Large Woody Debris
What is Large Woody Debris?
Large woody debris (LWD) is defined as a log having a mid-point diameter of at least 10 cm, a length of 2 m, and protruding into the bankfull channel. LWD is recruited into the stream channel by various processes such as direct tree mortality, channel migration, bank erosion, landslides, and snow avalanches. The adjacent riparian forests up to the headwaters provide the source of this wood, and the characteristics of these forest stands are closely related to the quality and quantity of wood in the stream. Once in the stream, this wood can store fine sediment, retain spawning gravels, form pools, provide cover and nutrients, and fulfill other functions that promote favorable fish habitat.
LWD and Fish Populations
Woody debris in rivers is a positive and necessary natural process in many situations. Woody material helps create diverse fish habitat, protect endangered species, enhance water quality, and sustain complex ecosystems.
Specifically, currents that flow through the logs scour deep pools where fish, particularly salmon, can rest, find refuge from predators, and reproduce. Logjams also shade the water so that it remains at a cool temperature suitable for spawning, and the decaying logs serve as a source of nutrients for plants and fish.
LWD and Bridges
Debris can be a boon to local fish populations, but a large problem for structures downstream. Debris accumulation at bridge sites during high water events can cause structural damage from impact, as well as increase the rate of local scour. Flow constriction can be an issue where large logjams exist upstream.
Debris Free advocates the use of LWD as a way to enhance local fish populations and protect against erosion at river banks. Our systems can provide the delicate balance required to maintain the delicate riparian ecosystem.
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