The rate and extent of windgap migration regulated by tributary
confluences and avulsions
Abstract. The location of drainage divides sets the distribution of discharge, erosion, and sediment flux between neighboring basins, and may shift through time in response to changing tectonic and climatic conditions. Major divides commonly coincide with ridgelines, where the drainage area is small and increases gradually downstream. In such settings, divide migration is attributed to slope imbalance across the divide that induces erosion rate gradients. However, in tectonically affected region, low-relief divides, windgaps, abound in elongated valleys, whose drainage area distribution is set by the topology of large, potentially avulsing side-tributaries. In this geometry, distinct dynamics and rate of along-valley windgap migration is expected, but this process remains largely unexplored. Inspired by field observations, we investigate along-valley windgap migration by simulating the evolution of synthetic and natural landscapes, and show that confluences with large side tributaries influence migration rate and extent. Such confluences facilitate stable windgap locations that deviate from intuitive expectations based on symmetry considerations. Avulsions of side tributaries can perturb stable windgap positions and avulsion frequency governs the velocity of windgap migration. Overall, our results suggest that tributaries and their avulsions may play a critical role in setting the rate and extent of windgap migration along valleys and thus the time scale of landscape adjustment to tectonic or climatic changes across some of the most tectonically affected regions of Earth, where windgaps are common.