INTERACTIONS AMONG VEGETATION, HYDROLOGY, AND GEOMORPHOLOGY AND THE DISCOVERY OF A HIDDEN FLUVIAL LAKE IN THE BRAZILIAN PANTANAL

Wetlands are seasonally or permanently flooded areas and are among the Earth’s most biologically productive environments. The relationships among geomorphology, hydrology, sedimentation, and vegetation cover are crucial for understanding how the landscape constrains the dynamic of wetlands. The Pantanal is one of the world’s largest freshwater wetland with area of ~150,000 km², most in Central-West Brazil. The wetland is settled in an active sedimentary basin where the interplay of megafans, interfans, and a trunk fluvial plain result in a complex diversity of processes and landforms.

Here, we present the results of a study in the Negro River interfan system (NRIS), southern Pantanal, using multidisciplinary approaches (i.e., remote sensing analysis, hydrological surveys, and geomorphological zonation). The use of multitemporal analysis of Landsat images (2000 – 2011), resulted in a flood frequency map that revealed a hidden fluvial lake of 1,100 km² of area in the central portion of the NRIS, not recognized before due to the accumulation of free floating mats and floating meadows of macrophytes.

The Negro and Aquidauana rivers are the perennial feeder channels, exhibiting remarkable differences in planform, water discharge, and sediment load, sourcing the lake. The Negro River acquires a distributary pattern with marginal levees that lose elevation and remains as a subaqueous landform conditioning the water flow downstream, forming a prograding deltaic lobe into the lake. On the other hand, the Aquidauana River crosses the entire lake on elevated levees built due to higher water discharge and sediment load. The lake outflow occurs mainly as sheet flow during flood seasons and by small tributary channels during dry months. By using the flood frequency and vegetation type maps, we recognized ENE and WNW lineaments in the borders of the lake and in rivers channel reaches, suggesting that the area is tectonically controlled.

Our results showed remarkable changes in the NRIS area in conjunction with possible neotectonic activity and climatic effects, which resulted in contrasting fluvial styles in rivers and diverse vegetation. Thus, the NRIS may be used as a model for understanding the evolution and functioning of wetlands, and the applied methods can be replicated to other seasonally / permanently areas worldwide.