Ecological Engineering 24 (2005) 503–508 Soil erosion and gully growth associated with cultivation in southwestern Hokkaido, Japan Akiko Nagasaka a,* , S. Yanai a,1 , H. Sato a , S. Hasegawa b a Hokkaido Forestry Research Institute, Kosyunai, Bibai 079-0198, Japan b Hokkaido Central Agricultural Experiment Station, Naganuma, Yubari 069-1395, Japan Accepted 10 January 2005 Abstract 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. © 2005 Published by Elsevier B.V. Keywords: Pyroclastic flow plateau; Agricultural development; Soil erosion; Gully expansion; Watershed conservation 1. Introduction The mountainous landscape of Hokkaido is char- acterized by many volcanoes, and this is reflected in the region’s geological features. Geological matter of volcanic origin is found in one-third of Hokkaido; its * Corresponding author. 1 Present address: Hokkaido Institute of Technology, Teine, Sap- poro 006-8585, Japan. distribution coincides with the locations of active vol- canoes. Approximately 71% of the mountainous area of Hokkaido is devoted to forestry, while only 16% is used for agriculture. Tokachi district in southeastern Hokkaido and Abashiri district in the northeast contain large, flat plains and plateaus of volcanic matter that are famous for vegetables and row crops. Nevertheless, volcanic-based soils readily erode. Although the area devoted to agriculture is small, soil erosion has been a noticeable problem in these regions since the 1960s. 0925-8574/$ – see front matter © 2005 Published by Elsevier B.V. doi:10.1016/j.ecoleng.2005.01.008