矢作川上・中流域における礫の移動

Abstract

Amounts of the gravel transport during a flood in September 2000 were estimated in the upper and middle reaches of the Yahagi River, central Honshu, Japan, from the change of sediment deposition n 5 reservoirs on the main stem, and from the observation of particle size of the sediments. The estimated amounts at influxes into the reservoirs are 17,000, 3,200, 41,000, 400, and 460,000 m^3 respectively from lower to upper reservoirs, and each amount deposited probably all in the reservoir. The transport below the lowest reservoir was also estimated at 5,000 m^3 by the change of riverbed cross sections. These amounts of gravel transport tend to be negatively correlated to the biomass of benthic invertebrates and the richness of riparian vegetation after the flood; i.e., a mass of gravel transport above the uppermost reservoir greatly destroyed the riparian vegetation and the benthic invertebrates, especially net-spinning caddis larvae, whereas both were well preserved even after the flood in the middle reaches. The amounts are positively correlated to the sediment yield in the tributaries estimated by previous studies. It suggests that the gravel transport in the main stem of the Yahagi River depends on the sediments from tributaries because the transport through the main stem is interrupted by the reservoirs.