Alteration of flood pulses by damming the Nenjiang River, China – Implication for the need to identify a hydrograph-based inundation threshold for protecting floodplain wetlands

Abstract

Floodplain wetlands of low-gradient rivers are especially sensitive to alteration of hydrologic conditions. Although many studies have investigated changes in downstream hydrologic regimes caused by river dam construction, no study has specifically quantified the long-term effect of altered flood pulse on floodplain wetlands. Such knowledge is needed to develop effective dam operation strategies and plans for downstream wetland protection and restoration. We applied data time series from 1984 to 2018 to delineate the inundation extent of floodplain wetlands during the pre- (1984–2005) and post-dam (2006–2018) periods. Four segments of the 360-km river reach were selected to investigate how inundation frequency of floodplain wetlands responded to changes in flow regimes and flood pulse for both periods associated with dam operation. The results showed that the water area slightly increased as a result of recharging water from dam, while the inundation frequency of adjacent floodplain wetlands overall reduced because of lower magnitude of flood pulse. Inundation area of four segments show less variation during the post-dam period compared to the pre-dam period. These findings clearly indicate that decreased flood pulses caused by dam operations reduce both inundation frequency and areal extent of floodplain wetlands. Based on the findings, we propose an inundation threshold that can be used for dam operation to protect and restore river corridor and floodplain ecosystems.