By Jeffrey Mount, Peter B. Moyle, Andrew L. Rypel, and Carson Jeffres

The current wet spell, made up of a parade of atmospheric rivers, is a welcome change from the last three years of record dry and warm conditions. For very good reasons, the focus during these big, early winter storms is first and foremost on flood management and public safety. There is of course also great interest in the potential of these storms to relieve water shortages for communities and farms. What is not always appreciated is the role of these early winter storms  in supporting the health of freshwater ecosystems.

For millennia, California’s biodiversity evolved strategies to take advantage of these infrequent, but critical high flow events. Benefits from recent storms are now being realized throughout the state, from temperate rainforests of the North Coast to semi-arid and arid rivers in the south. 

As an example, here is a sample of some of the vital ecological processes that take place during winter wet periods in the Central Valley and San Francisco Estuary:

• Shaping of rivers and their habitat. Floods are when the work of a river gets done.  Important geomorphic thresholds are crossed during these high flows, leading to erosion, transport and deposition of sediment, and channel migration and formation. This is essential to creating habitat heterogeneity, abundance, and quality. A healthy river is one that does not hold still, but is constantly adjusting its channel and floodplain.
• Dispersal of plants and animals. Large flow events are vital to moving everything, from fish to trees. For some fishes, such as endangered winter- and spring-run Chinook salmon, high flow events give an essential lift to juveniles, transporting them downstream, and enhancing outmigration survival rates to the Pacific Ocean. For riparian trees like willows and cottonwoods, these events send pieces of vegetation downstream, depositing them on newly-formed sandbars where they sprout in the spring.
• Increased spawning success of native fishes. Native fishes including Chinook salmon, white sturgeon, Sacramento splittail, Sacramento sucker, hitch, and other species generally show an increase in populations following wet years, in part from the increase in floodplain spawning and rearing habitat. Indeed, a high fraction of native fishes in the Central Valley evolved to take advantage of floodplain habitats when available, either for rearing or spawning. Splittail, for example, only spawn on floodplains.
• Access to more river margin habitat. During the summer and fall, low flows on rivers reduce the amount of available habitat. High flows open access to channel margin habitat, which are good places for fishes and other aquatic organisms to hold, feed, and escape predators. Increased flooding and access to river margin habitat, in turn, also generates a positive feedback cycle whereby these habitats become more likely to support riparian vegetation. And increased vegetation is of exceptional value to migratory songbirds, beaver, and other wildlife. Good things lead to more good things.
• Wetting of seasonal wetlands. Large storms play an essential role in delivering water to seasonal wetlands, whether through direct rainfall, overflow from rivers and streams, or irrigation canals. This helps spread out migratory waterbirds, increasing available habitat and food resources, and reducing disease transmission. Increasing the amount and duration of flooding in the Central Valley has long been a solid conservation goal for diverse practitioners. All this rain provides these restorative benefits for free.
• Priming the floodplain. Floodplains of the Central Valley are an essential part of river ecological productivity. As days grow longer and air temperatures increase, the water pushed onto floodplains in winter warms, slowly turns into a rich soup of aquatic insects. Numerous fishes—most notably juvenile Chinook salmon—make use of floodplains as a place to fatten up. Like packing a lunch for a long trip, salmon subsequently use these resources during their journey to the ocean and once they get there if resources are not plentiful.
• Groundwater recharge. Winter floods play a large role in maintaining shallow groundwater levels throughout the Central Valley. Perennial wetlands, floodplain lakes, and streams and rivers are fed by shallow groundwater, particularly over the dry season, making ideal, productive habitat for an array of plants and animals. Today, with many basins facing overpumping and groundwater level declines, there’s renewed interest in using floodwaters for recharge—a boon for water supplies and for nature.
• Estuarine rejuvenation. Life in the San Francisco Estuary—not the largest on the west coast, but a very important one—is adapted to and dependent upon pulses of fresh water, nutrients, and sediment that come from the watershed during winter floods. These pulses are especially important to building and maintaining tidal marsh habitat, which is the signature habitat of the Sacramento-San Joaquin Delta and much of the rest of the estuary. The biodiversity of this estuary is closely linked to these flood pulses.

These are just a few items from the long list of ecological benefits associated with large winter floods. The history of water and land management in California has muted these important processes. Reservoirs store floods and trap sediment. Thousands of miles of levees built to reclaim land for cities and farms have reduced or eliminated the historic connections that sustained wetlands, primed the productivity of the floodplain, and recharged groundwater. River channels have been straightened and simplified to speed water off the land for flood control. Overpumping of groundwater has disconnected many groundwater dependent wetlands. And all these changes have resulted in greatly diminished estuaries—most notably the San Francisco Estuary—that are no longer productive and have become home to numerous non-native species. This moment is also a reminder that the many efforts underway in California to improve freshwater ecosystems need to consider the potential value of winter flood pulses, and crafting strategies to restore these essential functions, such as including timing of flow releases and reconnecting water to land.

Still, even in our highly changed landscapes, high flow events like those unfolding this month (roughly once a decade on average, with the last big early winter flows in 2017) are very helpful in managing river and estuarine ecosystems. So while the news is rightfully focused on water supply and flood damages, it is worth keeping in mind that there are other important, often unseen benefits for our natural environment.

Jeffrey Mount is a senior fellow at the Water Policy Center, Public Policy Institute of California and founding director at the UC Davis Center for Watershed Sciences. Peter B. Moyle is a Distinguished Professor Emeritus at the University of California, Davis and is Associate Director of the Center for Watershed Sciences. Andrew L. Rypel is a Professor and the Peter B. Moyle and California Trout Chair of coldwater fish ecology at the University of California, Davis. He is a faculty member in the Department of Wildlife, Fish & Conservation Biology and Director of the Center for Watershed Sciences. Carson Jeffres is Field and Lab Director and Senior Researcher at the Center for Watershed Sciences.

Further Reading:

Cloern, J. E., J. Kay, W. Kimmerer, J. Mount, P. B. Moyle, and A. Mueller-Solger. 2017. Water wasted to the sea? San Francisco Estuary and Watershed Science 15(2).

Moyle, P., J. Opperman, A. Manfree, E. Larson, and J. Florsheim. 2017. Floodplains in California’s future. https://californiawaterblog.com/2017/09/10/floodplains-in-californias-future/

Rypel, A.L., P.B. Moyle, and J. Lund. 2021. A swiss cheese model for fish conservation in California. https://californiawaterblog.com/2021/01/24/a-swiss-cheese-model-for-fish-conservation-in-california/

Sturrock, A.M., Ogaz, M., Neal, K., Corline, N.J., Peek, R., Myers, D., Schluep, S., Levinson, M., Johnson, R.C. and Jeffres, C.A., 2022. Floodplain trophic subsidies in a modified river network: managed foodscapes of the future? Landscape Ecology 37(12): 2991-3009.

Torres, F., M. Tilcock, A. Chu, and S. Yarnell. Five “F”unctions of the Central Valley floodplain. https://californiawaterblog.com/2022/05/08/five-functions-of-the-central-valley-floodplain/

Yarnell, S.M., Petts, G.E., Schmidt, J.C., Whipple, A.A., Beller, E.E., Dahm, C.N., Goodwin, P. and Viers, J.H. 2015. Functional flows in modified riverscapes: hydrographs, habitats and opportunities. BioScience 65(10): 963-972.