The Geomorphological Processes of The Three Gorges Section of the Changjiang River PhD. Zhou Bin Prof. Yang Da-yuan Geomorphological Processes is knowledge of land development process. Its emphasis is the movement mode, rate and result of the earth s surface materials in the action of each outside motivity such as water, gravity, wind, ice and so on. It is basic content and important embranchment in the Geomorphology. Its research can make clear the principle and motivity of every geographical landscape s formation and evolution. It can farther post the evolution law of geographical environment. The Three Gorges of Yangtze River are composed of Qutang Gorge, Wuxia Gorge, and Xiling Gorge, the Three Gorges Section of the Changjiang River from Baidicheng, Fengjie to Nanjinguan, Yichang is 200km in length. Many scientists in the world have researched its origin, cause of formation and age. The Three Gorges Section of the Changjiang River in this research is from Chongqing to Yichang, and it is 660km in length, it is the result of the Three Gorges Reservoir, the Three Gorges Reservoir Area is the specifically geographical area formed by the building of Three Gorges Project. There are many geomorphological type such as summit level, terrace, scarp, hill slope, gorges and so on. Since 2003, we have the chance to research the Three Gorges Section of the Changjiang River, and study the geomorphological Processes of the Three Gorges Section of the Changjiang River. file:///c /Documents and Settings/lincadtaiw...My Documents/My Webs/RISEgeo/geo_page_08.htm 1 / 5 [2009/9/6 06:42:24]
River terraces in the Three Gorges reach of Yangtze River mainly occur in either the estuary of the tributaries or the wide valley. Up to 6 levels of terraces are recognized in Chongqing, up to 4 levels of terraces at most in lower reaches down to San Douping. Generally, up to 2 or 3 levels of terraces can be recognized. From Chongqing to Yichang, along the principal river of the Changjiang river, 1 levels of terraces can be found generally. Usually there are six terrace structure in the Three Gorges Section of the Changjiang River: 1) pedestal and coarse pedestal and fine particle pedestal; 2) pedestal and travertine layer and coarse pedestal; 3) pedestal and fine particle pedestal; 4) pedestal and coarse pedestal; 5) pedestal and alluvial deposit and secondary deposit; 6) pedestal. Based on the analyses of the sedimentary facies and sedimentary texture of the river terraces in the Three Gorges reach, we put forward an approach to estimate the incision depth during a specific period by taking account of the relative altitude of the river terraces. The height difference between the exampling position on the river terrace and the position of modern sediments of the same sedimentary facies at the same site is used as the incision depth in this study. Those terraces displaying any evidence of later period deformation, such as landslide, are discarded since their original position has been altered. From the analyse result of the heavy mineral component of the river terraces in the Three Gorges setion. In order to evaluate the incision time, we collected samples used for TL dating in deposits belonging to either the riverbed facies or the floodplain facies14 TL samples were collected from 8 sites that are situated in the Three Gorges reach from Chongqing to Yichang. file:///c /Documents and Settings/lincadtaiw...My Documents/My Webs/RISEgeo/geo_page_08.htm 2 / 5 [2009/9/6 06:42:24]
These examples were measured by the Thermoluminescence Laboratory, Institute of Crustal Dynamics, China Earthquake Administration. The obtained TL age data, ranging from 7 to 127 ka, are basically reliable since they are in accord with the stratigraphic sequence. We have drawn the conclusion that the sample of the river terraces is the terrace debris of the modern Changjiang River. By the result of the TL age and ESR age, 1): The TL age date is according to the grade of terrace. 2)The division, development imperfection, later damage of the terrace also can lead the a rather large difference of the TL age of up to 2 or 3 levels of terraces; 3)There are a rather large error in ESR age date. Because the ESR age of the same grade terrace has obvious difference, and the result of the ESR age is different from the reliable result. Using the incision depth and numerical dating results, including 14 C age data given by the previous study, incision rates of Yangtze River in the Three Gorges reach are calculated. The result shows that the incision rates are 84.56cm/ka in Chongqing, 93.9cm/ka in Fuling, 69.8cm/ka in Fengdu, 77.65cm/ ka in Zhongxian, 75.1cm/ka in Wanzhou, 83.8cm/ka in Fengjie and 74.2cm/ka in Yichang. The average incision rate of all studied sites is 81.2cm/ka. The incision rates among different sites in the Three Gorges reach are similar. However, the incision rate of the river across the anticlinal mountains is possibly higher than that across the wide valley in the past about 100ka, but this conclusion need to be verified in the future by applying more precise dating methods. (2) The incision rate of Yangtze River in the Three Gorges reach tends to become higher with elapsed time in the file:///c /Documents and Settings/lincadtaiw...My Documents/My Webs/RISEgeo/geo_page_08.htm 3 / 5 [2009/9/6 06:42:24]
past 0.4Ma, which implies that the new tectonic uplift, the climate change and the erosion base level change may accelerate the river incision. (3) The incision rate is also similar to that in the lower reach of Jinshajiang River. This finding suggests that the new tectonic uplift in these two regions is consistent. Scarps are found in the Three Gorges Section of the Changjiang. We have divided the geomorphological type into three types: Scarps, Scarps-colluvium debris, Scarps-denuded bedrock platforms. We try colluvium-rock means to estimate the retreat rate of the scarps in some sites the Three Gorges Section of the Changjiang River. we obtain a retreat rate of about 5 cm/ka and 3.5cm/ka in Yaquexi in Wushan and Daping in Badongt., and 2.77 m/ka and 6 m/ka in Puyakou and Chenjiaba. We drawn a conclusion that these Scarps retreat rates is more bigger, because the geomorphic process of the slope is faster. From the research of the deep troughs of the Three Gorges section of the Changjiang River. we have drawn the conclusion that the deep troughs distribute intensively in the river section that the incision rates are relative big, and the deep troughs distribute sparse in the river section that the incision rates are relative small. We think this phenomenon is according with the difference of the erosion effect of the river. In the area of Three Gorges, four slope types, namely landslide slope, the deep weathering slope, the colluvium-debris slope and thick thick slope, are identified according to our field survey from Chongqing to Yichan. We take 137 Cs technique to calculate rate estimation of geomorphic process of the slope. We make the context value of 137 Cs in the study area is 2026.21Bq/m 2 by standard cut plane testing data and relative study. The average denudation rate of the study area is file:///c /Documents and Settings/lincadtaiw...My Documents/My Webs/RISEgeo/geo_page_08.htm 4 / 5 [2009/9/6 06:42:25]
7020.55t/km 2. a. file:///c /Documents and Settings/lincadtaiw...My Documents/My Webs/RISEgeo/geo_page_08.htm 5 / 5 [2009/9/6 06:42:25]