Abstract

Rivers in western Brazil and northeastern Argentina are used to illustrate the complex adjustment of fluvial patterns and sediments to the shifting arid-humid climatic regimens of the tropics and subtropics. Many of these rivers show pronounced deviations from the sediment size-sinuosity relationships that were developed by studies of rivers in temperate semi-arid and humid environments. In the Amazon Basin, a broad range of sinuosity (P) and floodplain depositional features occurs in rivers that lack appreciable bedload. Rivers draining the tropical lowlands, e.g. the Jurua River, have very regular streamflow, nearly all solution and suspended load, numerous meander cutoffs, poorly developed point bars and scroll topography, and single channels of high sinuosity (P = 1.9–3.0). Major trunk rivers draining Andean source terrains, e.g. the Solimoes River, have pronounced anastomosing patterns of lower sinuosity (P = 1.2–1.5), prominent scroll topography on flood plains, and high suspended load. These differences result from the relative abilities of different rivers to rework coarse, relict alluvium that was deposited during the relatively arid full-glacial phases of the Pleistocene.

Rivers of the Gran Chaco of northeastern Argentina display a detailed record of altered hydrologic regimen during the Quaternary. Many streams show a late Quaternary reduction of channel-forming discharge that always produced greatly reduced channel widths but only occasionally produced reduced meander wavelengths. However, not all these changes need be attributed to temporal climatic change. Major changes in both sediment load and discharge can be achieved in this piedmont environment through stream captures that delete or add the Andean source terrain to a given river’s drainage basin.

Hydrologic calculations show that empirical formulae derived from studies of humid-temperate and semi-arid streams do not necessarily explain the regimen behavior of these complex tropical rivers. The low-sinuosity Amazon Basin streams are much less dominated by relatively coarse-grained sediment than are streams of equivalent sinuosity described for semi-arid regions by S. A. Schumm. Discharge characteristics and bank stability (related to both vegetation and lithology) appear to play the dominant role in accounting for channel pattern variations in the Amazon Basin.