Cleared of 20,000 Cubic Yards of Sediment and Debris, Devil’s Gate Reservoir Ready for Winter Storm

The Los Angeles County Public Works removed over 20,000 cubic yards of sediments that flowed into the Devil’s Gate Reservoir in the past storm season during the annual maintenance of the reservoir.

The annual maintenance of the reservoir immediately behind the nearly 100-year-old Devil’s Gate Dam, includes clearing vegetation within the reservoir basin, excavating excess sediment that washed in during the past storm season, and repairing erosion within the side slopes, according to the LA County Public Works. 

“This annual fall work [was] necessary to allow the dam to regulate high runoff flows that may occur during the upcoming storm season, mitigating flood risk to communities downstream,” the LA County Public Works said in a release. 

This annual maintenance area is approximately 36 acres at the bottom of the reservoir. 

“The reservoir bottom is currently generally clear of vegetation. However, vegetation is present on the side slopes and along the bottom sides that provide habitat for the wildlife,” Public Information Officer for the LA County Public Works, Steven Frasher said. 

According to Frasher, now that the maintenance work has been completed, “the reservoir can continue providing flood protection to the downstream communities, and storm flows from recent rain events were managed by the dam and released downstream in a controlled manner.”

“The Flood Control District’s water conservation facilities are ready to operate this storm season,” said Frasher. 

“The groundwater recharge facilities throughout the system have already captured 8,100 acre-feet of stormwater from October 1, 2022, [as of December 9]. That’s 2.6 billion gallons of water, enough for 64,800 residents of Los Angeles County for a year.”

From April 2019 to August 2021, LA County Public Works, as part of its Devil’s Gate Reservoir Restoration Project, removed 1.3 million cubic yards of sediment from the reservoir which has not been removed since 1994.

Sediment removal from the reservoir provides flood relief to the communities that have endured nearly a decade of elevated flood risk along the Arroyo Seco. 

After the Devil’s Gate Reservoir Restoration Project, the reservoir is to be maintained annually to reduce impacts to the community.

According to Frasher, the project resulted in a reservoir capacity of 3 million cubic yards. 

While it has positive impacts, the project, which has led hundreds of truck trips in and out of the Arroyo Seco daily, has come under fire by residents of Pasadena and nearby La Cañada Flintridge. 

It has also been assailed by the Arroyo Seco Foundation and the Pasadena Audubon Society, both of which filed a lawsuit to stop it.

While a larger maintenance crew and more earthmoving equipment and hauling trucks were used for the recent maintenance work at the reservoir, Frasher reported that it “was conducted without many complaints or comments from the community.”

There was one request received at the start of maintenance activities and that was to modify the backup alarms on the construction equipment to be more quiet. Frasher said LA County Public Works quickly heeded the request. 

“Public Works developed a Sediment Management Plan to indicate how the annual maintenance would be conducted using the lessons learned from the Devil’s Gate Reservoir Restoration Project and placing a priority on the community,” he added. 

According to Frasher, the maintenance work was planned and executed well, resulting in completion of the work earlier than anticipated. The annual maintenance began on September 1, 2022 and concluded on October 20, 2022.