Fate and rate of alluvial river captures

River capture occurs when a river is diverted into a neighboring drainage basin, changing the routing of water and sediment and altering the dispersal pathways of aquatic organisms. Captures are commonly used to explain drainage patterns and biogeographic distributions, yet the mechanisms that govern river capture are not well understood, especially for alluvial rivers. Field observations suggest that an alluvial river capture begins when a river’s flow is partially diverted into an adjacent basin, forming a bifurcation. We adapted a morphodynamical model of river bifurcation to study the evolution of an ongoing alluvial river capture and test how the relative slopes and discharges of the two branches determine whether the capture succeeds or fails. The results indicate that the branch with the smaller initial ratio of sediment supply to sediment transport capacity ultimately claims most of the discharge. This competition proceeds more rapidly if the branches differ more in their supply-to-capacity ratios. Applied to the ongoing capture of the upper Orinoco River by the Casiquiare River, a tributary of the Amazon River, the model predicts that the capture will succeed over thousands of years.