Cogswell Reservoir Reservoir Report

Last Updated: December 5, 2025

Cogswell Reservoir is a man-made reservoir located in the San Gabriel Mountains of Los Angeles County, California.


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Summary

The reservoir was built in 1954 by the Los Angeles County Department of Public Works as part of the San Gabriel River Watershed. It is a significant source of drinking water for the region and can store up to 8,000 acre-feet of water.

Cogswell Reservoir is fed by both surface flow and snowpack from the San Gabriel Mountains. The San Gabriel River, which originates from the mountains, is the primary surface flow provider to the reservoir. Snowmelt from the surrounding mountains also contributes to the reservoir's water supply.

The water from Cogswell Reservoir is used for both agricultural and recreational purposes. The water is mainly used for irrigation in the San Gabriel Valley, while recreational activities such as fishing, boating, and hiking are also popular in the area. The reservoir is managed by the Los Angeles County Department of Public Works and is an important source of water for the region.

Reservoir Details

Storage 24hr Change 0.0%
Percent of Normal 243%
Minimum 15.0 acre-ft
2021-09-06
Maximum 8,892.0 acre-ft
2023-01-18
Average 448 acre-ft
Dam_Height 266
Hydraulic_Height 239
Drainage_Area 38.4
Year_Completed 1935
Nid_Storage 8969
Hazard_Potential High
Primary_Dam_Type Rockfill
River_Or_Stream Wfk San Gabriel R
Surface_Area 146
Dam_Length 585
Nid_Height 266
       
Seasonal Comparison
Storage Levels
Pool Elevation Levels
Top Conserv Storage Levels
Inflow Levels

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.