Geomorphology and hydrological process and environmental change of rivers and bog pools in Ozegahara Mire

Abstract

The Ozegahara mire is the largest peatland (approximately 8 km 2 ) on the mainland of Japan. The mire is located on an amphitheater-shaped basin surrounded by mountains that rise 2,000 m asl, and the effects of climate change and flooding on the mire ecosystems were studied. The water level of Kaminoohorigawa was highest between 2016 and 2017, exceeding 7 m from the bottom of the stream after heavy rainfall in autumn 2016, and approximately 2 m from the bottom of the stream after heavy rainfall in early spring 2017. Water level of ground surface in Kamitashiro in snowmelt season was 0.2 m, and that at Kenkyumihonen was 1.6 m in snow coverage. The water level during the snowmelt season at Kenkyumihonen was very high at several times. The water level of the bog pool of KA1-08 increased in March 2018 due to surface water and groundwater inflow. The bog pool KA1-04 had early snowmelt due to groundwater inflow from the middle bog pool. Heavy snow flooding in torrents occurred in May 2019 in the Kamitashiro area. After the flooding, the distribution of the muddy water in the bog pool was obtained by air photography by UAV. A high content of inorganic particles was detected in the peatland sediment of 0.9 m depth layer and estimated by historical flooding a millennium ago. Radio cesium from the Fukushima Daiichi Nuclear Pwer Plant accident was detected as 5,919 Bq m -2 at Nakatashiro and 8,594 Bq m -2 at Kamitashiro. The radioactivity of Kamitashiro in the flooding area was higher than that of Nakatashiro in the non-flooding area because of contaminants from the hydrographic basin.