Hydrogeomorphology of the origin of the Amazon River, the confluence between the Marañón and Ucayali rivers

The origin of the Amazon River is formed at the confluence of the Marañón and Ucayali rivers. Remote sensing and detailed hydrodynamics, sediment transport and bed morphology analysis under different hydrological conditions have been applied to understand the control mechanisms of the modern confluence, thus informing about ancient confluences. Results showed that: 1) meandering-meandering confluences existed when bifurcated meandering channels from the Marañón River joined the meandering Ucayali River, 2) far-field scale provided the boundary conditions for the near-field scale processes. In the case of the Marañón River, the spatial frequency and displacement of incoming anabranching structures to the confluence location set the boundary conditions for the near-field scale. In the case of the Ucayali River, the incoming hydrodynamics and bed morphology are governed by far-field processes such as the occurrence of cutoffs, 3) high intensity secondary flows at large rivers were observed at far- and near-field scales, where previous studies have reported that secondary flows are weak or nonexistence or mainly found downstream of confluences. Finally, 4) even though the Marañón River is larger compared with the Ucayali River, the confluence hydrogeomorphology is governed by the Ucayali River because of more developed and stronger secondary flows.