Understanding Central Valley Floodplains

by Carson Jeffres, University of California, Davis

Salmon and steelhead populations in California experience a “death-by-a-thousand cuts”: dams block access to spawning grounds, water diversions reduce flows, irrigation pumps suck up juvenile salmon, and so on. As CalTrout works to reverse the downward trend in salmon and steelhead populations, we continually try to identify why fish populations are being limited. The answer changes depending on which watershed you study.

In the Central Valley, recent research suggests that the lack of access to historic Sacramento and San Joaquin Valley floodplains may be an underappreciated factor in limiting Central Valley salmon and steelhead populations.

CalTrout is collaborating with colleagues at the University of California, Davis to raise awareness of the importance of Central Valley floodplains for rearing healthy juvenile salmon and steelhead.

We are also working with a host of partners to identify priority floodplain restoration projects and to advocate in Sacramento for progressive statewide floodplain management.

The Cosumnes River floodplain (photo Carson Jeffres)

Fish Growth in Floodplains

UC Davis scientists have found that floodplains grow larger juvenile salmon and steelhead compared to juveniles that do not have access to floodplains. This is important because the larger a juvenile salmon is entering the ocean, the better chance it has of surviving to return as a spawning adult.

Juvenile Chinook salmon reared on a restored floodplain on the Cosumnes River (right) and in the main channel (left) (photo Jeff Opperman)

The Central Valley of California is home to one of the world’s largest and most complex water management systems.

Built over the course of a century, the network of levees and dams that regulates floods, generates hydropower, and provides water for urban and agricultural uses, has disconnected the Central Valley from its rivers.

The consequences of this disconnection for native fish, particularly steelhead and salmon, has been a steady decline in abundance, including multiple listings under the Endangered Species Act.

Most of the attempts to bolster wild salmonid populations have focused on improving riverine spawning and rearing habitat and increasing flow releases from dams to manage temperature and move juveniles to the sea. However, research has shown that floodplains may be an important missing ingredient in the efforts to restore these fish.

Floodplains Driving Fish Food Production

Rivers and their floodplains subsidize each other. When flows from rivers spill out across floodplains they carry along sediment, nutrients, and organic material, including the seeds of important riparian and wetland plants.

As water flows slowly across floodplains and through riparian wetlands and forests, several critical processes are initiated. First, the water warms and clears, allowing phytoplankton and algae to grow. This primary productivity, along with the abundant available organic matter, fuels rapid growth of zooplankton and aquatic insect populations which comprise an ideal food source for fish.

Salmon have evolved to take advantage of the historically abundant floodplains in California. During outmigration, juvenile salmonids follow increased flows (from an annual spring snowmelt) from the channel out onto the floodplain. The abundant food resources, relatively low velocities and warmer temperatures on the floodplain make for ideal rearing conditions, producing growth rates that are up to three times those of juveniles that remain in the channels. In addition, juveniles are able to disperse broadly across floodplains, reducing predation pressures.

As temperatures continue to warm and flows drop, the salmon leave the floodplain and return to the river channels, continuing their outmigration to the sea. These larger juveniles are more likely to survive the rigors of passing through the Delta and San Francisco Bay (along with several years in the ocean), and are thus more likely to return and spawn.

The benefits of floodplains are not just confined to the floodplain itself. As water drains off of floodplains, it carries with it both phytoplankton and zooplankton that help to fuel riverine food webs. In addition, these receding waters carry carbon in all forms: dissolved, particulate and in some cases, large wood. These forms of carbon feed the river and give it structural complexity.

California floodplains critical to salmon?

Floodplains Matter to Fish

Today, California is re-thinking its approach to flood management and ecosystem restoration. Reconnecting rivers to floodplains (by setting levees back and constructing flood bypass channels) reduces flood risk by storing or conveying floodwaters. This same reconnection restores the processes that drive floodplain and riverine biological integrity. This multi-purpose approach holds great promise for managing flooding (and thus public safety) in the Central Valley while also improving habitat for endangered salmon and steelhead.

Other Articles

The California Water blog recently published “The Benefits of Floodplain Reconnection” which details many benefits of floodplain reconnection.

About the Author

Carson Jeffres is a research associate at UC Davis Center for Watershed Sciences. He works closely with Dr. Jeff Mount and Dr. Peter Moyle. Carson has worked extensively with CalTrout on the Shasta River in the Klamath watershed assessing limiting factors to coho salmon production.