Geomorphic control of wetland distribution and sustainability in Japan

Many wetlands of various scales and origins are distributed from low to mountain regions in Japan, where the East Asian monsoon brings much precipitation through the year, and topographic changes associated with tectonic and volcanic activities, mass movements and river and coastal processes are active. This study clarifies the wetland distribution in Japan with a focus on the geomorphological features, then considers the sustainability of each type of wetland. We used the wetland database of the National Survey on the Natural Environment conducted by the Environment Agency in 1993, which covered natural inland wetlands with an area of 1 ha or more (total area for a wetland group). The wetland density (wetland area / surface area) at each elevation exhibited a bimodal pattern, high at less than 200 m asl and around 1600-2000 m asl. The former includes lagoons and peatlands on alluvial lowlands formed by the middle to late Holocene marine regression, and are mainly distributed in the northern district of Japan. It also includes the largest tectonic lake in Japan. The latter is dominated by group of snow patches, formed with increasing of snowfall since 12,000 yBP, on gentle slopes of Quaternary volcanoes in heavy snow areas. Wetlands at higher than 200 m asl includes many landslide wetlands in wide elevation and a few large tectonic lakes. On the Quaternary volcanoes, crater lakes stand at the top and wetlands recharged by artesian groundwater are around their foot. As mentioned above, geomorphic feature where wetlands are located at each elevation zone are different, and that is considered to control sustainability of wetlands. Snow patches in a higher elevation area mainly charged with snow melting water may be vulnerable to climate change with a decrease in snowfall amount. In contrast, ground water recharging wetlands such as inside of landslides and at the foot of the volcanoes may be resistant to climate change. Landslide wetlands can be greatly affected by topographic changes associated with landsliding and tend to be long lasting. More attention should be given to landslide wetlands that contribute to the stability of the local mountain wetland environment.