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Lake Hennessey is the major local water source for the City of Napa system. Located approximately 13 miles north of the City, the Lake Hennessey reservoir was formed in 1946 with construction of Conn Dam, allowing storage of water from Conn Creek, an upvalley tributary of the Napa River. The City’s water rights are secured through a permit with the State Water Resources Control Board. Lake Hennessey has an approximate storage capacity of 31,000 acre-feet (1 acre-foot = 325,851 gallons). Its tributary watershed area is about 35,000 acres.
Water level (lake elevation) is continuously measured and reported via the Napa OneRain web site. Elevation of 315 feet represents 100% storage capacity, 31,000 acre-feet. Storage volume at any elevation may be determined using the conversion chart (PDF) linked below.
Raw water from Lake Hennessey flows into a cylindrical concrete intake tower and is pumped up to the Hennessey Water Treatment Plant (WTP). Hennessey WTP began operation in 1981 and has a nominal treatment capacity of 20 million gallons per day (MGD). The facility provides complete conventional treatment, including flash mixing, coagulation, flocculation, sedimentation, filtration, and disinfection. Treated drinking water from the plant is conveyed into a buried 5.0 million gallon concrete clearwell tank on site. This treated drinking water is delivered to the distribution system through the 36-inch diameter Conn Transmission Main. The Conn Line is approximately 20 miles long and runs parallel to Conn Creek, Highway 128, and Highway 29. It travels along easements and right-of-ways before meeting the Jamieson Line in northwest Napa.