Effects of vanishing power-law tails of river flows caused by damming on downstream hydrological connectivity

Abstract

High-intensity dam operations have severely disturbed the power-law behaviors of natural river flows, which have important implications for the entire ecosystem’s integrity, diversity, and sustainability. This study investigated the effects of vanishing power-law tails of river flows caused by damming on downstream hydrological connectivity in the Wanquan river–floodplain (China) and determined the consequent ecological responses. A two-dimensional hydrodynamic model was established to estimate inundation regimes under different hydrological scenarios, and a statistics-based method was used to generate vanishing power-law tails. Connectivity assessments quantified the duration, magnitude, accumulation, and efficiency of connection events in the river–floodplain system for historical hydrological processes, current dam operating schemes, and simulated flow series with compressed power-law tails. The results showed significant reductions in downstream connection duration, magnitude, accumulation, and efficiency with compression of the natural power-law tail, especially for small (P < 20 %) and large (P > 90 %) connection events. Under a fully modified scenario, the cumulative large connection events were reduced by as much as 50 %. Damming also diminished the seasonal fluctuations of lateral exchanges and weakened the synergetic relationships among inflows, inundation extents and connectivity in the river–floodplain system. Furthermore, increasing damming led to an exponential and accelerated loss of downstream connectivity, with noticeable inflection points. Reduced or insulated hydrological exchanges would weaken material cycling, energy flow, and information transmission in the river–floodplain system, ultimately impacting the entire aquatic ecosystem. Our study emphasizes the importance of the power-law tail in the estimation of natural flows, which, although accounting for only a small part of the whole flow series (approximately 6 % in this case), is of great significance for maintaining the downstream connectivity necessary for healthy aquatic ecosystems.