Morphological response of the Lower Yellow River to the operation of Xiaolangdi Dam, China

The morphodynamics of the Lower Yellow River (LYR) has been impacted by intense dam-induced changes over the past few decades. Here we present results on the morphological impacts produced on the LYR by the construction and operation of Xiaolangdi Dam, whose construction was completed in 1999. We used multi-temporal remotely sensed images and geomorphological parameters, and developed a systematic and universal river morphology extraction framework to investigate changes in the planform of the LYR between 1987 and 2017. The morphological changes were assessed by identifying seven sub-reaches (R1 to R7) along similar to 860 km of the LYR. The multichannel upper reaches of the LYR exhibited more marked changes in anabranching and braid-channel ratio, sinuosity, and lateral migration rate than the lower reaches, which are characterized by a single channel pattern. Sandbars in the upper reaches (R1-R3) underwent degradation after the beginning of operation of Xiaolangdi Dam, while the reverse was seen in the lower reaches (R6-R7). The area of overlap between the 1987 channel mask and the channel mask at different times throughout the study decreased gradually over the study period, indicating intense channel migration. The operation of Xiaolangdi Dam prevented the river from running dry but also from suffering floods, while extending the duration of the low/medium stage flows. These actions account for the morphological changes in the LYR. Despite the fact that effective discharge did not change significantly after the Xiaolangdi Dam began operating, it became compressed into a shorter range of annual discharge variability (i.e., larger low-level discharges and shorter rising-smaller floods stages). Our integrated assessment shows a significant amount of water flow energy has been consumed by multiple planform adjustments in the upper reaches of LYR, while local siltation is the main issue in the sinuous but laterally stable lower reaches. (C) 2019 Elsevier B.V. All rights reserved.