By Ted Sommer
. . .
My most inspiring bike ride this past year was not on a mountain or in some exotic destination. It was sixteen flat and muddy valley miles under overcast skies. My destination was a new concrete structure designed to reconnect the Sacramento River with its adjacent floodplain, the Yolo Bypass. This habitat restoration project reflects decades of work by my team and represents one of the most important steps to save Central Valley salmon.
In the late 1990s, I worked full-time as a scientist for California Department of Water Resources while moonlighting as a U.C. Davis Ph.D. student. Like several other scientists in my cohort, I was trying to answer the question of why wet years are so good for many species in the Bay-Delta and its watershed. I chose to study the Yolo Bypass, the primary floodplain of the region. This system is essentially an overflow basin for the Sacramento River during big flood events—it is a 60,000 acre partially leveed feature that is designed to safely convey floodwaters away from Valley communities like Sacramento. My logic was that floodplain inundation was one of the most unique aspects of high flow years, so perhaps there was something special about the Yolo Bypass for the ecosystem.
We spent years developing methods to sample floods and their biota. Floodplains were especially challenging to study because they are dry for much of the year, then switch to a wild torrent during floods. Much of our focus was on studying juvenile Chinook Salmon, which migrate down the Sacramento River during their journey to the Pacific Ocean. We relied on a complex series of nets, traps, tags, sensors, and probes to understand what happened to these fish when they were swept into the adjacent Yolo Bypass floodplain during high water.
In the early 90s, much of the prevailing dogma was that off-channel habitats, like the Yolo Bypass, were too dangerous for migrating salmon because they would become trapped or eaten by predators. My team quickly discovered that seasonal floodplain habitat is nothing short of magic for young salmon—it is loaded with food and has thousands of acres of safe rearing habitat. Baby salmon in the floodplain grow much faster than fish that stay in the Sacramento River channel, helping them to survive the dangerous journey to the ocean.
Years of research demonstrated that the seasonal floodplain is, in fact, perhaps the single most important nursery habitat for young salmon and other native fish. In hindsight, this should not have been a surprise given the geography of California. But we also found that the heavily engineered Yolo Bypass floodway was far from perfect. The system only connects to the Sacramento River during flood events, so species such as salmon usually can’t access this rich seasonal habitat. It is also engineered to drain quickly, forcing young salmon to exit the Yolo Bypass with receding floodwater before they can receive the maximum benefits of nursery area. For similar reasons, the system is risky for upstream migrating adult salmon, sturgeon and steelhead trout, which frequently become trapped in Yolo Bypass when the floodplain rapidly disconnects from the Sacramento River.
In 2000, I led an effort to recommend ways that the Yolo Bypass could be made even better for salmon and other species. We had several ideas about how to make the system even more fish-friendly. The most important idea was to improve the connectivity between the Yolo Bypass and the adjacent Sacramento River, allowing flow and baby salmon to enter the floodplain during much more of the year. This relatively simple change had the potential to dramatically improve survival of several iconic and imperiled species.
Making the dream a reality took much longer than I ever expected. Twenty-five years to take this project from conception to completion may seem ridiculous, but this timeline is on par with other similar landscape-level habitat projects, such as the recent Klamath River dam removal. Engineering along rivers is always tricky, and the environmental permitting is challenging, especially given the broad range of interest groups in the state.
After our initial project proposal, a team of engineers and planners worked for many years to complete the Fremont Weir Big Notch project. It is basically a huge gated concrete ditch that can be opened to allow a modest amount of Sacramento River flow to spill into Yolo Bypass well before (and after) peak floods.
Just before Christmas of 2025, the gates were opened for the first time, reconnecting the Sacramento River to its floodplain. My original inspiration for studying the floodplain came on a cold, wet bike commute across the Yolo Bypass. It was only fitting that my visit to the new Fremont Weir project was via bicycle. I stood on the side of Fremont Weir, imagining thousands of baby salmon moving through the new notch to reach the rich Yolo Bypass nursery habitat. Although California salmon are in dire shape, this project and others give us hope that they can rebound with a little help.
Acknowledgements
The science behind the Yolo Bypass project was conducted over the past 28 years by dozens of dedicated staff at Department of Water Resources. Special thanks to Louise Conrad, Fred Feyrer, Jared Frantzich, Lenny Grimaldo, Bill Harrell, and Brian Shreier for their huge contributions. The talented engineering and design team included Manny Bahia, Marianne Kirkland, James Newcomb and many others. Peter Moyle, John Brennan, Carson Jeffres, Rene Henery, Josh Israel, and John Cain provided frequent inspiration and encouragement.
About the Author
Ted Sommer is the retired Lead Scientist for California Department of Water Resources. He is currently an Adjunct Fellow with Public Policy Institute of California.
Further Reading
Huntsman, B.M., Wulff, M.L., Knowles, N., Sommer, T., Feyrer, F.V. and Brown, L.R. (2024), Estimating the benefits of floodplain restoration to juvenile Chinook salmon in the upper San Francisco Estuary, United States, under future climate scenarios. Restor Ecol, 32: e14238. https://doi.org/10.1111/rec.14238
Sommer, T. R., W. C. Harrell, M. Nobriga, R. Brown, P.B. Moyle, W. J. Kimmerer and L. Schemel. 2001. California’s Yolo Bypass: evidence that flood control can be compatible with fish, wetlands, wildlife and agriculture. Fisheries 26(8):6 16.
Sommer, T. R., M. L. Nobriga, W. C. Harrell, W. Batham, and W. J. Kimmerer. 2001. Floodplain rearing of juvenile Chinook salmon: evidence of enhanced growth and survival. Canadian Journal of Fisheries and Aquatic Sciences 58(2):325 333
Sommer, T., W. Harrell, and M. Nobriga. 2005. Habitat use and stranding risk of juvenile Chinook salmon on a seasonal floodplain. North American Journal of Fisheries Management 25:1493-1504.
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