Geomorphic and Geologic Influences in a Tropical Semi-Humid Climate Ecosystem

Direct influences of geology and landforms in ecosystems are not always easy to identify. This influence seems to occur in the development of the Veredas, an ecosystem belonging to the Cerrado, the second biggest biome in Brazil. The Veredas ecosystem which can also be considered as a river system is one of the most striking and still not very well understood landscape feature of this biome. It occurs along a swamp type of shallow depressions that function as hydrological channels with permanent water flow but no terraces that could indicate depositional processes. They are characterized by a linear specific vegetation cover of hydrophytes including different types of palm wetlands (Melastomataceae), the most representative of which is the Mauritia flexuosa. The objective is to study these hydrological systems with the help of GIS techniques and field data in order to understand the main factors affecting their spatial distribution. The studied eco-fluvial systems comprise nine drainage basins distributed in 33,448.13 km2 in the Minas Gerais State, Brazil (1508'/180 S and 4805'/5105' W), located in a tropical semi-humid climate. Images of TM/Landsat 5 and geologic maps were used as well as an Elevation Digital SRTM-NASA-2001 Model processed using ArcGIS 10 and SPRING-INPE 4.3. Various morphometric indicators and longitudinal river profiles were also obtained complemented by field data. Results show that the Veredas present mostly slow flows what seems to be an important factor for maintaining the ecosystem as such. As a specific ecosystem as well as drainage systems they change characteristics along the geologic and geomorphic domains. They present a more representative cover of wetland palm trees accompanying shallow incised channels while running over two platos sequences elaborated on Cretaceous sandstones. In the higher portions of the Urucuia Plato (+600m) the river systems are less dense and geological control influences the occurrence of parallel patterns of the drainage. The lower Areado Plato (500-650m) is interconnected with the higher one through breaks of slopes. There are evidences that the erosion of the Urucuia Plato is causing the slow but inevitable lowering of the water table that tends to spring at the basis of Urucuia Plato, or at the top of the Areado forming a sequence of water heads. This areas have higher channel density index and a dendritic to trelice pattern with a very well represented herbaceous and forest swamp cover in which the palm trees are often seen as a continuous line following the water channel. These channels become more incised but present depositional sequences downstream in their contact with the geologic domains of Bambui Group formed by metapelitic rocks from the Mesoproterzoic. There the Veredas as ecosystem changes acquiring a more riparian type of vegetation with the predominance of species from the Fabaceae, Myrtaceae and Cyperaceae families. Landforms as well as part of the drainage system become associated to karstic processes.;