Clear Lake

The largest body of fresh water in the State, Clear Lake is one of the oldest lakes on the continent, with estimates of its age ranging between 100,000 and 4,000,000 years; however, its drainage has altered several times within the Recent epoch.

The basin is of sufficient depth to have held a lake of considerable size before the present barriers were formed and there are sedimentary deposits dating well into the Pleistocene. Portions of the basin may be graben, others volcanic in origin, and there are several maar volcanoes on the borders.

Before the Recent epoch the lake had two outlets, Cache Creek, flowing east to the Sacramento River, and Cold Creek, flowing west to the Russian River. Within the past several thousand years volcanic eruptions closed the Cache Creek drainage, sending the overflow through the Cold Creek canyon gorge. A few centuries ago a large landslide on the north slope of Cow Mountain filled the Cold Creek gorge northwest of the present Upper Blue Lake for a distance of 1,800 meters (6,000 feet) or more, thus backing up the waters until they flowed through a low point in the lava flow at the mouth of Cache Creek Canyon. The flow of water cut Red Bank Gorge and dropped the level of the lake 18 meters (60 feet).

The lake is fed by a number of streams and, particularly on the eastern side, by both warm and cold springs in the lake; one produces a roil 10 - 12 centimeters (4-5 inches) high at the surface. Much of the upper lake is shallow, averaging perhaps 8 meters (25 feet), while below the narrows the average is some 18 meters (60 feet), with holes to 40 meters (130 feet). Its shallowness and the prevailing winds prevent extended summer stratification; thus it is a uniformly warm (20 - 25° C) temperature from top to bottom.

Rich in nutrients, the lake supports a dense phytoplankton population. The eutrophic nature of the lake, which may be a climax condition, has resulted in a large fish population. A total of 13 native species and 16 introduced species have been recorded from the lake. Several of the natives, including the thicktail chub, Gila crassicauda, are probably extinct. Four species or subspecies are endemic to the lake, the rare Clear Lake splittail, Pogonichthys ciscoides, hitch, Lavinia exilicauda chi, tule perch, Hysterocarpus traski lagunae, and possibly the prickly sculpin, Cottus asper ssp.

A variety of birds breed on or near the lake. (See Blue Lakes and Clear Lake State Park.)

Several rare invertebrates occur in the lake but it is famous for the hordes of Clear Lake gnats, Chaoborus astictopus, and chironomid midges.

Integrity: In 1914 a dam was constructed on Cache Creek and the level fluctuates up to 3.5 meters (11 feet) above the old level. Some introduced fish have been detrimental. The lake is heavily used for recreation, and resorts crowd the shores. The use of DDD between 1947 and 1959, to control gnats, is a classic example of biological magnification of toxic materials. The lake appears to be recovering.

Use: Research, educational, recreational. Current studies are being made of the phytoplankton (and zooplankton) populations which may have evolved ecological strategies in this eutrophic lake that would be useful both scientifically and in the management of reservoirs.

Ref: Hopkirk, J. D. 1973. Endemism in Fishes of the Clear Lake Region. Univ. Calif. Pub. Zool. 96, 160 pp.
Hodges, C. --- Geomorpholoqy of Clear Lake. Unpub. Ph.D. Thesis, Stanford University, Stanford, Calif.

February 1977