HYDROLOGICAL PROCESSES Hydrol. Process. 23, 3225–3229 (2009) Published online 24 July 2009 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/hyp.7391 Tectonic control of erosion and sedimentation in the Amazon Basin of Bolivia Patrice Baby, 1,2 * Jean Loup Guyot 1,2 and G´ erard H´ erail 1,3 1 LMTG, Universit´ e de Toulouse, CNRS, IRD, OMP, 14 Avenue Edouard Belin, 31400 Toulouse, France 2 IRD, CP 7091 Lago Sul, 71635-971 Brasilia DF, Brazil 3 IRD, Casilla 18-1209, Lima 18, Peru Abstract: The western Amazon drainage basin, which extends from southern Colombia to northern Bolivia, comprises the Cordillera Oriental of the Andes and its adjacent foreland basin system. In northern Bolivia, the orogenic wedge of the eastern Andes is very large, and its forward propagation controls the morphology of the Madeira drainage basin. We consider here the erosion and sedimentation mass balance in this part of the Amazon Basin, estimated on the basis of recent sediment yield data, within the current tectonic and geomorphic framework. The total suspended sediment (TSS) flux exported from the present orogenic wedge of northern Bolivia has been estimated at 500–600 million t year 1 . More than 50% of the total sediment load crossing the Madeira foreland basin system is deposited. The rest of the sediments (less than 46%) reaches the eastern Amazon Basin, bypassing the Brazilian craton to the north. The average mass of sediment that has been deposited from the late Miocene to the present in the Madeira foreland basin sedimentation system is less than that intercepted today, by a factor of about 2Ð4. These results can be interpreted as an increase in Bolivian foreland basin flexural subsidence over time, associated with crust thickening and orogenic loading, and accentuated by the growing mass of retained sediments. They are consistent with the uplift rates of the Cordillera Oriental, obtained from fission-track dating, which began increasing significantly around 10–15 Ma. Copyright  2009 John Wiley & Sons, Ltd. KEY WORDS Amazon; Andes; Bolivia; erosion; foreland basin system; hydrology; sedimentation; tectonics Accepted 1 October 2008 INTRODUCTION The western Amazon drainage basin extends from south- ern Colombia to northern Bolivia (Figure 1). It comprises the Cordillera Oriental of the Andes and its adjacent fore- land basin system. The amount of sediment generated within this part of the Amazon Basin is considerable, and can be estimated from available sediment yield data. Ero- sion of the Cordillera Oriental and sub-Andean foothills supplies over 99% of the sediment load passing through the Amazon Basin (Aalto et al., 1999), and a large pro- portion of this is retained in the foreland system, which can be divided into several sub-basins that are delim- ited by structures resulting from north–south variations in Andean deformation (Gil Rodriguez et al., 2001). The Bolivian Madeira drainage basin includes the southern- most of these sub-basins, which corresponds to a geo- morphic region commonly referred to as the Beni plain (Figures 1 and 2). Geological and hydrological studies, carried out as part of the programme of the Institut de Recherche pour le D´ eveloppement (IRD) in Bolivia, have been combined to investigate the tectonic processes driv- ing current sediment transfers in the Madeira foreland basin. Here, we estimate the erosion and sedimentation * Correspondence to: Patrice Baby, LMTG, IRD, 14 Avenue Edouard Belin, 31400 Toulouse, France. E-mail: Patrice.Baby@ird.fr mass balance using recent sediment yield data, and con- sider them in the context of the current Andean tectonic and geomorphic framework. TECTONIC AND GEOMORPHIC FRAMEWORK In Bolivia, the Andean Mountain belt (Figure 1) is very large, deformed by thin-skinned tectonics (Roeder and Chamberlain, 1995; Baby et al., 1997; Rochat et al., 1999), and characterized by an elbow shape (the Boli- vian Orocline). The retroarc fold-thrust belt is formed by the Cordillera Oriental, characterized by high relief (several summits over 6000 m), and the sub-Andean foothills (Figures 2 and 3). This orogenic wedge has sup- plied the adjacent foreland basin with sediment since the Upper Oligocene. In the northern part of the Boli- vian Orocline, the horizontal Neogene shortening of the retroarc fold-thrust belt is about 177 km. The present axis of the Bolivian Orocline separates the Amazon and Paraguay Basins, between the Pilcomayo and Madeira Basins (Figure 1). The Amazon River established a connection to the Atlantic during the late Miocene (Hoorn et al., 1995), when both Andean deformation and tectonic uplift increased. The Madeira drainage Basin forms the south- western boundary of the Amazon Basin (Figure 1). It is limited to the south-west by the Cordillera Oriental Copyright  2009 John Wiley & Sons, Ltd.