Lake Hennessey is a

reservoir A reservoir (; from French ''réservoir'' ) is an enlarged lake behind a dam. Such a dam may be either artificial, built to store fresh water or it may be a natural formation. Reservoirs can be created in a number of ways, including contro ...

in the

Vaca Mountains The Vaca Mountains are a mountain range in Napa and Solano Counties, California that is one of the California Coast Ranges. They represent the easternmost of the Inner Coast Ranges in north−central California, and divide the Suisun Valley on ...

, east of

St. Helena Saint Helena () is a British overseas territory located in the South Atlantic Ocean. It is a remote volcanic tropical island west of the coast of south-western Africa, and east of Rio de Janeiro in South America. It is one of three constitu ...

and the

Napa Valley Napa Valley is an American Viticultural Area (AVA) located in Napa County in California's Wine Country. It was established by the Bureau of Alcohol, Tobacco and Firearms (ATF) on January 27, 1981. Napa Valley is considered one of the premier ...

, within

Napa County, California Napa County () is a county north of San Pablo Bay located in the northern portion of the U.S. state of California. As of the 2020 census, the population was 138,019. The county seat is the City of Napa. Napa County was one of the original co ...

. The reservoir is formed by Conn Creek Dam, built in across Conn Creek. Construction of the

earthen dam An embankment dam is a large artificial dam. It is typically created by the placement and compaction of a complex semi-plastic mound of various compositions of soil or rock. It has a semi-pervious waterproof natural covering for its surface and ...

was authorized by the

United States Congress The United States Congress is the legislature of the federal government of the United States. It is bicameral, composed of a lower body, the House of Representatives, and an upper body, the Senate. It meets in the U.S. Capitol in Washing ...

when it passed the

Flood Control Act of 1944 The Pick-Sloan Flood Control Act of 1944 (P.L. 78–534), enacted in the 2nd session of the 78th Congress, is U.S. legislation that authorized the construction of numerous dams and modifications to previously existing dams, as well as levees acros ...

in order to mitigate flooding downstream in

Napa, California Napa is the largest city and county seat of Napa County and a principal city of Wine Country in Northern California. Located in the North Bay region of the Bay Area, the city had a population of 77,480 as of the end of 2021. Napa is a major t ...

. 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 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 Basalt Rock Company was a multifaceted industrial operation that was founded in 1920. The company started as a rock quarrying operation located a few miles south of Napa, California near the Napa River. It later branched out into the ship build ...

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 the city of Napa, and it is its primary source of water. When the reservoir reaches its capacity, outflow reaches

San Pablo Bay San Pablo Bay is a tidal estuary that forms the northern extension of San Francisco Bay in the East Bay and North Bay regions of the San Francisco Bay Area in northern California. Most of the Bay is shallow; however, there is a deep water ch ...

via Conn Creek to the

Napa River The Napa River is a river approximately long in the U.S. state of California. It drains a famous wine-growing region called the Napa Valley, in the mountains north of the San Francisco Bay. Milliken Creek and Mt. Veeder watersheds are a few of ...

. 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 high and long containing of material. Its crest is above sea level. It is owned by the City of Napa. Lake Hennessey aerial

References

{{Reflist Hennessey, Lake Dams in California Dams completed in 1948 United States local public utility dams Napa, California Vaca Mountains Hennessey

Conn Creek Dam

See also

References