Henne Reservoir Report

Last Updated: December 5, 2025

Henne, also known as Lake Henne, is a private water resource located in Angwin, California, in Napa County.


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Summary

Built in 1959, this earth dam stands at a height of 49 feet and has a storage capacity of 109 acre-feet, serving primarily for water supply purposes, including irrigation and recreation. The dam sits on Angwin Stream, with a drainage area of 0.91 square miles, providing vital water resources to the surrounding area.

Managed by the California Department of Water Resources (DWR) under the Safety of Dams program, Henne is regulated, inspected, and enforced by the state to ensure its safe operation. With a high hazard potential but a satisfactory condition assessment as of September 2017, this dam plays a crucial role in water management in the region. Although there are no associated structures and limited information on its emergency action plan and risk assessment, Henne remains an essential component of the local water infrastructure.

Located within the Sacramento District and overseen by Congressman Mike Thompson, Henne presents a blend of historical significance and contemporary water resource management. As climate change continues to impact water availability and quality, understanding and maintaining dams like Henne becomes increasingly important for ensuring sustainable water management practices in the face of evolving environmental challenges.

Year Completed

1959

Dam Length

470

Dam Height

49

River Or Stream

Angwin Stream

Primary Dam Type

Earth

Surface Area

9

Hydraulic Height

45

Drainage Area

0.91

Nid Storage

109

Hazard Potential

High

Nid Height

49
       
Seasonal Comparison

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.