河岸段丘から推定した河床高度変化の歴史 River Deepening and Aggradation Estimated from Relative Heights of River Terraces
To estimate degradation processes and rates of river deepening, terraces and buried valley bottoms of the Sagami River, the Kabura River, and the Ara River in the Kanto region and the Shinano River in the Chubu region were studied. In the glacial epoch, each river aggraded at a rate of 0.2-0.7m/kyr by filling the valleys with sand-gravel sediments. Fill-top terraces and buried valleys were formed. In the interglacial epoch, each river deepened its bed and dropped its bed at a rate of 2-7m/kyr. Consequently, gradual degradation of the riverbeds occurred during the Quaternary. This process of river deepening and aggradation shows an almost reverse phase to that of sea level changes during the Quaternary.<BR>A linear relationship is found in the rivers studied when the heights of filltop terraces are plotted against their ages (=TT lines). A linear relationship is also found when the heights of buried valley bottoms are plotted against their ages (=BB lines). TT and BB are almost parallel in each river. These relationships indicate that each riverbed changed its level between TT and BB to reach the recent level. The parallel relationship of TT and BB indicates that each river deepened by almost equal values of uplift during the period of one glacial and interglacial cycle. The gradients of these lines indicate not only degradation rates but also the uplift rate of each drainage basin, i. e., in the rivers studied, the degradation rate almost equal to the uplift rate.
地學雜誌 109(3), 366-382, 2000-06-25
Tokyo Geographical Society