California water officials are easing restrictions intended to keep invasive golden mussels out of Lake Oroville, a decision that is drawing concern from scientists and water managers who warn the reservoir could become a new launch point for the destructive species.
The Department of Water Resources’ decision follows a state-funded risk assessment that found the mussels may pose less danger to Lake Oroville and related facilities than previously feared. State officials say that finding changes the cost-benefit calculation for expensive boat inspection and decontamination measures.
Golden mussels have not been detected in Lake Oroville, one of California’s largest and most important reservoirs. But invasive species experts say scaling back prevention efforts could increase the chances that the mussels enter the lake and then spread to other reservoirs on boats, trailers or in standing water. Experts differ, however, on whether stopping the spread is realistic at this point.
“California is under an epidemic of golden mussels,” said Anthony Ricciardi, a biology professor and director of the Bieler School of Environment at McGill University. “Like in any epidemic, you got to control the key hubs — or else the war is lost.”
Golden mussels were first discovered in the Sacramento-San Joaquin River Delta in October 2024, marking the species’ first confirmed appearance in North America. The fast-spreading mollusks can blanket hard surfaces, clog water intakes and damage dams, canals, power plants and other infrastructure.
The Delta is central to California’s water delivery system, including supplies that move south through aqueducts and canals to farms and cities, including parts of Southern California. State officials have said water infrastructure itself can carry mussel larvae downstream, creating risks for irrigation districts and water agencies far from the original infestation.
San Joaquin and Kern counties have declared local emergencies because of the mussels. Meanwhile, state agencies are working on changes at key facilities to limit damage along California’s complex water system.
At Lake Oroville, however, the state is ending a prevention program that required inspections and decontamination for boats launching at the reservoir and at the nearby Thermalito Forebay and Thermalito Afterbay. The change comes as warmer weather brings more boaters to the lake.
Department officials said one reason for the shift is that lakes and boat launches farther upstream in the Feather River watershed were not operating under the same restrictions. That means mussel larvae could still wash into Lake Oroville through river flows, even if boat inspections continued at the reservoir.
Cost also played a role. According to the department, the Oroville inspection program cost about $7.5 million to launch and would cost roughly $6.5 million annually to operate. By comparison, installing ultraviolet treatment to help prevent mussels from settling in pipes at downstream power plants is estimated at about $1 million.
“We severely impacted recreation at that lake,” said Tanya Veldhuizen, special projects section manager in the Department of Water Resources’ environmental assessment branch. “We also evaluated the risk to our infrastructure and what it would take to mitigate mussels — and that was much lower than expected.”
The new risk assessment, prepared by a Canada-based consulting firm that specializes in aquatic invasive species, examined Lake Oroville, nearby reservoirs, hydropower facilities, fish hatchery operations and the Upper Feather River Lakes.
The analysis found that while surface water at Lake Oroville can be warm enough for golden mussels to survive in shallow areas, deeper water is generally too cold for reproduction below about 60 feet. Veldhuizen also said Lake Oroville has lower nutrient levels than the Delta, limiting food for the mussels. Combined with colder water and fluctuating lake levels that can expose and dry out mussels along the shoreline, state officials do not expect the species to reach nuisance levels there.
The department also expects cold water released from Lake Oroville to slow the growth of any larvae that might reach the Feather River Fish Hatchery and power plants in the Oroville-Thermalito Complex.
But scientists say risk remains around shorelines, docks and watercraft — the very places where mussels can attach and travel.
“That’s where the action is. The boats will be moving them,” Ricciardi said.
Boats can transport mussels in ballast water, bilges and other standing water, or through plants and debris attached to trailers and hulls. That pathway is a major concern for reservoirs across the state, including those used heavily for recreation.
Demetrio Boltovskoy, a retired researcher formerly with Argentina’s National Scientific and Technical Research Council, said adult golden mussels are capable of surviving very cold conditions. A study in China found they can live for weeks at temperatures near freezing.
Still, Boltovskoy said that, while he is not specifically familiar with conditions at Lake Oroville, reducing precautions may be a reasonable policy choice if the species is likely to spread regardless.
“No matter what precautionary measures you take, sooner or later it will spread,” he said. “I don’t think that stopping their range expansion is actually feasible at all.”
Other invasive species specialists argue that even delaying new infestations can save money and protect ecosystems. Last year, the California Department of Fish and Wildlife told CalMatters that slowing the spread of golden mussels was worth the effort. This year, the agency referred questions about Oroville’s new approach to the Department of Water Resources.
“There’s so much to protect yet,” Martha Volkoff, environmental program manager for Fish and Wildlife’s Invasive Species Program, said last summer. “Yes, it’s a lot of work, but the long-term savings — to the environment and to all the other ways that it costs us — is investment well spent, even if we just delay new introductions.”
With inspections no longer required at Oroville, state officials are placing more responsibility on boaters to make sure their vessels are clean, drained and dry, especially after leaving infested waters such as the Delta.
If golden mussels are detected at Lake Oroville, Veldhuizen said, the department would begin inspecting boats as they leave the reservoir to reduce the chances of spreading the species elsewhere.
That strategy is already used at infested Southern California reservoirs, including Castaic Lake and Pyramid Lake, both important recreational destinations and water facilities.
Other Northern California reservoirs, including Folsom Lake, Lake Tahoe and Lake Berryessa, are continuing inspection programs. Drew Gantner, water resources manager for the Solano County Water Agency, which oversees the mussel prevention program at Lake Berryessa, said the Oroville decision raises concerns for other lakes.
“If Lake Oroville does surrender its program and becomes infested with golden mussels it creates an increased risk for all waterbodies,” Gantner said. “At that point, any watercraft travelling to Berryessa (or anywhere else) from Lake Oroville would essentially be no different than watercraft coming from the Delta.”
Ricciardi said the consequences could extend beyond Oroville’s infrastructure.
“There is another thing about invasions. They often surprise you,” he said. “Sometimes invaders don’t act the way they’re supposed to act.”
Original source: CalMatters
