The Parnaiba River Delta - from modern hydro and morphodynamics to sea level change
This study presents the first findings regarding the hydrography, morphodynamics, hydrodynamics and sea-level change on Parnaíba River (PR) and its delta. The PR is 1,400 km long and located in NE Brazil. It forms an asymmetric wave dominated delta, the Parnaíba River Delta (PRD). The delta region is approximately 100 km wide and has tidal channels forming an estuarine-lagoonal system, west of PR. The delta is located in a transition zone in terms of climatic and oceanographic conditions. Despite the existence of a river dam on its mid-course, the river has a moderate (approximately 91 mg/l, in average) suspended sediment load which is mainly attributed to the geology and the climatic conditions of its drainage basin. Rainfall data from 1965 to 2009 revealed seasonal variations on rainfall and suspended sediment concentration (SSC). Moreover, harmonic analysis of time series revealed a correlation of both, rainfall and SSC, with El Niño-Southern Oscillation (ENSO). The suspended sediment concentration was calculated from a set of 12 multispectral satellite images (CBERS 2-B and LANDSAT TM 5), which were calibrated using in situ measurements. Using averaged river discharge and remote sensed suspended sediment concentration it was possible to estimate the amount of sediment that is transported through the PR to the continental shelf. The PR is the primary sediment source to the adjacent coast and this is evidenced by the shoreline stability and progradation noticed at the vicinities of the river mouth (observed on decadal time scale using satellite images). The river runoff interferes on the direction of the offshore bottom currents on the continental shelf up to 4 km in front of the river mouth. It acts as a hydrodynamic groin preventing sediment bypass. Offshore bathymetry and seismic profiles depicted evidences of past sea level on the continental shelf offshore PRD in the form of incised valley and paleo-channels. Interpretations of architectural elements, seismic facies and seismic boundaries were related to extreme events of sea level such as sea level low- and highstand providing relative age control for the formation and complete burial of the paleo-channels. Relating worldwide reconstructions of sea level curves to the interpretations it is suggested that the valley incision occurred during the Late Pleistocene sea-level drop and lowstand (70,000 – 20,000 years BP). Moreover, the valley infill indicates the flooding of the shelf during deglacial sea-level rise that reached the mid-Holocene highstand around 5,500 years BP.
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