云梦泽与荆江三角洲的历史变迁

HISTORICAL EVOLUTION OF YUNMENG MARSH AND JINGJIANG DELTA

本文通过对大量地质地貌、考古和文史资料的综合考证与分析,讨论了(1)云梦泽的形成、范围与兴衰历史;(2)荆江三角洲变迁的过程、阶段与特征;(3)荆江三角洲与洪涝灾害;(4)江湖历史变迁与问题思考等。

On the basis of comprehensive research on the information of geology, geomorphology, channel hydrography, meteorology, historical documents and archaeology of Yunmeng Marsh and Jingjiang Delta, some conclusions are obtained as follows: ( 1 ) The Yunmeng Marsh was formed at the end of late pleistocene ice age. when the sea level was declining dramatically resulting in the cutdown of the river-bed of the Changjiang River. As the climate went warmer in the postglacial stage, the gradual rise of sea level made the gradient of the Changjiang River more gentle, the sedimentation against the stream took place and the river level began to rise. When the flood breached the bank after the original channel was mandated , the Yunmeng Marsh was formed in the Jingjiang River (middle reach of the Changjiang River) area. The early marsh limited mainly in the abandoned river valley, when constant deposits in the Changjiang River accumulated on the lower lands, the marsh was enlarged. The marsh was vast with lands and shallow water interlocked, including the areas of Changjiang River. Hanshui River and part of the Dongting Lake. (2) The Jingjiang Delta experienced three changes in history: the delta was a part of the developing Yunmeng marsh till Tang and Song dynasties; of the developing Jingjiang bend plain till the end of Qing Dynasty; of the developing Dongting Lake delta since the early 19th century. It was concluded that the migration of the Jingjiang Delta showed some gradient southward. (3) The evolution of the Jingjiang Delta has proved some characteristics of stages. The early delta was thin due to plane accretion on a big area, while later vertical accretion along the diversion channels increased its thickness. The depocemer position of sediments was the main factor affecting the delta's evolution . others were the neutectonic movement and human activities such as dike building , etc. ( 1 ) The development and migration of Jingjiang Delta in history led to the dramatic rising of flood level in Dongting Lake and Jianghan plains. Beginning from Pleistocene,the first rising period was from postglacier age to some 5000 years ago while the second in the late Ming or early Qing dynasties. Nowadays because of many discharge diversions of the Changjiang River, a great quantity of sediments deposited to the south of Dongting Lake .the lake bed rose and the surface area shrank gradually. Were some programs and counter-measures not employed, the third rising period of flood level would come to reality in no time,which would make the flood control of the two lake plains more difficult to deal with.