The Holocene alluvial plain of the middle Amazon River, Brazil

The fluvial belt of the Amazon River is a complex of Quaternary sedimentary units of different ages and formation conditions. Present position of the channel and the morphology and size of the alluvial plain are related to neo-tectonic lineaments. In the study area the main channel of the Solimُes is nearly straight. The river may be considered anastomosed, albeit with few islands and anabranches. The main Holocene units are the impeded floodplain and the channel-dominated floodplain. The channel-dominated floodplain is a complex mosaic of fluvial forms, including younger channels, active sandbars, islands, levees, scroll-dominated plains, and abandoned belts. The impeded floodplain is a widespread unit characterised by a very flat surface and round or irregularly shaped lakes. The impeded drainage flood plain is only partially affected by the present floods, and even those parts affected by floods receive little or no sediment from the main channel. More important sediment contribution is by alluvial plain channels from the Solimُes branches that generate deltas in the impeded floodplain lakes. The impeded floodplain is interpreted to be an earlier stage in the evolution of the Holocene alluvial belt. The formation was marked by widespread vertical accretion and a channel morphogenetically less active than at present. The later stage is associated with an active main channel and the paranلs, and generation of a scroll-dominated plain. ery large river system had a complex and delayed response to variations in the hydrological conditions induced by changes in the regional climate during the Late Holocene.