Downstream Control on the Stability of River Bifurcations

River bifurcations are prevalent features in both gravel‐bed and sand‐bed fluvial systems, including braiding networks, anabranches and deltas. Therefore, gaining insight into their morphological evolution is important to understand the impact they have on the adjoining environment. While previous investigations have primarily focused on the influence on bifurcation morphodynamics by upstream channels, recent research has highlighted the importance of downstream controls. In particular, in the case of rivers, current linear stability analyses for a simple bifurcation are unable to capture the stabilizing effect of branches length unless a confluence is added downstream. In this work, we introduce a novel theoretical model that effectively accounts for the effects of downstream branch length in a single bifurcation. To substantiate our findings, a series of fully 2D numerical simulations are carried out to test different branches lengths. Results from linear stability analysis show that bifurcation stability increases as the branches length decreases. These results are confirmed by the numerical simulations, which also show that, as the branch length tends to vanish, bifurcations are invariably stable. Finally, our results interestingly reveal that when a source of asymmetry, such as a free surface gradient or channel area advantage, is present at the node, there are scenarios in which the less‐favored branch becomes dominant over the hydraulically favored branch. Plain Language Summary This research looks at how rivers divide into multiple branches and how this process shapes the surrounding environment. While past studies mostly focused on factors upstream influencing these splits, recent research emphasizes the importance of downstream factors, such as branch length and tidal forces. The study introduces a new theoretical model to better understand how downstream branch length affects a single river split. We used computer simulations with different branch lengths and channel widths to test the model, discovering that shorter branch lengths result in more stable river splits. The theoretical model is also adapted to account for different shapes commonly found in nature, revealing results that are not always straightforward. Key Points A new formulation for the stability of river bifurcations based on energy balance at the node is proposed The range of bifurcation stability has been found to increase as the branches length decreases Despite the presence o