Croplands in the semiarid central part of the Snake River Plain are dependent on the availability of irrigation water, most of which comes from the Snake River. Allocation of irrigation water from the river requires that gains and losses be determined for American Falls Reservoir, Lake Walcott, and Milner Lake. From 1912 to 1983, average ungaged inflow to American Falls Reservoir , determined from monthly water budgets, was 2,690 cu ft/sec. About 94% of this inflow was spring discharge and groundwater seepage; the remainder was from small tributaries and irrigation-return flow. Ungaged inflow estimated from water budgets for various periods correlated favorably with measured discharge of two springs and water levels in two wells. Discharge of Spring Creek was a better indicator of ungaged inflow than groundwater levels. Therefore, correlation with Spring Creek discharge was used in estimating ungaged inflow to American Falls Reservoir in 1983. Daily water budget calculations of ungaged inflow to American Falls Reservoir are less variable when storage changes are determined by using three stage-recording stations rather than one. Water budgets do not indicate large amounts of leakage from American Falls Reservoir, but small amounts of leakage are indicated because flow in downstream springs increased about 25% after reservoir storage began in 1926. Water budgets for Lake Walcott and Milner Lake show average annual net gains (1951-83) to Lake Walcott and Milner Lake of 245 and 290 cu ft/sec. These amounts are verified by monthly water budgets when discharge in the Snake River is low, and measured and estimated sources of inflow. Gains and losses estimated from daily water budgets are variable, owing to inadequate determination of (1) changes in reservoir storage, (2) streamflow, (3) lake surface precipitation, and (4) lake surface evaporation. Backwater effects are accounted for in the process used to determine storage in Milner Lake.
