Flushing sediment from reservoirs triggers forestation in the downstream reaches

It was assumed that sediment release from a reservoir would trigger forestation and change the spatial pattern of tree recruitment in the downstream. This study tested these hypotheses by observation on a downstream sandbar of the Kurobe River. Prior to sediment release, Elaeagnus umbellata and Salix gilgiana were the most abundant tree species (33% and 64% of the total tree population) on the bar. The former colonized on elevated areas, whereas the latter mostly grew along the edges of river channels. After 2008 and 2009 sediment releases, a thick layer of fine sediment accumulated in active river channels and on low‐elevated areas of the bar. The particle size of the accumulated sediment was significantly lower (p < 0·01) and the moisture content was higher (p < 0·01) than in the underlying substrate. Total nitrogen, carbon and phosphorus concentrations of the accumulated sediment were higher than in the covered portions. After sediment release, a seedling flushing of both species occurred on the bar where 87% was composed of S. gilgiana and the remaining 13% was of E. umbellata. Although most of the S. gilgiana seedlings recruited on low‐lying areas, numerous individuals also grew on elevated margins of the channels and on areas dominated by E. umbellata. The accumulated sediment made changes in the substrate, which were favourable for the recruitment and spread of S. gilgiana. This study suggests that an alternative to sediment flushing for reservoir maintenance should be sought or the impacts on downstream environment should carefully be considered before an extensive sediment flushing operation.