Loon Lake Reservoir is a reservoir in the Eldorado National Forest of El Dorado County, California, United States. The 76,200 acre·ft lake is formed by Loon Lake Dam, completed in 1963 as part of the Upper American River Project by Sacramento Municipal Utility District to conserve spring snow melt runoff for use during the summer and autumn for hydroelectric power production. Loon Lake Dam impounds water at the headwaters of Gerle Creek which, prior to the dam, flowed intermittently through (natural) Loon and Pleasant Lakes. But most of the water now stored in Loon Lake Reservoir arrives from Buck Island Reservoir in the adjacent Rubicon River watershed by way of the Buck-Loon Tunnel. Nearby is Loon Lake Chalet, a popular winter recreation destination. In summer, a boat ramp for water sports and camping are available, but the area is less popular than nearby Union Valley Reservoir and Rubicon Trail.
It is popularly referred to as Loon Lake, although strictly speaking this is the name of the former lake now inundated by the dam. Boaters still refer to the northeast portion of the reservoir as the Pleasant Lake Arm, the portion over the inundated Pleasant Lake.