Dinâmica geomorfológica do complexo fluvial de Mariuá, rio Negro, bacia Amazônica

Abstract

The Mariuá Complex is one of the greatest geomorphological wonders of the world, it is located in the middle Negro River, in the Amazon basin, with an area of 4,000 km², approximately 400 km from the confluence with the Solimões River. It is considered the largest river archipelago in the world with an intricate anabranching pattern and river shapes ranging from islands, lakes, floodplain and river terrace. This object of this study is to understand the fluvial geomorphology of the river complex of Mariuá. For this, parameters of fluvial dynamics such as the water level regime, water discharge, flow velocity, suspended sediments and variation of the slope of the water mirror were analyzed. For geomorphological analysis, mapping of the landscape was carried out in the periods of low and high water at a scale of 1:800,000 and, segmenting the archipelago into five reach, 1:185,000, with this the geomorphological units were analyzed (sand bars, islands, lakes, internal plain, floodplain and river terrace), to support the analysis, the channel sinuosity index, the anabranching index were calculated and 9 bathymetric profiles were performed in different river environments. Finally, 30 sediment samples were collected at 16 points of analysis (sand bars, islands, internal plain and floodplain), granulometric analysis was performed using the pipette method. The dynamics of the quota regime in the complex presents a peak of flood between the months of June and July, while the peak of ebb in Serrinha (upstream) and Barcelos (inside) occurs between December-January and in Moura (downstream) in November. The average annual water discharge upstream of the complex (Serrinha) is 16,671 m³ s-1, while the low flow velocity and slope of the water surface indicate a low predisposition to erosion and sedimentation of fine material in the complex. It is possible to notice a great variation in the fluvial forms between the two hydrological extremes, with the islands losing a large part of their area and the lakes significantly increasing the area during the high-water period, while in the ebb one can map sand bars with extension greater than 3 km. The archipelago channels, analyzing the width-depth relationship, are deep and wide, and the complexity of the archipelago is high, especially in the last reach, where the location of the main channel becomes difficult due to the number of islands, width of the valley and main channel branches. The islands and the complex, in the Flemming diagram, are closer to the silty end, indicating that the fine material is the most present in the granulometric composition of these shapes. Some islands have a higher percentage of sandy material, this may indicate more recent stabilization, indicating that the islands of the complex are still in the process of expansion. The sand bars present predominantly fine sand in their composition, in addition to this, pebble is noted as a bottom material, it is hypothesized that currently the hydrodynamics of the Negro River does not have the capacity to transport this type of material. Two processes of sedimentological evolution of the islands of Mariuá stand out. The first is sidebar stabilization, which can join two different islands or increase the area of a pre-established island; and the second is the stabilization of bars downstream of the islands.