California's waterways could get clogged by a problem that didn't exist two years ago

When golden mussels were found in an international shipping channel in Stockton nearly two years ago, marking the first detection of the invasive shellfish in North America, state officials knew it was going to be bad.

Now those fears are being borne out.

The roughly 1-inch-long, golden-brown mollusks, native to Asia, have spread from the Sacramento-San Joaquin River Delta, where they were initially spotted, through canals and aqueducts to the Bay Area and Southern California. Along the way, they've blanketed boats and docks with their crusty colonies and gummed up pumps and pipelines at major water facilities.

"The mussels have the ability to clog an entire pipe," said Jennifer Allen, spokesperson for the Contra Costa Water District, which is dealing with the pest in its water intake fixtures. "Getting in to clean all the infrastructure is a massive amount of work and potentially requires shutting down parts of the system."

She added, "We're really only seeing the beginning of what the impacts are."

Across California, tens of millions of dollars are being spent to stop the mussels. But with no retreat in sight and increasing potential for disruptions to water delivery as well as flood control systems and hydroelectric operations, efforts to get a handle on the infestation are ramping up.

Congress has introduced legislation to bring more funding. The state is looking to add staff. New control methods are being tested. Boating restrictions at many lakes are tightening up to keep mussels from hitching rides to new spots, such as Lake Tahoe, where communities worry about the mollusks turning their famously blue water green.

"This is certainly a challenge for the state of California," said Martha Volkoff, environmental program manager for the invasive species program at the California Department of Fish and Wildlife. "We anticipate most water bodies throughout the state are susceptible to golden mussel survival. Time will tell."

The toll of the mussels on water quality and native ecosystems is yet to be fully determined.

Invasive species are not new to California's waterways, which are home to aquatic weeds, nonnative frogs and nutria and at least two other menacing mussels, but the golden mussel ranks among the worst.

These mussels withstand a wider range of conditions than similar quagga and zebra mussels: They tolerate both fresh and brackish water and spread just as easily. They can reside on almost any watery surface, from the walls of a canal to the inside of a pipe, and they reproduce swiftly, their larvae carried far and wide by currents.

California's sprawling water-delivery network, with its hub in the delta, has been optimal for the mollusk's advance.

"Any diversion out of the delta, most likely, is spreading this mussel," said Tanya Veldhuizen, environmental program manager at the California Department of Water Resources and the agency's lead in combating the invader.

The department, which coordinates statewide water deliveries alongside the federal government, has seen its infrastructure in and near the delta hit hard by the mussels. Perhaps the most infested site is the Skinner Delta Fish Protective Facility, a complex designed to keep salmon, smelt and other fish from being swept into the water exports.

The facility's screens and pipes are regularly covered in mats of mussels, despite weekly cleaning crews that remove the mollusks only slightly quicker than new ones emerge.

"On some of the walls out there, we're seeing layers of mussels 3 to 5 inches thick," Veldhuizen said. "We're now hand-scraping and using high-pressure water cleaners."

There are no easy ways to expel the mollusks. More aggressive chemical treatments are usually unsuitable not only because golden mussels are highly resilient, but also because the compounds can harm wildlife and taint water supplies where the mussels congregate.

"We don't have a lot of products right now that are fish-friendly and effective on the golden mussel," Veldhuizen said.

Some water agencies have reported limited success with targeted application of chlorine- and copper-based products. The state has begun experimenting with ultraviolet light and hot water at some facilities.

Without a sure treatment, the mussels are proceeding hundreds of miles to terminal points of the state and federal water projects, showing up at such spots as the San Joaquin Valley's giant San Luis Reservoir, the pipeline to Lake Perris in Riverside County and the South Bay Aqueduct in Alameda and Santa Clara counties.

From the water projects, the mussels have jumped to local water systems that receive the imported supplies. The Santa Clara Valley Water District reported detections this spring at two treatment plants. The Contra Costa Water District has been dealing with the mussels for longer, having spotted them coming from the delta into canals and reservoirs shortly after their discovery in the state in October of 2024.

Neither agency is expecting immediate problems with water quality or water deliveries, but they say the monitoring and disinfection has been time-consuming and expensive.

To date, the infestation has been largely limited to water systems that are linked to the state and federal water projects, particularly south of the delta. The mussels have not colonized project facilities to the north, including Lake Oroville and Lake Shasta. Most areas unconnected to the water projects also have been spared.

Since state officials don't expect to be able to eradicate the mussels, a top priority is keeping them from going elsewhere. This is the strategy used successfully with other invasive mussels, with the quagga mussels confined to Southern California and the zebra mussels confined to San Benito County. The more stalwart golden mussels, though, have already spread more broadly than their peers.

In many places, including Lake Tahoe, officials have enacted strict policies to prevent the mussel's overland introduction, including mandatory boat inspections and decontaminations. Last month, inspectors in the Tahoe basin detected golden mussels for the second time on a boat being towed in from outside the region, according to the Tahoe Regional Planning Agency. The vessel was quarantined.

Tahoe officials warn that establishment of golden mussels could hobble the lake's water clarity by ushering in more algae and plants. Such aquatic life often flourishes as the filter-feeding mussels facilitate more sunlight in the water and excrete nutrients. Mussel infestation would likely be limited to Lake Tahoe's shallow waters since the mollusks appear unable to mature and reproduce at colder temperatures.

At Lake Oroville, state officials recently dropped requirements for boat inspections and decontaminations, determining that the cumbersome regulation was unnecessary since the lake is too cold most of the year to sustain a significant mussel population.

The state is focusing its pest-control efforts on the delta and points south. Officials at the Department of Water Resources said recently they're thinking about declaring an emergency for the State Water Project, to marshal more assistance for problem areas. San Joaquin and Kern counties have already declared local emergencies to get help confronting infested water systems that serve farms.

California Democratic Sens. Adam Schiff and Alex Padilla introduced federal legislation last month called the Golden Mussel Eradication and Control Act of 2026, with a companion bill by Rep. Josh Harder, D-Stockton. The measure would establish a grant program with $15 million in each of the next four years, to help contain the mussels and research new ways to control them. Additionally, it would help track the infestation and coordinate the response.

The mussels are not believed to be established beyond California, though wildlife officials in Oregon recently intercepted a boat carrying golden mussels from the delta, and the state remains on high alert.

At least two bills currently in the state Legislature also address the issue. One by Assembly Member Diane Papan, D-San Mateo, seeks to tighten the rules on boat transport and inspections to prevent mussel spread while the other by Assembly Member Rhodesia Ransom, D-Stockton, makes it easier for water managers to do control work.

Additionally, the proposed state budget, which is scheduled to take effect July 1, includes new staffing for the Department of Fish and Wildlife to confront the mollusk.

"The time is now to deal with golden mussels," said Papan at a recent Assembly meeting in Sacramento. "We are on the precipice of an infestation the likes of which we may not be able to overcome."

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