Abstract:
The confluence of the Negro and Solimões rivers to originate the Amazonas 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 Chão 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 Solimões 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 Amazonas due to erosion or deposition. The so called Ponta do Catalão, 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 midchannel bars were formed in the river 15 km up from Ponta do Catalão.
Keywords:
Meeting of the Waters; Amazonas River; Solimões River; Negro River