Hillslope-controlled incision thresholds shape mountain range topography of the Northern Andes

Rivers have long been regarded as the drivers of mountain landscape evolution, with hillslopes following suite. In this view, rivers set the rate of base-level change at the bottom of hillslopes that passively adjust their geometry to attain a matching hillslope denudation rate. Hence, most large-scale landscape evolution studies focus on analyzing metrics of the river network, such as normalized river steepness (k_sn). More recently, it has been recognized that k_sn may depend on incision thresholds, a critical shear stress or unit stream power required to erode bedrock, which depend on sediment cover. In this study, we use previously published cosmogenic-nuclide derived erosion rate data from the Northern Andes to investigate how hillslope sediment delivery, controls incision thresholds and the first-order topography of two adjacent mountain ranges with different lithology.

Our results suggest that the exponent of the power law between k_sn and erosion rate, which we refer to as topographic insensitivity, is twice as high in the sedimentary rock-dominated Eastern Cordillera compared to the crystalline rock-dominated Central Cordillera. This generally means that in the Eastern Cordillera, spatial differences in erosion rate, e.g. induced by tectonic gradients, will only result in minor differences of river steepness compared to the Central Cordillera. We use river width measurements, discharge data, and channel grain size data to constrain a stochastic threshold incision model. Our results indicate that the difference in the erosion rate- k_sn relationship can be explained by a 26 times higher incision threshold in the Eastern Cordillera. This difference in topographic insensitivity caused by incision thresholds cannot solely be explained by factors such as discharge variability, river width to discharge scaling, or channel grain size. However, we find a significantly higher landsliding frequency in the Eastern Cordillera that causes transient channel covering and damming, leading to a lower sensitivity of k_sn to erosion rate. These findings highlight how hillslope-controlled sediment delivery can modify the stream response to tectonic uplift and exhibit a first-order control on the landscape evolution of adjacent mountain ranges.