Why fish love ‘large woody debris’

By Alan Moore

“Large woody debris.”

A local self-taught stream savant I know has mounted a campaign to do away with that phrase. Wood deserves better, he says, than to be referred to as “debris” in streams. That unsavory distinction should be reserved for shopping carts, Igloo coolers and other inorganic crap that shows up in scrap heaps after community stream clean-up days.

Wood in streams is a lot of things, but crap is rarely one. When it comes to trout and salmon, and the hundreds of other species intertwined in their existence, wood and water together form the recipe for life. And, of course, great fishing.

Water Life: Episode 2 – Water & Wood from Freshwaters Illustrated on Vimeo.

So what does wood really do for fish? Why do we seek exorbitant sums of money to do crazy things like hiring a $30 million heavy-lift “airship” (at that price,and with  more than one prop, they are no longer mere “helicopters”) to place whole trees across an otherwise perfectly clear, floatable and frankly, more easily fishable, stretch of water? Why put so much effort into pulling trees out of rivers and streams – “stream cleaning” we called it – only to come back a few decades later and start putting it all back?

The needs of people and fish collide on the streams we share thanks to our misguided  need for uniformity: clean lines and orderly appearance, clear of obstructions. Like our shipping channels, canals, irrigation ditches … even our yards and golf courses. Fish hate that stuff. They need randomness, disorder and crooked lines simply to survive. This is only one of the reasons why steelhead are no good at golf, but an important one.

Now here’s the rub: we also prefer our streams to have healthy fish runs in them, and once we realized that our preference for “clean” streams was detracting from that other outcome, we realized we needed to do something differently.

Large wood in streams—preferably whole trees with root wads and all—provides the randomness and dynamic environment that fish absolutely need to survive in the ever-changing waters they occupy. Wood breaks up the current and spreads water sideways across its natural floodplain, creating wonderful, dynamic and necessary diversity while also absorbing energy that could cause serious damage downstream otherwise, such as flooding or unnatural erosion. It sorts gravels during high flows, creating those beautiful spawning gravel beds laid out like blankets among bigger rock. It makes thosecurrent breaks downstream of log jams. It provides cooling shade and cover, and slow pools and edge habitat that baby fish need after emerging from those gorgeous gravels to ride out high flows, find food and hide from prying eyes. Decomposing wood and the nutrients it produces jumpstarts that the natural processes critical to insect, animal, amphibian and plant life.

Simply put, if we want adult fish to fish for, fight over or simply rest easy at night knowing they’re out there, we have to take care of the habitat and the conditions that make adult fish possible. Debris isn’t one of those key cogs in the wheel of life… but wood is.

Alan Moore is TU’ director of Northwest Habitat Programs. 

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