Defining Fluvial Megafans through Geomorphic Mapping and Metrics

Fluvial megafans are 'large' fan-shaped bodies of sediment that form from lateral migrations of a river as it exits a topographic front of a mountain belt. The criterion of what differentiates megafans from alluvial fans is an artificial scale divide. The scale divide varies in the literature, with the most common being a 100-km apex-to-toe length. Alternative values as little of 30-km apex-to-toe length have been proposed, as well as alternative metrics like coverage area. The question remains how should we define megafans, and what metrics differentiate them from smaller alluvial fans? To answer these questions, we catalog the known megafans of the world into a geodatabase delineating size and extent of basins both upstream and downstream from the apex. Through remote sensing, elevation modeling and geomorphic mapping, we populated the database with morphometric measurements, qualitative descriptions and basin parameters. We utilize the geodatabase to test a variety of criteria differentiating megafans from alluvial fans. The analysis draws into question a single parameter for defining megafans.