Mudança hidrossedimentológica na volta grande do Xingu influenciada pela Usina Hidrelétrica Belo Monte

Abstract

Dams profoundly alter the hydrological and sedimentary dynamics of rivers, directly affecting channel morphology as well as aquatic and floodplain ecosystems. Sediment retention in reservoirs, combined with reduced discharge, disrupts downstream sediment supply, potentially leading to both channel and bank erosion or to siltation, thereby compromising the environmental heterogeneity essential for sustaining biodiversity. Such reductions in habitat diversity directly influence habitat availability, as systems with greater physical variability tend to be more resilient to disturbances such as climate change or anthropogenic impacts. Within this context, the Belo Monte Hydroelectric Plant—one of the largest in Brazil in terms of installed capacity—was constructed in the Volta Grande do Xingu. This approximately 130 km stretch of the middle Xingu River is characterized by clear, shallow waters, sinuous reaches, and a complex mosaic of aquatic and seasonally flooded habitats. It also hosts rapids, islands, beaches, igarapés, and rocky-substrate channels that support rich and often endemic aquatic and riparian biodiversity. This study aimed to characterize the sedimentary deposits of the main channel and igarapés of the Volta Grande do Xingu adjacent to the Mïratu village (Paquiçamba Indigenous Land), and to assess their internal characteristics through facies and granulometric analyses. Sediment provenance was evaluated using optically stimulated luminescence (OSL) sensitivity, while areas undergoing erosion and siltation were mapped. We also investigated how variations in discharge drive changes in igarapés and channels associated with aquatic fauna habitats and seasonally flooded environments. Facies analysis revealed that the beds of the Igarapé Paraíso, Ilha do Zé Maria, and main channels comprise at least three sedimentary facies: compact massive mud with gravel, boulder gravel, and medium to very coarse sand. Bathymetric profiles showed marked variation in water depth, reflecting the hydrodynamic and erosional–depositional processes shaping the Xingu River bed, consistent with high environmental heterogeneity. Water-level gauge data indicated a positive correlation between the main channel and igarapé water levels. Grain-size distribution showed a predominance of medium to coarse sand (with gravel) and a lower occurrence of fine sand. OSL sensitivity analyses identified distinct sediment sources. Area 7 contained predominantly low-sensitivity quartz with a median below 50% (percentage in the first second of stimulation), while Area 3 (Igarapé Paraíso and adjacent channels) showed variable sensitivity, indicating mixed sources. The remaining areas were dominated by quartz grains with median sensitivity above 50%. These findings enhance our understanding of the impacts of flow regulation in the Volta Grande do Xingu, offering valuable insights for future environmental management strategies aimed at biodiversity conservation and floodplain protection, particularly in light of the changes triggered by the Belo Monte Hydroelectric Plant.