Morfoestrutura, formações superficiais e estado de conservação da bacia hidrográfica do rio Santa Cruz, sertões de Crateús, CE/Brasil

Abstract

The study of Hydrographic Basins (BH) in Northeast Brazil, notably in the State of Ceará, is a strategic action to understand the characteristics and environmental dynamics, as well as to improve the management of this system. In view of the limitations of the research that has been developed, it is necessary to expand the studies in BH and its fluvial plains to elucidate which factors and processes contributed to their formations and morphologies, to analyze the environmental configurations, including the uses and types of coverings, as well as determining the impacts of environmental legislation and which spaces are protected. Therefore, the BH of the Santa Cruz River, located in the center-west of Ceará, in the micro-region of Sertões de Crateús, was chosen as a sample object for its poorly studied and degraded environmental configuration, for being inserted in the semiarid and for composing the high course of the BH of the Poti River. Thus, this work aimed to carry out an environmental analysis, focusing on the geological, geomorphological, climatological and conservation dimensions of BH and its fluvial plains to expand knowledge about the fluvial systems located in the semiarid portions. For this purpose, bibliographic and geocartographic surveys, extraction and morphometric calculations of drainages and lineaments were performed in the TerraViewHidro 0.4.5 software and in SPRING 5.5.2 and construction of thematic maps in Qgis 3.4. In addition, Google Earth Pro images and measurement tools and rainfall data from the FUNCEME station in Independência-CE were used. Field work, collections and pedestrostratigraphic, physical, chemical and geochronological analyzes (Optically Stimulated Luminescence -LOE) were also carried out on sediments and soils in the river plains. Finally, environmental legislation and data from the Mapbiomas project were used to analyze and measure the types of use and coverage and whether the protected areas were in compliance with the legislation. Soon, it was detected that the BH is oriented from east to west due to the ruptural tectonics (faults, fractures and shearing) that generated the Poti river valley. The morphometric parameters are correlated and vary according to the environmental components of BH, they did not indicate susceptibility to flooding and drainage anomalies are derived from the Precambrian lithostructure. Lineaments imply the configuration of drainage patterns due to the structural conditioning that enables the occurrence of more or less dissected reliefs. The fluvial plains are made up of sandy silicycles deposited with massive structures in the last 4,500 A.P. Its genesis is linked to the lateral addition of bars and avulsion of the rivers during the hydrosedimentary pulses. The thorny deciduous forest is the most degraded vegetation in the studied area, between 1985 to 2019 and many of its species were conserved in riparian forests. The grazing class was the one that had the greatest spatial expansion due to deforestation and abandonment of agricultural fields. Environmental legislation has limitations as it does not protect the entire river plain and creates gaps for the exploitation of riparian permanent preservation areas (APPs) that are commonly deforested and used for agricultural fields.