The effect of the shortage of gravel sediment in midstream river channels on riparian vegetation cover

The vegetation cover in the steep, gravelly reaches of rivers was originally low. However, significant vegetation colonization can currently be observed in these riparian areas, and understanding the cause of this colonization is important for management. The objective of this study was to assess the effect of sediment deposition and erosion on vegetation colonization in gravel bed rivers. The delay in colonization by herbs and trees after sediment deposition or erosion was investigated using aerial photos of 6 Japanese rivers. A field study was conducted before and after a large flood at depositional and erosional locations. The colonization of vegetation after flushing was substantially delayed at sites where gravels were deposited compared with that at erosional sites, and it was faster in sandy reaches compared with gravelly reaches. Little tree colonization was observed at the depositional sites of gravelly sediments, whereas at erosional sites, new shoots sprouted from the collapsed live trees in the following spring, achieving a rapid increase in tree density. The nutrient and moisture contents of the sediment were significantly higher at the erosional sites. Gravels are deposited after washing and being segregated from fine sediment during floods. This coarse-sediment layer is low in moisture and nutrients compared with the erosional sites, at which the underlying sediments are exposed by flooding. However, moisture and nutrients are the primary requirements for vegetation colonization. Therefore, the reduction in gravelly sediments due to gravel mining, river regulation, and modification of river basins can have a substantial effect on vegetation colonization.