160-3 Mineralogical variability of fluvial sediments induced by mega-reservoir regulation (Yellow River, China)

Session: Advances in Fluvial Processes and Sediment Transport, Part I

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Unraveling the mineralogical variability of fluvial sediments is crucial to unambiguously trace detrital signatures from mountain belts to ocean floors. Through the last century, the construction of large dams and associated reservoirs has disrupted the continuity of fluvial sediment transport all around the world, and yet the effects of such a profound alteration of natural systems remains poorly documented. This study investigates these effects and their controlling mechanisms in the highly anthropically altered lower reaches of the Yellow River, the sixth longest river on Earth with the highest suspended-sediment concentration. Here the artificial regulation of the big Xiaolangdi Dam determines sharply different hydrological regimes through the year (pre-water regulation in May, water regulation in June, sediment regulation in July). The followed multi-technique approach employs multiple-window heavy-mineral analyses of all significant size classes by TESCAN Integrated Mineral Analyzer, coupled with bulk-sediment X-ray diffraction and grain-size analyses. The results show that reservoir regulation causes significant intersample and intrasample mineralogical variability. Suspension sorting controlled by size-density-shape relationships leads to different features in different hydrological regimes downstream of the Xiaolangdi reservoir. In the sediment-regulation stage, a clear downstream fining trend is associated with a notable decrease in quartz and faster-settling heavy minerals. Such a trend is not equally evident in the preceding pre-water and water regulation stages, when − in the lack of a sediment flux from upstream – fine sediment is generated by reworking of river banks downstream of the dam. In the water-regulation stage, stronger hydrodynamic conditions coupled with minimal sediment supply increase the possibility of selective-entrainment processes, leading to a lower percentage but coarser size of heavy minerals. In all three regulation stages, mica tends to increase with transport distance from the dam because of its platy shape and consequently low settling velocity. Settling-equivalence accounts for the majority of intrasample variability in all three stages (size-density correlations being 0.81 in May, 0.58 in June, 0.79 in July). This study illustrates general patterns of fluvial sediment transport controlled by different reservoir-regulation modes and provides a prominent case documenting the mineralogical effects produced by the strong anthropogenic modification of a natural sediment-routing system.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-5720

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