Efeitos de barragens em barras fluviais do baixo Rio São Francisco: dinâmica morfosedimentar e implicações para interpretação de rios antigos

Abstract

The world's major active rivers are dammed, resulting in reduced outflow and mostly siltation of the downstream channel, limiting the use of their sedimentary deposits as paleoenvironmental analogues. The So Francisco River, which would have been influenced by the last Xingó dam, is an example of a channel that would have experienced significant geomorphological changes in the last 30 years but has received little attention. The current study investigated the morphology and sedimentology of bars in a stretch of the lower course near Propriá (SE) in order to fill this gap. Photointerpretation of topographic maps and satellite images, as well as sedimentary facies analysis, were used as methods. The results show a wider river channel under natural flow (1854-1959-60), with insignificant accumulations of mid-channel bars and secondary channels, and downstream additions and clogging of shortcut channels during the bus construction period (1954-1994). The stretch's morphology did not change significantly in the 18 years following Xingó. The highest growth rates were observed in the last decade, corroborating the hypothesis of siltation influence, but were more noticeable in the 2010s. The geomorphological-sedimentary model suggests an ambiguous interpretation for paleorivers record in which the same evidence would be consistent with floodplain environments.