Input, storage and distribution of large woody debris along a mountain river continuum, the Drôme River, France

Large woody debris (LWD) input, storage and distribution were studied along the Drôme River, a French Alpine river with an active shifting channel and a well-developed riparian forest. LWD input from the floodplain is low: 669.6 mg year−1 between 1948 and 1971 and 569.3 mg year−1 between 1971 and 1991. Based on estimates of average LWD mass per study plot, a range of 766–2122 mg year−1 of LWD were stored within the active channel (e.g., unvegetated bars and low-flow channel; 60 km course covering 492 ha). LWD accumulations are mainly observed on gravel bars at a limited number of preferential sites. They are relatively rare and are randomly distributed in the low-flow channel. Consequently, LWD location is mainly associated with the decrease of flow level in shallow sectors. In the low-flow channel, LWD stop-en-route is primarily caused by in-channel structures such as boulders or vegetated islets. General geomorphological factors (e.g., pattern, slope, etc.) provide less explanation of LWD distribution. Moreover, the residence time of LWD accumulations on the Drôme River is short (LWD storage=1.3–3.7 times the annual LWD input from the floodplain) and their morphogenic role is negligible: few of the accumulations are buried or characterized by vegetation shoots and associated pools.