Changes in sediment discharge over the 20th century from a mountain catchment that has experienced severe anthropogenic disturbances

Mountainous areas are a source of sediment supply to downstream plains and coasts; thus, it is essential to understand the changes in sediment discharge that follow land cover change. In Japan, many mountain areas near populated lands that were degraded by overutilization before the beginning of the 20th century are now forested. However, little is known about how the sediment dynamics have changed during this period. The Shirasaka Experimental Watershed (88.5 ha) of the Ecohydrology Research Institute of the University of Tokyo was established in an area with bare hillslopes on granite bedrock, and sediment discharge has been monitored there since 1929. This study describes the details of the amazing long-term sediment monitoring program and demonstrates changes in sediment discharge. The catchment has a gauging weir where deposited sediment has been regularly removed to maintain water discharge measurements. This record of dredging and/or deposited volumes provides the sediment discharge data. Erosion control work started in the 1910s. Bare land covered 8.6% of the watershed area in 1930 but decreased to less than a few percent in the 1980s. In the 1930s, sediment yield was about 1,000 m³/km²/y, gradually declined over time, and became close to 100 m³/km²/y in the 1990s. Annual sediment yield was, for the most part, larger than the sediment production predicted for surface erosion at hillslopes between 1930s and 1990s. It took more than 60 years for the catchment's sediment yield to become close to the amount of sediment production at the hillslope of that time. The time lag between forest recovery and the decline in the catchment's sediment discharge demonstrates the lasting effect of anthropogenic disturbance and is attributable to channel storage. The long-term data also demonstrated that the effects of record-breaking rainfall events are smaller than those of past anthropogenic disturbances on catchment scale sediment dynamics.