Abstract
The riparian forests dominated by willow trees along the Ina River sustained heavy damage during a huge flood in the July 2011 “Niigata-Fukushima heavy rain episode”. Many hillsides in the Ina River basin collapsed and many trees fell into the river. This study clarified the influence of a large flood on riparian forest dominated by willow trees and the driftwood retention function of riparian forest. We established a research plot in a willow forest on a sandbar in the Ina River and clarified the circumstances of living and damaged trees after the flood, and the quantity of driftwood caught by the riparian forest on the sandbar. We found that willow forest growing on the relatively low sandbar had collapsed, but some trees remained alive and trapped new sediment and driftwood. In river management practices to date, trees growing on sandbars have been removed because they “caused” driftwood. However, this study showed that the willow trees on the sandbar in this river forest played an important role in catching a large quantity of driftwood from the upper basin in this large flood; the volume of driftwood was more than double the volume of living trees.
