Surface Water Streams draining the mountains and valleys to the west and north of the INEL (Fig. 2) are a source of irrigation water in agricultural areas adjacent to the INEL. Snowmelt and rainfall contribute to surface water, especially in the spring. The Big Lost River is an important source of groundwater recharge at the INEL. The Big Lost River flows southeastward in its valley past Arco, onto the Snake River Plain, and then turns northeastward through the INEL to its termination in three

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... rmination in three playas (Fig. 2) . The river loses water by infiltration through the channel bottom as it flows onto the plain. As flow approaches the playas, the channel branches into many distributaries, and the flow spreads over several flooding and ponding areas (Barraclough et al, 1967). Recharge to the Snake River Plain aquifer from flow in the river during wet years is significant. During dry periods, streamflow does not reach the INEL because of upstream diversions for irrigation. Mackay Dam, 30 mi upstream from Arco, and the flood-control diversion dam in the southwestern part of the INEL (Fig. 2) affect flow in the Big Lost River. The flood-control diversion dam was constructed in 1958 to reduce the threat of flooding at INEL facilities near the Big Lost River.