A study of sediment transport in the Madeira River, Brazil, using MODIS remote-sensing images

Abstract

The Madeira River may contribute nearly half of the Amazon River sediment discharge to the Atlantic Ocean, showing the highest erosion rates in the Amazon Basin. However, few studies have assessed the Madeira River sediment budget and the transport processes occurring in the main stem of the river. In this study, MODIS space-borne sensors were used to analyze the suspended sediment transport processes along the main stem of the Madeira River. Field measurements of suspended sediment concentration, spectral radiometry and granulometry were performed during 10 cruises from 2007 to 2011. The relationship between the spectral reflectance and the surface suspended sediment concentration (SSSC) was analyzed using both field radiometric measurements and satellite data. Ten-day SSSC samples acquired by the HYBAM monitoring network were used to match satellite observations with field measurements performed from 2000 to 2011. Over 900 MODIS images of 6 different locations were processed to monitor the SSSC dynamics in space and time. Satellite reflectance was found to be significantly correlated with the SSSC. However, a seasonal dependency was demonstrated, most likely caused by a variable granulometric distribution along the annual cycle. The ratio between the red and near-infrared bands was found to be free of the seasonal dependency (r = 0.79, N = 282), and a SSSC retrieval model was built from the satellite data using a bootstrap resampling technique. The satellite-retrieved SSSC time series showed excellent accuracy over the 11-year period and at two different stations located 800 km from each other. The satellite data were averaged to analyze the SSSC pattern temporally and spatially along the entire Madeira River, which provided evidence of significant sedimentation and resuspension. The backwater effect caused by the two-to-three-month lag between peak water in the Madeira and Amazon Rivers was used to predict local sedimentation near the Madeira River mouth. Our results facilitated a precise assessment of such sedimentation, which demonstrated an SSSC decrease 400 km upstream from the Madeira–Amazon confluence.