Geomorphic form and process of sediment flux within an active orogen: Denudation of the Bolivian Andes and sediment conveyance across the Beni Foreland

Mountains produce most of the sediment conveyed within continental river systems, and their associated foreland basins trap much of the material debouched from the range front. Modern erosion rates in the Bolivian Andes are correlated with hillslope angle and basin lithology, but not associated with basin runoff or precipitation. An empirical model was developed to estimate sediment production rates throughout the Amazonian Andes. When the predicted sediment efflux of ∼3.1 Gtonnes/yr was compared to records from the upper mainstem Amazon River, it appears that approximately half of the debouched material is intercepted by the foreland basins along the Andean range front.; To investigate sediment conveyance across a pristine foreland basin, the processes of sediment exchange and deposition were documented for the Beni River, northern Bolivia. Water and sediment discharge records gauged at the inlet and outlet of the Beni Foreland provided a hydrological framework for flux analysis. Channel and floodplain topography and granulometry were surveyed throughout the Beni Foreland, including transects into the forested floodplains. Sediment cores taken along these transects were dated to determine accumulation rates, utilizing a new method for 210Pb geochronology, developed here for floodplain environments to resolve episodic sedimentation events of heterogeneous grain size. These field data were combined with a GIS analysis of four decades of channel morphology and migration. A model for meso-scale fluvial geomorphology was developed and compared to the GIS results.; The flux analysis indicated that sediment is mostly trapped in the Beni Foredeep, where it is conveyed overbank during exceptional floods and deposited across the broad floodplain. A smaller amount of sediment is lost in the secondary back-bulge basin, and there is no significant trapping of sediment within the forebulge. The annual exchange of sediment due to channel migration is considerably larger than the total sediment efflux from the Beni Foreland. The geochronology and stratigraphy of floodplain deposits indicate episodic accumulation, probably due to crevasse splays produced during major floods associated with cold-phase ENSO events. In responding to such climate perturbations, mountains and foreland basins dominate intracratonal mass fluxes and modulate sediment supply to lower portions of the Amazon River.