EPSS Colloquium spring-2018

Rivers determine the maximum elevation of most tectonically active mountain belts, control the coupling between climate and tectonic processes, and archive the pace and tempo of fault-related rock uplift rates. Long-profiles of rivers in steep, non-glaciated landscapes are thought to be controlled by the bedrock incision rate, leading many to posit that channel discharge and slope determine the pace of river incision (hereafter referred to as the power-law incision rule). We tested the power-law incision rule in watersheds varying by four orders of magnitude in erosion rate (4.7 x 10-3 - 7.1 mm/yr mm/yr), and combined these with a global analysis of erosion rates and topography. Our data and analyses reveal that this rule breaks down in steep, rapidly eroding landscapes, in which river profiles reach a threshold steepness that is invariant in steep watersheds. This limit to the steepness of channels suggests the present and past overall topographic relief on Earth may be limited by the horizontal extent of active rock uplift, with higher relief resulting from longer channels of a given steepness, rather than the rate of uplift itself.