Soil erosion and gully growth associated with cultivation in southwestern Hokkaido, Japan

This study examined the processes underlying land and riparian degradation, by quantifying soil erosion and gully growth in slopes adjacent to cropland. The concentration of suspended sediments was consistently higher in cultivated catchments, where gully expansion causes 2-3 times more landslides than occur in forested catchments. Sediment from gullies contributed about 34% of the total sediment in the cultivated catchment. There has been increasing erosion and sedimentation on the valley floor over the past 20 years, both because of the expansion of land under cultivation and because of the mechanization of agriculture since the 1960s has reduced the infiltration capacity of cropland, making it easier for erosion to occur when it rains. Most of the finer sediment is transported to the sea, where it affects coastal ecosystems, while the coarser sediment, such as sand, remains in the stream and fills the spaces between gravel on the streambed. This eliminates habitat suitable for fish and invertebrates; the density of macro-invertebrates in cultivated catchments is only 10-20% of that in forested catchments. Effective stream restoration will require both construction measures to prevent slopes from eroding and the regulation of land use, including reforestation at the borders of agricultural land.