SEASONAL VARIABILITY OF SEDIMENT TRANSPORT AND PROVENANCE IN THE DISTAL CUIABÁ MEGAFAN, PANTANAL WETLANDS, BRAZIL

The Cuiabá megafan is the second-largest alluvial megafan (~15,000 km²) in the Pantanal basin. In the past few decades, the Cuiabá River watershed has been drastically modified by intense deforestation, urban growth, and the construction of hydroelectric dams. Yet much remains unknown about sediment transport and deposition in the modern megafan lobe. Here, we present results from two surveys, one in the high-water (April 2016) and another in low-water level (November 2016). We collected the primary data using a RiverRay® 600 kHz Acoustic Doppler Current Profiler and suspended sediment samples in nine locations of the lower Cuibá River. We applied the Van Rijn equation to estimate the rates of sediment transport, and the calculations revealed that suspended sediment transport was 80% higher and bedload transport was 19% higher during the low-water level period. Changes in particle size were also evident between the seasons, with average D₅₀ in the high-water period exceeding that of the low-water level (0.192 versus 0.143, respectively). Quartz is the dominant component of the bedload sands with minor metamorphic rock fragments, derived from the planalto hinterland lithologies composed mostly of Neoproterozoic, Devonian, and Cretaceous sandstones, shales, and metaconglomerates. Seasonal variability in hydrology and sediment transport are characteristic features of the Cuiabá River that are sensitive to environmental variations. Changes in catchment land use can lead to significant impacts in the megafan sedimentary dynamics. The results have implications for broad areas of the Pantanal lowlands including risks presented to indigenous and traditional community lifestyles, which are dependent on riverine resources.