Shifts in river-floodplain relationship reveal the impacts of river regulation: A case study of Dongting Lake in China

Better understanding of the dynamics of hydrological connectivity between river and floodplain is essential for the ecological integrity of river systems. In this study, we proposed a regime-switch modelling (RSM) framework, which integrates change point analysis with dynamic linear regression, to detect and date change points in linear regression, and to quantify the relative importance of natural variations and anthropogenic disturbances. The approach was applied to the long-term hydrological time series to investigate the evolution of river-floodplain relation in Dongting Lake in the last five decades, during which the Yangtze River system experienced unprecedented anthropogenic manipulations. Our results suggested that 1) there were five distinct regimes during which the influence of inflows and local climate on lake water level changed significantly. The detected change points were well corresponding to the major events occurred upon the Yangtze; 2) although the importance of inflows from the Yangtze was greater than that of the tributaries flows over the five regimes, the relative contribution gradually decreased from regime 1 to regime 5. The weakening of hydrological forcing from the Yangtze was mainly attributed to the reduction in channel capacity resulting from sedimentation in the outfalls and water level dropping caused by river bed scour in the mainstream; 3) the effects of local climate was much smaller than these of inflows; and 4) since the operation of The Three Gorges Dam in 2006, the river-floodplain relationship entered a new equilibrium in that all investigated variables changed synchronously in terms of direction and magnitude. The results from this study reveal the mechanisms underlying the alternated inundation regime in Dongting Lake. The identified change points, some of which have not been previously reported, will allow a reappraisal of the current dam and reservoir operation strategies not only for flood/drought risk management but also for the maintenance and restoration of the regional ecological integrity.