Mccoy Flat Reservoir Report

Last Updated: December 5, 2025

Mccoy Flat is a privately owned Earth dam located in Susanville, California, along the Susan River.


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Summary

Built in 1891, this dam serves primarily for irrigation and water supply purposes, with a storage capacity of 17,290 acre-feet and a maximum height of 21 feet. The dam has a low hazard potential and was last inspected in September 2020, with a satisfactory condition assessment reported in September 2017.

Managed by the California Department of Water Resources (DWR) under the Safety of Dams program, Mccoy Flat is regulated, permitted, and inspected by the state agency to ensure its safety and compliance with regulations. Situated in Lassen County, this dam plays a crucial role in water management and supply for the surrounding area. Despite its age, Mccoy Flat continues to function effectively, providing essential water resources for agriculture and local communities.

With a beautiful surface area of 1,800 acres and a drainage area of 110 acres, Mccoy Flat is not only a vital infrastructure for water supply but also a scenic spot for water resource and climate enthusiasts to appreciate. The dam's historical significance, coupled with its functional importance, makes it an intriguing site for those interested in the intersection of water management, climate resilience, and infrastructure development in California.

Year Completed

1891

Dam Length

650

Dam Height

21

River Or Stream

Susan River

Primary Dam Type

Earth

Surface Area

1800

Hydraulic Height

19.1

Drainage Area

110

Nid Storage

17290

Hazard Potential

Low

Nid Height

21
       
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.