Recent history of sediment dynamics in Lake Toro, Kushiro Mire, northern Japan

1. I ntroduction Human activities, such as deforestation, agricultural development, and stream channelization, have increased sediment production. The fine sediments eventually accumulate in lakes and wetlands, and sediment cores there reflect historical erosion rates in the catchments. Lakes have gradually become shallow due to a rapid accumulation of sediment delivered from the disturbed catchments. Thus, the historical record of lake sedimentation is important for understanding the lake environment with respect to land use in the catchment. Lake sediment cores are used for reconstructing sedimentation histories. Accurate sediment chronologies are crucial for interpreting historical sediment dynamics. Volcanic tephra dating is popular, but it is limited to landscapes that have preserved ash layers. Other radiometric dating using Cs-137 and Pb-210 profiles was widely used to examine the last 100 years in lake and reservoir environments. Many rivers and streams in Japan are channelized, and the floodplains and gentle hillslopes have been used for cattle grazing and crop cultivation since the 1960s. The Kushiro Mire, the largest wetlands complex in Japan, is home to a diversity of wetland wildlife. However, excessive production and transport of suspended sediment and nutrients are evident in association with deforestation, pasture development, and stream channelization. There are three lakes on the eastern margin of the mire, and recently they have suffered from accelerated sedimentation and a decline in water quality because of the influx of nutrient-rich, turbid water. In particular, Lake Toro, the largest lake, has suffered degradation and eutrophication from intensive pasture development in the catchment. Recent studies have indicated that water quality and aquatic plant species in Lake Toro are being rapidly degraded. However, there are no studies examining the recent history of sediment accumulations with reference to land use in the Lake Toro catchment. This study aimed to elucidate the recent history of catchment sediment yield using Cs-137 and tephrochronology. In particular, this study examined the relationship between the spatial variation of sedimentation rates and the sediment yields from the catchments together with changes in land use.