Ground water in the upper part of the Teton Valley, Teton Counties, Idaho and Wyoming

Abstract

The Teton Valley in eastern Idaho and western Wyoming is bordered by mountains on the east, south, and west. The climate is characterized by cool summers and cold winters. Precipitation on the valley floor averages about 16 inches and on the adjacent mountain ranges to as much as 70 inches. At the higher elevations the precipitation occurs mostly as snow. Rocks consisting principally of limestone, sandstone, quartzite, and shale of Paleozoic and Mesozoic ages crop out in the mountains and presumedly underlie the valley at depth. Silicic volcanic rocks of Miocene and Pliocene age are exposed across the northern end of the valley and also crop out discontinuously along the lower slopes of the mountain fronts. Basalt of Tertiary and probable Quaternary age overlies the silicic volcanic rocks at places in the northern part of the valley. Remnants of glacial drift of Pleistocene age occur at the mouths of canyons entering the valley from the east. Coalescing alluvial fans, of late Tertiary to Recent age, extend from the mountain fronts almost to the flood plain of the Teton River, which flows along the axis of the valley. Alluvium of Recent age underlies the channel and flood plain of the river. Loess of Pleistocene age blankets much of the basalt, silicic volcanic rock, glacial drift, and alluvial-fan deposits. Many small springs issue from the older rocks along the valley margin and many small to large springs rising in alluvium and along the toes of the alluvial fans feed the Teton River. The alluvial-fan deposits and alluvium are the principal waterbearing formations. Adequate yields for irrigation can be obtained from wells tapping these deposits in the eastern part of the valley. An aquifer test in well 4N-45E-13adl indicated that the alluvial-fan deposits in that area have a coefficient of transmissibility of about 550,000 gallons per day per foot and a coefficient of storage of 0.03. Wells pumped at 1,500 gallons per minute for 60 days and spaced a quarter of a mile apart would not seriously interfere with one another. Sparse data indicate that irrigation wells generally would be unsuccessful on the west side of the valley. The average annual precipitation on the basin is estimated to be about 725,000 acre-feet. Of this amount, about 285,000 acre-feet flows out of the valley in the Teton River, 25,000 to 50,000 acre-feet leaves the valley by underflow, and about 400,000 acre-feet is evapotranspired. Chemical analyses of water from 26 sources indicate that the water is moderately hard to hard but otherwise is of excellent quality for irrigation, domestic, and stock uses. The U.S. Bureau of Reclamation has considered a plan of development that would provide both a water supply for about 15,200 acres of land now dry-farmed 1 2 GROUND WATER, UPPER TETON VALLEY on the west side of the valley and a supplemental water supply for the presently irrigated land on the east side. Water for the west side of the valley would be obtained from a reservoir that would be created by construction of a dam on the Teton River, and that for the east side would be obtained from wells. Part of the water used for irrigation but not needed by the crops would flow overland to the river and part would recharge the ground-water reservoir. As a result of the development, consumptive use of water would be increased about 1 acre-foot per acre of newly irrigated land and 0.4 acre-foot per acre in the area already under irrigation. Thus, the annual amount of water now flowing from the basin, either as surface water or as underflow, would be decreased about 25,000 acre-feet. INTRODUCTION The Teton Valley is almost entirely in Teton County, eastern Idaho, in Tps. 3-7 N., Rs. 44-46 E., Boise base line and meridian, but it includes about 5,000 acres in Tps. 44 and 45 N., R. 118 W., sixth principal meridian and base line, in Teton County, Wyo. (fig. 1). The valley, which has an area of nearly 250 square miles, lies between 117° 116 115' 114' 113° H2° 111 FIGURE 1. Index map of Idaho showing area described in this report.