IWRA Proceedings

The islands of Japan are located between 20 25' and 30' north latitudes, or in temperate Monsoon Asia. Seventy percent of the land is covered with steep mountains, many of which are volcanic. The geology consists of weak soils. Temperature is affected by the southeastern wind from the Pacific in the summer and by the northwestern wind from the Eurasian continent in the winter owing to the location of the islands. Areas along the Sea of Japan are covered with world-class heavy snow in the winter. Both seasonal rain and typhoons hit the nation in the summer, often inducing downpours. In areas along the Pacific coast in particular, 50 to 60% of annual precipitation is concentrated during the summer. Rivers that catch water in their basin while flowing are greatly affected by the topography and climate described above, and exhibit the following characteristics as compared with large rivers on (a) Japanese rivers are steep-sloped Many of the Japanese rivers are steep-sloped and flow down mountains quickly to the sea from their headwaters over short (b) Water level rises quickly in Japanese rivers Japanese rivers have much smaller catchment areas than major European rivers. Even Japan's largest catchment 16,840 km2 of the Tone River is only one-fifth of that of the Seine. Heavy rains frequently fall locally in Japan. Water level rises in rivers with a small catchment area as soon as the entire catchment area receives heavy rains. Actually, once rain falls, the water level rises at a rate of dozens of centimeters per hour.