Preston Forebay Reservoir Report

Last Updated: December 5, 2025

Preston Forebay is a historic hydroelectric facility located in Ione, California, owned by a public utility.


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Summary

Built in 1892, this offstream dam stands at a height of 40 feet and has a storage capacity of 30 acre-feet. The primary purposes of the dam include flood risk reduction, hydroelectric power generation, and irrigation, making it a vital water resource for the region.

Managed by the California Department of Water Resources (DWR), Preston Forebay is regulated, permitted, inspected, and enforced by state agencies to ensure its safety and functionality. With a low hazard potential and a satisfactory condition assessment, the dam provides reliable water storage and power generation while maintaining minimal risk to surrounding communities. The dam's historical significance, combined with its modern regulatory oversight, makes it a valuable asset in managing water resources and climate challenges in Amador County.

As part of the San Francisco District, Preston Forebay plays a crucial role in the local water infrastructure, contributing to the overall resilience of the region. With a strong emphasis on safety and regulatory compliance, this dam serves as a testament to the intersection of traditional hydroelectric technology and modern water management practices. For water resource and climate enthusiasts, Preston Forebay offers a fascinating glimpse into the history and evolution of water infrastructure in California.

Year Completed

1892

Dam Length

176

Dam Height

40

River Or Stream

Offstream

Primary Dam Type

Earth

Surface Area

2

Hydraulic Height

36

Nid Storage

30

Hazard Potential

Low

Nid Height

40
       
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.