Lewiston Reservoir Report

Last Updated: December 5, 2025

Lewiston is a Federal-owned dam located in Trinity, California, along the Trinity River.


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Summary

Built in 1963 by the Bureau of Reclamation, this Earth-type dam stands at a height of 73 feet, with a structural height of 91 feet and a length of 745 feet. It serves a primary purpose of water supply, providing a storage capacity of 14,660 acre-feet and covering a surface area of 750 acres.

With a high hazard potential and a moderate risk assessment rating, Lewiston Dam plays a crucial role in managing water resources in the region. Despite not being state-regulated, the Bureau of Reclamation oversees its operations, inspections, and regulatory aspects. The dam's uncontrolled spillway, with a width of 66 feet, further underscores its importance for flood protection and water management in the area. Overall, Lewiston Dam stands as a significant infrastructure for water resource enthusiasts and climate advocates to monitor and study in Trinity County.

Year Completed

1963

Dam Length

745

Dam Height

73

River Or Stream

TRINITY RIVER

Primary Dam Type

Earth

Surface Area

750

Hydraulic Height

73

Drainage Area

719

Nid Storage

14660

Structural Height

91

Hazard Potential

High

Nid Height

91
       
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.