Monday August 24, 2026
California’s Central Valley was once a vast mosaic of seasonal wetlands, meandering rivers, riparian forests, native grasslands, and rolling hills blanketed in wildflowers. Together, these interconnected ecosystems supported an extraordinary diversity of fish, wildlife, and plants. During one of his journeys from San Francisco to Yosemite, naturalist John Muir described the San Joaquin Valley as the “floweriest piece of world,” where the land, “watered by the San Joaquin and Sacramento rivers, formed one flower-bed, nearly four hundred miles in length by thirty in width.” The fertile soils and abundant water that supported these diverse ecosystems also made the valley ideal for agriculture. As farms and communities expanded, rivers were confined within levees and dikes to reduce flooding. Over time, winding rivers were transformed into straighter, deeper channels that were easier to manage. While these changes benefited agriculture and flood control, they disconnected rivers from their natural floodplains and dramatically reduced the diverse habitats that once supported numerous fish and wildlife species.
Floodplains and side channels are seasonally inundated, shallow, vegetated, low-velocity areas known to benefit juvenile salmon. Historically, these productive, slow-moving environments provided refuge from aquatic predators, as well as increased forage, allowing fish to reach larger sizes before migrating downstream, increasing their chances of surviving in the ocean. Recent research has shown that restoring seasonal floodplain habitats in California’s Central Valley increases habitat complexity, encouraging salmon to remain in rivers longer while supporting a wider variety of migration strategies that contribute to more resilient populations. Floodplains are naturally dynamic, constantly shifting as rivers transport sediment and respond to changing flows. This variability creates a diversity of habitats that are difficult to recreate through engineering alone. Restoration practitioners have found that successful side channels should be designed with the expectation that they will continue to evolve over time rather than remain fixed in place. In California’s regulated rivers, floodplain restoration is especially challenging. Studies have demonstrated the benefits of reconnecting floodplains, but dams alter the natural timing and magnitude of flows in impounded rivers, trapping the sediment that replenishes these habitats. Scientists are still working to better understand how these altered flow regimes influence the long-term success of floodplain restoration.
Historic gold mining also negatively impacted floodplains. Hydraulic and dredge mining excavated river channels and deposited large piles of gravel across these areas. Restoring these sites often requires relocating thousands of cubic yards of gravel, making projects expensive and logistically complex. These mining remnants are also typically low in fine soils, making it difficult for native riparian vegetation to establish. Without healthy streamside forests, rivers lose valuable shade, protective habitat, and critical food sources for juvenile salmon.
Several restoration projects across California are reconnecting rivers with their historic floodplains. The Yolo Bypass Salmonid Habitat Restoration and Fish Passage Project, commonly known as the Big Notch Project, modified the Fremont Weir to improve fish passage. The project will allow Sacramento River salmon to access approximately 30,000 additional acres of seasonal floodplain habitat. The Honolulu Bar Restoration project on the Stanislaus River addressed decades of damage caused by gold dredging. Restoration crews relocated much of the gravel from large in-stream islands back into the river, rebuilding spawning habitat while lowering island surfaces for floodplain inundation. The 2.4-acre project also removed invasive vegetation and planted native species, restoring habitat that supports aquatic insects, provides cover for fish, and stabilizes the streambank. Recently awarded funding will support continued restoration efforts on the Stanislaus River. On Butte Creek, riparian restoration takes a different approach. Because the watershed is free of large reservoirs that trap sediment, restoration practitioners can work with more natural river processes to capture sediment, improve connections between the channel and floodplain, and encourage riparian forest regeneration. Rather than creating a fixed design, these projects help the river rebuild and maintain its own complex habitat over time.
Floodplains are increasingly recognized as one of the most valuable habitat types for juvenile salmon in California. Restoring these landscapes can increase habitat diversity and strengthen the resilience of salmon populations facing changing ecosystems. Rebuilding floodplains, however, is rarely straightforward, and restoration strategies vary from one watershed to another. Projects are often costly and technically challenging, particularly in rivers affected by altered flow regimes. Continued research and monitoring will help identify which restoration strategies provide the greatest long-term ecological benefits and can effectively rehabilitate the dynamic floodplain ecosystems that once made California rivers so productive.
Header Image Caption: Electrofishing the floodplain of the Stanislaus River.
This Fish Report is part of our Restoration miniseries, where we discuss how and why different freshwater habitats are being restored.