流域圏における水系を視点とした景観特性の分析―那珂川，霞ケ浦，鬼怒川，小貝川の各流域を事例として―

Abstract

Over the last 100 years, regional landscapes in Japan have undergone major changes in land cover and land use, and the relationships between human society and river have also changed dramatically. The purpose of this study is to investigate the characteristics of landscape changes over a 100-year period related to the river systems in four main watersheds—Nakagawa, Kasumigaura, Kinugawa, and Kokaigawa watersheds—in the North Kanto Region. The four watersheds are divided into 801 subwatersheds. Landscape attributes such as recent land use, landform, geology, and soil are obtained from digital national land information provided by the Ministry of Land, Infrastructure and Transport of Japan, and land use maps of the Meiji era were retrieved from old edition topographic maps of the Geographic Survey Institute. Data are aggregated at the subwatershed level, followed by GIS and multivariate analysis. First, the composite properties of the physical environment are examined by a principal component analysis, resulting in nine major components. The subwatersheds are classified into five groups by a cluster analysis using the major component scores as variables. Each group shows its unique characteristics of landform, geology, and soil. Next, the relationships between land use patterns and physical environment in the Meiji era and the present in each classified group are illustrated with the stream order proposed by Strahler. Finally, the steady tendency of landscape change in relation to the stream order of river in the four watersheds is identified. The results suggest that: (1) Each classified group in terms of physical environment has a unique land use pattern and direction of change. (2) There is a strong relationship between stream order and landscape characteristics. (3) In the Nakagawa watershed, development of the Nasunogahara area in the Meiji era had a great impact on the distribution of paddy fields. (4) Major changes of land use from forest to dry field on the tephra-covered terraces are observed in the Kasumigaura watershed. (5) Kinugawa and Kokaigawa watersheds are characterized by a paddy field landscape molded along the river, which was inherited from the Meiji era. (6) Rapid urbanization of the terraces and natural levees are observed in all watersheds.