Suspended sediment discharge from snowmelt: Ikushunbetsu River, Hokkaido, Japan

Sediment discharge generated from snowmelt was observed in the Ikushunbetsu River, Hokkaido, Japan (drainage basin area: 60 km(2)) in April-May 1992 and April 1993. Continuous records of water discharge Q and suspended sediment concentration C-s indicated synchronous temporal variations at a station on the riverbank and a downstream increase in sediment discharge Q(s)(= Q . C-s) between two stations along a river channel. The sediment suspended over 90% of water depth consisted of more than 95 wt.% of silt (3.91 < d less than or equal to 62.5 mu m) and clay (d less than or equal to 3.91 mu m). Sources of the fine suspended sediment were identified by comparing grain size distributions of riverbed and side gorge slope sediment samples, being supplemented by mineralogical analysis. It is concluded that the sediment discharge is generated by the fluvial erosion of weathered soils which are accumulated on side gorge slopes from the collapse of the top soil, and the amount of sediment eroded per unit area per unit time is approximately proportional to the fluvial shear stress.