|Location||Napa County, California|
|Primary inflows||Sage Creek, Chiles Creek, Moore Creek, Conn Creek|
|Primary outflows||Conn Creek|
|Catchment area||54 sq mi (140 km2)|
|Basin countries||United States|
|Max. length||2 mi (3.2 km)|
|Max. width||3,500 ft (1,100 m)|
|Surface area||790 acres (320 ha)|
|Water volume||31,000 acre feet (38,000,000 m3)|
|Surface elevation||322 feet (98 m)|
The lake is formed by Conn Creek Dam, built in 1948 across Conn Creek. Construction of the earthen dam was authorized by the United States Congress when it passed the Flood Control Act of 1944 in order to mitigate flooding downstream in Napa, California. Funding for the dam was never appropriated by Congress, so in 1946 the City of Napa took on the project and built it at a cost of $550,000 dollars plus $250,000 for the land. The cost of laying the 36 inches (910 mm) diameter pipeline from the dam to the city of Napa was $1.7 million. The 30 miles of pipe for the project was manufactured at the Basalt Rock Company plant located south of Napa. The design of the dam did not include a way to drain water from the reservoir when it comes close to full capacity. Once the lake is full, water drains from a spillway causing potential flooding dangers downstream. The reservoir and pipelines are maintained by Napa, and it is the primary source of water for the city. When the reservoir reaches its capacity, outflow reaches San Pablo Bay by way of Conn Creek, which feeds the Napa River. The lake was named after Edwin R. Hennessey. Hennessey was a local civic leader who played a role in the development of the Conn Valley reservoir.
Conn Creek Dam
Conn Creek Dam is an earthen dam 125 feet (38 m) high and 700 feet (210 m) long containing 500,000 cubic yards (380,000 m3) of material. Its crest is 330 feet (100 m) above sea level. It is owned by the City of Napa.
- List of lakes in California
- List of lakes in the San Francisco Bay Area
- List of reservoirs and dams in California
- U.S. Geological Survey Geographic Names Information System: Lake Hennessey
- "Dams Within the Jurisdiction of the State of California" (PDF). Retrieved September 1, 2010.
- http://www.countyofnapa.org/Pages/DepartmentContent.aspx?id=4294968277 County of Napa Flood Control District
- Courtney, Kevin (October 17, 2004). "Napa Pipe plant loads its final rail car". Napa Valley Register (Napa, CA: Lee Enterprises, Inc.). Retrieved September 24, 2011.
- Goetting, Jay (January 28, 2006). "Lake Hennessey adds to flood flows, but officials say changes to Conn Dam unlikely, unhelpful". Napa Valley Register. Retrieved October 17, 2010.
- http://www.napanow.com/chronhist.html/Napa Napa Now
- Brennen, Nancy (October 17, 2010). "Examining the life of one of Napa’s founding physicians". Napa Valley Register. Retrieved October 17, 2010.