Wednesday, 29 September 2010

Woody Debris

This is is not my favourite blues guitarist but the scientific term for fallen branches and trees in a river.

Large Woody Debris (or LWD, generally defined as timber greater than 0.1m in diameter and 1.0m in length) is a vital natural component of all rivers. However, due to human intervention over the millennia, it is now largely absent from many river systems. Traditional river management has included a presumption for the removal of LWD, on the grounds that it restricts angling access, collects debris around it andcould pose a risk of flooding. Many fishery interests have also had concerns that LWD can adversely restrict the upstream migration of pre-spawning salmonids.

However, more recently, research has shown that LWD is fundamental to many river processes, which are of direct and indirect importance to trout, with its influence particularly strong in headwaters. LWD causes localised changes in water velocity, with consequent downstream scouring of gravel substrate, improving its quality for spawning salmonids and some fast water loving coarse fish species. The lower water velocity occasioned upstream and within LWD bundles results in the detention of fine sediment in marginal zones where it can become colonised by emergent vegetation. The increased variability of water velocity also results in significant changes to the river’s water depth and width. Similarly, leaf litter tends to accumulate in and around LWD, providing an important food reserve for 'shredding' macroinvertebrates. LWD also provides shelter for a range of invertebrate and fish species, and reduces water temperature by shading.

Accumulations of LWD can cause the formation of so-called 'woody debris dams'. These can become remarkably stable, with some examples lasting for years. These can have particular value in riverine systems, becoming important structural features in their own right. However, careful monitoring of extensive woody debris dams is important.Although concerns regarding their impact on migrating fish are generally not well founded, in extreme circumstances, they can totally occlude channels, preventing access to spawning areas for brown trout and salmon.
In these unusual circumstances, it is usually possible to carefully remove a small section
of the dam, re-establishing a passage for fish.

So all in all wood is good for trout!

As part of ERTs Petteril Project we have formed a technical partnership with the conservation charity, the Wild Trout Trust to select and introduce LWD at a number of sites along the river. As I speak Tim Jacklin and Paul Gaskell (WTT) and Alison Reed (ERT) are busy with their chainsaws, creating much needed habitat on this once iconic trout stream. The support and guidance from WTT has been invaluable and anyone considering restoring a trout stream should contact them. Their advice and assistance is free and will ensure your project gets off the best possible start. see for more details.
Photo: Copyright Natural England