Lake Cynthia Reservoir Report

Last Updated: December 5, 2025

Lake Cynthia in Napa, California, is a privately owned water supply reservoir on Tr Soda Creek.


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Summary

Built in 1955, this earth dam stands at 20 feet high and has a storage capacity of 92 acre-feet, serving primarily for irrigation and water supply purposes. With a drainage area of 0.65 square miles and a surface area of 12 acres, Lake Cynthia is a vital resource in the region, providing crucial water storage for agricultural and domestic use.

Despite its relatively small size, Lake Cynthia plays a significant role in water management in the area. With a high hazard potential due to its location and structural characteristics, the dam is regularly inspected and maintained to ensure its safety and integrity. The most recent inspection in September 2017 deemed the dam to be in satisfactory condition, with a comprehensive emergency action plan in place. The dam's proximity to the city of Napa and its crucial role in water supply make it a key asset in the region's water resource infrastructure, highlighting the importance of proper management and maintenance for sustainable water usage and climate resilience.

Year Completed

1955

Dam Length

750

Dam Height

20

River Or Stream

Tr Soda Creek

Primary Dam Type

Earth

Surface Area

12

Hydraulic Height

14.6

Drainage Area

0.65

Nid Storage

92

Hazard Potential

High

Nid Height

20
       
Seasonal Comparison

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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.