Ground-water resources of Camas Prairie, Camas and Elmore Counties, Idaho

Abstract

Camas Prairie is an eastward-trending intermontane basin along the north flank of the Snake River Plain in southern Idaho. The basin is about 40 miles long and averages about 8 miles wide. It was formed as a structural depression in which a considerable thickness of alluvial and lake deposits accumulated behind basalt flows, which at times blocked the outlet to the east. Intrusive and extrusive rocks of Cretaceous to Quarternary age enclose the basin on the north, west, and east. The enclosing rocks yield small amounts of water to springs and wells from the weathered mantle and fractures. The principal aquifers are sand and gravel in the alluvial fill, and basalt. Water in the shallow deposits is not confined, and the water table generally is less than 10 feet below the surface at most places. Ground water in the deeper deposits occurs chiefly in two horizons that comprise the upper and lower artesian aquifers. Throughout much of the prairie, the pressure is sufficient that water will flow from wells in these aquifers. Recharge to the basin is from direct precipitation and percolation of stream runoff from the bordering mountains. Ground water moves from the higher areas at the base of the encircling mountains toward the center of the basin and the eastern outlet. The artesian aquifers leak by upward percolation through the imperfectly confining beds and help maintain the shallow water table. Basalt, which interfingers with the alluvial deposits, is an important aquifer near the southeast margin of the prairie and at the east end. Annual recharge to the artesian aquifers is estimated to be about 40,000 acre-feet. Discharge from the artesian aquifers is about equally divided between upward leakage to the shallow aquifers and underflow out of the prairie. Most of the underflow discharges into Camas Creek or Magic Reservoir east of the prairie; little of the underflow reaches the Snake River Plain. Wells drilled for irrigation generally yield 500 to 1,200 gallons per minute from the artesian aquifers. Better construction and development methods would result in considerably better yields. Wells drilled in the basalt will yield 2,000 to 3,000 gallons per minute with moderate drawdowns. Computations made using aquifer coefficients, estimated on the basis of data collected during the investigation, suggest that 12,000 acre-feet of ground water might be withdrawn annually. However, the aquifers are limited in areal extent, and productivity of the alluvial aquifers is not great. Consequently heavy development would result in large drawdowns in wells, and there would be much interference between wells. The postulated large withdrawals from wells on the prairie would be supplied in part by a reduction in underflow from the prairie and in part by a decrease in leakage from the artesian aquifers, which in turn would cause a decline in the shallow water table.