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In the past several years, torrential rainfall has occurred and triggered serious slope failure disasters caused large socio-economic damage due to loss of life and property Japanese nationwide. Watershed sediment management is highly needed to conserve river eco-system and dam reservoir management. In August 2003, torrential rains caused severe damage in Hidaka subprefecture, Hokkaido Prefecture, and triggered more than 5,000 slope failures in the basin of the Nukabira River, a tributary of the Class-A Saru River. The Saru River basin is located west side of the Hidaka collision zone. This area covered with various geologic unit such as cretaceous sedimentary rock, accretionary complex, and serpentinite. Field investigations were made in 2004 and 2005 at the basin of the Soushubetsu River, a tributary of the Nukabira River. The slope failure ratio has increased from 0.4% to 2.2%, when the torrential rainfall disaster of 2003. The investigations found the following: 1) The Cretaceous sedimentary rock in the investigated area is gently sloping. Most part of the slope failures of this area was formed rotational landslides, at which most of the sediment remained on the slope. The basement rocks from failed slopes become less than a few millimeters cause of rapidly slaking. This makes the sediment readily transportable in the river channel even with light rainfall. 2) The accretionary complex is steeply sloping. Although most of the failed sediment is supplied to the river channel, sediment movement is not active.

THE INFLUENCE OF GEOLOGICAL COMPOSITION ON SEDIMENT FORMATION AND MOVEMENT IN MOUNTAIN WATERSHEDS