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Blog Post · July 29, 2019

The Yolo Bypass: It’s a Floodplain! It’s Farmland! It’s an Ecosystem!

photo - Yolo Wetlands

This is part of a series on issues facing California’s rivers.

California’s biggest river—the Sacramento—needs a lot of room to spread in big water years. A floodplain project called the Yolo Bypass allows it to flood naturally, while also providing habitat for waterbirds, fish, and other aquatic species. We talked to Ted Sommer, lead scientist for the Department of Water Resources (DWR), about this versatile landscape.

photo - Ted Sommer

PPIC: What is the Yolo Bypass, and what does it do?

Ted Sommer: It’s California’s primary floodplain. The Sacramento River drains a massive portion of the state, and the bypass gives it a large area—about 60,000 acres—to spill in high flood events. Then it drains safely back into the river and the Delta. You can see the bypass from highway 80 heading into Sacramento—it often looks like an inland sea in winter. It stayed flooded this year for months on end. If not for that floodplain a lot of people’s back yards would have been flooded in Sacramento. It’s a unique and effective part of the state’s flood management system.

The bypass also provides other benefits. Although its primary purpose is for flood management, it’s used extensively for agriculture. And it’s also managed heavily for wildlife. Its seasonal wetlands provide great bird habitat. And when flooded it serves as a nursery area for young fish in the Delta and good habitat for salmon migrating downriver to the bay. It’s a great example of balancing ecosystem benefits with other needs.

PPIC: What are some of the challenges in managing this landscape?

TS: One reason we started working on the bypass was the finding that a lot of key species do really well in wet years. So we started looking at this habitat that usually only gets flooded in wet years. Over 20 years of research on the bypass, we learned a lot about its role as a fish nursery.

In wet years this floodplain provides better habitat and more food for fish. But because it was designed as a floodway, the water shuts off quickly at the end of the wet season, and fish can get stranded or forced out of this habitat before they can get all the benefits from it.

We also found that the bypass is also a superhighway for many migrating adult fish—sturgeon, adult salmon, and others. The problem is that it is a dead-end in dry years. Even in wet years, as it starts to dry up, fish can get stranded as they move upstream in receding floodwaters.

Based on these areas of research, we’ve identified three priorities. The first is to improve fish passage. We’ve created a fish collection area in the bypass so we can help them get where they need to go in drier conditions. We also built a sophisticated fish ladder for fish migrating upstream during wetter years.

The second priority is improving connectivity between the river and its floodplain. A proposed notch in the top of the bypass would help extend the duration of flooding, giving baby fish more time in the nursery habitat. We hope to move forward with this project in the next several years.

The last improvement is to enable the floodplain to act as a foodbank beyond wet times. In recent years we’ve been working with water users, particularly rice farmers, to get pulses of water in summer and fall to move plankton downstream. Much of this water has already been used to grow rice and is returning to the system. Local farmers and water users have been great partners in this project.

PPIC: Is this model replicable elsewhere?

TS: Yes! Locally, the success of Yolo Bypass has helped fuel interest in restoring floodplain habitat in both the Sacramento and San Joaquin valleys. And it’s inspired people around the state and beyond to consider creating floodplains as part of their flood and ecosystem management strategies. People come from all over the world to visit the bypass. As the climate changes, such places can help manage flooding from extreme events while also providing many other beneficial uses.


California rivers Floods Freshwater Ecosystems Water, Land & Air