Fluvial transport of sediment across a pristine tropical foreland basin: channel-flood plain interaction and episodic flood plain deposition

摘要

We present the results of study of the Beni River, a large, sand-bedded river which drains 68 000 km~2 of the northern Bolivian Andes into the Amazon River basin. The fieldwork included surveys of channel, bank, and flood plain topography, granulometry, stratigraphy, and system-wide flood plain accumulation rates measured with ~(210)Pb geochronology. These results were evaluated within the spatial and temporal framework of a GIS analysis of four decades of channel migration, derived from Landsat and aerial imagery. The resulting flux model of sediment interchange between the channel and flood plain agrees with a net foreland accumulation of ～100 Mt year~(-1), as independently determined using three decades of stream sampling and gauging records. The majority of this sediment is distributed throughout the distal flood plain, while channel-proximal and bed accumulation, lateral migration, and the formation and filling of oxbow lakes all play a relatively minor role. This predominantly distal flood plain accumulation occurs in remarkable episodic pulses of decadal recurrence interval, linked to major floods associated with cold phase ENSO events.