Dam-induced changes in multi-fraction sediment recovery in the middle-lower Yangtze River

Abstract

Hydropower dams induce downstream sediment starvation, influencing fluvial morphology. With the focus commonly on morphological changes, an aspect of sediment starvation that has received much less attention is the impact of changes in the sediment grain size distribution (GSD) on these morphological changes. In this study, we investigate the effects of the Three Gorges Dam (TGD) on the multi-fraction sediment transport and bed recovery in the middle-lower Yangtze River. Based on long-term field data (1987–2021), we evaluate how fine (d < 0.031 mm), medium (0.031–0.125 mm), and coarse (d > 0.125 mm) fractions differentially respond to dam regulation. Our findings reveal a progressive coarsening of suspended sediment and identify three distinct dam-induced sediment regimes: static armored gravel bed, active bed armoring, and strong erosion. Within the first ~350 km downstream of the TGD, erodible sediments, especially fine and medium fractions, have been almost entirely depleted. In contrast, the subsequent 750 km reach has emerged as the dominant sediment source, increasingly characterized by medium and coarse fractions over time. In addition, tributaries now supply fine-grained sediment during the wet season, whereas lakes, acting as long-term sediment storage zones, release previously deposited material during the dry season. Both sources are playing an increasingly important role in modulating the GSD of the middle-lower Yangtze. These findings shed lights on the dam-induced multi-fraction sediment recovery, offering valuable guidance for the sustainable management of river systems influenced by upstream dams.