Runoff controls on stream network branching

Abstract

The geometry of stream networks varies systematically with climate [1,2]. In humid regions diffusive processes seem to dominate the branching geometry of stream networks, resulting in wider branching angles close to 72 degrees, which is the theoretical angle for growth in a diffusive field [1,3]. In arid climates, on the other hand, channel networks display much narrower angles [1,2].Here we show that the narrower angles in arid regions can be related to the higher frequency of extreme runoff events, which are more common in arid landscapes than in humid ones [4]. Erosion due to overland flow leads to incision which is more focused in the direction of regional topographic gradients and thus resulting in narrower branching angles as the influence of diffusive processes becomes weaker and weaker. Our analysis is based on flow frequency distributions derived from USGS gauging stations across the United States [4] and branching angles obtained from the USGS medium resolution National Hydrographic Dataset [1]. Our measurements show, that the tails of the flow frequency distributions become systematically heavier with aridity in the same way as branching angles become narrower.This result suggests that the relative impact of diffusive network growth systematically decreases with increasing aridity as the landscape's Peclet number changes across a landscape with varying climate.