LARGE WOODY DEBRIS AND RIVER GEOMORPHOLOGICAL PATTERN - EXAMPLES FROMSE FRANCE AND S ENGLAND

The study of accumulations of dead wood within the fluvial environment has been mainly undertaken in mountain streams and rivers within the Northwestern United States, and particularly in hydrosystems which hav e experienced little riparian vegetation cutting or disturbance by man . Appraisals of the spatial variability in the physical character of a ccumulations of dead wood has mainly highlighted the volumes of large woody debris (LWD) accumulations and the local channel morphological p roperties induced by their presence. The spatial variability in the ac cumulation and processing of organic material forms one of the central concepts of the River Continuum Concept, which characterises the occu rrence and processing of organic material, of which LWD is an importan t component, according to a longitudinal gradient along a river's a co urse. Some studies have extended the concept by illustrating the impor tance of the lateral dimension, particularly in large rivers with exte nsive floodplains, and by relating the occurrence of dead wood to fluv ial morphodynamics. However, to date there has been no synthesis of th e relationship between LWD and the geomorphic pattern of the river cha nnel. Although the research literature shows that the routine clearanc e of wood from water courses is not an environmentally-sympathetic str ategy, within Europe LWD accumulations are usually seen as a river man agement problem and are routinely cleared from river channels. This pa per addresses these physical and applied aspects of the role of LWD. I t presents an analysis based upon semi-natural hydrosystems in S.E. Fr ance and S. England. The forested corridors discussed are currently or have recently been maintained. They are essentially young and so prod uce relatively small amounts of woody debris in relatively small-sized individual pieces in comparison with the rivers studied in North Amer ica. Using observations from these example river corridors, the relati onship between rivers of a particular size and geomorphic pattern and the dynamics of dead wood is described and evaluated. Major constrasts in the role of LWD are found between small, single thread rivers, and larger, piedmont, braided and wandering rivers. Some points of synthe sis concerning the ecological, hydraulic and morphological impacts of dead wood are drawn from these examples, and are used as a basis for p roposing some simple maintenance rules.