Hydrological and Geochemical Dynamics of the Mixing Water Zone of the Rios Solimões and Negro in the Amazon Basin

In the framework of the ANEEL-CNPq-IRD-UnB joint project, the HiBAM program carried out a systematic sampling survey of the Solimões/Negro mixing water zone near Manaus in September 1997 using an Acoustic Doppler Current Profiler (ADCP). The waters of the Negro river differ from those of the Solimões river with respectively lower speed (0.3 and 1 m.s^–1), conductivity (8 and 80 μS.cm^–1 at 25°C), turbidity (5 and 80 NTU), and pH (5.5 and 7.0), and higher temperatures (1°C). Because of their higher density, the waters of the Solimões River slide under those of the Negro. The contact line between the waters changes from regular and vertical to horizontal and diffuse as they move downstream. After 4 hours and 12 kilometers, the waters of the Rio Negro have mixed with those of the Solimões, albeit incompletely. Complete mixing takes more than 30 hours and 100 kilometers, with a decreasing gradient from a ‘Negro' pool on the surface of the left bank, toward a ‘Solimões' pool at the bottom of the right bank.