Monday June 9, 2025
Floodplain habitat provides valuable benefits to juvenile fish, offering shelter from predators and increased nutrient availability and prey abundance. These benefits are especially critical for juvenile anadromous salmonids like Chinook salmon (Oncorhynchus tshawytscha) and steelhead (Oncorhynchus mykiss) in California’s Central Valley. As they migrate to the ocean, these young fish must traverse a gauntlet of hazards on their journey through the Sacramento-San Joaquin River Delta. Floodplains give juvenile salmonids an opportunity to avoid predation and grow larger while they fill up on the plentiful food sources found in these habitats. As the Central Valley and San Francisco Bay Area have become more populated, rivers have been diverted and disconnected from historic floodplains to provide irrigation for agriculture and flood protection for surrounding urban areas. Although this process has successfully sustained and protected local communities, it has significantly reduced the availability of floodplain habitat for native fish species.
The loss of floodplain habitat and its associated benefits is one of the many contributing factors to the ongoing decline of Chinook salmon populations in the Central Valley. In recent years, efforts have been initiated to enhance and restore floodplain habitat throughout California. These projects typically involve leveling sloped or eroded banks to create an area adjacent to the river for more frequent inundation. Seeding of native plants is also a common practice in restoration projects to provide cover and shade, enhance bank stability, and house insects that are a valuable food source for young salmonids.
A study published in the Restoration Ecology journal highlighted a recent restoration project on a major floodplain in the Sacramento River watershed known as the Yolo Bypass. The researchers used this project to evaluate how restoring floodplains may benefit Chinook salmon in the Sacramento River under current and projected climate change conditions. The study ultimately revealed that this restoration project is expected to increase the frequency and duration of flooding in the Yolo Bypass and allow more Chinook salmon to enter the floodplain, which could lead to higher growth and survival rates of all regional runs of Chinook salmon that use the Sacramento River.
The Yolo Bypass is a managed floodplain designed to divert the Sacramento River away from residential and commercial communities during high water events. It now serves as the main floodplain for the entire San Francisco Bay Estuary and has become a major target for habitat restoration efforts aimed at benefitting salmonids and other native fish species. The study focused on a recent restoration project modifying the low-head dam at the top of the floodplain known as the Fremont Weir. The project plans to add a notch to the weir to improve fish passage and connectivity to the Sacramento River and increase seasonal flooding of the Yolo Bypass.
Researchers used daily flow projections at Yolo Bypass to summarize the annual flooding patterns, also called the flood regime, of the area. They created a series of statistical models to estimate Chinook salmon production and survival under several climate change scenarios (Hot/Dry, Hot/Wet, Warm/Dry, and Warm/Wet) using flow, temperature, and Chinook salmon migration route selection as variables. This model was further informed by rotary screw trap catch data collected at Knights Landing by the California Department of Fish and Wildlife, which were used to estimate the starting population size for salmon run. Modeling results showed that notch construction in the Fremont Weir will have major impacts on the flood regime in Yolo Bypass at lower flows (less than 113 m3/s). These changes could be beneficial during low precipitation and drought years as less water may be needed to inundate the floodplain after restoration improvements. Their results also projected an increase in production for all Chinook salmon runs with the addition of the weir notch.
Although the Fremont Weir restoration project is not yet complete, the potential benefits for Central Valley salmonid populations highlighted by this study are encouraging. In addition to increased Chinook salmon biomass and production, the study also predicted that the improved floodplain would increase size and age class diversity in Chinook salmon, making these fish populations more resilient to changing temperatures and flow conditions. Restoration projects such as these provide a unique opportunity to achieve floodplain and native fish habitat enhancement while still maintaining adequate water supply and flood protection for the surrounding communities.
Header Image Caption: Restored floodplain on the Stanislaus River.
This post was featured in our weekly e-newsletter, the Fish Report. You can subscribe to the Fish Report here.