By Peter B. Moyle

My local stream, Putah Creek, looks like a river these days. Water is pouring down the Glory Hole of Lake Berryessa and rushing in muddy turmoil from the ‘dry’ creeks that are its main tributaries. The creek’s deeply incised and leveed channel is containing the flows that once would have spread across the landscape. As a fish biologist, I am often asked: what’s going on with the fish? Are they all being flushed downstream into the Sacramento River and Delta?

My response is this. Some unlucky fish are being flushed into places they would rather not be, causing freshwater fishes to be found in places normally too salty for them. But most fish manage to stay near home or are carried to places they want to be, especially to food-rich floodplains.

For stay-at-home fish, the key is to get out of the main channel and into side channels, among the trees and bushes, where they shelter from the current. A few days ago, I led a small expedition (4 enthusiastic fish people) to look for fish in the backwaters of Putah Creek. We used a seine and pulled it on flooded roadways, where the currents were weak and the bottom free of logs and vegetation. In an hour of somewhat difficult sampling, we caught eight species of fish: largemouth bass, bluegill, green sunfish, black crappie, golden shiner, fathead minnow, Sacramento pikeminnow, and threespine stickleback. With the exception of the single stickleback, these are all fishes that we have collected sampling during low summer flows. The stickleback probably washed in from upstream; it is common just below the Putah Creek Diversion Dam. What we did not find were juvenile Chinook salmon; I am hopeful that the small juveniles (alevins) will emerge soon from where they are buried deep in the gravel below the dam, and will migrate downstream. We will be looking for them.

The juvenile salmon are an example of native fish that can benefit from being carried downstream by high flows, if they wind up on the flooded fields of the Yolo Bypass. Previous studies from the UCD Center for Watershed Sciences have shown that young salmon thrive on the floodplains, growing at rapid rates, and then moving off the floodplains as the water recedes (Holmes waterblog, March 20, 2016). A five year study of the Cosumnes River showed that other native fishes, like the salmon, arrived on the floodplain early and then bailed out as it drained (Moyle et al. 2007). Many of the non-native fishes were not so fortunate; they arrived on the floodplain late, moving up and in from the permanent sloughs, and then became food for pelicans after they were trapped in shallow floodplain ponds.

The key concept here is that stream fishes have evolved to handle high flows, which most streams experience. Native fishes are especially well adapted for the fairly predictable timing of high flows in California. Even the non-native fishes, such as bass and sunfish, show behavioral adaptations to move to sheltered locations during floods, but they appear to be less well equipped to handle the particular seasonal flow regimes found in California streams..

As the flows recede in Putah Creek and elsewhere, native fishes like Sacramento sucker and pikeminnow move upstream and start spawning in the emerging riffles. Their young will rear in the flooded vegetation at the edges, which should be in abundance. The non-native fishes, in contrast, will have their reproduction delayed as high flows keep water temperatures cool and prevent them from spawning in quiet pools and vegetated areas.

In short, stream fishes are adapted for living with floods. The flood of 2017 should favor native fishes, from salmon to suckers. We expect to see a big increase in the abundance of juvenile natives in the coming summer. How well they survive to reproduce themselves will depend on how well we manage our dammed rivers in the coming years.

Peter Moyle is a UC Davis Professor Emeritus of fish biology and an associate director of the Center for Watershed Sciences.

Further reading

Moyle P.B., Crain P.K., and Whitener K. 2007. Patterns in the use of a restored California floodplain by native and alien fishes. San Francisco Estuary and Watershed Science 5(3):1-27.

Jeffres, C. A., J. J. Opperman, and P. B. Moyle. 2008. Ephemeral floodplain habitats provide best growth conditions for juvenile Chinook salmon in a California river. Environmental Biology of Fishes 83: 449-458.

Ribeiro, F., P. K. Crain, and P. B. Moyle. 2004. Variation in condition factor and growth in young-of-year fishes in floodplain and riverine habitats of the Cosumnes River, California. Hydrobiologia 527:77-84.

Little, Jane B. We should better understand how to live with floods. Sacramento Bee. March 6, 2017.