Scope of Devil’s Gate Dam sediment removal downgraded by county supervisors vote

The Los Angeles County Public Works Department’s plan to remove 2.4 million cubic yards of sediment from behind Devil’s Gate Dam at Hahamongna Watershed Park was downgraded Tuesday, after county supervisors voted to cap removal at 1.7 million cubic yards.

The Los Angeles County Board of Supervisors was to decide whether portions of an environmental review for the project should be recertified, after a Los Angeles County Superior Court judge deemed the county’s document insufficient while considering a lawsuit brought against the agency by environmental groups the Arroyo Seco Foundation and Pasadena Audubon Society.


The panel was also asked to decide whether they’d support the sediment removal plan moving forward.

Supervisor Kathryn Barger, who represents the 5th district, proposed a motion to reduce the total removal amount by 700,000 cubic yards and to commit $2.5 million toward completion of an Arroyo Seco Ecosystem Restoration Study by the U.S. Army Corps of Engineers. That study would guide efforts to mitigate the biological impacts of the project along a 10-mile stretch of the Arroyo Seco downstream of Devil’s Gate Dam.


First district supervisor Hilda Solis proposed a motion requiring the county to work with local cities to repair roads impacted by sediment hauler truck trips from Hahamongna to Manning Pit spreading basins in Irwindale and calling for additional outreach to residents downstream of the dam.

Both motions were approved by a unanimous vote, according to a public works spokesman, who said Tuesday that the work would not likely begin before 2019.

Mitchell Tsai, a Pasadena attorney representing the environmental groups, said Tuesday’s concessions are a good start, but are not enough to automatically dismiss his clients’ lawsuit against the county.

“It doesn’t give them everything they want, but it’s a step in the right direction,” he said.

The plaintiffs, he added, are hopeful now that the sediment removal amount has been reduced, the county’s Flood Control District will be open to proportionately reducing other impacts, including the proposed 400 daily round-trip truck hauler trips.

Another point of contention is the 50 acres of Arroyo Seco land that would remain denuded as a permanent maintenance area. Environmental groups have said the move would degrade a biologically rich area.

“They’d like it restored to a river rather than a flood channel,” Tsai said. “That’s really the ultimate vision for the Arroyo.”

The Devil’s Gate Dam removal itself is not likely begin before 2019, the county spokesman said.