Characteristics of the stream network composition of drainage basins in the Japanese Islands

This research is to analyze the composition of stream networks of drainage basins in Japan and to discuss the characteristics of drainage basins in the Japanese Islands. Study areas were selected for 180 drainage basins in the entire country. In this study, a topographic map with a scale of 1:50,000 was used and inserted into the valley line by V-shaped-contour methods in each drainage basin. The streams are ordered according to Strahler's system and measured by the stream number, stream length, stream slope, drainage area, and stream fall in each drainage basin. Horton's law of drainage network composition holds good in most drainage basins in Japan. The average values of the bifurcation ratio, stream length ratio, stream slope ratio, drainage area ratio, and stream fall ratio are 4.24, 1.91, 1.98, 4.52, and 1.04, respectively. The mean bifurcation ratio of drainage basins in Japan is 4.24, and this ratio is slightly greater than in other countries. It is also greater than the theoretical value of 4.0. This implies that the slope of drainage basins in Japan is so steep and the relief energy so large that excess streams have developed to a rather marked degree. The average stream fall ratio is 1.04 for drainage basins in Japan. It suggests that drainage basins are generally considered to be in a stage of maturity in stream channel development for the Japanese Islands. The stream fall ratio, however, is slightly greater than 1.0. Although, the stream beds are nearly in dynamic equilibrium in most cases, it tends to be in a slightly aggrading state.