Impact of the Xiaolangdi Reservoir Operation on Water–Sediment Transport and Aquatic Organisms in the Lower Yellow River During Flood Events

The operation of reservoirs has prompted rivers to transition from natural ecosystems to “natural–artificial” composite ecosystems, which has not only altered the water–sediment processes but has also affected river ecology in the downstream river channels. To reveal the impact of the Xiaolangdi Reservoir (China) on sediment transport and aquatic organisms in the Lower Yellow River (LYR), this article analyzes the changes in the water–sediment processes and sediment transport characteristics prior to and following the reservoir construction, based on measured water–sediment data of 688 floods from 1960 to 2023. It derives a theoretical formulation for the sediment delivery ratio (SDR) of flood events based on the sediment transport rate equation and evaluates the living environment of aquatic organisms in the LYR. The results indicate that after the construction of Xiaolangdi Reservoir, the frequency of floods with an average flow discharge below 1000 m3/s increased from 26.08% to 37.42%, and the frequency of floods with an average sediment concentration below 20 kg/m3 increased from 46.34% to 89.03%. The SDR of flood events significantly correlates positively with the average flow discharge and the water load variation coefficient. Conversely, it negatively correlates with the average sediment concentration and the incoming sediment coefficient. The sediment transport capacity of various river reaches in the LYR gradually increases along the direction of the river channel. The use of Xiaolangdi Reservoir has enhanced sediment transport in the upper LYR reach while decreasing it in the lower reach, aligning the overall sediment transport capacity of the downstream river channel. Additionally, the water–sediment process of the flood events following the completion of the Xiaolangdi Reservoir construction has improved the living environment for aquatic organisms, which is conducive to restoring biodiversity and improving the ecological environment of the river. The research results have enriched the understanding of the impact of reservoir construction on downstream water–sediment transport and aquatic organisms in sandy rivers, providing technical support for the health and sustainable development of rivers.