Construction of Cache Creek Dam, which is the outlet from Clear Lake, began in 1912 by one of the District’s predecessors, the Yolo Water and Power Company. Construction of the concrete, gravity fed dam was completed in 1914. The outlet works consist of fifteen 6’x7’ openings controlled by hydraulically operated slide gates. The slide gates are located along the base of the dam and are operated by hydraulic rams positioned directly overhead. The dam was quite cleverly designed to incorporate the technology of the time, and uses pressure generated by raising and lowering weight to actuate the hydraulic rams to raise the slide gates for dam discharge.
In 1997 one of the discharge bays was equipped with a pneumatic overpour gate which is operated with electric power; this gate was later retrofitted with SCADA controls, thus allowing District staff to make increases or decreases in flows at any time of the day or night from anywhere that has an internet connection.
The Grigsby Riffle
An interesting and important aspect of Cache Creek Dam operations is that it is not the limiting factor when releasing water for flood control purposes on Clear Lake. A rock ledge called the Grigsby Riffle crosses Cache Creek approximately three miles upstream of Cache Creek Dam and acts as a natural dam, limiting the amount of water that can flow past it. Cache Creek Dam is outfitted with an overpour spillway with a capacity of just over 29,000 cubic feet per second (cfs), but only 4,700 cfs can flow past the Grigsby Riffle when Clear Lake is at 11 feet on the Rumsey Gauge. The constrained outflow caused by the riffle is illustrated by the fact that the highest recorded lake level occurred before the dam was built in 1890 when Clear Lake reached 13.66 feet on the Rumsey Gauge.