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The middle Big Wood River-Silver Creek area, lying between the mountains of central Idaho and the Snake River Plain and drained in part by the Big Wood River and in part by Silver Creek a tributary of Little Wood River, Includes about 48,000 acres of lowland in Blaine County. About 20,000 acres of the lowland is irrigated, chiefly with surface water. Wells supply water to 575 acres and supplemental water to about 3.000 acres. About 7,500 additional acres of arable land could be irrigated if sufficient water were available. About 20,500 acres of the area is nonarable swamp, waterlogged land, and gravelly river bottom. Nearly all the surface water in the area has been appropriated for irrigation within, and downstream from, the area. The available natural flow is not adequate for late-season water demands within the area. Reservoir sites are available in the valley of the Big Wood River upstream from the middle Big Wood River-Silver Creek area, but the cost of large dams there probably would be too high to warrant their construction. Water to supplement the supply for the presently irrigated land is wanted, and additional land might be irrigated if water could be obtained for it. The U. S. Bureau of Reclamation is considering proposals to install additional irrigation wells in the area, and to construct a small upstream storage reservoir. The river, the reservoir, and the wells then might be managed so that consequent reduction of the amount of water available for downstream storage in Magic Reservoir would be offset by delivery of pumped ground water to the reservoir. This report is a general evaluation of the ground-water resources available for the irrigated area and of the relation of the ground-water supply to the discharge of streams and existing water wells. Pre-Tertiary basement rocks form the relatively impermeable floor of a basin which is partially filled with fluvioglacial sediments, alluvium, and basalt, all of Quaternary age. The Quaternary materials yield both artesian and unconfined ground water copiously to wells and springs. The ground-water reservoir receives recharge by underflow from the north (upstream), by direct infiltration of precipitation and irrigation water, by seepage from certain reaches of the channel of the Big Wood River, and by mass percolation from surrounding highlands. Ground water is discharged by springs which, give rise to Silver and Spring Creeks, by seepage into some reaches of the Big Wood River, by evapotranspiration, by wells, and by underflow through a southeastern outlet from the basin. 2 GROUND-WATER RESOURCES, BLAINE COUNTY, IDAHO Among 28 representative irrigation wells the average yield of pumped wells is 1,875 gallons per minute (gpm) and of flowing artesian wells, 825 gpm. The depth to the water table ranges from 70 feet below the land surface in the northern part of the area to zero in the southern part. The average depth is about 18 feet. The piezometric surface of the artesian water in the small area where it was measured averages 27 feet above the land surface. Watertable wells range in depth from 23 to 81 feet and average 59 feet. Flowing artesian wells range in depth from 118 to 174 feet and average 139 feet. The estimated withdrawal of ground water for irrigation in 1954 was about 7,500 acre-feet Ultimately, new development might increase the yearly withdrawal to about 60,000 acre-feet. Of the total of 60,000 acre-feet, probably about 21,000 acre-feet a year would be consumptively used about 10 percent of the estimated total supply of ground water. That amount of depletion probably would not infringe materially on existing use of surface water from springfed streams, if pumping were restricted to strategic times and places. Some water that otherwise would escape by underflow southeastward out of the basin might be salvaged by pumping. The ground-water reservoir is capable of yielding a large 'amount of water to wells. Increased pumping would cause general lowering of the water table and of the artesian pressure in the lowland, but the lowering probably would be small if pumping were properly managed. Nonartesian wells that yield 2,000 to 3,500 gpm with an average drawdown between 10 and 20 feet, and artesian wells that yield 500 to 1,500 gpm by free flow, could be developed in the central part of the area, where pumping lifts probably would range between 20 and 70 feet. The most suitable area for pumping supplemental irrigation water is in the central part of the area, but other areas are described in which such pumping would be feasible. Construction and operation of a small storage reservoir upstream from the irrigated area might be desirable in a balanced program of water management. One alternative plan of the Bureau of Reclamation proposes that the full additional water supply be obtained from wells. Water from well fields might be distributed through existing canals, but single wells or dispersed small groups of wells would be advantageous under some circumstances. If increased pumping from wells in the central part of the area should deplete the surface-water supply to a greater extent than is desirable, the depletion might be offset by pumping water for replacement from the southeastern part of the basin, where estimated unused underflow out of the basin is about 30,500 acrefeet yearly. The pumped water would be discharged for downstream surface diversions. Additional exploration and study of the southeastern area is advisable. By recognized standards the ground water and surface water in the basin are chemically suitable for irrigation.

Ground-water resources of the middle Big Wood River-Silver Creek area, Blaine County, Idaho