Types de lits fluviaux en Amazonie brésilienne - article ; n°473 ; vol.86, pg 1-54

Jean L.F.Tricart

Description

Annales de Géographie - Année 1977 - Volume 86 - Numéro 473 - Pages 1-54
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.
vallée du Juruá offre des exemples particulièrement démonstratifs de tels méandres (voir figures).
A l'aval, les nappes alluviales plongent vers un niveau marin bien inférieur à l'actuel. La coïncidence dans le temps de la transgression flan drienne et de la phytostabilisation par la forêt a provoqué un déficit alluvial général, mais d'importance inégale suivant les cours d'eau et suivant les tronçons :
- Le déficit maximum est réalisé le long du cours inférieur des rivières locales, ne drainant que le Néogène. Elles se terminent par des rias fluviales, fonds de vallées inondés en lacs dont le niveau fluctue considérablement sous l'effet des variations de hauteur d'eau des grands fleuves (15-20 m sur le Solimóes). Aucun delta ne s'observe à la tête de ces rias fluviales ce qui témoigne de l'extrême faiblesse des apports solides. Ces cours d'eau, par manque d'éléments minéraux, ont des eaux extrêmement acides et des écosystèmes d'une extrême pauvreté (rios da fome).
- Un déficit un peu moindre apparaît sur des cours d'eau disposant d'une certaine charge solide mais installés dans des angles de faille, comme le bas Rio Negro.
- Sur l'Amazone, vers Obidos, une certaine charge solide est disponible grâce aux sapements des versants néogènes de la vallée par certains bras. Mais elle suffi à peine à compenser la rapide transgression flandrienne. Des levées d'altitude croissante s'observent le long des bras successifs de plus en plus récents. De vastes cuvettes marginales persistent entre le pied des versants et les levées relativement hautes bordant le cours actuel. Des deltas de débordement s'y édifient. Les eaux de l'Amazone contiennent des minéraux dissous allogènes. Leur pénétration dans les cuvettes lors de la montée des crues y permet le développement d'un phytoplancton, producteur primaire, qui, avec les prairies flottantes, rend possible le développement de biocénoses aquatiques plus variées et formant une biomasse plus importante. Les images du satellite ERTS-1 ont permis d'enregistrer le phénomène.
- Enfin, les cours d'eau suffisamment chargés, comme le Japurá et le Solimòes ont pu édifier, pendant la transgression, des plaines alluviales caractérisées par des systèmes de levées très importants, généralement méandres, parfaitement visibles sur les mosaïques de radar.
54 pages
Source : Persée ; Ministère de la jeunesse, de l’éducation nationale et de la recherche, Direction de l’enseignement supérieur, Sous-direction des bibliothèques et de la documentation.

Sujets

• Sciences et techniques

Informations

• Publié par: ANNALES_DE_GEOGRAPHIE
• Publié le: 01 janvier 1977
• Nombre de lectures: 62
• Langue: Français
• Poids de l'ouvrage: 9 Mo

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