Ralston Afterbay Reservoir Report

Last Updated: December 5, 2025

Located in Auburn, California, the Ralston Afterbay is a concrete dam structure built in 1966 primarily for irrigation purposes.


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Summary

However, this reservoir serves multiple functions including hydroelectric power generation, recreation, and water supply. With a height of 89 feet and a storage capacity of 2,780 acre-feet, the Ralston Afterbay plays a crucial role in water management in the region.

Managed by the local government and regulated by the Federal Energy Regulatory Commission, the Ralston Afterbay is subject to regular inspections to ensure its structural integrity and safety. With a controlled spillway width of 200 feet and a maximum discharge of 159,000 cubic feet per second, the dam has a high hazard potential and is categorized as a very high-risk structure. Despite its age, the dam continues to play a vital role in supporting agriculture and water resources in the area.

The Ralston Afterbay's location on the Rubicon River provides a picturesque setting for outdoor recreational activities while also serving as a critical water storage facility for the surrounding communities. As water resource and climate enthusiasts, understanding the infrastructure and management of dams like the Ralston Afterbay is essential for ensuring sustainable water usage and environmental conservation in the face of changing climate patterns.

Year Completed

1966

Dam Length

560

Dam Height

89

River Or Stream

Rubicon River

Primary Dam Type

Concrete

Surface Area

83

Hydraulic Height

79

Drainage Area

214

Nid Storage

2780

Structural Height

89

Outlet Gates

Tainter (radial) - 5

Hazard Potential

High

Nid Height

89
       
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.