A study on the ecological effects of water and sediment regulation by Xiaolangdi Reservoir

<p indent="0mm">This paper evaluates the ecological effects of water and sediment regulations at the Xiaolangdi Reservoir by combining monitoring data before the 2023 flood season with historical research results. The main conclusions are as follows. (1) During the clear water discharge phase, ammonia nitrogen concentrations increased from Xixiayuan to Aishan owing to the sediment resuspension from river erosion. During the centralized sediment discharge stage, ammonia nitrogen, total phosphorus, and total nitrogen levels between Xixiayuan and Huayuankou were affected by the pollutant release from sediment discharge. (2) During the centralized sand removal stage, dissolved oxygen concentrations in the water dropped to <sc>0.2–4.6 mg/L.</sc> Specifically, between the Yiluo River estuary and Huayuankou, levels were <sc>0.2–0.9 mg/L,</sc> significantly lower than in other periods, mainly owing to oxygen consumption by reducing substances released from sediment resuspension. (3) In 2023, 58 species were surveyed in the lower reaches of the Yellow River, a significant increase from 2009–2010 and 2013–2017. This rise is linked to Xiaolangdi Reservoir’s water and sediment regulation, which stabilizes river flow and enhances ecological flow during fish breeding seasons. (4) “Flowing fish”, a natural phenomenon unique to the Yellow River, is usually caused by high sediment content and low dissolved oxygen. The occurrence of the “flowing fish” phenomenon during the 2023 water and sediment adjustment period was mainly concentrated in the river section from Xiaolangdi Dam to Xixiayuan Hydrological Station. Compared with historical periods, the river section where “fish migration” occurred has significantly shortened. (5) The Yiluo River and Qin River estuaries maintained low sediment zones suitable for fish survival. During the centralized sediment discharge stage, 26 fish species were surveyed at the Yiluo River estuary, more than before adjustments, highlighting the protective effect of low sediment concentration zones at these estuaries.