Efeitos da deposição de sedimentos e dinâmica fluvial no Pantanal Marimbus.

Abstract

The Marimbus Pantanal, located in the floodplain of the Santo Antônio River, at the municipal boundaries of Andaraí and Lençóis, constitutes an extensive permanently flooded area of approximately 57,5 km². This region features ecotone characteristics, where various ecosystems meet and interrelate. Previous publications have suggested that the flooded area was formed due to channel obstructions caused by accelerated sediment deposition along the Santo Antônio River channel and at its mouth. These sediments, originating from the rudimentary and mechanized mining activities in the Sincorá Mountains and on the terraces of the Paraguaçu and Santo Antônio Rivers between the 19th and 20th centuries, are considered directly responsible for altering the river dynamics and forming the Marimbus. The specific objectives of this study include: assessing whether there is a topographic gradient between the Santo Antônio River and the Paraguaçu River through topographic surveys, discussing the water flow behavior in the flooded area, mapping the spatial arrangement of alluvial deposits directly related to the Marimbus Pantanal, investigating the influences of mining activities on potential changes in flow direction, and dating the sedimentary material to determine if the deposition ages are consistent with the mining period. Data from the Fertém and Andaraí hydrometric stations, historical sources, topographic surveys, supervised classification of remote sensing images, sediment dating using optically stimulated luminescence, and digital elevation models obtained through aerial surveys with remotely piloted aircraft combined with PPK technique provided the necessary inputs to understand the silting phenomena of watercourses and were useful in elucidating the fluvial dynamics and interactions that possibly gave rise to what is known today as the Marimbus Pantanal. The methodological application of these inputs led to results demonstrating a very high concentration of alluvial deposits in the areas directly influenced by mining, which become sparse as the distance from these exploitation zones increases. The topographic survey revealed that the water surface level of the Marimbus Pantanal is not lower than that of the Paraguaçu River, thus not favoring natural drainage from the Paraguaçu to the Santo Antônio. The same result was found from the aerial survey using RPA combined with PPK technique, which, besides confirming the absence of a gradient at the Santo Antônio River outlet, exposed various backwater effects, especially during Paraguaçu's flood periods. Digital elevation models from the precision aerial photogrammetry survey demonstrated that the effects of accelerated sedimentation caused a relative narrowing of the Santo Antônio River mouth, particularly noticeable during dry periods, yet without completely obstructing the flow of this drainage section. On the other hand, the results of dating the sedimentary material from the place where the rivers confluence were not sufficient to identify relationships between the age of deposition and the period of occurrence of rudimentary and mechanized mining. The proposals developed in this work can contribute to more precise academic studies on the genesis of the Marimbus Pantanal while providing scientific inputs to the managers of the Marimbus Iraquara Environmental Protection Area for more assertive decision-making.