local and State governments in 

understanding the complex issues associated with the SVRP 

aquifer. To this end, a numerical ground-water flow model will 

be developed to simulate the flow in the SVRP aquifer and 

its relation to surface water. The intended users are WADOE 

and IDWR. The intended use of the model is to test future 

management scenarios. The results of this report will be used 

by the modelers to help define specific characteristics of 

streamflow and scope of additional data needed for use in the 

model.

Purpose and Scope

This report documents the results of analysis of 

streamflow data obtained for streams located within the 

extent of the SVRP aquifer. Existing streamflow data for 

these streams were compiled and reviewed. Data from gaging 

stations with 10 or more years of record were analyzed for 

trends during the expected low-flow months of July through 

December. During periods for which statistically significant 

trends in streamflow were observed, other data sets, such 

as temperature and precipitation, were analyzed to look for 

relationships. Where possible, exchanges between ground 

water and surface water also were analyzed.

Description of Study Area

The study area is comprised of the Spokane River 

Valley, Rathdrum Prairie, and surrounding highlands included 

within the extent of the SVRP aquifer (fig. 1) and includes 

parts of eastern Washington and northern Idaho. The valley 

is relatively flat with altitudes ranging from about 1,500 to 

2,600 ft. The valley floor is composed primarily of alluvial 

deposits, and the surrounding highlands primarily are 

composed of bedrock. Land use across the valley floor is 

mostly agricultural, while the highlands are covered primarily 

with coniferous forests and residential housing (Caldwell and 

Bowers, 2003). Development in the area increased rapidly 

between 1990 and 2000. During that period, population 

increased 16 percent in Spokane County, Washington, and 

almost 56 percent in Kootenai County, Idaho (U.S. Census 

Bureau, 2004).

The climate ranges from semiarid to subhumid; summers 

are warm and dry; winters are cool and moist. Most of the 

precipitation falls as snow during the winter months of 

November through March (Molenaar, 1988). Mean annual 

precipitation (1971–2000) across the study area ranges from 

about 16.7 in/yr at the Spokane Airport to about 26.6 in/yr in 

Coeur d’Alene (Western Regional Climate Center, 2004).

Several lakes are located along the edges of the study 

area. The largest of these are Coeur d’Alene Lake and Lake 

Pend Oreille in Idaho. Two perennial streams—the Spokane 

River, which drains Coeur d’Alene Lake and flows westerly, 

and the Little Spokane River. which originates in the highlands 

north of Spokane—flow across the study area. Streamflow 

in the Spokane River is regulated by Post Falls Dam. During 

most years, the dam gates are left open during the months 

of December through June, and streamflow often mimics 

inflows into the lake from snowmelt runoff. During most of 

the summer, however, the gates are set to maintain specific 

water levels in Coeur d’Alene Lake (Gary Stockinger, Avista 

Corporation, oral commun., 2004).

The SVRP aquifer is a basin-fill aquifer composed of 

unconsolidated coarse-grained sands, gravels, cobbles, and 

boulders primarily deposited by a series of catastrophic glacial 

outburst floods during the Pleistocene Epoch (Molenaar, 

1988). Because the sediment is coarser grained than that 

of most basin-fill aquifers, the aquifer is one of the most 

productive, in terms of total withdrawals relative to the size 

of the aquifer, in the United States. Wells located with in the 

aquifer typically yield several thousand gallons per minute 

(Bolke and Vaccaro, 1979). More detailed information on 

the SVRP aquifer can be found in the report by Caldwell and 

Bowers (2003).

Available Data

Ten USGS continuous-record gaging stations either are 

operated currently or were operated previously on streams 

located within the study area (table 1; fig. 2). These include 

seven gaging stations on the Spokane River, one on Hangman 

Creek, and two on the Little Spokane River. The seven gaging 

stations that are currently in operation have at least 10 years 

of record are available for statistical analyses. Many of these 

gaging stations, however, were not operated continuously 

(fig. 3); thus, trend analyses and streamflow comparisons were 

not always possible.

2 Streamflow Trends in the Spokane River and Tributaries, Spokane Valley/Rathdrum Prairie, Idaho and Washington
Figure 1. Location of study area, general extent of basin-fill aquifers and Spokane Valley/Rathdrum Prairie sole-source aquifer, Idaho and Washington.

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Available Data 3
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Figure 2. Location of gaging stations in the Spokane Valley/Rathdrum Prairie, Idaho and Washington.

4 Streamflow Trends in the Spokane River and Tributaries, Spokane Valley/Rathdrum Prairie, Idaho and Washington
Gaging station name Gaging

station No.

Period of record

(water years)

Total years of 

record

Spokane River near Post Falls, ID 12419000 1913–present 92

Spokane River above Liberty Bridge, near Otis Orchard, WA 12419500 1929–83, 1999–present 61

Spokane River at Greenacres, WA 12420500 1948–52, 1999–present 11

Spokane River at Trent, WA1 12421000 1912–13 2

Spokane River below Trent Bridge, near Spokane, WA 12421500 1948–54 7

Spokane River below Green Street at Spokane, WA 12422000 1949–52 4

Spokane River at Spokane, WA 12422500 1891–present 114

Hangman Creek at Spokane, WA 12424000 1948–present 57

Little Spokane River at Dartford, WA 12431000 1929–32, 1947–present 62

Little Spokane River near Dartford, WA 12431500 1948–52, 1998–present 12

Table 1. Summary of existing continuous streamflow records for gaging stations located within the extent of the Spokane Valley/Rathdrum Prairie 

aquifer.

[Locations of gaging stations shown in figure 2. Abbreviations: ID, Idaho; WA, Washington]

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Figure 3. Period of records for gaging stations located within the extent of the Spokane Valley/Rathdrum Prairie Aquifer, Idaho and 

Washington.

(Locations of sites shown in figure 2.)

Available Data 5

1 Trentwood was known as Trent when the gaging station was established.
Trend Analysis

The purpose of the data analyses was to identify any 

long-term trends in streamflows, primarily during baseflow 

periods. Streamflow data for the months of July through 

December were used in each of the analyses. The Kendall’s 

tau hypothesis test was used to test for the presence of trends 

in the data. This test measures the strength of the monotonic 

relation between two data sets (Helsel and Hirsch, 1992). 

In this study, a strong monotonic relation would indicate an 

increase or decrease in streamflow with respect to time. The 

null hypothesis for each test was that no trends existed in the 

data. The significance level (α) defines the acceptable error 

in determining if a trend actually does exist and the p-value is 

the probability that the computed tau value could be obtained 

when the null hypothesis is true (no trend). The acceptable 

error was limited to 5 percent (α = 0.05) for this study. Thus, 

for data sets in which trends were detected, there is less than 

a 5-percent chance that the test was wrong and trends do not 

exist.

Four significant events in the basin that affected 

streamflows during the periods of data collection were noted 

during the analyses. These included (1) the completion of Post 

Falls Dam in 1906; (2) the operation of the Spokane Valley 

Farms Canal, which began in 1924 and diverted water from the 

Spokane River upstream of the Post Falls gaging station; 

(3) the change in operating practices of Post Falls Dam in 

1941 to raise the summer levels of Coeur d’Alene Lake; and 

(4) the discontinuation of irrigation withdrawals through the 

Spokane Valley Farms Canal in 1967.

The trend analyses were completed using data from four 

gaging stations: Spokane River near Post Falls, Spokane River 

at Spokane, Hangman Creek at Spokane, and Little Spokane 

River at Dartford. Data from the other three gaging stations 

with 10 or more years of record (table 1) were not analyzed 

for trends because most of the data were not collected during 

continuous years (fig. 3). Data for the period 1968–2002 

from the Spokane River gaging stations were used because 

operation of the Spokane Valley Farms Canal had ceased and 

these data likely represent the most current conditions for the 

Spokane River.

Spokane River near Post Falls, Idaho (12419000)

Statistically significant decreasing trends in monthly 

mean streamflow were detected for August and September 

(fig. 4). Analyses of data for other months revealed no 

statistically significant trends.

Because of the proximity of this gaging station to Post 

Falls Dam, and the lack of any significant inflows or outflows 

between the gaging station and the dam (Rick Backsen, 

USGS, oral commun., 2004), any trends in the streamflow data 

from the gaging station essentially represent trends in releases 

from the dam. Woods and Beckwith (1997) also determined 

that inflow or outflows within this reach were negligible 

during their hydrologic budget analysis for water years 1991 

and 1992.

It is normal operating procedure to keep lake levels 

steady between late May and early September for recreational 

purposes (Gary Stockinger, Avista Corporation, oral commun., 

2004). August and September monthly mean stage of 

Coeur d’Alene Lake for the post-canal period is shown in 

figure 5. Lake levels were relatively consistent during August 

throughout the entire period. Lake levels during September, 

however, were more variable; mean lake levels increase during 

September throughout the period of interest. This increase 

could be partly the reason for the trends in the September data 

for the Post Falls gaging station.

Given that mean August lake levels have been relatively 

consistent during the post-canal period, the decreasing trends 

in streamflow in August could be related to decreases in 

inflows to or increases in outflows from the system upstream 

of Post Falls Dam (for example, water balance). Inflows 

result primarily from precipitation over the lake, surfacewater inflows to the lake, or ground-water inflows to the 

entire system. A Kendall’s tau analysis of total monthly 

precipitation data (Western Regional Climate Center, 2004) 

for Coeur d’Alene, Idaho, from 1968 to 2002 indicated a slight 

downward trend for both August and September, but neither 

trend was statistically significant based on the criteria used in 

this study (α = 0.05).

Streamflow data from gaging stations on the major 

rivers flowing into Coeur d’Alene Lake (Coeur d’Alene 

River at Cataldo, 12413500, and St. Joe River at Calder, 

12414500) also were analyzed for trends. Data from these two 

gaging stations indicate that these rivers account for about 

92 percent of the total inflows to Coeur d’Alene Lake (Woods 

and Beckwith, 1997). No trends were found in the data for 

either of these gaging stations for water years 1968 through 

2002 based on the criteria used in this study (α = 0.05). Any 

changes in ground-water influences upstream of the lake likely 

would be seen in the streamflow data for the Coeur d’Alene 

River. Lack of trends in these data during the study period 

likely proves that ground-water conditions upstream of the 

lake also were essentially unchanged. Ground-water levels 

between Post Falls Dam and the Post Falls gaging station are, 

for the most part, as much as 100 ft below land surface, and 

there is believed to be little if any interaction between the 

aquifer and the river in this area (MacInnis and others, 2000).

6 Streamflow Trends in the Spokane River and Tributaries, Spokane Valley/Rathdrum Prairie, Idaho and Washington
Figure 4. Trend analysis of August and September monthly mean streamflows for the Spokane River near Post Falls, Idaho, 

1968 –2002.

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Figure 5. Monthly mean stage of Coeur d’Alene Lake, Idaho, August and September, 1968–2002.

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Trend Analysis 7
Outflows from the system that could affect long-term 

trends include surface-water and ground-water withdrawals 

and evaporation. Detailed water-use data for the study area 

were available only for the period from about 1990 to 2004. 

Data before 1990 were compiled using different methods and 

comparisons of these data to current data are difficult (Molly 

Maupin, U.S. Geological Survey, oral commun., 2004). Thus, 

additional analyses, beyond the scope of this study, are needed 

to determine whether water-use changes are affecting longterm trends in streamflow.

Because of the size of Coeur d’Alene Lake and the lack 

of trees along long distances of the rivers that feed the lake, 

evaporation could be a significant cause of water losses. 

Woods and Beckwith (1997) estimated that evaporation losses 

from Coeur d’Alene Lake ranged from 1.5 to 2.8 percent of 

the total outflows from the lake during water years 1991 and 

1992. Trend analyses of monthly August and September and 

annual temperature data (Western Regional Climate Center, 

2004) for Spokane and Coeur d’Alene revealed no trends 

during the post-canal period, with the exception of the annual 

mean temperature at Spokane which increased slightly more 

than 1oF during that period. Additional analyses, beyond the 

scope of this study, are needed to determine whether small 

increases in temperature could increase evaporation to the 

extent that it would affect long-term trends in streamflow.

In addition to the monthly data, annual mean streamflow 

data for the Spokane River near Post Falls gaging station 

also were analyzed for the post-canal period. No statistically 

significant trends were found. This lack of trends, like the 

lack of trends in the analysis of system inflows and outflows, 

indicates that the total available water in the system upstream 

of the Post Falls gaging station has remained relatively 

constant during any given year since 1968. Therefore, the 

decreasing trends in monthly streamflow in August and 

September would be expected to be offset by increases 

in streamflow during other months. However, analyses of 

monthly mean streamflows for the other months, including 

those for January through June, revealed no statistically 

significant trends. This likely means that any changes in 

long-term streamflow during the other months were small 

enough that they were not found to be significant in the trend 

analyses, but the cumulative changes were enough to offset the 

decreases in August and September.

Finally, annual 7-day low streamflows for two different 

periods of record 1914–2002 and 1968–2002 at the Post Falls 

gaging station were analyzed. Analysis of the data for the 

period 1914–2002 (fig. 6) reveals “breaks” during years in 

which the previously mentioned significant changes in the 

basin occurred. These include a decreasing trend in annual 

low streamflow during the operation of the “Spokane Valley 

Farms Canal” in the 1920s and 1930s, which corresponds 

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Figure 6. Annual 7-day low streamflows for the Spokane River near Post Falls, Idaho, 1914–2002.

8 Streamflow Trends in the Spokane River and Tributaries, Spokane Valley/Rathdrum Prairie, Idaho and Washington
Figure 8. Trend analysis of annual 7-day low streamflows for the Spokane River near Post Falls, Idaho, 1968 –2002.

Figure 7. Monthly mean streamflows for the Spokane Valley Farms Canal at Post Falls, Idaho, June, July, and August, 1911–1966.

to a general increase in summer streamflow being diverted 

into the canal until the late 1930s when the amount leveled 

off at about 260 ft3/s (fig. 7); a significant decrease in annual 

low streamflows in the early 1940s when efforts were made 

to keep Coeur d’Alene Lake levels higher; and a significant 

increase in annual low streamflows in the late 1960s when 

the operation of the Spokane Valley Farms Canal was 

discontinued.

The trend analysis of the annual 7-day low streamflows 

for the post-canal period 1968–2002 (fig. 8) revealed a 

significant decrease in annual 7-day low streamflows. This 

would be expected because 7-day low streamflows often occur 

in the late summer months of August and September where 

the decreasing streamflow trends were found.

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Trend Analysis 9
Figure 9. Trend analysis of August and September monthly mean streamflows for the Spokane River at Spokane, Washington, 

1968–2002.

Spokane River at Spokane, Washington 

(12422500)

Monthly mean streamflow data for the Spokane River 

at Spokane gaging station for the months of July through 

December during the post-canal period, 1968–2002, were 

analyzed for trends. Statistically significant decreasing 

trends in monthly mean streamflow were detected for the 

month of September (fig. 9). Visual inspection of the August 

monthly mean streamflows at Spokane (fig. 9) also revealed 

a decreasing trend; however, this trend was not statistically 

significant based on the criteria used in this study (α = 0.05).

A trend analysis of the annual 7-day low streamflow for 

the Spokane River at Spokane gaging station for the period of 

record 1891–2002 indicates an obvious decrease in the lowflow characteristics of the river (fig. 10). The annual 

7-day low streamflows until about 1930 often exceeded 

1,500 ft3/s; annual 7-day low streamflows after about 1985 

rarely exceeded 1,000 ft3/s. Results of a trend analysis of the 

annual 7-day low streamflows at this gaging station for the 

post-canal period of 1968–2002 (fig. 11) were similar to those 

for the Post Falls gaging station: a significant decrease in 

annual 7-day low streamflows during the post-canal period.

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10 Streamflow Trends in the Spokane River and Tributaries, Spokane Valley/Rathdrum Prairie, Idaho and Washington
Figure 10. Annual 7-day low streamflows for the Spokane River at Spokane, Washington, 1891–2002.

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Figure 11. Trend analysis of annual 7-day low streamflows for the Spokane River at Spokane, Washington, 1968 –2002.

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Trend Analysis 11
Figure 12. Trend analysis of September and October monthly mean streamflows for the Little Spokane River at Dartford, 

Washington, 1930–32, 1947–2002.

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Hangman Creek at Spokane, Washington 

(12424000)

Monthly mean streamflow data for the Hangman Creek 

at Spokane gaging station, for the months of July through 

December during the periods 1949–2002 and 1968–2002, 

were analyzed for trends. No trends were found in the monthly 

mean data for any of the months that were analyzed. This 

would be expected because analyses of precipitation and 

temperature data revealed little if any changes that may have 

affected streamflow. Hangman Creek also is unaffected by 

human factors (regulation, diversions, water use, etc.) and 

typically is a good indicator of natural streamflow conditions 

within the lowland portions of the study area (Raymond 

Smith, USGS, oral commun., 2004).

Little Spokane River at Dartford, Washington 

(12431000)

Monthly mean streamflow data for the Little Spokane 

River at Dartford gaging station, for the months of July 

through December during the periods 1930–32 and 

1947–2002, were analyzed for trends. Statistically significant 

decreasing trends in monthly mean streamflow were detected 

for the months of September and October (fig. 12). No trends 

were detected for the other months. Monthly precipitation data 

from the Spokane Airport (Western Regional Climate Center, 

2004) for the period 1914–2002 were analyzed for trends. 

A Kendall’s tau analysis indicated that there were no trends 

in monthly precipitation for the months of September and 

October during the period of record.

Annual 7-day low streamflow data for this gaging 

station for the periods 1930–32 and 1947–2002 (fig. 13) also 

were analyzed for trends. Although visual inspection of the 

data revealed a slight downward trend, the trend was not 

statistically significant. For comparisons with data from the 

Spokane River gaging stations, data from the period 

1968–2002 also were analyzed. No statistically significant 

trends were detected in any of the monthly mean data or the 

7-day low streamflow data for this period.

12 Streamflow Trends in the Spokane River and Tributaries, Spokane Valley/Rathdrum Prairie, Idaho and Washington
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Figure 12. Continued.

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Figure 13. Trend analysis of annual 7-day low streamflows for the Little Spokane River at Dartford, Washington, 1930–32 and 

1947 –2002.

Trend Analysis 13
Ground-Water / Surface-Water 

Exchanges

Results from previous studies and analyses of existing 

streamflow data were used to summarize ground-water/

surface-water exchanges on both the Spokane and Little 

Spokane Rivers. As shown in figure 3, there are very few 

periods for which data for adjacent locations exist for use 

in estimating ground-water/surface-water exchanges. For 

example, estimates can be calculated for the overall reach 

from Post Falls to Spokane over an extended period; however, 

only limited data are available at intermediate locations for use 

in making more refined estimates within the reach.

Spokane River

Previous investigations of the interactions between the 

Spokane River and the SVRP aquifer have shown that the 

Spokane River consistently loses streamflow to the aquifer 

in the upstream reaches where ground-water levels are below 

that of the streambed. Conversely, the Spokane River gains 

streamflow from the aquifer in areas farther downstream near 

the city of Spokane, where ground-water levels intersect the 

streambed. Because of the presence of coarse-grained, highly 

transmissive deposits and the high productivity of the aquifer, 

these losses and gains are quite large (hundreds of cubic feet 

per second).

Caldwell and Bowers (2003) reported consistent and, in 

most cases, significant streamflow losses in the river reach 

between the Spokane River gaging stations (fig. 2) near Post 

Falls, Idaho, and near Otis Orchard, Washington, and in the 

longer reach between the Spokane River gaging stations near 

Post Falls, Idaho, and at Greenacres, Washington, during 

water years 2000 and 2001. Their estimates of differences in 

monthly mean streamflow between the Post Falls and Otis 

Orchard gaging stations ranged from a loss of 69 ft3/s to a loss 

of 810 ft3/s; the median loss was 255 ft3/s. Their estimates of 

differences in monthly mean streamflows between the Post 

Falls and Greenacres gaging stations ranged from a gain of 

42 ft3/s to a loss of 858 ft3/s; the median loss was 288 ft3/s.

Differences similar to those reported by Caldwell and 

Bowers (2003) for water years 2000 and 2001 were computed 

following a seepage study completed by the USGS during 

September 2004. Losses between the Post Falls and Otis 

Orchard gaging stations averaged 177 ft3/s and losses between 

the Post Falls and Greenacres gaging stations averaged 287 

ft3/s. In addition, gains computed for the reach between the 

Greenacres and Spokane gaging stations averaged about 

557 ft3/s (Raymond Smith, U.S. Geological Survey, written 

commun., 2004). 

Gearhart and Buchanan (2000) estimated losses in the 

reach of the Spokane River between the Idaho-Washington 

State line and the Sullivan Road Bridge, which is about 2 mi 

downstream from the Greenacres gaging station (fig. 2). They 

used streambed areas and Darcy’s Law to estimate losses of 

104 ft3/s during low-flow conditions and 571 ft3/s during highflow conditions.

Streamflow data for various periods of record (fig. 3) at 

four gaging stations located between the long-term gaging 

stations Post Falls, Idaho, and Spokane, Washington, (fig. 2) 

were available for analysis. Differences in monthly mean 

streamflow values for July through December were analyzed 

for periods for which data comparisons were possible. Based 

on all available monthly mean data for the reach between the 

Post Falls and Otis Orchard gaging stations, the median July 

through December losses ranged from 37 ft3/s in November 

to 78 ft3/s in December. Data analyses indicated that for 

the reach between the Otis Orchard and Greenacres gaging 

stations no calculated gains or losses were outside the range 

of measurement error. The reaches between the Greenacres 

and the Spokane River below Trent Bridge gaging stations 

and between the Trent Bridge and Spokane River below 

Green Street gaging stations both generally gained streamflow 

during the periods of time data were collected. Based on all 

available monthly mean data, median July-through-December 

gains between the Greenacres and Trent Bridge gaging 

stations ranged from 330 ft3/s in December to 754 ft3/s in 

July; median July through December gains between the Trent 

Bridge and Green Street gaging stations ranged from 259 ft3/s 

in November to 447 ft3/s in July. Data analyses indicated that 

for the reach between the Green Street and Spokane gaging 

stations no calculated gains or losses were outside the range of 

measurement error.

Differences in monthly mean streamflow between the 

Post Falls and Spokane gaging stations for the months of 

July through December during 1968–2002 were analyzed 

for trends. Although the upper parts of this reach generally 

lose streamflow to the aquifer, the overall reach historically 

has gained streamflow. Trends detected for the months 

of September, October, and November were statistically 

significant. The analyses showed that streamflow gains within 

this reach decreased over time during the period 1968–2002 

(fig. 14). This long-term decrease could indicate a decrease in 

ground-water inflows to the river channel within this reach; 

however, additional analyses of ground-water levels and the 

ground-water/surface-water interactions within this reach 

would be necessary to better understand the reason for the 

decrease in streamflow gains.

14 Streamflow Trends in the Spokane River and Tributaries, Spokane Valley/Rathdrum Prairie, Idaho and Washington
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Figure 14. Trend analysis of differences in September, October, and November monthly mean streamflow between 

the gaging stations at Spokane River at Post Falls, Idaho, and Spokane River at Spokane, Washington, 1968–2002.

Ground-Water / Surface-Water Exchanges 15
Little Spokane River

Nearly all streamflow in the Little Spokane River during 

the summer and autumn is from ground-water discharge from 

the SVRP aquifer (Cline, 1969). Cline (1969) also estimated 

the annual mean ground-water discharge to the Little Spokane 

River upstream of Dartford to be at least 160 ft3/s. A seepage 

study completed by the USGS during September 2004 resulted 

in estimates of streamflow gains between the gaging stations 

at Dartford and near Dartford of about 255 ft3/s (Raymond 

Smith, U.S. Geological Survey, oral commun., 2004).

Limited data are available to compare the data from the 

Little Spokane River at Dartford and the Little Spokane River 

near Dartford gaging stations. Differences in monthly mean 

streamflow values for the months of July through December 

were analyzed for periods for which data comparisons were 

possible (1947–52 and 1998–2003; table 1, fig. 3). The 

differences were very consistent and ranged from an average 

gain of 244 ft3/s in July to an average gain of 249 ft3/s in 

October and December. The consistent increase in streamflow 

along the lower part of the Little Spokane River also has 

been attributed to ground-water discharge from the SVRP 

aquifer (Cline, 1969). Close inspection of the differences in 

streamflow during 1998–2003 revealed a possible decreasing 

trend in streamflow gains between the two gaging stations. 

This was an observation based on 6 years of data, however, 

and no statistical analyses were completed.

Summary

The Spokane Valley/Rathdrum Prairie aquifer (SVRP) 

is the sole source of drinking water for more than 400,000 

residents in Spokane County, Washington, and Kootenai 

County, Idaho. Recent and projected urban, suburban, 

industrial, and commercial growth has raised concerns about 

potential future impacts on water availability and water 

quality in the SVRP aquifer, and the Spokane River and its 

tributaries. Water resource demands are increasing at a time 

when aquifer and river dynamics are not well understood. 

This understanding is essential in making proper management 

decisions concerning ground water and surface water 

appropriations in the SVRP area.

Existing streamflow data for all streams located within 

the extent of the aquifer were compiled and reviewed. Trend 

analyses were performed on data from gaging stations with 

10 or more years of record. During periods when statistically 

significant trends in streamflow were observed, trend analyses 

were completed on other data sets, such as temperature and 

precipitation. Where possible, exchanges between ground 

water and surface water also were analyzed.

Ten USGS continuous-record gaging stations either are 

operated currently or were operated previously on streams 

located within the study area. Seven of the ten gaging stations 

have at least 10 years of record available for statistical 

analyses. Many of these gaging stations, however, were not 

operated continuously over the years, making trend analyses 

and upstream to downstream data comparisons difficult.

Statistically significant decreasing trends in monthly 

mean streamflow between 1968 and 2002 were detected for 

the months of August and September at the Spokane River 

near Post Falls, Idaho gaging station. Because of the proximity 

of the gaging station to Post Falls Dam, and the lack of any 

significant inflows or outflows between the gaging station 

and the dam, the data essentially represent releases from the 

dam. Analyses of long-term lake-level, precipitation, tributaryinflow, temperature, and water-use data revealed no significant 

trends that would indicate a direct relation to the decreasing 

trends in streamflow at Post Falls. A significant decreasing 

trend in the annual 7-day low streamflow at the Post Falls 

gaging station also was observed.

A significant decreasing trend in monthly mean 

streamflow between 1968 and 2002 was detected for the 

month of September at the Spokane River at Spokane, 

Washington gaging station. A significant decreasing trend in 

the annual 7-day low streamflow at the Spokane gaging station 

also was observed.

Hangman Creek is unregulated, has only minor diversions 

for irrigation purposes, and is typically a good indicator of 

natural streamflow conditions in much of the study area. No 

trends were detected in the monthly mean data for any of the 

months that were analyzed.

Significant decreasing trends in monthly mean 

streamflow between 1968 and 2002 were detected for the 

months of September and October at the Little Spokane River 

at Dartford, Washington gaging station. No trends in monthly 

precipitation that might explain the decrease in streamflow 

were detected for these months.

16 Streamflow Trends in the Spokane River and Tributaries, Spokane Valley/Rathdrum Prairie, Idaho and Washington
A net gain in streamflow occurs in the reach of the 

Spokane River between the gaging stations near Post Falls and 

at Spokane. Significant losses do occur in the reach between 

Post Falls and Greenacres, Washington, while most, if not all, 

of the gains occur downstream from Greenacres. Statistically 

significant decreasing trends in the net gain between Post 

Falls and Spokane were detected for the months of September, 

October, and November, between 1968 and 2002.

Nearly all streamflow in the Little Spokane River during 

the summer and autumn is from ground-water discharge. 

Visual inspection of the limited data available revealed a 

possible decrease in ground-water discharge to the Little 

Spokane River between 1998 and 2003; however, no statistical 

analyses were completed to verify this observation.

References Cited

Bolke, E.L., and Vaccaro, J.V., 1979, Selected hydrologic data 

for Spokane Valley, Spokane, Washington, 1977-78: U.S. 

Geological Survey Open-File Report 79-333, 98 p.

Caldwell, R.R., and Bowers, C.L., 2003, Surface-water/

ground-water interaction of the Spokane River and the 

Spokane Valley/Rathdrum Prairie aquifer, Idaho and 

Washington: U.S. Geological Survey Water-Resources 

Investigations Report 03-4239, 60 p.

Cline, D.R., 1969, Ground-water resources and related 

geology north-central Spokane and southeastern Stevens 

Counties, of Washington: State of Washington, Water 

Supply Bulletin no. 27, 195 p.

Gearhart, Christina, and Buchanan, J.P., 2000, The hydraulic 

connection between the Spokane River and the Spokane 

aquifer gaining and losing reaches of the Spokane River 

from State line, Idaho to Spokane, Washington: Cheney, 

Wash., prepared for the Spokane County Water Quality 

Management Program, 106 p. 

Helsel, D.R., and Hirsch, R.M., 1992, Statistical methods 

in water resources: New York, Studies in Environmental 

Science 49, Elsevier Science, 522 p.

MacInnis, J.D., and others, 2000, The Spokane ValleyRathdrum Prairie Aquifer Atlas: Boise, Idaho Department 

of Environmental Quality, 24 p.

Molenaar, Dee, 1988, The Spokane aquifer, Washington—Its 

geologic origin and water-bearing and water-quality 

characteristics: U.S. Geological Survey Water-Supply Paper 

2265, 74 p.

Stoffel, K.L, Joseph, N.L., Waggoner, S.Z., Gulick, C.W., 

Korosec, M.A., and Bunning, B.B., 1991, Geologic map 

of Washington-northeast quadrant: Olympia, Washington 

Division of Geology and Earth Resources, Geologic Map 

GM-39.

Tuck, L.K.; Briar, D.W.; Clark, D.W., 1996, Geologic history 

and hydrogeologic units of intermontane basins of the 

northern Rocky Mountains, Montana and Idaho: U.S. 

Geological Survey Hydrologic Atlas 738-A.

U.S. Census Bureau, 2004, State and county quickfacts: 

accessed June 2004, at URL http://www.census.gov/

 Western Regional Climate Center, Historical Climate 

Information: accessed May 2004, at URL http://www.wrcc.

dri.edu/CLIMATEDATA.html.

Woods, P.F., and Beckwith, M.A., 1997, Nutrient and traceelement enrichment of Coeur d’Alene Lake, Idaho: U.S. 

Geological Survey Water-Supply Paper 2485, 93 p.

References Cited 17
Manuscript approved for publication, January 11, 2005

Prepared by the U.S. Geological Survey Publishing staff, 

Idaho Water Science Center, Boise, Idaho

 Barbara Kemp

 Richard Helton

Washington Water Science Center, Tacoma, Washington

 Linda Rogers

 Sharon Wahlstrom

For more information concerning the research in this report, contact the 

Director, Idaho Water Science Center

U.S. Geological Survey, 230 Collins Road

Boise, Idaho 83702

http://id.water.usgs.gov
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