Lake Hennessey Reservoir Report

Last Updated: December 5, 2025

Lake Hennessey is a freshwater reservoir located in the Napa Valley of California.


°F

°F

mph

Wind

%

Humidity

Summary

The lake was named after one of the first settlers in the area, Charles Hennessey. The primary source of the lake's water is the Conn Creek watershed. The lake also receives water from the Rector Creek and Milliken Reservoir. The lake is managed by the Napa County Flood Control and Water Conservation District. The lake's water is used for agricultural purposes, including vineyards and orchards in the Napa Valley. The lake is also used for recreational activities such as fishing and boating. The lake's depth, size, and water quality make it an ideal location for water sports and fishing. The lake's hydrology is characterized by a surface flow and snowpack providers. The surface flow is a result of rainfall and snowmelt, while the snowpack providers are the Sierra Nevada Mountains.

Reservoir Details

Storage 24hr Change 0.0%
Percent of Normal 240%
Minimum 60.0 acre-ft
2022-12-26
Maximum 33,166.0 acre-ft
2024-02-19
Average 11,921 acre-ft
       
Seasonal Comparison
Storage Levels
Pool Elevation Levels
Top Conserv Storage Levels
Inflow Levels

Weather Forecast

Dam Data Reference

Condition Assessment

Satisfactory
No existing or potential dam safety deficiencies are recognized. Acceptable performance is expected under all loading conditions (static, hydrologic, seismic) in accordance with the minimum applicable state or federal regulatory criteria or tolerable risk guidelines.
Fair
No existing dam safety deficiencies are recognized for normal operating conditions. Rare or extreme hydrologic and/or seismic events may result in a dam safety deficiency. Risk may be in the range to take further action. Note: Rare or extreme event is defined by the regulatory agency based on their minimum
Poor A dam safety deficiency is recognized for normal operating conditions which may realistically occur. Remedial action is necessary. POOR may also be used when uncertainties exist as to critical analysis parameters which identify a potential dam safety deficiency. Investigations and studies are necessary.
Unsatisfactory
A dam safety deficiency is recognized that requires immediate or emergency remedial action for problem resolution.
Not Rated
The dam has not been inspected, is not under state or federal jurisdiction, or has been inspected but, for whatever reason, has not been rated.
Not Available
Dams for which the condition assessment is restricted to approved government users.

Hazard Potential Classification

High
Dams assigned the high hazard potential classification are those where failure or mis-operation will probably cause loss of human life.
Significant
Dams assigned the significant hazard potential classification are those dams where failure or mis-operation results in no probable loss of human life but can cause economic loss, environment damage, disruption of lifeline facilities, or impact other concerns. Significant hazard potential classification dams are often located in predominantly rural or agricultural areas but could be in areas with population and significant infrastructure.
Low
Dams assigned the low hazard potential classification are those where failure or mis-operation results in no probable loss of human life and low economic and/or environmental losses. Losses are principally limited to the owner's property.
Undetermined
Dams for which a downstream hazard potential has not been designated or is not provided.
Not Available
Dams for which the downstream hazard potential is restricted to approved government users.