Recent Changes in Sedimentation Rate in Three Lakes of Ishikari Wetland, Northern Japan Determined by 210Pb Dating

Resumo

Historical changes in lake sedimentation for the last 150 years and the lifetime of lakes of Ishikari Wetland are reconstructed in this study. The floodplain wetlands surrounding the lakes have gradually converted to farmlands since the 1890s. ²¹⁰Pb activity in lake profiles was found to have a non-exponential decrease with depth due to increased and variable sediment flux from agricultural activities. ²¹⁰Pb dates of these lakes were calculated in this study using constant rate of supply model, which was in good agreement with the date based on ¹³⁷Cs method. The average total sediment mass in the lakes under natural conditions were 54–112 ton/year until 1890s. It was increased to 111–1055 ton/year after 1890s due to conversion of floodplain to agricultural lands. Especially, lakes after the 1940s had the highest sediment accumulations due to sediment influx resulting from intensification of agriculture via increasing farm size and destroying swamp buffers surrounding the lakes. The time forecasted for filling lakes based on post-1940s sediment accumulation mass was about 15 to 23 years. Restoring swamp buffers surrounding the lakes and converting to low intensity agricultural activities are needed to restrict lake shallowing.