Abstract:

There are several old river courses located in the lower reach of the Jingjiang.The area includes two national natural reserves and two national breeding bases of natural parent fish,which make it very important to conserve the special ecosystem.In the study,the distribution and connection characteristics of old river courses are analyzed.There are seven old river courses in the lower Jingjiang.The old courses mainly distributed in left bank of the Jingjiang and their areas range from 32 km2 to 219 km2.Though the old courses all are connected with the Jingjiang,only two of them,Heiwawu and Shangchewan,still keep the natural connectivity with the river.Northern Nianziwan,Tianezhou and Laojianghe have been controlled by the sluice gate.Among the old courses,the connected ones have bigger average area than the obstructed ones.It illustrates that the connectivity property protect the old courses hard to be transferred to the cultivated fields.The old courses form from 1980s with the five natural curving cutoffs and two humaninduced engineering ones.The natural curving cutoffs mostly occur one time in 22-23 years,and now it is a potential dangerous period for 18 years to the last naturally curving cutoff.According to the form and change history of old river courses,the flow and sediment dramatic variation is a key factor to cause the river regime shifting.So the flowsediment data series of the Jingjiang from 1950 to 2008 are involved to explain the variation scale after the three gorge project (TGP) operation.There are not more than 5% runoffs differences between the preTGP and postTGP in the Jingjiang.The sediment transportations in Shashi and Jianli decrease 81% and 72% respectively after TGP operation,indicating that the dramatic sediment discharge reduction occurred.The river bathymetric data before and after the TGP operation of the lower Jingjiang also suggests that the river channel is in the instability.The scouring depth of Shishou and Jianli are correspondingly 008 m and 007 m in Sept.1998-Oct.2002,and change to 113m and 074 m in Oct.2002-Oct.2008.The souring rates of two ranges in the Jingjiang are increased from 2 cm/a and 1.8 cm/a to 18 cm/a and 12 cm/a after TGP operation.It means that the great scouring function would decrease the Jingjiang river water stage 18 cm and 12 cm with the same runoff,and reduce the possibility of the connection between old river courses and the Jingjiang.For the controlled old courses,the scoring process could prohibit its achievement of floodin from the river and reduce the production of the national breeding bases of natural parent fish.The naturally connected ones also would be impacted by the process and interfered with their protection of some rare species such as river dolphin and David′s deer.Though the embankment of the Jingjiang started from East Jin dynasty and has a development history more than 600 years,only the Jingjiang bank is the first grade bank,Jingnan bank and Honghu bank are the second grade bank.All the old courses are not located at the protected area of these great banks.The limited control effect of recent banks in the Jingjiang is difficult to keep the river regime safely near the old river courses.The potential evolvement trends of the Jingjiang could cause the connection possibility of the old river course decrease,which will accelerate the obstructed process of the old river courses.To keep the ecological security of the old river courses,it is necessary to investigate the hydraulic and hydrologic evolvement of the Jingjiang and develop the suitable engineering project to protect them.〖