Mega capture of the Rio Negro and formation of the Anavilhanas Archipelago, Central Amazônia, Brazil: Evidences in an SRTM digital elevation model Raimundo Almeida-Filho a, ⁎ , Fernando P. Miranda b a National Institute for Space Research-INPE, C. P. 515, São José dos Campos-SP, Brazil b Petrobras Reseach Center-CENPES, Ilha do Fundão, Q-7, Rio de Janeiro-RJ, Brazil Received 11 December 2006; received in revised form 12 March 2007; accepted 17 March 2007 Abstract In the Central Amazônia (Brazil), the Solimões River and the Negro River (“Rio Negro”) merge to form the Amazonas, the largest river in the world. Analysis of a digital elevation model produced by the Shuttle Radar Topographic Mission (SRTM) in that region reveals the relicts of a large ancient drainage system hidden by the tropical rain forest. Interpretation of this unreported terrain feature in the context of the regional geological framework indicates that the present lower course of the Rio Negro results from a mega fluvial capture driven by an active tectonic regime prevailing in the Amazonian basin. The diversion of the Rio Negro was controlled by pre-existing NW–SE structural features, in which the arrangement of fault blocks compelled the river to funnel down into a strait. Partially blocked by this natural barrier, the Rio Negro widens upstream, assuming a lake-like appearance (up to 20 km-wide). Free space availability and low energy favored the deposition of sediments to form the Anavilhanas Archipelago, the largest group of freshwater islands on the world, refuge of a diversified fauna and flora. © 2007 Elsevier Inc. All rights reserved. 1. Introduction Digital elevation models produced from interferometric synthetic aperture radar (InSAR) data during the Shuttle Radar Topographic Mission (SRTM) (Rabus et al., 2003) opened unprecedented opportunities for regional terrain analysis. By highlighting subtle surface features, this new class of space- borne data is of particular interest for studies in densely vegetated tropical areas, where details of the terrain are mostly blurred in currently used remote sensing images, and where no topographic maps are available. In regions of tropical rain forest like the study area, without soil exposures, the response of the radar at the C-band frequency (5.6 cm) results from the interaction of the radar signal with components of the vegetation canopy (trunks, branches, twigs and leaves). Thus, the inter- ferometric digital elevation models obtained by the SRTM in such areas correspond to the mean height of the vegetation canopy, which at a regional scale follows terrain features below. This article discusses the use of an SRTM digital elevation model in the region of the confluence of the Solimões River and Negro River (“Rio Negro”), Central Amazônia, Brazil. In this region, SRTM data revealed terrain features that are not discernible in L-band synthetic aperture radar images of the Japanese Earth Resources Satellite (JERS-1) and only barely suggested in the infrared bands of the Landsat Thematic Mapper (TM). Analysis of the SRTM data, complemented with previous geological information and field observations, indicates that the study area has experienced remarkable landscape changes, as a result of neotectonics. 2. Study area and geological setting Covered by tropical rain forest, the study area (Fig. 1) is located in the heart of the Brazilian Amazônia, where the Solimões and Negro rivers converge to form the Amazonas, the largest river in the world. At the confluence, the muddy, tan- colored waters of the Solimões and the dark waters of the Rio Negro remain unmixed for several kilometers downstream, constituting a famous attraction for tourists from all over the Remote Sensing of Environment 110 (2007) 387 – 392 www.elsevier.com/locate/rse ⁎ Corresponding author. E-mail address: rai@dsr.inpe.br (R. Almeida-Filho). 0034-4257/$ - see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.rse.2007.03.005