Ground water in the vicinity of American Falls Reservoir, Idaho

Abstract

Analysis of groundand surface-water relationships suggests1 that increasing the capacity of the American Falls Reservoir by raising the height of the dam 15 feet would increase leakage from the reservoir by less tl in 0.2 percent of the average inflow to the reservoir, or less than 10,000 acre feet per year. This amount is less than one-tenth of the evaporation rate from tl 3 reservoir. The American Falls Reservoir lies near the south margin of the Snake River Plain in southeastern Idaho. The Snake River Plain is about 200 miles long and averages nearly 60 miles in width. It is underlain by a thick sequence of basaltic lava flows, interbedded pyroclasties, and sedimentary deposits. The uppermost few thousand feet of this sequence is the Snake Plain aquifer, one of the great aquifers of the United States. Recharge to the aquifer is chiefly by water percolating from the T^ake River, its tributaries, and irrigated tracts, and by underflow from surrounding areas. Ground water moves generally southwestward and discharges to t^re Snake River through springs in the American Falls Reservoir reach and in the Hagerman Valley reach (between Twin Falls and Bliss).Total discharge from the aquifer is about 9,000 cfs (cubic feet per second). The occurrence and movement of ground water in the vieinit^ of American Falls Reservoir are controlled by the local geology. Silt and tuff in the Neeley Formation and the Walcott Tuff and silt and fine sand in the FT ft Formation and American Falls Lake Beds have a low permeability. These rocks transmit little ground water compared with the basalt and intercalated pyroclasties and gravels of the Snake Plain aquifer. The less permeable deposits underlie the reservoir area and act as a barrier to the movement of ground witer. Under present conditions the water table on the periphery of the reservoir slopes toward the reservoir, except within 3 or 4 miles of the dam, where the water table slopes away from the reservoir. Most of the springs discharge at altitudes above 4,370 feet, some 15 feet above the maximum r-^servoir stage. Thus, reservoir stage has little effect on ground-water inflow to the reservoir. A fairly close relationship exists between the annual amount of surface water diverted for irrigation of lands up the Snake River from the reservoir and the annual ground-water discharge through springs for the period 1911-60. After about 1952, greatly increased ground-water withdrawals from wells, which increased consumptive use, virtually balanced increased diversions from the surface-water^ system for irrigation, so that ground-water inflow to the reservoir remained about constant. 2 GROUND WATER, AMERICAN FALLS RESERVOIR, IDAHO All or nearly all the seepage loss from the reservoir reappear* at the surface as seeps or springs along the reach between American Falls Dam and Minidoka Dam. Rough estimates of seepage loss were made from water-budget calculations below the reservoir and by extrapolating spring-flow data below the reservoir. Although both estimates are in close agreement, neither is probably reliable except as an order of magnitude. The best estimates are as follows: Existing seepage loss: average, 60 cf s; maximum, 80 cf s. Seepage loss if dam is raised 15 feet: average, 70 cfs; maximum, <100 cfs. The net effect of raising the maximum reservoir stage by 15 feet would be twofold: seepage loss would be increased by about 10 cfs, and some areas in the vicinity of Sterling and Springfield would become waterlogged. The decrease in annual inflow to the reservoir would probably be negligible. INTRODUCTION American Falls Reservoir (fig. 1) is on the Snake Eiver in southeastern Idaho. It is the largest reservoir for storage of irrigation water in Idaho (1.7 million acre-ft. capacity at an altitude of 4,354.50 ft. above sea level). American Falls Dam is not only downstream