By Amy Bentley Special Sections Writer A small mollusk is proving to be a growing problem in California’s water delivery system. Golden mussels are freshwater mollusks native to the rivers and lakes of China and Southeast Asia. They were unintentionally introduced to South America in the 1990s, likely by ship ballast water discharges in shipping ports. Since then, golden mussels have rapidly spread through South America, and in 2024 were detected in California’s Sacramento–San Joaquin River Delta, Water Delivery Infrastructure Golden Mussels Challenge
Gardner added that treatment chemicals capable of limiting golden mussels are already being used safely to control algae. They pose no danger to people or the environment, she said. Jose Reynoso, General Manager of the San Gabriel Valley Municipal Water District (SGVMWD), a direct State Water Project (SWP) contractor, said, “It’s important to note that the water continues to flow. The golden mussels will ultimately increase the cost of water but the intrusion won’t prevent us from delivering water to the region. The golden mussel has brought all the agencies in the San Gabriel Valley together for a common cause and we are working together toward a solution.” The Main San Gabriel Basin Watermaster, SGVMWD, Upper Water, Three Valleys, and the Los Angeles County Public Works Department are working together to devise a Golden Mussel Control Plan – science-based, effective pretreatment measures to prevent golden mussel propagation. The water agencies are
“Eradication is not possible once they are in these water systems.” Tom Love, General Manager, Upper San Gabriel Valley Municipal Water District
“Eradication is not possible once they are in these water systems. The State Water Project is too big. So we will come up with a plan to control the population and spread, and mitigate the impacts on local infrastructure,” Love said. “As long as we have an effective control plan, there won’t be other environmental impacts.” Love said the Metropolitan Water District of Southern California
Photo Source: California Department of Water Resources
(Metropolitan) – where the Upper District gets its water – is adding chlorine to the water as it leaves the Colorado River to go into pumping stations. Chlorine won’t kill the mussels but it does kill 80 percent of the larvae. Metropolitan is also inspecting pipes during routine maintenance shutdowns to look for large clusters of mussels to remove them if necessary. Love said Los Angeles County in September 2025 suspended Upper Water’s new water deliveries pending submission of their acceptable golden
make sure this gets under control. Challenges come in all shapes and sizes but how we band together as a region shows how resilient we are,” said Kelly Gardner, Executive Officer of the Main San Gabriel Basin Watermaster, the agency which oversees the allocation of water rights in the Main San Gabriel Basin. The biggest challenge: golden mussels can cause damage to water infrastructure. They can rapidly colonize in water infrastructure by attaching to pipes, gates and pumps, multiplying quickly and forming dense colonies, clogging water infrastructure and restricting water flow. They do not affect water quality, said Matt Litchfield, General Manager and Chief Engineer for Three Valleys Municipal Water District (Three Valleys), a wholesale water agency serving over half a million people in northeast Los Angeles County. “It’s important for people to know that there is no impact to the quality of treated water,” Litchfield said. “It’s still new to us and we will have to adapt to this over time, but it’s not like we haven’t dealt with different challenges before in water treatment.”
“Our goal is to make sure that water is at the tap when it is called up and we are working together to make sure this gets under control.” Kelly Gardner Executive Officer, Main San Gabriel Basin Watermaster
mussel control plan. Upper Water has plenty of water in the meantime stored in the San Gabriel Basin and expects to have its plan done within a few months, with the delivery issue resolved soon. Love anticipates the district will also use chlorine close to where it takes delivery of water, at the San Gabriel River just below Morris Damn, near Azusa. Adult golden mussels have been detected at the Three Valleys’ surface water treatment plant in Claremont, which takes in water from the SWP, Litchfield said. Three Valleys also has a plan to pretreat that water as soon as it is delivered, he said. Reynoso said his agency is working on an individual control plan as well to protect their infrastructure and could have water delivered to isolated storage basins. SGVMWD owns a 36-mile pipeline from San Bernardino to Azusa Canyon that will be treated against the mussels, Reynoso said.
developing their own control plans, too. They are required by law to submit such a plan to the California Department of Fish and Wildlife if their water system operations will result in the transportation of golden mussels; any local water district taking water from the SWP is either doing this already or will have to, Love said.
which supplies drinking water to more than 25 million people. It’s likely they got in the Delta and the California Aqueduct the same way – from a cargo ship emptying its bilge water into the bay system. This spring, they were found locally in Claremont. “They are now at our back door and we need to address this,” said Tom Love, General Manager of the Upper San Gabriel Valley Municipal Water District (Upper Water), whose service area includes the cities of Arcadia, Azusa, Baldwin Park, Bradbury, City of Industry, Covina, Duarte, El Monte, Glendora, Irwindale, La Puente, Monrovia, Rosemead, San Gabriel, South El Monte, South Pasadena, Temple City and West Covina.
“It’s important for people to know that there is no impact to the quality of treated water.” Matt Litchfield, General Manager and Chief Engineer, Three Valleys Municipal Water District
“It’s important to note that the water continues to flow. The golden mussels will ultimately increase the cost of water but it won’t prevent us from delivering water to the region.” Jose Reynoso, General Manager, San Gabriel Valley Municipal Water District
“Our goal is to make sure that water is at the tap when it is called up and we are working together to
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