What is the Rumsey Scale? In 1872, a pioneer named Captain Rumsey established the low point of the Grigsby Riffle Sill, a rock ledge located at the confluence of Cache and Siegler creeks in Lower Lake, as "Zero Rumsey." All subsequent measurements of Clear Lake's level are based on this elevation above sea level. Clear Lake is considered full at 7.56 Rumsey. Flood levels vary, however, warnings are issed at 9 Rumsey. The highest recorded lake level was 13.66 Rumsey in 1890, while the lowest was -3.5 Rumsey in 1920. Typically, the lake level varies from an average of 1.6 Rumsey to 6.75 Rumsey each year. When Clear Lake is full, it has a surface area of 43,790 acres and contains 1,155,00 acre-feet of water. At Zero Rumsey, the surface area is 39,170 acres with a capacity of 842,00 acre-feet.