Ecohydrological Impacts of Two Large Cascade Reservoirs in the Middle Yellow River, China

The Yellow River is the second largest river in China, providing precious water resources to ecosystems in northern China. The Sanmenxia and Xiaolangdi are two large cascade reservoirs in the middle Yellow River that have significantly altered the regional ecohydrological conditions. Multiple methods, including the sequential Mann–Kendall test, range of variation approach, and ecoflows based on flow duration curves, were used to assess the ecohydrological impacts of the two reservoirs. Results indicated that (1) in detecing impact years by the sequential Mann-Kendall test, the minimum daily discharge of each year performed better than the maximum, mean, and median daily discharges of earch year; (2) the delay in the date of minimum flow caused by the Sanmenxia Reservoir and the advance in the date of maximum flow caused by the Xiaolangdi Reservoir had a serious effect on some native fish species; (3) the integrated hydrological changes caused by the Sanmenxia Reservoir and Xiaolangdi Reservoir were 30.91% and 51.39%, respectively; (4) the Sanmenxia Reservoir amplified the annual ecodeficit, whereas the Xiaolangdi Reservoir reversed the annual ecosurplus from a decreasing to an increasing trend and the annual ecodeficit from an increasing to decreasing trend; and (5) the autumn ecoflows experienced the greatest impact, with accompanying deterioration, although the Xiaolangdi Reservoir reduced the increasing rate of autumn ecodeficit and radically improved the previous unfavorable conditions in other seasons. The results shed new light on the ecohydrological impacts of large dams and are beneficial for the restoration of the aquatic ecosystems in the lower Yellow River.