Fluvial fans are large, low-gradient cones of sediment that can appear wherever rivers lose lateral confinement. They are some of the largest depositional features on Earth, host significant populations and geohazards, record paleoclimate and tectonic signals from erosional uplands, and are thought to make up much of the terrestrial sedimentary record. However, while there are many rivers that exit mountain-fronts around the world, only some rivers build fluvial fans. Decades of study have focused on explaining why this is the case, but there has been comparatively little focus given to explaining the underlying mechanics of how and why fluvial fans grow into the wide variety of shapes observed around the world.

I focus on understanding fluvial fan development as emerging from river avulsions. Avulsions, where rivers suddenly change course to a new one on their floodplains, are some of the deadliest natural disasters in human history. I will first present cellular modeling results demonstrating how abandoned channels, the relics of previous avulsions, control where, when, and why future avulsions occur. Then, I will show that by parameterizing only a few, simple avulsion mechanics (trigger period, abandoned channel repulsion and attraction), the model can reproduce a wide variety of fluvial forms in alluvial basins by controlling the rate at which sediment propagates away from mountain-fronts. As a result, basin-scale factors (like tectonics) that set the available space for aggradation determine whether or not a fluvial fan will form, but river-scale mechanics (including those controlled by climate) determine the shape of fluvial fans. I propose to reconcile previous contradictory community hypotheses by suggesting that fluvial fans are more recognizable in environments that frequently trigger avulsions and preserve abandoned channels. Finally, I will present future opportunities for testing the extent to which mountain-front environments record upstream tectonic and climatic signals in their landscapes and stratigraphic deposits.