Hydro‐Sedimentological Dynamics in a Compound Bifurcating System of a Large Anabranching River: A Case Study of the Paraná River

Abstract

Bifurcations in large anabranching rivers are key controls on lateral and longitudinal hydro‐sedimentological connectivity, governing the interactions among flow, sediment transport and channel morphology that drive continuous morphological adjustment and reorganization. Although substantial advances in understanding bifurcation dynamics have been achieved through theoretical formulations, numerical modelling, laboratory experiments and field observations, the behaviour of natural compound bifurcation–confluence systems with multiple interacting nodes remains comparatively less understood. The Paraná River exemplifies such complexity, comprising an extensive anabranching network in which successive bifurcations and confluences interact over large spatial extents and under highly variable hydrological and sediment‐supply conditions. Here, we investigate the dominant hydro‐sedimentological and morphological controls governing the evolution of a natural compound bifurcation unit and examine how external boundary conditions, including upstream sediment inputs and downstream flow diversion through a distributary channel, interact with its internal dynamics. We combine multi‐decadal planform analysis, field measurements of flow and suspended‐sediment transport and hydrodynamic modelling. Results show that flow partitioning within natural compound bifurcation systems reflects a shifting balance among channel morphology and conveyance, branch slope and flow inertia, whose relative importance changes with both morphological adjustment and hydrological conditions. Progressive aggradation and loss of conveyance in individual branches can modify this balance, whereas high‐flow conditions increase the relative influence of flow inertia and temporarily redistribute discharge among branches. A key finding is that sediment partitioning remains strongly decoupled from water‐discharge partitioning, because transverse gradients in suspended‐sediment concentration inherited from upstream tributary inputs persist over long downstream distances and preferentially route sediment through particular branches, promoting localized aggradation and bar–island development. Numerical experiments further show that flow diversion through a downstream distributary influences discharge partitioning at upstream bifurcation nodes, revealing hydraulic coupling across the compound system. Overall, the evolution of large compound bifurcation systems is governed by the combined effects of local hydraulic and morphological controls, hydrological variability, inherited sediment distributions and external boundary conditions. These findings highlight the need to view compound bifurcation systems as interconnected systems rather than as collections of independent bifurcation nodes.