THE GEOMORPHIC EVOLUTION AND CHARACTERISTICS OF THE RIVER BED IN THE LOWER REACHES OF JINSHAJIANG RIVER
金沙江下段河槽地貌特征与地貌过程*

Based on the field investigation and survey data, the longitudinal section, and 18 cross sections near Jinjiang, Yuzha, Longjie, and Aoga, the geomorphy of the lower reaches of Jinshajiang River were studied. Cross sections have shown a typical deep-incision V-shape valley in these reaches of Jinshajiang River. The longitudinal section displays obvious river bed gradient changes crossing the Tiger-leaping Gorge, the Wudongde Gorge, and the Baihetan Gorge. The gradient between Qizong and Tiger-leaping Gorge is relatively easy, of about 0.83 ‰ . The average gradient in the Tiger-leaping Gorge section is 14 ‰ , and those between the Tiger-leaping Gorge and Wudongde, Wudongde and Baihetan, Baihetan and Yibin are respectively 1.1 ‰ , 0.93 ‰ , 0.56 ‰ . Two categories of river cross sections are recognized, the one of broad valley, with width of 700~900m, the other of typical gorge such as the Tiger-leaping Gorge and the Wudongde Gorge, whose bank cliff could be more than 35°.Based on incision scales of low terrace and its age, the average incision rate is calculated to be 0.71~1.18m/ka according to the incision scale of T3 and to be 0.75~1.36m/ka according to the incision scale of T2. It is suggested that the downcut speed of the lower reaches of Jinshajiang River is about 0.9~1.0m/ka. Deposits have been formed by alluvial and rockfall process on the Jinshajiang river bed, with the thickest up to 173m at the Shigu cross section. In other sections, the thickness of the loose deposits is often over 10m. The watercourse change of the Jinshajiang River is the result of integrated action of fluvial incision and bank slop development. There are three change models: 1) A change caused by largescaled rockfall blocking. 2)A typical ancient river bed-cut off meander core-new channel series change. 3) A slow lateral watercourse migration.