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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.

Plain Language Summary Despite the intricate geometry of river networks, their basins are characterized by a surprising regularity in the ratio between their width and length (i.e., aspect ratio). This regularity captures the pattern of landscape partitioning into river basins and hence the partitioning of water and life across Earth's surface. The causes for this regularity, however, remain unclear. This study presents a model that explains this regularity through the geometric configuration of adjacent river basins, as well as the geometry of the river profile. The agreement between model predictions and observations from natural and simulated landscapes suggests that small basins that bound larger ones play a key role in determining the aspect ratio of the larger basins and that the properties of the river profile (and hence of the erosive processes that carve this profile) are inherently linked to the aspect ratio of the river's basin.