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

Abstract

The lower Henrys Fork region includes the plains and low benches between Ashton and the junction of Henrys Fork and Snake River in eastern Idaho. The northwestern and western parts of the area are part of the Snake River lava plain. The central part of the area is occupied by alluvial plains of the Snake, Teton, and Falls Rivers, and Henrys Fork. The southeastern part of the area is a bench (Rexburg Bench), chiefly on silicic and basaltic volcanic rocks, which rises gradually to mountain peaks southeast of the area. The basalt, and the sand and gravel under the alluvial plains are good aquifers and yield large amounts of water with small drawdowns. The silicic volcanic rocks and the interbedded ash, pyroclastics, and sedimentary deposits generally yield much less water than the basalt and alluvium. The regional water table slopes southward beneath the basalt and alluvial plains. Seepage from stream channels and irrigated tracts has resulted in an extensive body of ground water perched above the regional water table. 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 alluvial plain. 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. Recharge to the regional water table is estimated to average 725,000 acre-feet annually. 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 were estimated to be 25,000 acre-feet and caused no significant decline in the water table. In the Ashton area surface-water irrigation has cause water to be perched in basalt above the silicic volcanic rocks and much of this perched water contributed to streamflow. The flow of Henrys Fork would be decreased by the amount of pumped ground water consumed by crops. Ground-water prospects for irrigation in the Falls River area are not encouraging.