1 Introduction
A river is regarded as a whole system in the principle of river continuum, and the channel connectivity is the basis of judging the regional continuity (Robin L, et al, 1980; Leigh C, 2009). The connectivity of the water system refers to the smoothness of the interconnection between water system units, which is the basic attribute of river systems and plays an important role in the ecological environment, material transport and energy cycle of rivers (Pringle C, 2003; Pringle C, 2001). Rivers with good connectivity are conducive to the development of various functions, such as hydrological regulation and storage, water and sediment transport, and ecological environment (Michaelides K,2009; Williams, C. J.,2015).
Due to the influence of reservoir construction or soil and water conservation, the sediment load of the Yichang hydrological station is greatly reduced, which leads to significant changes in the channel connectivity of the Jingjiang River. In particular, the impoundment of the Three Gorges reservoir (TGP) in 2003 has a great impact on the runoff, sediment load and channel erosion of the Jingjiang River. The average annual runoff and sediment load of the Yichang hydrological station were 395.8 billion m3 and 46.6 million t from 2003-2013, which were 9% and 90% smaller than before operation of the TGP project, respectively. After the operation of the TGP, the sediment concentration decreased by 75%, the suspended load and bed load became coarse quickly (Zhu, 2012), and the water level changed significantly in the Jingjiang River (Geng, 2012). In addition, the Jingjiang River was in a balance of erosion and deposition basically before the impoundment of the TGP, while the erosion-accretion variation was severe after that (Xia, 2016; Chang, 2010; Chen, 2010; Xu, 2007; Dai, 2013; Yang, 2014). The scour amount of the Jingjiang River accounts for 78.9% in the Yichang-Chenglingji reach, and the scour intensity is relatively large.
At present, the study of the Jingjiang River mainly focuses on sediment variation, riverbank failure or channel evolution, but a comprehensive study of channel connectivity is still lacking. Therefore, combined with the river’s basic functions, we set up an evaluation model of the channel connectivity using the AHP method described in this paper. Based on data of the runoff, sediment load, cross-section morphological parameters, and volume of scour and fill, the variation in processes of the channel connectivity of the Jingjiang River was evaluated and analyzed from 1955-2015.
In addition, there are some problems with channel connectivity in other basins; for example, human activities and climate change cause river channel cut-off in the Rhone, Rhine, Ain and Doubles rivers (Bornette, 2002). Due to the changes in the forest landscape pattern, landscape connectivity decreased by an average of 54.74% in the Manwan Basin (Yang, 2014). In addition, the dam is a strong disconnecting factor in the longitudinal sediment connectivity in the Kaja river (Bertsch R, 2012). The flow connectivity, water discharge, and flood pulse changed in the Paraná River after closure of the Porto Primavera Dam (José C, 2009). After the implementation of the water-sediment regulation by the Xiaolangdi Reservoir, the lower reaches of the Yellow River are scoured, which leads to a change in boundary connectivity (Liu, 2013). The evaluation of the channel connectivity and the analysis of the influencing factors in this paper can also provide useful information for the connectivity of other rivers in the world.