Intentionally inundated fish food fields at River Garden Farms near the Sacramento River. Photo by Mike Weir
Scientists in the Central Valley are honing a novel way of giving young salmon the nourishing benefits of wintertime floodwaters without undertaking costly floodplain restoration work.
The method, being practiced along the Sacramento River, mimics the flood patterns of natural Sacramento Valley wetlands by diverting water onto floodplain farm fields, retaining it there for three weeks, and finally flushing the water—now rich with zooplankton and invertebrate protein—back into the river. Onsite studies have shown that salmon smolts grow faster when provided with this supplemental nutrition source, giving the method promise as a tool for boosting survival rates of outmigrating juveniles and, ultimately, helping sustain imperiled Chinook runs.
Jacob Katz, a California Trout biologist helping lead the project, explains that flushing river water over dry land and then back into the river effectively “reenergizes the food web,” allowing terrestrial carbon to flow into the water where it can build aquatic biomass. For small salmon, this means they can reap the benefits of floodplain food synthesis even when confined to the leveed mainstem of the river, where food resources tend to be scarce. In a 2019 experiment that involved 5,000 acres of flooded land in the Colusa basin, Katz and his colleagues anchored cages containing Chinook smolts both upstream and downstream of the floodwater release sites. The results were striking: Caged salmon a mile downstream of the flushing site grew three times faster than the salmon held upstream. In a 2021 repeat of the experiment, smolts as far as six miles downstream grew 4.5 times faster than their upstream cohorts.
Katz says the plan is to scale up the program to 20,000 acres for the coming winter, and he believes that within five years, the land dedicated by private owners to this project could amount to 100,000 acres. The project, Katz says, does more than simply feed fish: “It also demonstrates that a healthy river is intimately connected to the landscape through which it flows.”