Características morfológicas da confluência dos rios Negro e Solimões (Amazonas, Brasil)

The confluence of the Negro and Solimoes rivers which originates the Amazon River is located in the transcurrent neotectonic belt which extends from east to west in the central part of the Amazon Basin. The last reach of the Negro River is defined by neotectonics, and it is carved in the Cretaceous rock of the Alter do Chao Formation which forms red cliffs on its north side that continue on the side of the Amazon after the confluence. The Negro River at the confluence is 3 km wide and 90 m deep. At this point the Solimoes is 2 km wide and 35 m deep. The junction angle is also defined by neotectonics. The confluence flow separation bar, called Xiborena Island, is formed by long, curved banks of modern sediments. An analysis of nautical charts from three decades 1978, 1988, 1998 and bathymetrical cross sections obtained from these charts permitted an examination of the morphology and an evaluation of the modification that have occurred in this period on the bed of the Amazon due to erosion or deposition. The so called Ponta do Catalao, that is, the outer bar of Xiborena Island increased in surface area and changed in shape. The scour hole at the confluence changed in place and in depth from 34 to 43 m, some long bars were formed by lateral accretion on the south side of the Amazon and some post-confluence mid-channel bars were formed in the river 15 km up from Ponta do Catalao.