The sedimentology of river confluences

Abstract

Channel confluences are key nodes within large river networks, and yet surprisingly little is known about their spatial and temporal evolution. Moreover, because confluences are associated with vertical scour that typically extends to several times the mean channel depth, the deposits associated with such scours should have a high preservation potential within the rock record. Paradoxically, such scours are rarely observed, and their preservation and sedimentological interpretation are poorly understood. The present study details results from a physically‐based morphodynamic model that is applied to simulate the evolution and alluvial architecture of large river junctions. Boundary conditions within the model were defined to approximate the junction of the Ganges and Jamuna rivers, Bangladesh, with the model output being supplemented by geophysical datasets collected at this junction. The numerical simulations reveal several distinct styles of sedimentary fill that are related to the morphodynamic behaviour of bars, confluence scour downstream of braid bars, bend scour and major junction scour. Comparison with existing, largely qualitative, conceptual models reveals that none of these can be applied simply, although elements of each are evident in the deposits generated by the numerical simulation and observed in the geophysical data. The characteristics of the simulated scour deposits are found to vary according to the degree of reworking caused by channel migration, a factor not considered adequately in current conceptual models of confluence sedimentology. The alluvial architecture of major junction scours is thus characterized by the prevalence of erosion surfaces in conjunction with the thickest depositional sets. Confluence scour downstream of braid bar and bend scour sites may preserve some large individual sets, but these locations are typically characterized by lower average set thickness compared to major junction scour and by a lack of large‐scale erosional surfaces. Areas of deposition not related to any of the specific scour types highlighted above record the thinnest depositional sets. This variety in the alluvial architecture of scours may go some way towards explaining the paradox of ancient junction scours, that while abundant large scours are likely in the rock record, they have been reported rarely. The present results outline the likely range of confluence sedimentology and will serve as a new tool for recognizing and interpreting these deposits in the ancient fluvial record.

An Integrated Approach for Constraining Depositional Zones in a Tide-Influenced River: Insights from the Gorai River, Southwest Bangladesh

“…Briefly, the average channel depth of each transect was calculated, and then a 10 km running average was applied to the trend to neutralize the effects of localized bathymetric irregularities, such as bed scouring from channel confluences (e.g., Ref. [57]). Bathymetric data were not corrected for water level changes induced by tides or river discharge.…”

“…Localized geomorphological elements, such as channel confluences, can also impact hydraulic conditions and resultant channel margin sedimentology (e.g., Refs. [57,70]). Both of the tidal point bars investigated in this study (DAC and BTG) are situated adjacent to major tidal channel confluences ( Figure 1B), which may generate irregular hydrodynamic conditions (e.g., eddying and rapid flow reversals [71]) and stratal architecture.…”

Large‐Scale Particle Image Velocimetry Reveals Pulsing of Incoming Flow at a Stream Confluence

“…Confluences form where individual rivers within branching networks join one another. As runoff from watersheds is conveyed through river networks, the convergence of separate incoming flows within confluences produces complex hydrodynamic conditions that affect sediment transport (Best, 1987; Martín Víde et al., 2015; Rhoads & Johnson, 2018; Rhoads, 1996; Yuan et al., 2017), channel morphodynamics (Best, 1987; Rhoads & Kenworthy, 1995; Rhoads et al., 2009), sedimentology (Best & Rhoads, 2008; Biron et al., 1993; Sambrook Smith et al., 2019) and mixing (Biron et al., 2004; Chen et al., 2017; Lewis & Rhoads, 2015; Lewis et al., 2020). The hydrodynamics of confluences consist of highly complex spatial patterns of flow (De Serres et al., 1999; Rhoads & Kenworthy, 1995, 1998; Rhoads & Sukhodolov, 2001, 2004, 2008).…”

Coupled Morphodynamics of River Bifurcations and Confluences

“…This condition is typically fulfilled in anastomosing rivers, where the individual channels, often separated by vegetated islands, are usually persisting for decades or centuries (Nanson & Knighton, 1996). Conversely, more caution needs to be taken when applying our approach to braided channels, where the lateral channel and confluence migration can be relatively fast (see Ashmore, 2013; Dixon et al., 2018; Sambrook Smith et al., 2019).…”