Golden Mussel Presents Widespread Threat To Major California Reservoir

Here are the morning’s top stories on Monday, June 30, 2025…

• Lake Oroville in Northern California’s Butte County is one of the state’s  most important reservoirs. It’s part of a state-spanning system that delivers water to 27 million people and hundreds of thousands of acres of farmland. Now officials are working to keep an invader out of the reservoir — a freshwater bivalve mollusk called the golden mussel. 
• Governor Gavin Newsom has conditionally approved a budget deal to close the state’s $12 billion deficit. The deal includes a controversial plan to scale back Medi-Cal access for undo***ented immigrants in the state, which the governor once supported.
• The American Civil Liberties Union says the federal government is breaking its promise to protect migrant families who were forcibly separated at the U.S.-Mexico border during the first Trump Administration.

The invasive two-inch wide golden mussel showed up near the Port of Stockton last fall. Since then, it’s spread south, extending to other waterways in the Delta and some in the San Joaquin Valley. Now, eyes are looking north to Lake Oroville, where the mussels could pose a large threat if they’re introduced.

The reservoir is the second largest in California. It’s part of the state’s system that delivers water through a network of pipes and pumps to more than two-thirds of residents and hundreds of thousands of acres of farmland.

Golden mussels like to stick to hard surfaces — like those pipes and pumps — and that has state officials concerned. “They’re going to grow, they’re going to settle out, attach onto the walls, and then begin forming those colonies,” said Tanya Veldhuizen, an environmental program manager with the Department of Water Resources. “They just keep adding layers and layers and mussels until that pipeline is completely blocked.”

The mussels also pose a significant environmental threat. Eric See is with the Department of Water Resources. He said Lake Oroville feeds water into the Feather River Fish Hatchery through small diameter piping. It raises steelhead trout and chinook salmon. Chinook populations are threatened, and the state is currently trying to bring them back. If that pipe gets blocked, it cuts off water to the fish. “Within just a few minutes you can lose millions of fish,” See said. The mussels could also create large algae blooms that can kill fish and filter water, increasing aquatic weed growth. That makes it harder for fish in the water to navigate and find food.

California Gov. Gavin Newsom signed on Friday a budget that pares back a number of progressive priorities, including a landmark health care expansion for low-income adult immigrants without legal status, to close a $12 billion deficit.