Onuma. 03/07/12; Hokkaido; 1,236 ha; 41°59'16"N 140°40'28"E. Quasi National Park. The site includes the Onuma, Konuma, Junsainuma freshwater lakes at the centre of the Oshima Peninsula, connected by waterways called "sebatto" (literally: 'narrow doors'). The lake system formed as a result of the damming of rivers following the great eruption of Mt. Komagatake in 1640. More than 120 islands called "Nagareyama" (small lava cones) were formed within the ponds, creating a unique landscape. After the volcanic eruption, vegetation slowly colonized the site and the forest is now dominated by Fagus crenata, of which the site is the northernmost limit. The site is also known for the diversity of shellfish species from the boreal regions and from Honshu (mainland Japan). The wetland provides flood control and acts as a reservoir used for agriculture, power generation, ecotourism and fisheries. Around 2 million visitors visit the site annually. It is currently threatened by eutrophication caused by agriculture and stockbreeding effluent and by the invasion of the alien plant species Rudbeckia laciniata. Ramsar Site no. 2058. Most recent RIS information: 2012.