Change in riverine suspended sediment concentration in central Japan in response to late 20th century human activities

The effects of human activities on suspended sediment concentration (SSC) during the late 20th century were examined at 57 sites along major rivers in central Japan using governmental data and GIS. Analysis of the relationships between mean SSC and catchment properties, such as land use, has revealed that patterns in suspended sediment concentrations in these rivers are primarily linked to agricultural land, construction work, and domestic and industrial effluent sources in populated areas, whereas natural erosion in steep areas plays only a limited role. This observation contrasts with the abundant bed load supply due to natural erosion in mountainous areas of central Japan. Measurements, made over approximately 20 years, indicate that the majority of the measurement sites underwent decreases in SSC with time. In contrast, river discharge tended to be constant, indicating that the supply of fine sediment decreased during the last two decades. This observation reflects reductions in area of agricultural land due to urbanisation, as well as water pollution control and erosion mitigation measures introduced since the 1970s. At a quarter of the measurement sites, however, the supply of fine sediment tended to be constant or even increased. Many of these sites experienced high rates of population growth over the last two decades, indicating that increases in sediment supply due to increased human activity obscured the decreasing trend in SSC. The temporal change in SSC in western Tokyo was also affected by artificial landform transformation.