ACCOUNTING FOR BOTH OROGRAPHIC EFFECTS AND STOCHASTIC RUNOFF ALTERS INTERPRETATION OF TECTONICS FROM TOPOGRAPHY (Invited Presentation)

Two independent lines of research suggest that inferring tectonic signals in bedrock river longitudinal profiles require consideration of: (1) orographic gradients in precipitation and runoff, and (2) the interaction between runoff variability and incision thresholds. Yet these complications to simple stream power models are rarely considered together. We argue that linking these dynamics is essential in settings where the growth of mountain topography simultaneously increases mean runoff due to orographic effects and decreases daily runoff variability due to the increasing importance of snowmelt. Using a global hydrologic model, WaterGAP3, we develop relationships between mean runoff, daily runoff variability, snowmelt fraction, and topography for mountain landscapes around the world. Regional relationships between hydroclimatic variables and topography are used to drive a 1D fluvial profile model, which we refer to as spatialSTIM. This new model modifies a widely used stochastic-threshold incision model by allowing for mean runoff and daily runoff variability to vary spatially along the river profile. Running a suite of numerical experiments at a daily timestep, but over geological time, reveals that feedbacks among mean runoff, daily runoff variability, snowmelt fraction, and topography can lead to highly non-linear channel steepness erosion rate relationships. Model results also highlight the critical, and often underappreciated, role of the spatial extent of runoff generating events in dictating the forms of steepness-erosion rate relationships. When different probability events are not synchronized along the river profile (i.e., spatially independent of each other), river profiles dramatically steepen and produce more non-linear steepness-erosion rate relationships than when runoff events are synchronized along the river profile. These results suggest that empirical relationships between channel steepness and erosion rate may embed the ratio between the characteristic spatial scale of runoff generating events and watershed area, provoking the need for understanding these spatial scales when trying to interpret tectonics from topography.