Loon Lake is a reservoir in the Eldorado National Forest of El Dorado County, California, United States.

But most of the water now stored in Loon Lake arrives from Buck Island Reservoir in the adjacent Rubicon River watershed by way of the Buck-Loon Tunnel.

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.

In 1884, Loon Lake Dam was constructed to provide summer and autumn water supply to Georgetown Ridge for mining.

Also, GDPUD exercised pre-1914 rights for diversion of water from several tributaries of Pilot Creek in the South Fork American River.

The watersheds that supply the UARP, encompassing some 674 square miles, are characterized by the mountains in the east and incised canyons in the west.

[6] The storage at Loon Lake typically follows an annual cycle, increasing in the spring, reaching its highest point in the summer months.

Through meeting in-stream flow commitments, whitewater boating requirements, and electrical system reliability needs over summer, the reservoir gradually lowers.

[7] An appendix to the 2014 California Water Quality Certification[8] details the mitigation associated with Loon Lake's impact on the geologic, aquatic and terrestrial resources of the former ecosystem as it was prior to the 1962 dams.

Some of these impacts include: varying water level, modified streamflow patterns affecting aquatic life communities, aesthetic resources, and human pathogen concentration due to recreation.

Loon Lake Main Dam is a rockfill structure that is 0.4 mi (0.64 km) long by 108 ft (33 m) high.

It is collected at four times during the year, first major rain, spring runoff, summer low flow and fall turnover.

[10] The Upper American River Project reservoirs including Loon Lake have vegetation that is considered upland type.

The specific vegetation and terrain that is common there are mixed/conifer and fir trees, Huckleberry Oak, barren/rocky areas along the shoreline, wet meadows scattered throughout and Lodgepole Pines.