The influence of tributaries and their avulsions on the tempo and extent of valley-confined divide migration

Abstract:

Drainage reorganization occurs via drainage divide migration and is a primary mechanism for landscape adjustment to tectonic and climatic changes. Divide migration is often viewed as a continuous process, driven by erosional imbalance across drainage divides, that can eventually lead to a stable topographic configuration, adjusted to the new tectonic and climatic conditions. In tectonically influenced areas, drainage divides often migrate along valleys that are confined by geologic structures or preexisting topography. Valley-confined divide migration is often associated with significant topographic gradients, and it thus affects erosion rate and sediment flux in some of the most rapidly changing landscapes on Earth. Despite its importance for landscape evolution, the processes that control the rate of this divide migration and the parameters that govern divide (in)stability remain largely unexplored.

Here we propose that the tempo and extent of valley-confined divide migration are governed by the drainage area distribution along the valley, and avulsions at the outlet of tributaries that drain proximal to the divide. We find that without avulsions, the spacing and relative drainage area of side tributaries control the rate of divide migration and that the spatial arrangement of these tributary-junctions facilitates multiple configurations of stable divide location along the valley. Avulsing tributaries can perturb divide stability by abruptly shifting discharge across the divide and triggering a burst of erosion and rapid divide migration. We identify field evidence of this process across scales, and present model experiments, based on natural settings, where avulsions set the tempo and extent of divide migration. Our results suggest that unlike the common perception, that views divide migration as a continuous process that leads to a stable topographic configuration, valley-confined divides are susceptible to abrupt tributary avulsions that can disrupt episodes of stable topography and trigger further divide migration. This implies that drainage reorganization in this setting is governed not only by tectonic and climatic changes, but also by the abrupt and somewhat stochastic dynamics of avulsion processes.