Cache Creek Dam Flows
Approximately 1 1/2 miles from Clear Lake, a rock ledge called the Grigsby Riffle crosses Cache Creek. The Riffle is at a narrow point in the creek and limits the amount of water which may flow past it. The Riffle controls outflow from Clear Lake. The Riffle maintained lake levels prior to the construction of the Dam in 1914. In fact, the highest recorded lake level was in 1890 when the lake reached 13.66 feet Rumsey. There were no dams controlling the outflow of Clear Lake between 1868 and 1914.
Cache Creek Dam is designed to release water at 21,000 cubic feet per second (cfs). However, Cache Creek can deliver less than 4,700 cfs when Clear Lake is at 11 feet Rumsey. In perspective 2,700 cfs will fill a typical in-ground, backyard swimming pool in one second. These flows show that the dam can release almost 4 1/2 times as much water as can reach it during severe flood events on Clear Lake.
In January 1995, peak flow into Clear Lake was estimated at over 50,000 cfs, resulting in the lake rising over 1 1/2 feet in a day. At the same time, only 1,000 cfs could flow out of Clear Lake down Cache Creek due to the low lake levels. Peak outflow from Clear Lake in January was approximately 3,350 cfs. After the rains ceased and Clear Lake peaked at 8.84 on February 1, it took 18 days of dry weather for the lake to drop to 7.13 on February 19. Higher lake levels in March resulted in peak outflows from Clear Lake of approximately 4,500 cfs.
Prolonged rainfall result in the lake level staying above flood stage for extended periods. The prolonged wet winter of 1983 required full releases from Clear Lake Dam for 105 days, with an average outflow of 3,850 cfs. In 1986, the heavy rainfalls were of shorter duration, requiring full releases from the Dam for 50 days, with an average outflow of 3,230 cfs. In 1998, full releases from the Dam occurred for 71 days, with an average outflow of 4,640 cfs.
Flow tests were conducted during the winter of 1937-38 to determine what would happen if there was not a dam on Cache Creek. The Dam was fully opened on January 31, when Clear Lake was at 6.30 Rumsey to allow Clear Lake outflows to be completely controlled by Cache Creek. During this flow test, Clear Lake rose to 10.25 on February 15, fell to 8.6 by March 11, rose to 9.7 feet on March 25, and receded to 7.30 feet on April 18, when the gates were closed. During this flow test, peak inflows were estimated to be as high as 41,000 cfs, while the peak outflow of Clear Lake was 4,255 cfs. This clearly demonstrated that the Clear Lake Dam does not cause the flooding of Clear Lake.
Instead of the Dam having all the flood gates open during flood events, it is operated with a limited number of gates open and maintaining the water level at the face of the dam near Zero Rumsey. The dam has 17 flood gates, each 6 feet wide and 7 feet tall. The tops of eight of the gates are at -6 Rumsey, with the tops of the remaining 9 gates at -11 feet Rumsey. Operating the dam at Zero Rumsey reduces the chances of floating debris, such as tree branches and docks, from blocking the gates. Floating debris is still a problem, and significant resources are utilized to keep floating debris from plugging the dam.
Flow tests in 1983 determined this was an optimal level to maximize the release of water through the dam and to minimize plugging of the flood gates. A flow test was conducted during the floods of 1983 and it was determined that lowering the water at the dam did not increase the flow of water through it. On March 1, 1986, with the gates only partially open, and a lake level of 10.17 Rumsey, the release from the dam was 4,320 cfs. In comparison, on February 17, 1938, all the flood gates on the dam were open, the water surface at the dam was approximately -10 Rumsey, the lake was at 10.22 Rumsey, the flow through the dam was 4,240 cfs, slightly less than the flow measured in 1986 when the dam was partially open. This clearly shows that operating the dam with the gates only partially open and maintaining the water surface at a higher level maintains the quantity of flow down Cache Creek at its maximum, while reducing the chances of plugging the flood gates.