Wood is Good -- The importance of woody debris in streams

Aug 03, 2016
Adding woody debris like large logs, trees, and branches to streams has incredible habitat benefits for fish like salmon and trout.

Large woody debris piled high. Photo: Land Trust.
Large woody debris piled high. Photo: Land Trust.
If you have visited the new addition to Whychus Canyon Preserve in the last year, you may have noticed several hundred trees, most with roots intact, stockpiled near the creek. This 15-foot wall of trees will play an important role in the restoration at the Preserve, helping to improve salmon and steelhead habitat in Whychus Creek.

For decades, removal of trees and snags in rivers and streams was seen as a helpful process; one that would clear waterways and provide more favorable navigation conditions for recreation and transport. Removal of downed logs was also considered an improvement to the beauty of a stream, and was driven by thoughts of property protection and flood control.

Over time, research has shown that removal of large woody debris from streams and creeks has been detrimental to fish populations. Including woody debris into the stream restoration design is essential to improving the health and structure of Whychus Creek.

So, what about that large wall of trees at Whychus Canyon Preserve? They have a star role to play in improving creek conditions for salmon and steelhead. Here are 7 ecological functions of woody debris in rivers, creeks and streams, from Hannah Ettema and the National Forest Foundation:

Large woody debris stabilizes streambanks. Photo: Jay Mather.
Large woody debris stabilizes streambanks. Photo: Jay Mather.
1.    Large woody debris helps slow the flow of water,
making it easier for adult fish to move upstream and for juveniles to rear. This in turn helps them expend less of their limited and valuable energy fighting strong currents.

2.    Gravel is an important material for salmon and steelhead to build redds and lay eggs. Adding wood debris to streams helps slow water flow allowing larger sediment like gravel to fall to the streambed instead of continuing downstream.

3.    Large woody debris can help decrease water temperature by providing shaded areas along streams and creating pockets of cooler water for cold-water loving species.

4.    Large woody debris creates places for fish to hide and seek refuge from predators.

5.    Woody debris helps trap organic material like leaves and twigs that provide nutrients for insects and invertebrates (critters without spines), which in turn provide food for fish.

6.    Large woody debris helps reinforce streambanks and channels by preventing erosion of soil along banks.

7.    Pools of water and “steps” created by woody debris can also provide habitat for fish during periods of low water flow.

The addition of large woody debris into the restoration design for Whychus Creek is vital to the success of reintroduction of salmon and steelhead. We have already seen the benefit it can have with the return of the first steelhead to Whychus Creek in 50+ years this past spring (Read more about Stella). Stella, as we call her, was hiding under one of the log jams at Camp Polk Meadow Preserve made of large woody debris!

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