Antecedent Topography and Sediment Dispersal: The Influence of Geologically Instantaneous Events on Basin Fill Patterns

Favorable topographic gradients and channel bed aggradation are often cited as primers for river channel avulsion. Sylhet Basin in Bangladesh subsides relatively rapidly and presents a topographically favorable channel course for the Brahmaputra River. Holocene occupations of this region remained routed along the basin's western margin, depositing tens of meters of amalgamated channel sands but leaving the central basin under-filled. The localized backwater effect from a seasonal lake that forms in Sylhet Basin, referred to as a "hydraulic barrier," has been proposed as a mechanism for keeping the channel pinned to the western margin. Using a 1-D channel profile model and a 2-D depth-averaged hydrodynamic model, we test the preferred flow path between two possible channel courses under a range of physical conditions. Results show that the hydraulic barrier interpretation is implausible unless water depths are increased beyond the physical dimensions of Sylhet Basin. Reducing topographic slope along the two pathways does not sufficiently enhance the local backwater to steer the channel to the basin center. Only the introduction of a scoured antecedent channel along the western margin induces a strong preference for bypass of the central basin. These results corroborate field evidence that Holocene outburst floods scoured a paleochannel that became the preferred course for the Brahmaputra within Sylhet Basin in the mid to late Holocene. We conclude that modification of basin topography by large, infrequent flood events can exert a persistent influence on channel steering that is comparable to previously documented effects of monsoon variability and tectonic deformation.