California storms help salmon by reviving habitat

August 2017

Heavy rains and runoff from last winter’s near-record snows in California have done more than end the state’s devastating drought, they have also helped rejuvenate salmon streams. Swollen rivers in recent months have deposited a renewed supply of what biologists call “woody debris,” an essential ingredient of healthy salmon habitat.

For most people woody debris means fallen trees, logs, or broken limbs deposited in a stream and along its banks during a flood. For salmon it means hiding places, deep pools to grow, food, and perhaps even a jump-start for other vegetation beside rivers.

“Of all the actions to improve salmon habitat, increasing woody debris is a priority action in all of our Endangered Species Act recovery plans for salmon,” said Dan Free, a fisheries biologist with NOAA Fisheries West Coast Region. “Stream restoration projects that increase woody debris import wood from elsewhere and are expensive, but the great thing about this resource is that it’s free and naturally introduced into the system.”

Woody debris provides extensive benefits. Water rushing past logs scours river bottoms, creating deep holes that provide habitat for juvenile salmon to hide and grow.  The wood also fosters growth of algae and insects for the fish to eat, helping them gain strength and size before migrating to the ocean.

Sediment deposited by heavy river flows can also bury wood alongside streams, giving other vegetation a foothold. Buried logs retain water that other trees can access through their roots, enabling them to survive long dry spells. Groves of willows and cottonwoods and other riparian vegetation along the river bank often have logs buried beneath them that helped support their initial growth.

Flood waters pick up woody debris by uprooting trees, snagging dead logs and stumps, and transporting old stores of wood from riparian areas. Eventually the wood settles in the streambed, on a gravel bar, or washes out to sea.

The recent drought in California and the common practice of removing wood from streams has left many watersheds without much woody debris, especially in northern and central California. Fortunately, this year’s storms have reversed the trend by bringing a significant amount of woody debris to most streams.

“With all the rain we’ve had, a lot of wood like old-growth timber, smaller limbs, and trees have come down the streams – which is a good thing,” said Free. “Unfortunately, some people may believe the wood deposited in our rivers and on gravel bars is available to supplement their next winter’s woodpile or may even remove larger wood for sale.”

Both NOAA Fisheries and the California Department of Fish and Wildlife highly discourage people from removing wood from streambeds, since it diminishes fish habitat quality and quantity.

“Wood is inextricably linked to providing a healthy habitat for salmon” said Free. “Leaving this naturally occurring resource in the streams and on the gravel bars for fish so they can gain strength is one of the best things we can do for their habitat.”

Homepage photo: large wood in stream by Patricia F. McDowell, Andrew Marcus, and Michael Hughes, Department of Geography, University of Oregon .