During the 2024-2025 fall run, 35,450 adult Chinook salmon returned to the Lower Mokelumne River to spawn. This broke the previous 2023-24 season’s record of 28,698. The combination of successful and innovative hatchery operations, habitat enhancement, and the management of cold-water flows from Camanche and Pardee reservoirs account for the consecutive record-breaking returns. East Bay Municipal Utility District (EBMUD) funds the Mokelumne River Fish Hatchery and runs it in partnership with the California Department of Fish and Wildlife (CDFW).
Chinook salmon make their way up Ship Creek near Anchorage, Alaska to spawn. Credit: Ryan Hagerty, USFWS
Mokelumne River hatchery personnel produce and release millions of fall-run salmon smolts each year. CDFW personnel release the majority of the smolts in the Delta or coastal harbors, where survival rates are up to ten times higher than they are for smolts released in the lower Mokelumne River near the hatchery. Read more about Chinook salmon in the Lower Mokelumne River.
Each year in the fall, EBMUD releases pulse flows from Camanche Dam in order to simulate storm flows that attract adult Chinook salmon into the lower Mokelumne River system. Additionally, EBMUD has increased natural spawning habitat by adding gravel to the reaches downstream of Camanche Dam and the hatchery. In years such as 2023 and 2024 with big returns of adult salmon, the hatchery cannot process all of the returning salmon. An increasing number thus spawn naturally, in the river.
Release of 23 million fall-run Chinook salmon near the Richmond-San Rafael Bridge in 2023. Credit: CDFW
One of the more complex elements of the success of Chinook salmon in the Lower Mokelumne River involves EBMUD’s management of the cold-water pools within Pardee and Camanche Reservoirs. The availability of cold water for fall-run Chinook salmon downstream of Camanche Dam depends on the movement of cold water between the reservoirs.
EBMUD Lower Mokelumne River Project. Credit: EBMUD
Snowpack in the Sierra Nevada Mountains melts and flows downstream in the forks of the Mokelumne River and their tributaries into Pardee Reservoir. As the surface of Pardee Reservoir starts to warm in the spring, the reservoir starts to stratify, with colder water on the bottom. The cold water flowing into Pardee during the spring flows under the less-heavy warm water at the surface, adding to the “hypolimnion” (pool of cold water) at the bottom of Pardee Reservoir.
Pardee Reservoir at its deepest is about 300 feet deep, about twice as deep as the deepest point in Camanche Reservoir. The greater depth of Pardee provides insulation from the sun’s heating in the summer for the cold water at the bottom. Moreover, Pardee has a higher ratio of reservoir volume to surface area than Camanche. This makes it more suitable for storing cold water.
EBMUD releases cold water from Pardee Reservoir in order to replenish releases from Camanche Reservoir to the Lower Mokelumne River. EBMUD has used this strategy to manage the cold water since 1990. Maintaining thermal stratification in both reservoirs is a major priority for EBMUD. Pardee and Camanche Dams have low-level outlets, allowing releases of cold water from their respective hypolimnions. In addition, EBMUD uses different intakes to the powerhouse at Pardee Reservoir to control the temperature of the water it releases. That way, it can save cold water in the spring to release into Camanche Reservoir in the summer and fall.
By monitoring and operating the reservoirs in tandem, EBMUD tries each year to optimize available cold water for the Chinook salmon in the fall. It also does so for the juvenile steelhead that spend a year in the raceways of the Mokelumne River Fish Hatchery.
Recent returns of adult fall-run Chinook salmon to the Mokelumne River are a rare bright spot for salmon populations in the Central Valley. CSPA appreciates EBMUD’s thoughtful and concerted efforts to contribute large numbers of Chinook salmon to California’s salmon fishery.
EBMUD’s Mokelumne River Hatchery downstream of Camanche Dam. The “raceways” are behind the buildings. Credit: CDFW