Water Resources of Idaho
Water Resources of Idaho |
STUDY AREA DESCRIPTION
The Northern Rockies Intermontane Basins study area encompasses 31,500 square miles (mi2) in western Montana, northern Idaho, and northeastern Washington and lies entirely within the Northern Rocky Mountains physiographic province. The study area is comprised of two major river basins: the Clark Fork-Pend Oreille River Basin, which contains about 24,900 mi2, and the Spokane River Basin, which contains about 6,600 mi2. The population of the study area was about 725,000 in 1990 with about 350,000 in Washington, 255,000 in Montana, and 120,000 in Idaho.
The Clark Fork originates near Butte in southwestern Montana and flows 350 miles to Lake Pend Oreille in northern Idaho. Outflow from the lake is the Pend Oreille River, with an average annual flow of about 28,000 cubic feet per second, which flows northward to join the Columbia River in Canada. Major tributaries to the Clark Fork are the Blackfoot, Bitterroot, Flathead, and Priest Rivers. The Spokane River originates in northern Idaho as the outflow from Lake Coeur d'Alene which has two principal tributaries, the Coeur d'Alene and St. Joe Rivers. The Spokane River has an average annual streamflow of about 7,000 cubic feet per second at Spokane, Washington.
Annual precipitation ranges from less than 15 inches in many of the intermontane basins of western Montana to more than 100 inches near the Continental Divide in northwestern Montana. Average annual runoff, primarily a result of snowmelt, ranges from more than 50 inches at higher elevations to less than 5 inches at lower elevations. The average annual temperature in the valleys ranges from about 40 to 55 degrees Fahrenheit, depending primarily on elevation. The topography varies from high, mountainous areas to large, flat- lying valleys. The study area has numerous large natural lakes and reservoirs, including Flathead Lake (the largest natural fresh-water lake in the western United States), Lake Pend Oreille (one of the deepest lakes in the United States), Lake Coeur d'Alene, Hungry Horse Reservoir, and Priest Lake.
Land use in the study area includes agriculture, mining-related activities, timber production, livestock grazing, urban and low- density residential development, and recreation, and varies with elevation. The Flathead, Kalispel, Coeur d'Alene, and Spokane Indian Reservations are either wholly or partly within the study area and constitute about 10 percent of the area. The average water use in the study area in 1990 was about 1,600 million gallons per day (Mgal/d). About 80 percent or 1,280 Mgal/d is from surface- water sources. Ground-water use in the study area is about 320 Mgal/d with about 60 percent used for municipal, domestic, and commercial supplies. Ground-water use is increasing as new development extends to rural areas and as communities replace or augment existing surface-water supplies with water from underground sources.
Quaternary and Tertiary basin-fill deposits contain the major aquifers in the study area. The aquifers, which are present along most reaches of the large rivers and main tributaries, are the principal sources of ground water in the study area. The Quaternary deposits, which generally are alluvial or glacial in origin, occur in valleys throughout the area, commonly are less than 200 feet thick, and consist of gravel, sand, silt, and clay. Thick, primarily fine- grained Tertiary deposits, which are fluvial or lacustrine in origin, generally occur in the southern part of the study area and may contain some permeable gravels in the upper sections. Igneous, sedimentary, and metamorphic rocks of Precambrian to Quaternary age generally are not a source of large amounts of ground water. However, because of their large extent, they probably contribute substantial amounts of recharge to the basin-fill aquifers.
U.S. Department of the Interior | U.S. Geological Survey
URL: id.water.usgs.gov/nrok/descript.htm
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Page Last Modified: Wednesday, 22-Apr-2009 14:16:31 EDT