DRASTIC CHANGE OF FLUVIAL ENVIRONMENT IN RESPONSE TO FOREST CLEARANCE SINCE LAST 100 YEARS IN THE TOKACHI PLAIN, NORTHERN JAPAN

Hokkaido, the northernmost island of Japan is well known as the region where deforestation and agricultural land use had drastically progressed only since last ca. 100 years. Therefore this change must have influence fluvial processes. On the basis of the sediments facies, chronostratigraphy using Cs137, marker tephra, tree ring and buried can with age, monitoring of present flooding as well as some documents such as settlement (deforestation) history, change of cultivation method, topographic maps of different ages, weather data etc. we have so far following remarks. 1. Topographic maps 1: 50,000 issued in 1920, 1950, 1975 and 1995 are the good documents showing the change of land use and the timing from the natural forest to agricultural land. 2. Fluvial sediments associated with the forest clearance and cultivation are significantly coarser, predominantly sand size, than those of pre-settlement. 3. The timing of grain size change of sediments has clearly coincided with the deforestation in each drainage basin. This fact implies that the fluvial processes respond rapidly to the human environmental disturbance, particularly to the deforestation. 4. The 137Cs analysis clearly shows the horizon of sedimentation in 1963. 5. A buried can found in sediments means that the sedimentation rate has been accelerated particularly during the last several years following 1963. 6. The main factors contributing to this acceleration should be the widespread use of tractors beginning in 1960s and the construction works of river channel since 1970s. 7. Monitoring of river water level and rainfall from 2000 to 2002 recorded the over bank flooding 4 times, when the total amount of rainfall attained to ca. 100 mm or more. 8. Suspension load during the over bank flooding contains the characteristic coarse particles, which is also composed of over bank sediments since the beginning of forest clearance and agricultural land use. 9. We will refer to these coarse sediments as the PWA (Post Wajin (Japanese) Alluvium).