History of Clear Lake
The Clear Lake Basin was shaped by a variety of processes over the last 1 to 2 million years. Scientists have recovered a nearly continuous sequence of lake sediments dating back 475,000. Other lake sediments in the region that date back to the Early Pleistocene, approximately 1.6 to 1.8 million years ago.
There is an excellent climate record from these cores for the last 127,000 years. The record documents a shift from pine dominated to oak dominated forests at the end of the Pleistocene Glacial Period 10,000 years ago, indicating a warming trend. The diatom sequence in these cores indicate that Clear Lake has been a shallow, productive system, essentially similar to the modern lake since the end of the Pleistocene Period.
The basin was created primarily from the stresses of the San Andreas Fault System, the eruption and subsidence of the Clear Lake Volcanics, and the erosion and deposition of the parent rock. The east-west extension of the fault system and vertical movements of the faults created and maintained the basin. Downward vertical movement within the basin created by these processes is at a rate approximately equal to the average sedimentation rate of 1/25 inch per year in the lake basin.
Since these rates are essentially equal, a shallow lake has existed in the upper basin for at least the last 475,000 years. If sedimentation rates were significantly different from the downshift, then either a deepwater lake or a valley would have resulted. Although the lake has changed shape significantly over this period, it has generally been located in the same area as the existing Upper Arm.
The Lower and Oaks Arms were created by a combination of faulting and subsidence resulting from volcanic activity. The Lower Arm was a shallow marsh from 9,500 to 40,000 years ago. The Oaks Arm was probably less than 6.5 feet deep from 11,000 to 36,000 years ago. The downshifting of the lake bottoms in the two arms is probably a combination of faulting and volcanic activity. It has been theorized by the locations of springs on the lake bottom that calderas (collapsed volcanoes) have formed significant portions of the Lower Arm, Oaks Arm and the Narrows area. Lava flows have also changed the shoreline in the Narrows, and Lower and Oaks Arms.
The basin is located on a topographic divide between the Russian River system to the west and the Sacramento Valley to the east. The lake initially drained to the Sacramento River, then into the Russian River, and currently drains into the Sacramento River. The change in drainage is thought to be primarily due to the faulting predominant in the Coast Mountain Range, although other forces have played a part in Clear Lake's history. For instance, the basin's outlet to the Russian River was blocked about 10,000 years ago by a landslide immediately west of Blue Lakes. Blue Lakes now exists in the flooded outlet canyon.
The natural level of Clear Lake has been maintained by the Grigsby Riffle, which is a rock sill located at the confluence of Cache and Seigler Creeks near Lower Lake. After a significant flood in 1938, the Riffle was excavated to an average depth of 2.3 feet Rumsey, however, further excavation was stopped by the courts and is now prohibited by the "Bemmerly Decree," 1940.
In 1914, the Cache Creek Dam, located approximately 3 miles downstream of the Riffle, became operational and has regulated the level of Clear Lake. Operation of the dam is now controlled by two court decrees, known as the "Gopcevic Decree" (1920), and the "Solano Decree" (1978, revised 1995).
These decrees attempted to balance the desire to store water in the lake for downstream water supply and local recreation with the desire to minimize local flooding. The Gopcevic Decree regulates winter water levels by setting a lake stage below which water may not be released and above which water must be released to reduce flooding. Because of the limited discharge capacity of the Cache Creek channel, it is physically impossible to prevent the lake from flooding during extended periods of heavy rainfall. Prior to construction of the dam, the highest recorded lake level was 13.66 feet Rumsey in 1890, with the lake level exceeding 10 feet Rumsey 9 times between 1874 and 1914. After the dam was constructed in 1914, the highest recorded lake level has been 11.44 feet Rumsey in 1998, with the lake level exceeding 10 feet Rumsey 9 times.
The Solano Decree regulates summer water levels by establishing the allowable releases based on the spring water level. If the lake level equals or exceeds 7.56 feet Rumsey on May 1, up to 150,000 acre-feet of water may be released through the dam. However, if the lake does not reach a level above 3.22 feet Rumsey on May 1, then no water may be released.