Abstract
The tidal to fluvial transition (TFT) of estuaries and coastal rivers is one of the most complex environments on Earth with respect to the transportation and deposition of sediment, owing in large part to competing fluvial and marine processes. While there have been recent advances in the stratigraphic understanding of the TFT, it is still unclear whether these findings are site-specific or representative of mixed tidal-fluvial systems worldwide. Yet, research from this depositional domain holds profound societal and economic importance. For instance, understanding the underlying stratigraphic architecture of channel margins is critical for assessing geomorphic change for fluvio-deltaic settings, which are generally vulnerable to lateral channel migration and resultant erosion. Findings would also benefit paleo-geographic reconstructions of ancient tide-influenced successions and provide an analog for hydrocarbon reservoir models. In the Ganges-Brahmaputra Delta of Bangladesh, the Gorai River is one of two Ganges distributaries actively connected to the Bay of Bengal. With fluvial input from the Ganges and meso-scale (2–4 m range) tides at the coast, the Gorai exhibits a variety of hydrodynamic regimes across its 350-km reach, providing a unique opportunity to investigate along-channel depositional patterns across the TFT. This study integrates multiple datasets—core sedimentology, river channel bathymetry, and remote sensing—to provide a process-based framework for determining the relative position of sedimentary deposits within the tidal-fluvial continuum of the Gorai River. The results of this investigation reveal coincident, abrupt shifts in river channel morphology and sediment character, suggesting the occurrence of backwater-induced mass extraction of relatively coarse sediments (i.e., fine sand). Despite being situated in an energetic tidal environment, evidence of tidal cyclicity in cored sediments is relatively rare, and the bulk stratigraphy appears strongly overprinted by irregularly spaced cm- to dm-scale sediment packages, likely derived from monsoonal flood pulses. Such findings differ from previously-studied mixed tidal-fluvial systems and underscore the site-specific complexities associated with this depositional domain.
Highlights
Rivers, estuaries, and other transitional bodies of water are subjected to varying degrees of terrestrial and marine influences on their journey from headwater source to open ocean, resulting in a complex spectrum of hydrodynamic and depositional conditions (e.g., Ref. [1])
Shuttle Radar Topography Mission (SRTM) elevation data have a spatial resolution of 1 south-southeast for ~350 km through Quaternary alluvial and tidal delta plain sediments before arc-second (~30 m) and were downloaded from the open source USGS EarthExplorer database emptying into the Bay of Bengal (Figure 1A).ofInthe theDEM
We postulate that similar hydrodynamic conditions lead to the inconsistent down-core grain size trends observed in the present study (Figure 5), especially in light of recent work demonstrating that the Ganges-Brahmaputra tidal delta plain is characterized by a tidal prism that is both seasonally and directionally asymmetric [69]
Summary
Estuaries, and other transitional bodies of water are subjected to varying degrees of terrestrial and marine influences on their journey from headwater source to open ocean, resulting in a complex spectrum of hydrodynamic and depositional conditions (e.g., Ref. [1]). Estuaries, and other transitional bodies of water are subjected to varying degrees of terrestrial and marine influences on their journey from headwater source to open ocean, resulting in a complex spectrum of hydrodynamic and depositional conditions River currents, those produced during flood events, represent the dominant hydrodynamic control on sediment transport and deposition Water 2019, 11, 2047 their currents are tempered by waning hydraulic gradients and the increasing influence of marine processes, namely waves and tides. Rivers connected to active tidal coasts are characterized by reaches of tidal dominance, mixed influence, and fluvial dominance, together referred to in the literature as the tidal to fluvial transition zone Rivers connected to active tidal coasts are characterized by reaches of tidal dominance, mixed influence, and fluvial dominance, together referred to in the literature as the tidal to fluvial transition zone (TFT, e.g., Ref. [10])
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