Mecanismos evolutivos de bacias de drenagem em diferentes escalas de tempo: Rio Japuíba (Angra dos Reis, RJ)

Abstract

Understanding the evolution of a drainage basin requires expressing qualitatively and quantitatively the forms, processes and their interrelationships. The analysis of the arrangement of river systems elucidates geomorphological issues, since watercourses are active agents in landscape modeling. The research seeks to investigate the evolutionary mechanisms through the integration of different methodologies, considering the analysis of tectono-structural control, the quaternary sedimentary dynamics proposing an evolutionary model for the Japuíba river basin, in addition to the recognition of technogenic terrains. The detailed analysis of the basin, located in Angra dos Reis and formed by the Japuíba and Cabo Severino rivers, with a predominance of steep mountain ranges in contact with fluvial-marine plains, was carried out from the division into three sub-basins (Banqueta-Japuíba, Cabo Severino and Japuíba do Meio) and revealed distinct characteristics, shaped by geological and anthropogenic processes over time. The Banqueta-Japuíba sub-basin stands out for its high tectonic influence, evidenced by high values of the Tectonic Activity Index (IAT), with abrupt slopes that increase the risk of erosion and landslides. The colluvial rock morphodynamics, with confined and partially confined valleys, gives the sub-basin high susceptibility to changes. On the other hand, the anthropogenic changes that predominate are technogenic equiforms. The Banqueta-Japuíba sub-basin stands out for its high tectonic influence, evidenced by high values of the IAT. The relief has steep slopes that increase the risk of erosion and landslides. The colluvial rock morphodynamics, with confined and partially confined valleys, gives the sub-basin high susceptibility to changes. Anthropogenic changes in the terrain are rare, with a predominance of technogenic equiforms, and quaternary sedimentation rates are lower, indicating the predominance of erosive processes and material transport. The Cabo Severino sub-basin also presents tectonic-structural control, with very high values of IAT. The relief exhibits a mature drainage system due to intense erosion. The morphodynamics are similar to the Stool, but with fewer drainage elbows and a non-confined section, making it a little more susceptible to changes. Anthropogenic changes include equiforms, scars, depressions, elevations, and technogenic superpositions. Quaternary sedimentation is small, given environments of high erosive energy. The Japuíba do Meio sub-basin, in turn, has a high tectonic influence, but less intense than the previous ones. The relief exhibits a lower risk of erosion and landslides, with areas of sediment accumulation. The anthropized alluvial morphodynamics, with an unconfined fluvial style, show a strong human influence on the modification of the channel and fluvial processes. The changes in the terrain are numerous, with the presence of equiforms, scars, depressions, elevations and technogenic superpositions, reflecting the greater population occupation. The sub-basin has the largest area of Quaternary sedimentation, with recent and continuous sedimentary activity, in addition to being the stretch of the basin that suffers the most from siltation due to human action. The evolutionary analysis of the basin in different time scales, long and short, allowed us to understand the fluvial dynamics and anthropogenic interferences. On the long time scale, tectonic-structural compartmentalization and Quaternary sedimentation showed a strong influence on the conformation of the basin, as well as on the distribution and retention of Quaternary age sediments. On the short time scale, the current erosive processes and the classification of fluvial styles (confined, partially confined and unconfined) indicate different degrees of morphodynamics of fluvial channels and the growing influence of human action, especially in the Japuíba do Meio sub-basin. The investigation of geometry and geomorphological processes demonstrated how the basin was driven by climatic events, tectonics, by the variation of vegetation cover and human activity. In fact, the increasing degradation of river systems has generated significant impacts on the rates of geomorphic processes in the landscape. This problem requires a more efficient environmental management, which involves understanding the characteristics and behaviors of the relief and river system.