Clear Lake is the largest, natural freshwater lake in California and perhaps the oldest lake in North America. It has a surface area of 43,790 acres (~68 square miles) and contains 1,115,000 acre-feet of water when full. Clear Lake stretches diagonally across the landscape from northwest to southeast and is composed of three "arms" (Upper, Lower, and Oaks) joined by a narrows.

Rumsey Gauge

The lake level is measured in reference to the Rumsey Gauge which was established by Captain Rumsey at Lakeport in 1873. Zero Rumsey is equal to 1318.256 feet above mean sea level and is considered to be the natural low water level of Clear Lake. By definition, a "full lake" is one that measures 7.56 feet (1,325.816 feet above mean sea level) on the Rumsey Gauge. 

The United State Geological Survey (USGS) maintains a sampling station on the District's pier that records the level of Clear Lake every 15 minutes. You can see the current level of Clear Lake here and the predicted level of the lake over the next week here.

Prior to the building of the Cache Creek dam in 1914, outflow from Clear Lake into Cache Creek was controlled by a rock ledge called the Grigsby Riffle. The riffle is a rock located at the confluence of Cache and Seigler Creeks, about three miles from Clear Lake. Before the dam was built, water would normally cease to flow over the riffle during the summer months.

Captain Rumsey decided to register the lake level. He decided that when water ceased to flow over the riffle it would be called zero Rumsey. When water was above the riffle it would be called plus Rumsey. Below the riffle, the lake level would be measured as minus Rumsey. He also installed an actual gauge in Lakeport and it's used as a reference to the actual depth at the riffle, not the depth at Lakeport. The actual location of the Rumsey Gauge is on our pier, here at the Lake County Vector Control District (USGS 11450000). This is the only gauge on the entire Lake. The gauge was read and recorded each day by county personnel, but nowadays the U.S. Geological Survey Agency (USGS) reads the gauge by remote telemetry.

Since 1914, the Cache Creek Dam has regulated the level of Clear Lake. The Dam is owned and operated by Yolo County Flood Control and Water Conservation District. Two court decrees, Gopcevic Decree (1920) and Solano Decree (1978, revised 1995), outline the operation of Clear Lake. The Gopcevic Decree regulates winter water levels by setting a lake level below which water may not be released and above which water must be released to reduce flooding (0 - 7.56 feet Rumsey, with exceptions). 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, Yolo County may withdraw 150,000 acre-feet. If the lake level is below 3.22 feet Rumsey on May 1, then no water may be released by Yolo County. Additionally, Yolo County can not take its full allotment at once and no water can be taken after Oct 31.  

References

"History of Clear Lake." County of Lake, California. 27 May 2009. Web. 9 Dec 2013.

Knight, Terry. "It's called the Rumsey Gauge for a reason." Record-Bee. 2 Feb 2010. Record-Bee Outdoors Web. 9 Dec 2013.