Water resources in the Big Lost River Basin, south-central Idaho

The Big Lost River basin occupies about 1,400 square miles in-south-central Idaho and drains to the Snake River Plain. The economy in the area is based on irrigation agriculture and stockraising. The basin is underlain by a diverseassemblage of rocks which range, in age from Precambrian to Holocene. The assemblage is divided into five groups on the basis of their hydrologic characteristics: Carbonate rocks, noncarbonate rocks, cemented alluvial deposits, unconsolidated alluvial deposits, and basalt. The principal aquifer is unconsolidated alluvial fill that is several thousand feet thick in the main valley. The carbonate rocks are the major bedrock aquifer. They absorb a significant amount of precipitation and, in places, are very permeable as evidenced by large springs discharging from or near exposures of carbonate rocks. Only the alluvium, carbonate rock and locally the basalt yield significant amounts of water. A total of about 67,000 acres is irrigated with water diverted from the Big Lost River. The annual flow of the river is highly variable and water-supply deficiencies are common. About 1 out of every 2 years is considered a drought year. In the period 1955-68, about 175 irrigation wells were drilled to provide a supplemental water supply to land irrigated from the canal system and to irrigate an additional 8,500 acres of new land. r ; Average annual precipitation ranged from 8 inches £ on the valley floor to about 50 inches at some higher eleva^ :. tions during the base period 1944-68. The estimated water yield of the Big Lost River basin averaged 650 cfs (cubic feet per second) for the base period. Of this amount, 150 cfs was transpired by crops, 75 cfs left the basin as streamflow, and 425 cfs left as ground-water flow. % map of precipitation and estimated values of evapotranspiration were used to construct a water-yield map. /' »j '.* , A distinctive feature of the Big Lost River basin, is the . .large interchange of water from surface streams into the ground and from the ground into the surface streams. Large quantities of water disappear in the Chilly, Darlington, and other sinks and reappear above Mackay Narrows, above Moore Canal heading, and in other reaches. A cumulative summary of water yield upstream from selected points in the basin is as follows: Water Surface Ground Crop yield water water evapotrans(cfs) (cfs) (cfs) piration ' ______________________________________(cfs) Above Howell Ranch 345 310 35 Above Mackay Narrows .450 325 75 50 Above Arco 650 75 425 150 Ground-water pumping affects streamflow in reaches, where the stream and water table are continuous, but the effects of pumping were not measured except locally. Pumping depletes the total water supply by the amount of the pumped water that is evapotranspired by crops. The part of the pumped water that is not consumed percolates into the ground or runs off over the land surface to the stream. The estimated 425 cfs that leaves the basin as ground-water flow is more than adequate for present and foreseeable needs. However because much" of the outflow occurs at considerable depth, the quantity that is salvageable is unknown. Both the surface and ground waters are of good quality and are suitable for most uses. Although these waters are low in total dissolved solids, they tend t6 be hard 0* very hard.