Conservation of the natural ecosystem should be an essential requirement to any kind of river works because rivers are fundamental elements of our natural environment. However, Japan’s river works, which were supported by technological innovation and a powerful economy driven by high economic growth, destroyed river ecosystems considerably. Riverbeds became unstable due to the sediment deposit by dams and the sediment extraction from the riverbed for responding to aggregate needs during the construction boom. In the postwar period, there was the problem of increased sedimentation at the mouths of rivers due to the series of large floods and were a cause of concern. However, a more recent problem is the decreasing sediment transport which causes coastal erosion around river mouths.