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

Large woody debris (LWD) input, storage and distribution were studied along the Drome River, a French Alpine river with an active shifting channel and a well-developed riparian forest. LWD input from the floodplain is low: 66 9.6 mg year(-1) between 1948 and 1971 and 569.3 mg year(-1) between 1971 an d 1991. Based on estimates of average LWD mass per study plot, a range of 7 66-2122 mg year(-1) of LWD were stored within the active channel (e.g., unv egetated bars and low-flow channel; 60 km course covering 492 ha). LWD accu mulations are mainly observed on gravel bars at a limited number of prefere ntial sites. They are relatively rare and are randomly distributed in the l ow-flow channel. Consequently, LWD location is mainly associated with the d ecrease 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, e tc.) provide less explanation of LWD distribution. Moreover, the residence time of LWD accumulations on the Drome River is short (LWD storage = 1.3-3. 7 times the annual LWD input from the floodplain) and their morphogenic rol e is negligible: few of the accumulations are buried or characterized by ve getation shoots and associated pools. (C) 1999 Elsevier Science B.V. All ri ghts reserved.