The U.S. Army Corps of Engineers Sacramento District is preparing to refill Lake Isabella to pre-construction levels, after completing dam safety work on Isabella Dam in California.

Isabella Dam is located about 40 miles northeast of Bakersfield. The reservoir is impounded by two earthen dams on the Kern River and Hot Springs Valley. Isabella Lake and its dams reduce flood risk for Bakersfield and the surrounding region and is a primary water source for Kern County water users. It impounds water for the 11.95-MW Isabella hydro project.

The final environmental impact statement for Isabella Lake Dam, released by the Corps in October 2012, proposed a number of improvements to “reduce the risk of dam failure or catastrophic downstream flooding during a large storm.” A panel found in 2006 that the dam qualifies for Corps’ Class I designation, meaning there is an “urgent and compelling” reason to believe that it might have a high risk of failure.

The Isabella Lake Dam Safety Modification Project addressed overtopping, seismic and seepage issues identified with Isabella Lake’s Main and Auxiliary dams to reduce the likelihood of dam failure. Construction of the dam modifications began in 2017, and the project achieved substantial completion in 2022.

The lake has been restricted to 361,000 acre-feet since dam safety issues were identified in 2006. Its pre-construction volume, or gross pool, was 568,000 acre-feet, and the Corps is preparing to request a deviation from the operating restriction to complete the refill.

The deviation request, when approved, would grant the district a temporary exemption to the operating restriction of 361,000 acre-feet, so Corps engineers can monitor dam performance and validate the dam safety work conducted during the Isabella Dam Safety Modification Project. The operating restriction won’t be permanently removed until the fill plan has been executed and the Corps changes the dam’s safety rating, said David Serafini, a senior engineer with the USACE Dam Safety Modification Mandatory Center of Expertise.

Since the dam’s construction in 1953, the Corps has limited the lake’s volume to 170,000 acre-feet from Nov. 1 to Jan. 31 for flood conservation space and to accommodate runoff from the Sierra Nevada snowpack in the spring. The plan to fill the lake is heavily dependent on snowpack. Snowpack levels are currently above normal, but uncertainty remains.

The goal is to have the fill plan and deviation request approved before April 1, 2023, according to Mike Ruthford, a lead engineer with the South Pacific Division Dam Safety Production Center. “This would allow us to proceed with reservoir filling and project monitoring above the operating restriction and potentially up to gross pool depending on snowpack volume,” he said. “The Fill Plan sets reservoir filling rate targets, but since we can’t control snowmelt volume, we will have to make adjustments in real time.” coordinate these adjustments with downstream water users as we’ve always done.”