Types de lits fluviaux en Amazonie brésilienne
Authors: Tricart Jean L. F. 
Published: October 2008

Abstract. The types of valley bottoms are relation with the prevailing Neogenous detritai deposits accumulated in two subsiding areas : the Amazon rift-valley, below Manaus, and the Andine miogeosyncline in the west; with recent tectonic movements, especially the play of faulted blocks (Solimòes above Coari, lower Rio Negro) ; with joint fluctuations of the climate and of the sea-level during the middle and recent Quaternary. The weak resistance of the Neogenous enabled head erosion to go far upstream, up to the vicinity of Porto Velho on the Madeira, and of the confluence of the Japurà on the Solimòes. It fell in with dry tropical climate, which made the rain-forest be replaced by the savanna, which favours water running. The Flandrian transgression concurred with the reconquest by the forest and so with some phytostabilization. On local rivers alluvial sheets have developped during the dry periods and have been incised ups tream during the phytostabilization. At Tarauacá, the R. Tarauacá and the R. Muru show two such sheets, the tops of which are almost at the same level. They are boxed into one another and presently partly floodable.

The incision of these alluvial sheets in valley-bottoms often made by very unstable meandering beds, with numerous abandoned meanders and cut-offs, slowly clogged by the vegetation which can root under one or two metres of water. The evolution of the meanders is speeded up by the proportion of silt and clay in the alluvial sheet of the valley-bottom : effectively, only the sands are left at short distance on convex banks while the thinner specks are brought along far in suspension. It results that the re treat of the concave banks of the meanders is more quick than the deposits on the corresponding convex banks, hence hernia-like broadenings which allow cut-offs. The attack of banks is partly due to the undermining of the rising flood - classic mechanism -, but also to the spluttering of watersaturated banks during the river-subsidence and under the effect of violent whirls due to the fall of undermined trees. The only superficial rooting of the rain-forest, due to the scarcity of assimilable minerals, favours this process. The Juruá valley gives peculiarly explicite examples of such meanders (see figures).

Downstream, the alluvial sheets plunge down to sea-level well below the present one. The coincidence in time of the Flandrian transgression and of the phytostabilization by the forest caused general alluvial shortage which is however inequal according to the rivers and to the sections :

- The biggest shortage is seen along the lower local rivers which drain only the Neogenous. They end with the so-called "rias fluviales", i.e. lake-like flooded valley-bottoms, the level of which changes considerably with the variations of the water-level of the big rivers (15 to 20 metres on the Solimòes). No delta can be seen at the head of these "rias fluviales", which shows the extreme weakness of solid deposits. These rivers, for lack of minerals, have extremely acid waters and very poor ecosystems ("rios da fome").

- A less important shortage appears on rivers displaying certain solid load, but which are located in fault-angles, like the lower Rio Negro.

- On the Amazon, near Obidos, certain solid load is available thanks to the undermining of Neogenous valley-sides by some arms. But it was hardly sufficient to compensate the rapid Flandrian transgression. Dikes at increasing heights can be seen along the more and more recent arms. Large side basins remain between the feet of the slopes and the rela tively high dikes which run along the present bed. Deltas due to overflowing develop. The Amazon waters contain allogenic dissolved minerals. Their penetration into the basins during the rise of floods allows in them the growth of phytoplankton, primary producer which, with the floating meadows, enables the development of more varied aquatic ecosystems which form more important biomass. The pictures taken from the satelli te ERTS 1 enabled to record this phenomenon.

- At last, the enough loaded rivers, like the Japurá and the Solimòes could build during the transgression some alluvial plains characterized by very important systems of dikes, usually with meanders, one can perfectly see on radar mosaics.