Reservoir A is an offstream reservoir about 7 miles southeast of Lewiston, Idaho, that has a total capacity of 3,000 acre-feet (active 3,000 acre-feet). Safety of Dams work was completed at Reservoir A in 1999. A stability berm with drainage features was constructed along the downstream toe of the lower embankment, the upstream face of the lower embankment was reworked, and minor modifications were made on the upstream face of the upper embankment and to the outlet portal. In addition, the reservoir operating elevation was restricted to 1800 feet reducing the reservoir’s total capacity from 3,000 acre-feet to 1,960 acre-feet. This work was accomplished under the Bureau of Reclamation’s Safety of Dams Program. An Issue Evaluation was completed in 2009, which indicated that the reservoir restriction could be eased to allow storage to elevation 1804.0 ft, 2,440 acre-feet.
Topography probably was the major factor influencing the selection of the site for Reservoir A Dam. The reservoir basin is a shallow, round-bottomed, erosional valley eroded into a gently undulating lava plateau by Lindsay Creek. Most of the surface of the plateau is covered with a clayey, silty soil, which is as much as 65 feet thick. The soil is thinnest in drainages where part of the soil cover has been removed by erosion. A few small outcrops of basalt are present on the plateau surface surrounding the reservoir basin and in some of the deeper drainages in the area. Clayey, silty soil with some fine sand constitutes most of the foundation and abutments of Reservoir A Dam. The soil was derived from weathering of basalt. It shows no evidence of stratification and is interpreted as a residual deposit. Near the right abutment of the dam, thick layers of silty clay, lime clay, silty sand and thin alternating layers of sandstone, clay, sand and boulders were penetrated by drilling. Stratification indicates this material has been transported and is interpreted as an alluvial deposit.