Characteristics and temporal variability of large woody debris trapped in a reservoir on the River Rhone (Rhone): Implications for river basin management

Woody debris is a structural element of river systems, which provides habitats for aquatic communities but may enhance flooding frequency and damage infrastructure. An abundant scientific literature highlights the role of the wood, particularly in the fields of ecology and geomorphology. In order to find a balance between woody debris preservation and reintroduction for ecological purposes, and the need for channel clearing for risk management, more research is now needed to understand woody debris delivery residence time and transport dynamics in regulated rivers. This work uses reservoirs, which trap woody debris, to determine its geographical origin and temporal variability in relation to the flow regime. This approach is illustrated by data collected on the Genissiat dam on the upper Rhone river (France), which traps all the woody debris even during high floods. The results show that the wood input increases with flood frequency but also depends on the position of the flood event in the hydrological series. The wood load is significantly high and removed from the reservoir when the flood recurrence attains one in 1.5 years. Qualitative description of the wood extracted from the reservoir indicates that 83% of the pieces, whatever the size, have a natural origin (broken, cut by beaver) and 17% has been cut by humans. The large pieces of wood (diameter > 12.5 cm) more frequently have a human origin (40%). Large pieces with roots are infrequent (10%) underlining the low contribution of bank erosion. Most of the pieces are strongly smoothed, without any branches, roots and bark confirming the effect of physical breakage of high energy rivers. Amongst 503 samples, 45% have a riparian origin (Populus, Fraxinus, Alnus). Copyright (C) 2003 John Wiley Sons, Ltd.