Channel Profile and Plan-View Controls on the Aspect Ratio of River Basins

The ratio between the width and length (i.e., aspect ratio) of river basins is a fundamental attribute of Earth's surface. Topographic analyses showed that the aspect ratio of high-order basins along linear mountain fronts is surprisingly consistent despite different tectonic, lithologic, and climatic conditions. Additionally, analysis of simulated landscapes showed that the aspect ratio depends on the concavity of the river profile. These observations remain difficult to explain because of the intricate shape of river networks. Here I present a simple geometric model that computes the aspect ratio from the location of a triple divide junction with neighboring basins, the concavity of the river profile, and the exponent that relates river length and drainage area (i.e., Hack's exponent). The model suggests that these seemingly independent empirical observations are interdependent and that small basins that bound larger ones play a key role in determining the aspect ratio of the larger basins.