Stormwater Engineering Projects

Picture of Cogswell Dam


Cogswell Dam is a vital component of the Los Angeles County Flood Control System and is critical to water conservation efforts, serving multiple beneficial uses including sustaining the downstream habitat and groundwater recharge. During the August 2009 Station Fire, approximately 90 percent of the watershed tributary to Cogswell Dam was burned. The increased rate of debris production from the denuded watershed has hindered the operability of the Dam's valves and reduced the Reservoir's storage capacity.

The Cogswell Reservoir Restoration Project proposes to remove the accumulated sediment from Cogswell Reservoir up-to approximately 2.5 million cubic yards. The Station Fire resulted in an additional 1.1 million cubic yards of debris. The total debris and sediment in the reservoir is 3.1 million cubic yards. This material must be removed as soon as possible to comply with permitting requirements and to maintain the Dam as a fully functional component of Los Angeles County Flood Control System.

Cogswell Sediment Placement Site, Details
Removed debris and sediment from the reservoir will be placed at the adjacent permitted Cogswell Sediment Placement Site (SPS) located at the southern edge of the reservoir. The use of the SPS for the maintenance of Cogswell Reservoir was evaluated in an Environmental Impact Report (EIR) pursuant to the California Environmental Quality Act (CEQA). The EIR was certified by the County Board of Supervisors in 1993. The Cogswell Reservoir Restoration Project will pursue necessary permitting from Regional Water Quality Control Board, California Department of Fish and Wildlife, The United States Army Corps of Engineers, and United States Forest Service.

Construction is currently planned for Fall 2022.

Cogswell Sediment Placement Site, Artistic Rendering Cogswell Sediment Placement Site, Artistic Rendering 2 Cogswell Reservoir Sediment Removal Project, Scope of Work


Location Angeles National Forest along the west fork of the San Gabriel River and off of Devil's Canyon Dam Truck Trail. (Thomas Guide Page 508, A5)
Fire Event(s)* 2009 Station Fire
Watershed Burned 90%
Amount of Sediment to be Removed Up to 2.5 MCY
Removal Method Mechanical Excavation
Disposal Location Cogswell Sediment Placement Site (adjacent to site)
Required Permits U.S. Army Corps of Engineers
California Dept. of Fish and Wildlife
Regional Water Quality Control Board
U.S. Forest Service
Environmental Documents Addendum to 1993 EIR. Project is Phase 3
Community Meeting Dates None currently scheduled
*Heavy sedimentation is expected for the five years following the fire event. With 7-10 years expected for full recovery under current drought conditions.


For more information on the project please contact
Updated on: 11/05/2019