TRIBUTARY SLOPES RECORD REGIONAL CLIMATE AND CONSTRAIN BRANCHING ANGLES IN U.S. RIVER NETWORKS

Abstract

The branching angles of 100 m - km scale tributary streams vary systematically between river networks in different climate regimes. Humid regions are characterized by wide angles of 72◦ while arid regions are typified by narrow angles of 45◦ or smaller. This relationship suggests a strong climatic control on terrestrial river network organization, and has been extrapolated to characterize the ancient climate under which Martian valley networks formed. Previous stud- ies proposed that this phenomenon results from groundwater seepage, which diffusively erodes humid landscapes at small scales, initiating valley growth at a theoretically characteristic 72◦. However, that theory cannot adequately explain why such small-scale, short-lived influences would be preserved over the large scales and long timespan of continental drainage evolution. Using three overlapping datasets derived at 4 cm–30 m resolutions, I show that meter scale branching angles in arid southeast Utah network display the same expected branching angles as large scale humid regimes. I propose that the ratio of tributary slopes is a primary control on branching angles across scales, and show that this slope ratio (SR) varies systematically with climate and branching angles across the contiguous United States. Networks in humid regimes are characterized by a mean SR below 0.5 (more than 50% difference in tributray slopes), while networks in arid regimes are characterized by a mean SR over 0.5 (less than 50% difference in tributray slopes). After proposing mechanisms of erosional and sediment transport processes that may result in the identified climate signatures over the observed scales, I present a simple method for predicting climate directly from the distributions of slope ratio in river networks.