Multitemporal Landsat data applied for deciphering a megafan in northern Amazonia

Abstract

Abstract In this article, we present a protocol for the recognition of channels and waterbodies and for estimating their hydroperiod to both characterize a megafan system in northern Amazonia and discuss its dynamics in the late Quaternary. Our approach is based on analysis of a multitemporal Landsat Thematic Mapper/Enhanced Thematic Mapper Plus (TM/ETM+) data set consisting of 13 scenes acquired from 2000 to 2009. The images were primarily preprocessed using standard techniques for geometric and radiometric corrections. Following numerous tests with the most common water classifiers, we determined that a simple threshold on an infrared band, which provided a binary image containing both water and non-water classes, would adequately meet the goals of this article. The sum of all 13 classifications produced a new image, the hydroperiod product, with pixel values corresponding to the relative frequency of standing water. The results indicate that there is no permanent drainage network in the Viruá megafan, which displays only temporary channels with medium flood frequency and palaeochannels. These are arranged as complex distributary networks, as is typical of megafan depositional systems. The southern part of the megafan contains distributaries that are seasonally flooded, while the central and northern parts display an increased volume of palaeochannels. These data lead us to conclude that the locus of sediment deposition along this megafan system changed from north to south through time. Acknowledgements This work was sponsored by FAPESP-Research Funding Institute of the State of São Paulo (grants 2009/00672-3 and 2010/09484-2). Logistic support during fieldwork was kindly provided by CPRM-Brazilian Geological Survey and ICMBio-Chico Mendes Institute for Biodiversity Conservation. We thank the two anonymous reviewers for the several comments and suggestions that contributed in significantly improving the early version of this article.