Brazilian Estuaries: A Geomorphologic And Oceanographic Perspective

Estuaries are primarily sites where rivers meet the sea, where fresh water is mixed with sea water by the tidal flow and where salinity gradients are set up along a mixing zone. In this chapter, we consider that the estuary is an accommodation space landward of the coastline, where fluvial sediments are largely retained with no appreciable effect on the sedimentation processes of the open coast. Here we recognize 41 large estuaries along the Brazilian coast, ranging in size from 40 km(2) to more than 10,000 km(2), encompassing areas with different topography, climate, geology, tidal ranges, and alongshore drift. The majority of these estuaries are located along fault lines or within grabens, and although scattered, structural and stratigraphic evidences suggest that land subsidence has caused sea level to locally rise, which has been an important factor in the preservation of these coastal features. A larger number of estuaries exist in the macrotidal and humid north coastal sector where large-scale coastal subsidence occurs. The estuary sizes along the coast bear no relation with tidal ranges, but have a positive linear correlation with river discharge in the east-southeast and northeast coastal sectors. Based on the existing hydrographic data, it could be stated that the large Brazilian estuaries are hypersynchronous, ebb-dominated, and well mixed. In the deeper estuaries of the east-southeast sector, the longitudinal density gradient is an important driver for water circulation, causing vertically stratified residual flows that end up retaining suspended sediments within the estuary and promoting the accumulation of organic, muddy sediments.