All About Woody Debris
What is woody debris?
Pieces of wood such as logs, stumps, branches, and twigs that are sitting in the stream and along the bank are referred to as woody debris. Wood can be over 10 cm in diameter and several meters long or it could be as small as a twig to be classified as woody debris.
How does woody deris affect water flow?
While woody debris can change flow patterns in localized pools; woody debris does not significantly impact full stream flow. Looking at the full cross section of the stream channel (bank to bank and bottom to water surface), water always seeks that lowest point, and woody debris does not have the energy to stop the overall flow of water.
What generally happens to woody debris during peak flows (floods)?
Flood waters have tremendous energy and force. Most woody debris is loosely anchored and is easily dislodged and carried down stream. At the same time, new debris from up stream will replace the old debris. However, when excessive amounts of woody debris is collected on a bridge, especially small bridges, flow can be impeded.
What are probable consequences of removing woody debris?
Altering flow pattern, by removing woody debris, will cause a change in bank erosion. An increase in erosion causes an increase in silt deposition down stream. This in turn, casues the channel to be shallower, and more prone to flooding.
Woody debris has always, and will always exist in a stream, regardless of the amount of time and money put into removing it. Extensive labor, time, and capitol will always be necessary, with little effect on flooding.
Does the literature suggest that removing woody debris will help to reduce flooding?
No, but we are interested in any liteature that suggests the contrary. Often, removing woody debris can seem like a logical solution, but watersheds are extremely complex and many factors work together to cause flooding. Woody debris is usually a minor contributor.
Why is woody debris important to the stream?
Woody debris is important, if not absolutely essential, to the health of the river's ecosystem. Research has shown that woody debris improves habitat by increasing pool types and sizes, sediment storage, and local scour.
If this wood is anchored in the stream back or in the stream bed, it serves several purposes:
- Fish hide under wood to be in the shade and to hide from predators. Additionally, large debris provides home for the beetles, the stoneflies, the mayflies, the caddis flies, and even the black flies. Woody debris collects fine organic material where the invertebrates feed, the shredders shred, the collectors collect, and the predators (damselflies, dragonflies, and trout) eat them all. This has a direct impact on water qualitiy which determines our quality of life.
- Woody debris decreases the channel slope, produces channel roughness, increases channel diversity (otherwise known as pools and riffles), stores and contains sediment, helps the stream retain organic matter, and deepens the water to both cool the water and provide protection for fish. Woody debris is in a constant cycle of loss and replenishment. As some pieces are lost or moved down stream, new ones take their place.
- Wood eventually breaks down into nutrients in the stream. As this wood decomposes over time, nutrients are added to the watershed, encouraging the growth of many organisms in and around it. This process is the foundation of our food chain, on which we all depend.
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