Bifurcation of Amazonian meandering rivers, interaction of main and side channels

Abstract

Amazonian meandering rivers are characterized by their migratory dynamics. During this process, they frequently capture paleochannels within the alluvial plain or bifurcate into different channels, thereby establishing competition between main and side channels for persistence and dominance. The fate of side channels depends on multiple conditions, but it is primarily determined by their capturing capacity of flow and sediment. These channels may capture the entire flow and sediment discharge, become abandoned, or achieve balanced conditions with the main channel. Three natural meandering bifurcation systems (Ichoa, Juruá and Ucayali–Puinahua) in the Amazon basin were selected based on their high dynamism, environmental impacts, and potential future impacts. Morphometric analysis of these natural cases identified two key planform characteristics that distinguish their evolutionary behavior: the bifurcation point location along the main channel bend (upstream, apex, or downstream) and the side channel relative length (shorter, equal, or longer than the main channel). These natural-derived morphological parameters were then used to design thirteen idealized numerical scenarios using a depth-averaged hydrogeomorphic model, allowing systematic investigation of how these geometric configurations control bifurcation morphodynamics under controlled conditions. Results highlight that channel length plays a fundamental role in flow and sediment capture capacity by side channels. Shorter channels capture more flow and sediment volumes compared to equal or larger lengths; however, in natural systems, these shorter channels progressively modify their morphology (elongating and widening) until they reach equilibrium conditions, as observed in the Ucayali–Puinahua system. This equilibration process can be rapid, as in the Ichoa case (lasting only two years), or prolonged, as in the Puinahua–Ucayali system. Once equilibrium is achieved, the channel with reduced flow discharge loses width while maintaining its sinuosity, as exemplified by the Juruá case. This transition process reduces the mobility of the channel that loses flow, leaving it in a frozen-like state. When bifurcated channels have similar lengths, highly unstable partitioning conditions arise where the bifurcation point location plays an essential role. The system’s fate is thus determined by the complex interplay between initial hydraulic advantage, bifurcation geometry, and dynamic evolution toward morphological equilibrium.