A stone-covered embankment dam surrounded by mountains.

Lake Isabella Dam Safety Modification


Our Client’s Challenge

Lake Isabella offers Southern Californians 568,000 acre-feet of water-based recreation opportunities and serves as a primary water source for Kern County residents. The lake was formed in 1953 when the U.S. Army Corps of Engineers (USACE) built the earthfill embankment Isabella Dam, consisting of a main and auxiliary dam, across the lower Kern River and Hot Springs Valley. Located 40 miles northeast of Bakersfield, the dam and lake provide flood-risk management for 300,000 downstream residents and irrigation for 350,000 acres of farmland.

Due to safety concerns, the lake’s volume was restricted to approximately 60% of total capacity for years, impairing the USACE’s ability to accommodate spring runoff from the Sierra Nevada snowpack. Despite the low water levels, the unpredictability of extreme weather events made flooding a potential threat to surrounding farmland and downstream communities.

The risk of overtopping, combined with seismic concerns and seepage issues, elevated the dam’s risk classification and spurred the USACE to undertake a three-phase dam safety modification project. The agency selected Gannett Fleming to support phase two, the main effort to modify the dams and spillways.

Our Solution

To reduce the risk to life and property from flood events, earthquakes, and seepage, phase two of the Lake Isabella Dam project included:

  • Raising both dams – main and auxiliary – by 16 feet.
  • Improving the filtering and drainage systems on both dams.
  • Adding a new 900-foot emergency spillway.

One of the project’s most visible aspects is the new three-story labyrinth weir in the emergency spillway. The accordion-like shape maximizes the concrete weir’s surface area, slowing the water flowing in the spillway and resulting in a controlled release.

Tapping into our best-in-class dam and levee engineering expertise, Gannett Fleming provided geotechnical engineering services during construction when an immense amount of materials were moved to create the dam addition and new spillway. Crews excavated over 3.5 million cubic yards of earth to make room for the new spillway and reinforce the main and auxiliary dams with over 67,000 cubic yards of concrete and almost 2.5 million cubic yards of rock.

Gannett Fleming provided earthwork construction observation, including excavation, fill placement, moisture conditioning, and compaction, to confirm design specifications and design requirements were met. We observed work activities at the borrow site and oversaw blending existing materials with the borrow soils, including identifying undesirable soils that did not meet the specifications.

Key Features

  • A wider and deeper emergency spillway supplements the existing service spillway.
  • Reinforcement materials were excavated onsite, reducing the need to unload materials from elsewhere.
  • The weir’s sawtooth shape slows the rushing water in the emergency spillway.


  • The modifications addressed overtopping, seepage, and seismic concerns.
  • USACE lowered the dam’s risk level from “highest urgency and risk” to “low urgency.”
  • Operating the lake at full capacity boosts the local economy by increasing recreation opportunities that attract tourists.
  • Operating at capacity also increases the amount of electricity produced by the privately owned hydroelectric plant on the main dam’s south side.

Awards & Recognition

  • Awards. This web part is hidden.


U.S. Army Corps of Engineers, Sacramento District


Kern County, Calif.


Geotechnical Engineering

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