The impact of Three Gorges Dam on the hydrological connectivity of “off-stream” floodplains

The construction of dams has led to concerns about ecological deterioration due to the weakening of hydrological connectivity. However, existing assessments of hydrological connectivity have focused on the damming effect on upstream–downstream connectivity, neglecting the impact on "off-stream" floodplains and resulting in an underestimation of dam's overall impact. This study took the Three Gorges Dam (TGD), the world's largest hydroelectric dam, as an example to assess its impact on the hydrological connectivity of the Yangtze-connected floodplain, Poyang Lake, through hydrodynamic modeling. The results revealled significant alterations caused by the dam construction, particularly during the receding period. The connectivity function (CF), representing the ability of water movement in different directions, was found to decrease due to the TGD. The average reduction in the North-South CF was 17.38 %, while the West-East CF decreased by an average of 13.62 %. Moreover, the dam's most pronounced impact was observed during specific periods. The average shrinkage of the connected object (CONNOB) was 55.17 % during drought years, 83.25 % during normal years, and 64.61 % during flood years. Additionally, these CONNOBs exhibited fragmentation, indicating a disrupted hydrological network. Furthermore, the dam-induced changes affected the timing of the longest connected period in the seasonal isolated lake. On average, during normal years, this period began 69.41 days earlier and ended 70.36 days earlier. The intensification of autumn drought caused by the emptying effect of the TGD is the underlying mechanism of the weakened hydrological connectivity in Poyang Lake. This study provides new insights into assessing the impact of dams on hydrological connectivity and can improve the current assessment framework, leading to better ecological and environmental benefits of water management related to hydrological connectivity adjustment.