Circle S Reservoir Report

Last Updated: December 5, 2025

Circle S is a privately owned dam located in Napa, California, along the Tr Milliken Cr river.


°F

°F

mph

Wind

%

Humidity

Summary

Built in 1979, this earth dam stands at a height of 28 feet with a hydraulic height of 24.7 feet, serving primarily for water supply purposes. With a storage capacity of 131 acre-feet, Circle S has a significant hazard potential but is currently assessed to be in satisfactory condition as of September 2017.

Managed by the Department of Water Resources (DWR) and regulated by the Safety of Dams agency, Circle S undergoes regular inspections, with the last one conducted in December 2019. Its location in a high-risk earthquake zone raises concerns, but the dam meets guidelines for emergency action plans, although specific details on preparedness and risk management measures are not outlined in the data. Despite its age, Circle S continues to play a vital role in irrigation, recreation, and water supply within the region.

For water resource and climate enthusiasts, Circle S presents an intriguing case study of a crucial infrastructure in California's water supply network. Its design and construction details, along with regulatory oversight and hazard assessment, offer valuable insights into the challenges and responsibilities associated with maintaining dams in earthquake-prone regions. As the debate around water resource management and climate change intensifies, understanding the complexities of structures like Circle S becomes increasingly important for ensuring the safety and sustainability of our water systems.

Year Completed

1979

Dam Length

412

Dam Height

28

River Or Stream

Tr Milliken Cr

Primary Dam Type

Earth

Surface Area

9

Hydraulic Height

24.7

Drainage Area

0.6

Nid Storage

131

Hazard Potential

Significant

Nid Height

28
       
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.