Lin Chengkun
The Dongting Lake,the second largest freshwater lake in China is located in the middle reaches of the Changjiang River.Up to 1974,It covered an area of 2740 km2,the total storage capacity was 27.4 billion m3,the average annual runoff flowing into the Dongting Lake was 352 billion m3,of which 48.6% came from four rivers (the Xiangjiang River, the Yuanjiang River,the Zi River and the Li River),33.9% from three diversion waterways of the Jingjiang River (Ouchi waterway,Songzi waterway and Hudu waterway)and the rest 17.5% from others.Hydrological data from 1958 to 1978 showed that the average amount of sediment transported into the Dongting Lake was 184.83 million tons per yeax,of which 82.1% was from three diversion waterways of the Jingjiang River,and 17.9% from the four rivers.The grain diameter of sediment from the three diversion waterways was coarser than those from the four rivers.According to balance principle of sediment amount,it was calculated that deposition rate of the Dongting Lake was 134 million tons per year,42.41 million,tons in the west and 92.39 million tons in the east of the Dongting Lake.Comparing 1951-1958 with 1972-1978,deposition rate decreased by 46.3%.The reasons were 1)the deposition of the three diversion waterways of the Jingjiang River led to the decrease of the sediment transport capacity,2)the flood channel formed by the deposition of the Dongting Lake led to the increase of sediment transport capacity.The essential measures to control and administrate the deposition of the Dongting Lake should be taken to build Sanxia reservoir in the Changjiang River,to control flood,to build floodgates in three diversion waterways to lower inflow of sediment into the Dongting Lake.But at present,it is necessary to administrate flood channel to increase sediment transport capacity.
