矢作川のアーマ化した河床における砂礫粒径の特徴

Abstract

Size of the bed material was measured at riffles, pools, and glides (transition zone between riffle and pool) of six stations in the Yahagi River, Aichi Prefecture, Honshu, Japan. River beds at the uppermost station and the lower two stations are deemed to be in natural condition with frequent downstream transport of the material, but the beds are armored and the material probably moves seldom at the intermediate three stations, according to the data on periphytic algae and benthic invertebrates. The material was sampled from two layers of the bed : 1) the extreme surface layer, 2) the second layer beneath the extreme surface. The thickness of each layer was adjusted at the depth of maximum diameter of the gravel at riffles and glides, but it was fixed at 10cm at pools. The material at intermediate three stations was very similar to that at the uppermost station, but the former was distinguished from the latter and that of the lower two stations by little ratio (less than 2%) of sand in the second layers at the riffles, and by small ratio (less than 4%) of sand and fine gravel (under 32mm diameter) in the extreme surface layers at the glides. The material at the pools of the intermediate three stations exhibits no distinctive characteristic from that of the uppermost station and the lower two stations. These characteristics of the material may be formed by washouts of fine gravel and sand even from the second layers at riffles and from the surfaces at glides in armored beds, whereas the washouts are probably limited to the surfaces of riffles in natural beds by frequent movement of material.