金沙江下段河槽地貌特征与地貌过程*

文章利用金沙江河谷地貌调查与水利工程选址勘察地质资料,根据河槽纵剖面与18个河谷横剖面的资料以及攀枝花金江河段、会理鱼鲊河段、元谋龙街河段、禄劝凹嘎河段的阶地调查结果,对金沙江下段河槽地貌进行了初步分析,金沙江下段的河谷为典型的深切"V"形河谷,在虎跳峡、乌东德、白鹤滩峡谷段河槽比降发生明显变化。根据河谷阶地下切幅度与形成时代,以T₃阶地计算金沙江下段河槽平均下切速率达到0.71~1.18m/ka,以T₂阶地计算金沙江下段河槽平均下切速率为0.75~1.36m/ka,因此,近十几万年来,金沙江下段河谷平均下切速率达到0.9~1.0m/ka。现代金沙江河槽底部均堆积了一定厚度的冲积-崩积物覆盖层,最厚的石鼓剖面河床底部冲积物盖层达到173m,一般河段堆积物盖层均达10m以上。金沙江河槽的迁移是河槽下切过程与岸坡崩塌过程综合作用的结果,形成原河槽崩塌阻塞,河槽被动迁移或原河槽改道下切,形成古河槽-离堆山-新河槽的地貌组合,以及河槽侧向迁移3种模式。

THE GEOMORPHIC EVOLUTION AND CHARACTERISTICS OF THE RIVER BED IN THE LOWER REACHES OF JINSHAJIANG RIVER

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.