Gravel-bed river morphodynamics and large wood dynamics

International audienceThe gravel-bed rivers of the Gaspé Peninsula, Québec (Canada), a coastal drainage system of the St. Lawrence estuary, receive and transport vast quantities of large wood. The rapid rate of channel shifting caused by high-energy flows and noncohesive banks allows wood recruitment that in turn greatly influences the river morphodynamics. The delta of the Saint-Jean River has accumulated wood since 1960, leading to frequent avulsions over that time period. The wood raft in 2014 was more than 3-km long, which is unusual but natural. The jam configuration allows a unique opportunity to estimate a wood budget and to better understand the interactions between river morphodynamics and large wood flux at the basin scale. Airborne and ground photo/video images are used to evaluate channel changes, to determine the wood transport rates and the wood volume introduced by erosion and the wood deposited in the raft. Annual surveys were carried out from 2010 to 2013 to locate and describe more than 1000 jams and 2000 individual pieces of wood along the upstream 60 km long river section. Analysis of the morphodynamic trajectory of the river since 1960 (eroded floodplain, channel width, gravel-bar evolution, stream power…) combined with repeated field surveys allowed defining the morphological characteristics as well as the jam configuration that control wood mobility and deposit. The results indicate that the volumes of woods deposited along the 60 km section are 4 times higher in 2013 than in 2010. Increase in wood amount occurs mainly in upper alluvial sections whereas decrease is observed in the semi-alluvial sections. Wood transport rate and raft lengthening are only partly linked to peak flows so that some threshold conditions can control wood transfer within the basin. The four years analysis of the large woods dynamics in Saint-Jean river can assist river managers and develop management tools and strategies to deal with large wood in medium to large rivers and in rivers of cold areas