California removes protection against mussels in Lake Oroville

from Rachel BeckerCalMatters

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The state of California is withdrawing protection measures designed to prevent destructive golden clams from Lake Oroville, one of the biggest and most important water bodies in the state.

The move follows a new state-funded risk assessment that the invasive species poses less risk to the lake, which water managers say is changing the state’s calculus on expensive and difficult measures aimed at containing the invaders.

No state agencies or scientists have yet found clams in Oroville. But invasive species experts say the Department of Water Resources’ revised policy makes it more likely that golden clams will invade Lake Oroville and be boated to other lakes. However, they disagree on whether preventing such an invasion is even possible.

“California is under an epidemic of golden clams,” said Anthony RicciardiProfessor of Biology and Director of the Bieler School of the Environment at McGill University. “As with any epidemic, you must control the key centers – or else the war is lost.”

Reopening of Lake Oroville

California Water Managers first discovered golden clams invading California’s Sacramento-San Joaquin River Delta in October 2024 — marking their first discovery in North America.

The voracious and fast-spreading mussels can cover surfaces so thoroughly that they clog water supplies and damage dams and power plants.

Now they are invading critical infrastructure in the Delta. And so are the very pumps, canals and aqueducts that keep water flowing across much of the state targeting larvae to irrigation districts and downstream water suppliers.

San Joaquin and Kern Counties have declared a state of emergency, and state officials are upgrading key facilities along the state’s nature-defying water system to reduce clam damage.

With the hot summer weather coming, state water utility managers said they are ending program to prevent clams and their larvae from coming home on boats to invade Lake Oroville, one of California’s largest reservoirs.

The department no longer requires inspections and decontamination for boats launching into Lake Oroville and nearby reservoirs — Thermalito Forebay and Thermalito Afterbay.

The Department of Water Resources says upstream ponds and outfalls in the Feather River watershed have not taken similar precautions, raising the risk of golden clam larvae being washed into the reservoir by river flows, regardless of boat inspections.

The cost of the lake inspection program was also about $7.5 million to start and $6.5 million a year to continue. Installing UV treatment to prevent mussels from settling in power plant pipes downstream from Oroville, by contrast, would cost about $1 million.

“We’ve seriously impacted recreation on this lake,” said Tanya Veldhuizen, manager of the Special Projects Division of the Environmental Assessment Branch of the California Department of Water Resources. “We also assessed the risk to our infrastructure and what it would take to mitigate the clams – and it was much lower than expected.”

Cold water, less clams?

The decision reflects the findings of a new risk analysis commissioned by the department for these reservoirs and associated hydropower and fish hatcheries, as well as for Upper Feather River Lakes.

Conducted by a Canadian based consulting firm specializing in aquatic invasive species, the assessment reported that while surface temperatures are high enough for mussels to survive in shallower waters of Lake Oroville, they are too cold further down for mussels to reproduce at depths greater than 60 feet below the surface.

Unlike the delta, Lake Oroville’s waters are also low in nutrients, Veldhuizen said. Between the scarce food, cold temperatures and water levels dropping enough to dry out the clams along the shoreline, Veldhuizen said he doesn’t expect the clams to reach nuisance levels.

The department also expects the cold water released from the reservoir to slow the growth of any larvae that reach the Feather River Fish Hatchery and Oroville-Thermalito Complex power plants downstream.

But Oroville’s shoreline, boats and docks remain at risk — and that’s what worries Ricciardi.

“That’s where the action is. The boats will move them,” Ricciardi said — because the boats, the water used for ballast in them, the aquatic weeds clinging to them and their trailers can carry clams from one lake to another.

And adult clams can actually survive even in very cold water, says Demetrio Boltovskoy, a retired researcher and former member of Argentina’s National Council for Scientific and Technical Research. A study in China found they can live for weeks at temperatures close to freezing point.

Still, Boltovskoy said that while he’s not specifically familiar with Lake Oroville, easing precautions may be wise.

“No matter what precautions you take, sooner or later it will spread,” he said. “I don’t think stopping their range expansion is actually possible at all.”

But invasive species experts are sharply divided on the topic. This is especially true in California.

Last year, the California Department of Fish and Wildlife said CalMatters that delayed invasions translate into saved money. This year, the wildlife department referred inquiries about Oroville’s new strategy to the Department of Water Resources.

“There’s still so much to protect,” said Martha Volkoff, environmental program manager for the California Department of Fish and Wildlife’s Invasive Species Program last summer. “Yes, it’s a lot of work, but the long-term savings – for the environment and for all the other ways it costs us – is an investment well spent, even if we’re just delaying the introduction of new ones.”

Relying on the boaters: Clean, drain, dry

The onus is now greater on boaters to ensure their boats are clean, drained and dry – especially when leaving an infested body of water, such as the Delta.

If state water managers find clams in Lake Oroville, she said, the department will begin inspecting boats as they leave the lake.

It’s a strategy already in use in other infested lakes, including Castaic and Pyramid.

Managers of other Northern California lakes told CalMatters they will continue their inspection programs, including at the lakes Folsom, Tahoe and In Berry.

Drew Gantner, water resources manager for the Solano County Water Agency, which oversees the mussel program at Lake Berryessa, called Oroville’s decision troubling.

“If Lake Oroville abandons its program and becomes infested with golden mussels, it creates an increased risk for all bodies of water,” Gantner said. “At this point, any vessel traveling to Berryessa (or anywhere else) from Lake Oroville will be essentially no different than a vessel coming from the Delta.”

Ricciardi agreed that the stakes extend far beyond Oroville Dam and downstream facilities.

“There’s another thing about invasions. They often surprise you,” Ricciardi said. “Sometimes invaders don’t act the way they’re supposed to.”

This article was originally published on CalMatters and is republished under Creative Commons Attribution-NonCommercial-No Derivatives license.