Scientific Investigations Report 2005-5227
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2005-5227
The Spokane Valley–Rathdrum Prairie (SVRP) aquifer is the sole source of drinking water for over 400,000 residents in Spokane County, Washington, and Bonner and Kootenai Counties, Idaho. Recent and projected urban, suburban, and industrial/commercial growth has raised concerns about potential future effects on water availability and water quality in the SVRP aquifer and the Spokane and Little Spokane Rivers. This report presents geologic, hydrologic, and ground-water flow modeling information compiled by the U.S. Geological Survey, in cooperation with the Idaho Department of Water Resources and Washington Department of Ecology for the SVRP aquifer. Descriptions of the geologic history, hydrogeologic framework, surface geophysical studies, water-budget components, ground- and surface-water interactions, computer flow models, and further data needs are provided.
The SVRP aquifer consists primarily of thick layers of coarse-grained sediments—gravels, cobbles, and boulders—deposited during a series of outburst floods resulting from repeated collapse of the ice dams that impounded ancient Glacial Lake Missoula. Sources of recharge to the aquifer include infiltration from precipitation, return flow from water applied at land surface, seepage from the Spokane and Little Spokane Rivers and adjacent lakes, and surface- and ground-water inflow from tributary basins. The aquifer discharges into the Spokane and Little Spokane Rivers and through withdrawals from wells.
A simplified geologic model of the Rathdrum Prairie and Spokane Valley includes the ancient Rathdrum-Spokane River valley being filled with generally unknown amounts of Miocene basalts and interbedded sediments followed by a period of downcutting, repeated cycles of glacial and interglacial sedimentation, and finally the repeated and catastrophic cycles of outburst flooding from Glacial Lake Missoula. The Missoula Floods eroded much of the existing basalt and older sediments, significantly altering the geologic features that developed during the Miocene. To date, relatively little is known about the thickness of the flood deposits or the location and extent of older deposits beneath them, including the location of the buried bedrock surface. Depth to bedrock estimates are available at selected locations in the aquifer from surface geophysical studies that were conducted over 50-years beginning in the early 1950s. Methods used include seismic refraction and reflection, gravity, and microgravity.
In most places, the SVRP aquifer is bounded by bedrock of pre-Tertiary granite or metasedimentary rocks, or Miocene basalt and associated sedimentary deposits. The lower or bottom boundary of the aquifer is largely unknown except along the margins or in shallower parts of the aquifer where wells have penetrated the entire aquifer thickness and reached bedrock or silt and clay deposits. Reported ground-water divides approximately represent the aquifer boundary within Hoodoo and Spirit Valleys and near Careywood, Idaho. Upgradient areas of the aquifer also are bounded by tributary lakes, including Pend Oreille, Spirit, Twin, Hayden, Coeur d’Alene, Hauser, Liberty, and Newman. Streams tributary to the aquifer include Lewellen, Sage, and Rathdrum Creeks in Idaho and Chester and Saltese Creeks in Washington. Streams tributary to the Spokane River in the aquifer extent include Hangman (Latah) Creek and the Little Spokane River, both in Washington. Subsurface outflow occurs at the lower end of the aquifer near Long Lake at the confluence of the Spokane and Little Spokane Rivers.
Recharge to the SVRP aquifer occurs through precipitation, irrigation, canal leakage, septic tank effluent, inflow from tributary basins, and flow from the Spokane River. Discharge from the aquifer occurs through withdrawals from wells, flow to the Spokane and Little Spokane Rivers, evapotranspiration, and underflow to the Long Lake area. A compilation of estimates indicates that these estimated values should be compared with caution due to variability in the area boundaries and time period of interest, as well as the methods used to make the estimates.
Numerous studies have documented the dynamic ground- and surface-water interaction between the SVRP aquifer and the Spokane and Little Spokane Rivers. Gains and losses vary throughout the year, as well as the locations of gains and losses. Analysis of historical and recent streamflow data illustrates the magnitude and variability of these relations. September 2004 streamflow measurements indicated that the upper reach of the Spokane River between Post Falls and downstream at Flora Road lost 321 ft3/s. A gain of 736 ft3/s was calculated between the Flora Road site and downstream at the Greene Street Bridge. A loss of 124 ft3/s was calculated for the reach between the Greene Street Bridge and the Spokane River at Spokane gaging station. The river gained about 87 ft3/ s between the Spokane River at Spokane gaging station and the T.J. Meenach Bridge. Overall, the Spokane River gained about 376 ft3/s between the Post Falls, Idaho gaging station and the T.J. Meenach Bridge. Estimated gains of 254 ft3/s were calculated for the reach between the Little Spokane River gaging stations at Dartford and near Dartford (a distance of about 7 river miles).
Four regional ground-water flow models have been constructed for the aquifer, three on the Washington part of the aquifer and one for the entire aquifer. In the early 1980s, a two-dimensional computer flow model of the Washington part of the SVRP aquifer indicated that pumping at the current rate (1977) had little effect on water levels in the Washington part of the SVRP aquifer. During a one-year simulation, pumping at twice the 1977 rate of 227 ft3/s resulted in calculated water-level declines of about 3 feet. Spokane River streamflow was calculated to decrease about 150 ft3/s in the summer and about 50 ft3/s during the rest of the year. The increased pumping rate had a more significant effect on Spokane River discharge than on the change in water levels in the aquifer. In the late 1990s, a 3-dimensional flow model of the Washington part of the SVRP aquifer was constructed as part of a wellhead protection program and designed to represent scenarios for September 1994 and April 1995. The calibrated model was used to estimate ground-water capture zones using particle tracking. Also in the late 1990s, the first ground-water flow model of the entire SVRP aquifer was constructed. The finite-difference, single-layer, steady-state model was designed as a tool for understanding the overall water balance. In 2004, a modeling report was completed for the Little Spokane River and Middle Spokane River watersheds, Spokane County, Washington. The model, representing water years 1994–99, was constructed for use in planning and management of watershed hydrologic resources.
Data are needed that would provide a more comprehensive data set for the construction and calibration of a regional flow model and better understanding of the SVRP aquifer. These data needs include, but are not limited to: quantification of the ground-water inflow from the surrounding bedrock boundaries and peripheral watersheds; an understanding of the surface-water/ground-water interaction of lakes adjacent to the aquifer; characterization of the cross sectional area and hydrostratigraphy of the aquifer at key locations; updated ground-water withdrawal values and estimates of the consumptive use of water; additional and frequent aquifer-head measurements near the rivers in conjunction with stage and discharge measurements; additional temporal and regional aquifer-wide head measurements; an understanding of possible ground water discharge out of the basin; the continued measurement of streamflow and (or) river stage at existing and (or) additional river locations; and determine the flow direction of water to or from the rivers using chemical tracers.
For more information about USGS activities in Washington, visit the USGS Washington District home page .
U.S. Department of the Interior |
U.S. Geological Survey
URL: https://pubsdata.usgs.gov/pubs/sir/2005/5227/section9.html
Page Contact Information:
Page Last Modified: Thursday, 01-Dec-2016 18:45:53 EST
