THE FIVE-YEAR-DROUGHT COULD hardly have been worse for some of California’s fish populations. The Sacramento River’s winter-run Chinook, for example, were nearly extinguished by low water supplies and sloppy handling of reservoir releases during the endangered salmon’s spawning season. The delta smelt, too – a key biological indicator species – is now closer to extinction than it has ever been. On the Klamath River, potentially deadly parasites that thrive in low-flowing rivers infected most of the Chinook born in 2014 and 2015.

After weeks of heavy rains and a mounting snowpack in the Sierra Nevada, California’s drought is easing, and according to the U.S. Drought Monitor, Northern California is drought-free. That should be good news for fish.

“All this rain is definitely a good thing,” says Dave Hillemeier, the director of the Yurok Tribe’s fisheries department. He says fast, gushing flows could potentially wash out of the river system a species of worm that serves as a host to the problematic fish-killing parasite Ceratonova shasta, which has been linked to population declines of Klamath steelhead and salmon.

“One day of huge flows would make life miserable for these polychaete worms,” Hillemeier says, explaining that rapid currents can not only sweep away the creatures themselves but also the algae on which they feed.

In the Central Valley, high flows are also a boon to fish, especially when rivers spill their banks. Research has consistently shown that numbers of young fish spike in the months following wet winters – probably because they create valuable, if only ephemeral, floodplain habitat for the fish.

However, too much rain at once can spell trouble for a river and its fish. High flows can wash away gravel beds containing incubating eggs – what biologists call “scour.” Rapid increases in flow can also bury and suffocate eggs with fine sediment or even sweep young salmon prematurely out to sea. These impacts are especially problematic in river valleys that have been overhauled by human activities such as logging, levees, dams and development.

Full article