The Little Lost River basin, an elongated, northwest trending structurally formed intermontane valley, drains an area of about 900 square miles into a closed depression near the northwestern edge of the Snake River Plain. Runoff from snowmelt and rainfall on the Lost River Range on the west and the Lemhi Range on the east maintains the flow of the Little Lost River, and recharges the ground-water reservoir, Both mountain ranges are complexly faulted and are underlain by a variety of rocks, dominantly limestone, of Paleozoic age The principal aquifers are highly transmissive alluvial fill in the middle and upper valley and alluvial fill interfingered with basalt in the southernmost part of the valley.