RIVER TYPES, SITE EVOLUTION AND SUCCESSIONAL VEGETATION PATTERNS IN PERUVIAN AMAZONIA

Lateral migration of Amazonian lowland rivers causes a vegetation succession on recently deposited fluvial sediments. The forest ground is formed gradually in sequence, giving rise to age-zonation of the successional stages. Using Landsat MSS maps and field observations, we studied variations in fluvial landform evolution and river character. The widely used characterizations of the rivers according to their suspension-load character and channel pattern were evaluated in relation to the vegetation succession. Sequential successional forests appear extensively along meandering white-water rivers, which are rich in suspended sediments and are characterized by mobile channels. Conversely, vegetation zonation is less pronounced at the margins of slowly eroding suspension-poor rivers. Line transects were established to document meander development along eight different rivers. The chemical composition of the recently deposited alluvium differs markedly both among rivers and along the transects. Concentrations of organic carbon and nitrogen increase toward the meander neck along with the acid reaction of the soil. The rate of riverine forest regeneration in Peruvian lowland Amazonia was extrapolated on the basis of erosion data from four different rivers. and suggests that approximately 130 km2 of forest is annually eroded and replaced by successional vegetation, the equivalent of 0.2% of the present floodplain area of this region.