4.1 Boundary index
The average stability coefficients of the Zhicheng, Shashi, Jianli and Luoshan hydrological stations in the Jingjiang reach are 0.34, 0.29, 0.27 and 0.28, respectively, which are higher than the critical value of the stability coefficient of the curved reach. There is a great difference in the change process of each hydrological station (Fig. 3a). For example, the stability coefficient of the Zhicheng hydrological station decreased slightly from 1991 to 2012, and this decrease was caused by river channel sedimentation. The stability coefficient of the Jianli hydrological station has an obvious increasing trend after 2007, which is caused by channel erosion and changes in cross-section morphology.
The Jingjiang River section belongs to a classical meandering river in the plain area, and there is no dam. Therefore, there is continuity along the river boundary, and the longitudinal connectivity index displays little change as a whole. Fig. 3b shows the variation in the stream gradient along the Jingjiang River in recent decades. The stream gradient has a significant decreasing trend after 2003.