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The City of Napa began offering water service in 1923 following its purchase of the privately-owned Municipal Water Works. This purchase coincided with the construction of Milliken Dam, which allowed storage of water from Milliken Creek, a tributary of the Napa River. The resulting Milliken Reservoir served as the City’s sole water source until Lake Hennessey was created in the 1940’s. Located approximately five miles northeast of the City, Milliken Reservoir is now a seasonal source of supply used in the high-demand summer and early fall period when turbidity levels in the reservoir can be effectively treated.
The City’s water rights are secured through a license with the State Water Resources Control Board. The storage capacity of the reservoir is limited to 1,390 acre-feet (1 acre-foot = 325,851 gallons) due to a State seismic evaluation that necessitated the boring of five holes in the dam, lowering storage elevation by 16 feet. Tributary watershed area is about 6,000 acres.
Water level (reservoir elevation) is continuously measured and reported via the Napa OneRain web site. Elevation of 907 feet represents 100% storage capacity, 1,390 acre-feet. Storage volume at any elevation may be determined using the conversion chart (PDF) linked below.
Raw water is not taken directly from the reservoir, but is instead released into Milliken Creek by a manually operated valve system at the base of the dam. About two miles downstream, a diversion dam directs water into a 16-inch diameter aboveground raw water line. That line then runs approximately one mile down to the Milliken Water Treatment Plant (WTP). This treatment facility was constructed in 1976 and has a treatment capacity of 4.0 million gallons per day (MGD). It is a direct filtration plant with a contact/reaction tank and four horizontal, dual-media pressure filters operated in parallel. Treated water is stored in a 2.0 million gallon clearwell tank located above the treatment plant site. The treated water is delivered to the distribution system via the Milliken Transmission Line. Approximately three miles long, the line serves customers in the Silverado Resort/Hillcrest area before its joins the main system at the intersection of Silverado Trail and Monticello Road.