IMPACTS OF OFF-STREAM DAMS ON RIVER SEDIMENT CONTINUITY

Abstract

Off-stream reservoir is a unique water management system that has diversion weir(s) only in the mainstream, which may reduce adverse impacts of storage dams on river environment such as water quality change, reservoir sedimentation, downstream degradation and river discontinuity. Off-stream dams were defined and extracted from a dam database and were classified into several groups. To clarify environmental effects of off-stream reservoir, satellite images analysis and field survey were conducted on selected rivers with off-stream dams. There was almost no difference in bar characteristics between the upstream and downstream reaches in the selected four rivers with off-stream dams. Grain size distribution, dimensionless shear stress and taxonomic composition of benthos were also similar between the upstream and downstream reaches in the river with Dodairagawa dam (off-stream dam). Our study suggests that sediment continuity and river environment are maintained in rivers with off-stream reserving system.