Channel changes in meandering rivers naturally exhibit complex behaviour, and understanding the river dynamics can be challenging in environments also subject to cumulative human impacts. Planform changes were analysed on four reaches of the lower course of the Peixe River, Brazil, at decadal scales over the period 1962-2008 from aerial photographs and satellite imagery, complemented by a historical map from 1907. Analysis of the spatial and temporal patterns of channel change mechanisms and morphometry of bends and of the sinuosity and morphodynamic variations of the reaches demonstrates major changes in planform characteristics. Sinuosity in all reaches decreased from~2.6 to~1.7, average wavelength of bends has increased from~200 to~500 m, and the planform has become much simpler. Changes have been progressive from downstream to upstream, with higher intensities of processes, particularly cutoffs first in downstream reaches then more recently in upstream reaches. It is suggested that channel changes represent a morphological adjustment to human interventions, such as reservoir construction and land use. However, evidence of the autogenic behaviour of meanders is highlighted in which the existence of compound meanders reveals control over the spatial variation in the reaches. The results suggest that geomorphic thresholds associated with the compound meander formation and the bend evolution should be considered, even in impacted meandering rivers, because they exert primary controls on the spatial-temporal adjustment of channels.
Resumo:Este estudo teve como objetivo identifi car e caracterizar as formas e os processos fl uviais dominantes associados ao padrão de canal meandrante no vale aluvial do baixo curso do rio do Peixe, estado de São Paulo. As interpretações geomorfológicas foram baseadas em imagens orbitais, fotografi as de sobrevoo, dados hidrológicos e levantamentos em campo. No vale aluvial do baixo curso do rio do Peixe foram caracterizadas unidades de terraços, planície de inundação e canais, ordenados como 1º táxon. Nessas unidades encontram-se sub-unidades denominadas de unidades geomórfi cas, como por exemplo, na planície de inundação ocorrem paleocanais, lagos em ferradura e bacias de inundação, estas últimas, portanto ordenadas como 2 º táxon. Também foram descritos como de 2 º táxon a faixa de meandros, leque de espraiamento e leque aluvial. Na unidade do canal destacamse as barras fl uviais, principalmente, as barras em pontal, características dos rios meandrantes, além de barras centrais e laterais. Essas informações integradas em uma perspectiva hierárquica permitiram associar algumas morfologias como indicadoras de estágios evolutivos desse sistema fl uvial. A dinâmica fl uvial do padrão meandrante do rio do Peixe construiu formas, resultando em um mosaico de áreas úmidas de distinta morfogênese. O estudo demonstra que tanto fatores naturais como antrópicos exercem infl uência nas morfologias do vale aluvial do baixo curso do rio do Peixe.
Abstract:The aim of this study is to identify and characterize forms and processes associated to the meandering channel pattern in the alluvial valley of the Lower Peixe River, São Paulo State, Brazil. Geomorphological interpretations were based on orbital images, overfl ight photos, hydrological data and fi eld works. In the alluvial valley
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