A Propósito de meandros

Resumo

The complex pattern formed by the sinusitis, branchings and tangencies of natural levels, built-up along stream banks in alluvial valleys, partitions the floodplain into a number of shallow basins, occupied by drainage branch works, lakes or swamps. It is suggested that the predominantly circular or elliptical depressions, characteristic of the Pantanal, Mato Grosso, might have a similar origin. True, these so-called "baías", encompassed by sandy swells which stand above the flood crests and not infrequently occupied by salt lakes  apparently do not occur in the Hood plain proper and have been noted almost exclusively in the higher tracts of the Pantanal. In this r8spect, attention is direct to conditions obtaining in the youngest deposition terrace of the Mississippi Alluvial Valley, where old meander patterns, levees and back swamps have been preserved on the broad uplifted plain and reflect aggradational processes similar to those prevalent on its modern counterpart, the present floodplain.

            Certain patterns observable in the younger sedimentary deposits of the Pantanal indicate that ancient fractures may have remained active during Recent times; even" minor adjustments of the blocks could perhaps, simultaneously, explain the aspect of alluvial drowning found in some and (if their presence should be established) the existence of features derived from floodplain morphology on tracts now rising above the present high water stage. The supposition put forward should be investigates in the light of a series of well-placed boring and the granulometric analysis of the ensuing cores.

            Discussion of the factors involved in the formation of abnormally large or small individual meander loops leads to the proposition the cutting-off a meander does not merely depend on a ripening, as it were, of the loop itself, but rather on the development of the adjacent meander loops. Underdevelopment of the upstream and downstream loops (which can be brought about by the presence of more resistant materials) favors over development of the intermediate loop. An excessive enlargement of adjoining loops, on the contrary, leads to a premature cutting through the intervening meander neck. Major departures from ideal, smoothly rounded, meander loops occur when extraneous causes inhibit free meandering. The deformities of False River channel, in the Mississippi Valley, are mentioned in connection with the inhibiting effects on meandering of the compact plugs of fine-grained materials, which, eventually, fill the central and deepest part of many ox-bow lakes.

            It is a known fact that flanking 'clay plugs" tend to confine the stream to the meander belt, but, when the river escapes the barriers, hinder re-entry to the old course, which tends to be definitely abandoned. Such appears to be the case of a deteriorated channel which extends along the Brazilian-Bolivian border; on the basis of its meander pattern, one is led to suspect that this channel was formerly occupied by waters now flowing some tens of miles to the east.