Balancing Water and Wildlife: How Science Guides the State Water Project

As a series of atmospheric rivers bring rain and high elevation snow to communities throughout California, the winter storms are crucial for the State Water Project, helping fill reservoirs and support water deliveries.

The State Water Project (SWP) is part of the backbone of California's water infrastructure - a multi-benefit project that supplies water to 27 million Californians and 750,000 acres of farmland, provides flood protection, generates hydropower, offers recreational opportunities and provides environmental benefits.

The SWP must balance water supply reliability with the protection of listed endangered fish species at the same time climate change alters how and when rain and snow reach California.

Current State of the System

California's recent water years have been defined by weather whiplash, with dramatic swings from extremely wet to extremely dry conditions, sometimes within the same season. With the 2026 water year already underway, DWR is looking to take advantage of the storms in December following a relatively dry second half of fall. Lake Oroville, the SWP's largest reservoir, and San Luis Reservoir currently hold above-average storage levels. For some threatened and endangered fish species, signs are pointing towards another strong rebound year. The adult winter-run salmon return this past summer was the third-highest observed in 40 years. On the Feather River, the adult spring-run salmon return was the fourth-highest observed in two decades. DWR is expecting growth in the population of adult longfin smelt as well, following a positive trend of in recent years.

What's in Store for SWP Operations?

Generally, when fish populations are doing well, the SWP sees higher numbers of listed species at the fish screens near its water export pumping facility in the south Delta. This does not necessarily mean that the SWP is harming the population. In fact, new data is showing the effects of salvaging and relocating fish at the pumps may not impact the subsequent next generation as long as appropriate pumping safeguards are in place. To continue this important work of protecting native fish species, scientists at DWR developed a genetic tool that differentiates a listed salmon from a non-listed salmon within hours. This new technology allows the SWP to better balance fish protection and water supply.

To help young salmon survive their migration through the Delta, DWR installed a bioacoustic fish fence at the junction of the Sacramento River and Georgiana Slough. The fence uses a combination of bubbles, light, and sound to discourage migrating salmon from entering the interior Delta that could lead them to the export pumping facility. The system operates from mid-November until the end of May while salmon make their way to the ocean.

DWR also recently celebrated the opening of the Yolo Big Notch facility, a key infrastructure investment to create rearing habitat for salmon on floodplains. Scientific studies have shown that juvenile salmon using habitat in the Yolo Bypass grow faster, and the habitat offers an alternative route to the Pacific Ocean that avoids migration routes leading to the pumps in the interior Delta.

In the last few years, DWR completed more than 7,400 acres of new habitat restoration in the Delta. These restoration projects cover wide swaths of wetlands that provide food and habitat for fish and wildlife, including listed species. Some sites, such as the recently restored Lookout Slough, also reduce local flood risk.

Finding Flexibility in Weather Whiplash

In December, DWR received approval from the California Department of Fish and Wildlife for an amendment to the Incidental Take Permit (ITP) that helps guide operations of the SWP. This amendment allows for flexibility in decision-making around fish protection actions from December to June, including for a pumping reduction known as a "first flush" action. First flush is triggered when early winter storms swell river flows. DWR analysis shows that the rules triggering these actions can sometimes be relaxed without impacts to listed species. This week's storms will bring more water into the Delta, benefiting SWP water supply while creating better conditions for native fish.

Rain and Snow Matter Most

Ultimately, California water supply is driven primarily by the timing, location, and volume of rain and snow - coupled with the dams, canals, and pumping plants necessary to move that water and store it for inevitable dry times. As it operates that infrastructure through the wet and potentially dry days ahead this winter, DWR is committed to using the best available science and forecasting to deliver water to communities while protecting threatened and endangered species.