. Feedbacks among river channel incision, threshold hillslopes, and runoff generation during rare events in the Colorado Front Range, USA

Abstract
The Colorado Front Range, USA is known for a persistent spatial pattern in floods and landslides whereby the largest events are generated at low to intermediate mountain elevations due to intense rainstorms. This orographic gradient in runoff generation during rare events differs from patterns in mean runoff, which instead directly reflects the increased contribution of snowmelt runoff as a function of elevation. In this work, we first assess whether the inverse relationship between mean runoff and the magnitude of rare events is a direct consequence of sub-daily rainfall statistics. We find that orographic gradients in rainstorm properties are weak and thus insufficient to explaining spatial patterns in the hazards of interest, namely floods and landslides. Instead, we show that properties of the Critical Zone like soil depth and exposed bedrock may act to amplify runoff generation at low to intermediate mountain elevations in this setting by decreasing the effective storage capacity of water in the landscape. Such elevation-dependent properties of the land surface suggest intriguing feedbacks between the geomorphic evolution of fluvial landscapes and their associated hydrologic responses, especially during the transient evolution of the landscape in response to an increase in base level fall. Sustained increases in base level fall are propagated from lower elevations to higher elevations in fluvial landscapes as the river network adjusts to increased erosion rates. This can lead to persistent gradients in flood frequency and mass wasting, independent of the climatological forcing, as long as topographic relief, soil production rates, soil thickness, and soil water capacity co-vary with erosion rate. This suite of feedbacks are not typically considered in either longer-term landscape evolution models or shorter-term assessments of hazard.