Análise de anomalias na rede de drenagem da bacia hidrográfica do rio Balsas, Maranhão

Abstract

Drainage anomalies correspond to local disagreements in drainage patterns that indicate topographic deviations, resulting mainly from morphostructural and morphotectonic processes. Understanding the geomorphological, geological and tectonic context within a drainage basin is essential for studies on the effects of deformation on the crust and the morphogenic processes that act on the configuration of the drainage network. This research aims to investigate the morphotectonic and morphostructural influence on the arrangement of channels in the Balsas drainage basin, located in the Southern Mesoregion of the state of Maranhão, based on the identification and analysis of drainage anomalies. The Balsas drainage basin is located in the Parnaíba Sedimentary Basin, comprising an area of approximately 25,600 km2

, with an elongated polygonal shape in the SW-NE direction. The research was developed based on the geosystemic theoretical and methodological approach, the technique was supported by geoprocessing from the manipulation of remote sensing data in a Geographic Information System environment, where the following were used: Digital Elevation Model from the TOPODATA/INPE project (2008), with a resolution of 30 m, images from the RapidEye satellite, from 2011, with a resolution of 5 m, in addition to vector data acquired through official government agencies and a manual vectorization process. Geomorphological variables of the basin and drainage network, as well as morphometric indices, were applied to analyze geomorphic anomalies in the channels, in order to investigate evidence of structural and/or tectonic control. The morphometric indices applied were: Drainage Density (Dd), Hydrographic Density (Da), Average Basin Slope (Dm), Transverse Topographic Symmetry Factor (FSTT), Drainage Basin Asymmetry Factor (FABD), Stream-Gradient Index (RDE index) and preparation of Topographic Profiles in the process of analyzing the

geomorphological evolution of the study area. The subdivision of the Balsas basin into sub- basins was carried out to better understand local differences during the analysis of

morphometric parameters. The FSTT and FABD results show tectonic influence in the Balsas basin, which has a moderate level of tipping, causing a displacement of the main river towards the right bank, more accentuated in the middle course. In the Coité drainage sub-basin, the highest rates of asymmetry were found, expressing high tilting that migrates the main channel to the left bank in relation to the central axis of the sub-basin. The Dd of the total area of the Balsas drainage basin was 0.59 km/km2

, the Dh is 0.18 channels per km2

, with medium Dd and low Dh being considered. The Dm identified in the Balsas basin coincides with the predominance of gently undulating and flat reliefs, accounting for almost 80% of the basin's surface. The Dm found is mainly associated with extensive flat areas on the tops of plateaus whose edges have significant areas of scarps that reach a slope greater than 75%. The application of the RDE index showed a high density of knickpoints indicative of first-order anomalies in the Balsas II, Neves and Cocal sub-basins, respectively. Based on the analysis of the geomorphological evolution of the basin, it was possible to hypothesize the occurrence of river capture, where the Balsas basin possibly captured drainage from the Itapecuru drainage basin and had channels and areas incorporated into the Tocantins drainage basin.