Antecedent Topography and Sediment Dispersal: The Influence of Geologically Instantaneous Events on Basin Fill Patterns. Issue 6 (23rd June 2022)

• Record Type: Journal Article
• Authors: Sincavage, Ryan; Liang, Man; Pickering, Jennifer; Goodbred, Steven; Passalacqua, Paola
• Is Part Of: Journal of geophysical research. Volume 127:Issue 6(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-06-23
• Subjects: Geomorphology -- Periodicals; 551.3
• DOI: 10.1029/2021JF006539
• Languages: English
• ISSNs: 2169-9003

Abstract

Abstract: 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. Plain Language Summary: River channels often steer toward sinking or subsiding areas due to the formation of steepened land surface gradients. The Brahmaputra and Ganges Rivers create the world's largest delta in the Bengal Basin, a region with tectonic deformation capable of steering the main river channels. During the mid‐Holocene, the Brahmaputra River occupied a course flowing to Sylhet Basin, an area of the delta with relatively rapid subsidence, yet the channel was not steered into the central basin despite its steeper gradient. We use numerical models to test whether a persistent lake in the basin or different topographic slopes could prevent the river from migrating into the center of the sinking basin. The model results suggest that only the presence of a pre‐existing scoured channel could prevent the river from steering into the low‐lying center of the basin. Field data reveal that large outburst floods in the early Holocene created such topographic channel scours and are proposed here as a mechanism that has influenced the channel pathway for thousands of years. We therefore conclude that large but infrequent flood events can have long‐term and far‐reaching effects on the river path selection and distribution of sediments in large river deltas. Key Points: Based on stratigraphic evidence, the Brahmaputra River did not follow the steepest path of descent during mid‐Holocene avulsions Numerical modeling indicates neither a local backwater nor a lower (pre‐subsidence) cross‐stream gradient are sufficient to exclude flow Scours from geologically instantaneous events can control millennial‐scale sediment dispersal patterns