Ground-water aspects of the lower Henrys Fork region, eastern Idaho

Abstract

The lower Henrys Fork region in eastern Idaho includes the plains and low benches between Ashton and the junction of Henrys Fork and Snake River. The northwestern and western parts of the area are part of the Snake River basalt plain. The central part of the area is occupied by alluvial plains of the Snake, Teton, and Falls Rivers and of Henrys Fork. The alluvial deposits are underlain by basalt. The southeastern part of the area is a bench (Rexburg Bench), chiefly on silicic and basaltic volcanic rocks, which rises gradually to mountain peaks (Big Hole Mountains) southeast of the area. Irrigation wells open to the basalt under the Snake River Plain and the basalt and sands and gravels under the alluvial plains yield large amounts of water with small drawdowns. Irrigation wells in the silicic volcanic rocks and the interbedded ash, pyroclastics, and sedimentary deposits beneath the Rexburg Bench generally yield much less water. The regional water table slopes southwestward beneath the basalt and alluvial plains. It is recharged by precipitation that infiltrates into the ground in the headwaters of Henrys Fork and Falls, and Teton Rivers and by water that moves downward from an extensive perched water body caused by seepage from stream channels and surface-water irrigation. The perched water in part moves vertically down to the regional water table and in part laterally to the streams. Ground water beneath the Rexburg Bench moves generally northwestward to join the regional ground-water body beneath the alluvia,1 and basalt plain, but this area contributes very little recharge to the main aquifer body. Recharge to the regional water table is estimated to average 725,000 acre-feet annually. The regional water table is below the level of the streams in the area, and ground water in the main aquifer, therefore, is not tributary to the streams. Pumping from the regional ground-water reservoir for irrigation or other uses would have no effect on streamflow or surface-water rights within the study area. However, depletion of the underflow would eventually reduce the inflow to American Falls Reservoir, unless the depletion was offset by additional recharge. Total withdrawals of ground water for irrigation in 1962, principally in the Rexburg Bench, were estimated to be 25,000 acre-feet. About 10,000 acre-feet was withdrawn for domestic, municipal, and stock supplies. These withdrawals caused no significant decline in the water table. In the Ashton area, surface-water irrigation has caused water to be perched in basalt above the silicic volcanic rocks, and much of' this perched water contributes to streamflow. Some ground water can be pumped from the basalt for irrigation and other uses. If ground water were pumped for irrigation, the flow of Henrys Fork would be decreased by the amount of pumped water consumed by crops. Water pumped for nonconsumptive use would have little effect on streamflow. Ground-water prospects for irrigation in the Falls River area are not encouraging.