Hydro-geomorphological characteristics in response to the water-sediment regulation scheme of the Xiaolangdi Dam in the lower Yellow River

Water-sediment regulation schemes (WSRS) enacted by dams drive downstream hydrological and geomorphic processes. However, few studies have been conducted to reveal the seasonal variation in the relationship between hydrological and geomorphic processes and the main factors driving river channel geomorphology. In this study, the effects of the WSRS of Xiaolangdi Dam (XLD) on the hydro-morphological characteristics of the lower Yellow River were assessed using data from Huayuankou (HYK) and Jiahetan (JHT) hydrological stations for the years 1983–2019. The monitoring data included water levels, flow rates, suspended sediment concentrations (SSC), and cross-sectional profiles; data analysis was based on the methods of indicators of hydrologic alteration and range of variability approach (IHA-RVA) and random forest (RF) models. After its completion in 2001, the operation of the XLD greatly altered water levels in the lower Yellow River (90.40% at HYK; 44.04% at JHT). The WSRS of dam scoured the riverbed and lowered the water level, causing the main channel to be narrower and deeper than during the pre-dam period. However, dam operation had little effect on seasonal variation in water levels when considering the following three hydrological seasons: pre-WSRS (March-May), mid-WSRS (June-August), and post-WSRS (September-November). In contrast, the SSC varied among seasons during the pre-dam period, but not after dam construction. Additionally, in the post-WSRS season, the cross-sectional geomorphic coefficients (CSGC) was lower in the post-dam period, while the average depth of river cross-sections increased. RF models revealed that water levels and the SSC were the main factors influencing channel geomorphic evolution in the pre-WSRS season at HYK and JHT, respectively. The SSC was the most important factor at both stations in the post-WSRS season. These results provide basic theoretical support for river regulation under the influence of intense human activities.