USACE Lowers Isabella Dam Risk Rating, Lifts Operating Restrictions After Unveiling Dam Improvements

The U.S. Army Corps of Engineers (USACE) Sacramento District held a ribbon-cutting ceremony April 4, 2023 at Isabella Dam in Lake Isabella, California, to mark the end of Phase 2 construction on the Isabella Dam Safety Modification Project.

The purpose of the project is to reduce flood risk for the southern Central Valley, including Bakersfield, and to provide water storage for downstream water users.

“The amount of technical expertise on display behind me is matched only by the incredible and collaborative relationships between our partners,” said Lt. Gen. Scott A. Spellmon, USACE commanding general, as he stood in front of the new 28-foot-high labyrinth weir at Isabella Dam. “I want to extend a very special thanks to the community who came together to help make today a reality.”

Isabella Dam consists of a main and auxiliary dam located just north of Lake Isabella, California. In 2006, a USACE study found the dam vulnerable to three potential failure modes: overtopping, seepage, and seismic damage. The Isabella Dam improvements, begun in late 2017, were designed to address each of these failure modes.

Over the next five years, the USACE Sacramento District raised both the main and auxiliary dams by 16 feet, excavated a new emergency spillway, improved the dam’s filtering and drainage systems, and installed the labyrinth weir. These safety improvements were substantially completed in November 2022.

Speaker of the U.S. House of Representatives Kevin McCarthy (CA-20); Brig. Gen. Antoinette Gant, commanding general of the USACE South Pacific Division; and Col. Chad Caldwell, commander of the USACE Sacramento District, also spoke at the ceremony. The speakers lauded the partnership between federal, state, and local agencies to move the project forward.

Col. Chad Caldwell, left, commander of the Sacramento District, and Lt. Gen. Scott A. Spellmon, USACE commanding general, provide a sense of scale as they pose for a photo in front of the 28-ft. high walls of the labyrinth weir at Isabella Dam in Lake Isabella, California. USACE PHOTO BY LUKE BURNS

The Sacramento District has coordinated closely with Kern River Water Master Mark Mulkay, who also spoke at the ceremony, and downstream water users to determine how best to balance flood risk reduction with irrigation needs.

“Substantial completion of the dam safety features could not have come at a more opportune time,” said Caldwell at the ceremony. “Just last week we approved a deviation plan that allows the lake to fill to its originally authorized gross pool – 568,000 acre-feet – this year.”

In July, USACE updated the dam’s risk level from “highest urgency and risk” to “low urgency” following the completion of dam safety improvements. This means that Isabella Dam operators can once again allow the lake to reach full capacity.

Dam Safety Action Classification (DSAC) 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 Phase 2, the dam’s risk rating is now a DSAC 4, signifying a low risk.

In summer 2023, Isabella Lake reached about 98% of its 568,100 acre-feet capacity, also known as gross pool. That number made the lake about 50% fuller than it’s been allowed since 2006 when USACE implemented an operating restriction of 361,000 acre-feet.

“This is the culmination of more than 15 years of effort to reduce the risk to down-stream 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.”

The labyrinth weir at Isabella Dam in Lake Isabella, California, June 28, 2023. USACE PHOTO BY JEREMY CROFT

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 is a multi-phase project centered on the dam safety modifications completed in Phase 2. Follow-on construction in Phase 3 will include building a new U.S. Forest Service visitor center at Lake Isabella and a permanent operations building for the USACE staff that oversee the daily operations of Isabella Dam. This phase is scheduled to be complete in 2025.

Completed in 1953, Isabella Dam is located approximately 40 miles northeast of Bakersfield. The reservoir is impounded by two earthen dams on the Kern River and Hot Springs Valley. Today, Isabella Lake and its dams reduce flood risk for Bakersfield and the surrounding region and is a primary water source for water users throughout Kern County. The Isabella Dam Safety Modification Project addresses 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 with the relocation of facilities within the project footprint, and the project achieved substantial completion in 2022.


To view the project documents or for more information on the Isabella Dam Safety Modification Project and the USFS visitor center relocation, visit www.spk.usace.army.mil/Missions/Civil-Works/Isabella-Dam/.


This article is featured in the 2023-2024 edition of America’s Engineers: The People, Programs, and Projects of the U.S. Army Corps of Engineers

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