Clear Lake

Clear Lake is a natural freshwater lake in Lake County in the U.S. state of California, north of Napa County and San Francisco. It is the largest natural freshwater lake wholly within the state, with 68 square miles of surface area. At an age of 2.5 million years, it is the oldest lake in North America. It is the latest lake to occupy a site with a history of lakes stretching back at least 2,500,000 years.

The expansive, warm water of Clear Lake makes it popular for watersports, such as swimming, water skiing, wakeboarding, sailing, boat races, and jet skiing.

The California Office of Environmental Health Hazard Assessment has issued a safety advisory for any fish caught in Clear Lake due to elevated levels of mercury.

Clear Lake is 19 miles by 8 miles at its widest point, with a surface area of 43,785 acres and a 1,155,000 acre?feet capacity. Average depth is 27 feet, maximum is 60 ft, lake elevation is 1,329 feet, and average water temperature is 40 °F (4 °C) in winter and 76 °F (24 °C) in summer.

Clear Lake is believed to be one of the oldest lakes in North America. The lake sits on a huge block of stone which slowly tilts northward 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.

At one time Clear Lake was even bigger than it is now, and included the Blue Lakes (to the northwest of Clear Lake). Volcanic eruptions and subsequent landslides changed the landscape dramatically, forever separating Clear Lake from the Blue Lakes and from its former westward drainage into the Russian River.

Clear Lake lies within the Clear Lake Volcanic Field, as designated by the United States Geological Survey. The volcanic field contains The Geysers, a steam field which powers a network of the world’s most productive geothermal power plants. There are numerous small faults present in the south end of the lake as well as many volcanoes, ranging in age from 10,000 to 2.1 million years, the largest being Mount Konocti, sitting at the middle of the lake's south shore. There have been no eruptions from the Clear Lake Volcanic Field for thousands of years, but there are indicators currently being monitored that suggest there is the potential for future eruptions. These indicators include volcanic-type earthquakes, hot spring activity and seepage of volcanic gas.