USACE lowers Isabella Dam risk rating, lifts operating restrictions

U.S. Army Corps of Engineers Sacramento District
Published July 20, 2023
Updated: July 20, 2023
a labyrinth weir in the foreground with a lake and mountains in the background

The labyrinth weir at Isabella Dam in Lake Isabella, California, June 28, 2023.

Operators at Isabella Dam can once again allow the lake to reach full capacity after the U.S. Army Corps of Engineers updated the dam’s risk level from “highest urgency and risk” to “low urgency” following the completion of dam safety improvements in 2022.

Dam Safety Action Classification ratings identify the risk each dam in the USACE inventory poses. When the risk rating system was initially developed in 2005, Isabella Dam received the highest risk rating—DSAC 1.

With the completion of work to construct a new labyrinth weir and emergency spillway, raise the main and auxiliary dams by 16 feet, and improve the dam’s filtering and drainage systems, the dam’s risk rating is now a DSAC 4, signifying a low risk.

“This is the culmination of more than 15 years of effort to reduce the risk to downstream communities against catastrophic flooding from a potential dam breach and return the project to normal operation,” said Mike Ruthford, lead engineer for the Isabella Dam Safety Modification Project. “This is incredibly exciting for this team and this district.”

In April 2023, USACE leaders joined elected representatives and federal, state, and local officials to celebrate the completion of the project’s second phase, which was designed to address overtopping, seepage, and seismic concerns at the 70-year-old facility that put large downstream populations, including the City of Bakersfield, at risk of catastrophic flooding.

“Substantial completion of the dam safety features could not have come at a more opportune time,” said USACE Sacramento District Commander Col. Chad Caldwell during the April ceremony, referring to the significant rain and snowpack that was inundating the region.

Isabella Lake currently has more than 540,500 acre-feet of water—about 95 percent of its 568,100 acre-feet capacity, also known as gross pool. That number also makes the lake about 50 percent fuller than it’s been allowed since 2006 when USACE implemented an operating restriction of 361,000 acre-feet.

In the coming months, the district will continue monitoring the dam’s performance to validate the accomplishments of the previous five years of construction.

“We’ve achieved our project safety goal,” said Ruthford. “With this year’s snowmelt, we are able to see how the dam performs in the first year after the new improvements were finished.”

The Isabella Dam Safety Modification Project still has one phase left with construction of a new U.S. Forest Service visitor center in Lake Isabella and a permanent USACE operations building. This phase is scheduled to be complete in 2025.

Ken Wright
Jeremy Croft

Release no. 23-026