INTERACTIONS BETWEEN LARGE WOODY DEBRIS AND MEANDER CUTOFF (EXAMPLE OF THE MELLON SITE ON THE AIN RIVER, FRANCE)

The Ain River, a 6th order tributary of the upper Rhone River, is char acterized by free-meandering and a corridor of riparian forest. Along the concave banks, notably in the upstream part of overbank flow chann els, sedimentation and jams of large woody debris (LWD) are interactiv e. This study focuses on the meander bend of Mellon which has shown do wnstream migration (10-15 m/yr) since 1963. Four annual field measurem ent campaigns were conducted on a sampling plot of 4,000 m(2) located in the upstream part of an overbank flow channel. Topography, sediment mosaics, LWD jam masses, forms and orientations have been studied. A GIS raster was used to quantify phenomena and interannual trends. We o bserve a strong regeneration of the floodplain: an annual bank mobilit y of 10-20 m/yr and gravel bar migration into the forest. Nevertheless , the main forms are stable from one year to another and the annual lo wering of the forest surface is slight (2-6 cm/yr). Although LWD masse s are stable during the period studied (15.6-19 t/ha), the debris line formed by the jams is very mobile, being broken up and restored diffe rently during each flood. Total LWD mass and mean mass per jam decreas e from the bank to the forest. Each jam modifies the topography and in creases the granulometric variability around it. Nevertheless, LWD jam s and the presence of the stands of trees also explain the interannual stability of the main floodplain forms. Finally, the interannual modi fication of the main channel pattern greatly influences the orientatio n of microforms and LWD jams.