Abstract
The annual sediment transport in the Shirakawa and Chikugogawa rivers that drains the Ariake Bay was investigated in every particle size such as sand, silt, and clay. The changes in the forest area in the basins, the suspended sediment load in the river channel, and the longitudinal shape of the river bed during the last 50 years were investigated. The results revealed that the forest area had increased and the sediment load in the river channel had decreased. These facts suggest that the sediment yield in the river basins is decreasing. The variation in the river-bed topography and the sediment volume removed away from the river channel by public works such as river improvement, gravel quarrying, and reservoir construction for the last 50 years were calculated. The results reveal that the variation in the shape of the river-bed is due to the dredged sediment by the public works, and the sediment quantity removed away has exceeded the natural production of sediment. The sediment load during flood conditions and normal conditions was measured for one year. The river mouth delta had progressed by deposition of flood sediment such as sand, silt, and clay. After the flood, the deposited silt and clay had been eroded by tidal current, intruded into the river mouth estuary, and finally settled on the banks. The amount of fine sediment transported during one year was equivalent to the amount of sediment discharge carried by floods occurring few times a year. These results suggest the decrease in the sand supply from the river to the tideland and the progress of deposition of silt and clay on the tideland.
