NATURE, MASS AND PREFERENTIAL SITES OF COARSE WOODY DEBRIS DEPOSITS IN THE LOWER AIN VALLEY (MOLLON REACH), FRANCE

Coarse woody debris (CWD) has been examined in a section of the Ain, a sixth order piedmont river with an actively meandering channel and a wooded floodplain. The spatial distribution of CWD, its mass and forms of accumulation are controlled by the hydrodynamics and the retention capacity of the forest. A typology shows the relative importance of w oody debris in the mosaic of patches and the essential role of the eco tonal zones. The mass of debris varies from 0.001 tha(-1) to more than 200 tha(-1), but is lower than those observed in certain American riv ers. Most of the material is deposited in the margins and forms a narr ow debris line. The restocking in woody debris is recent in Europe and tends to diversify the environment. This affects the researcher and t he planner. The first considers this transit of material as a useful h ydromorphodynamic and biodynamic tool which is easy to evaluate, and t he second considers it as a restoring and generative vector, the ecolo gical functions of which are recognized. Its effect is stronger today as the watershed area tends to be subjected to a decrease in agricultu ral activity.