Time scales of river bifurcations

Abstract River bifurcations are the fundamental building blocks of a variety of fluvial environments such as braiding and anastomosed rivers, alluvial fans, and river deltas. Their long‐term equilibrium configurations have been widely explored, together with the influence of several external forcing factors, whereas less attention has been devoted to investigate the characteristic timescale with which bifurcations evolve over time. In this work, we address this issue by combining the results of a 1‐D numerical model with those obtained through a linear stability analysis that accounts for the length of bifurcates. Numerical results show that the timescale of the adaptation of water and sediment partition at the bifurcation node is much shorter than the time required to achieve the long‐term equilibrium of the bifurcates. We find that the nodal point evolution becomes faster as the value of the width‐to‐depth ratio increases above the critical threshold for the bifurcation stability, while it gets slower as the length of the bifurcates increases. The timescale becomes independent of the branch length when this length exceeds a threshold value above which the effect of the downstream boundary condition no longer affects the evolution of the bifurcation node. The analysis of a large dataset of gravel‐bed bifurcations reveals that the evolutionary timescale of most of them is larger than that of natural flow variations. Moreover, the rate at which the water and sediment partitioning at bifurcations changes over time is generally smaller than the fluctuation rate of sediment transport caused by the migration of bars in the upstream channel, especially for bifurcations with long branches.