Clear Lake is a Lake in Lake, California.

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Clear Lake is the largest natural lake entirely in California, and has the largest surface area of any freshwater lake entirely in California, the tenth largest by capacity. It is located in Lake County and is fed by many streams, but its sole outlet is Cache Creek. There is a dam on Cache Creek to increase the lake's capacity and to regulate outflow.

Clear Lake is 19 miles (30.6 km) long, 8 mi (12.8 km) wide at widest point, with surface area of 43,785 acres (177.19 km²) and a capacity of 1.42 km³ (1,155,000 acre-feet). Average depth is 27 ft (8 m), maximum is 60 feet (18 m), lake elevation is 1,318 feet (401.7 m) minimum, average water temp is 40 °F (4 °C) in winter and 76 °F (24 °C) in summer. Location [show location on an interactive map] 39°03′N 122°49′W / 39.05, -122.817 (per USGS).

Clear Lake is believed to be one of the oldest lakes in North America, due to a geological fluke. The lake sits on a huge block of stone which slowly tilts in the northern direction at the same rate as the lake fills in with sediment, thus keeping the water at roughly the same depth. Core samples of the lake's sediments, taken by U.S. Geological Survey geologists in 1973 and 1980, indicate that the lake is at least 480,000 years old. Some experts feel that Mono Lake, to the east of the Sierra Nevada in California, is older than Clear Lake. However, the sedimentary history of Clear Lake is unbroken, while Mono Lake's sediments have been disturbed by past eruptions of the Long Valley Caldera and associated volcanoes.