The continuous feedbacks among tectonics, surface processes, and climate are reflected in the distribution of catchments on active mountain ranges. Previous studies have shown a regularity of valley spacing across mountain ranges worldwide, but the origin of this geomorphological feature is currently not well known. In this work, we use a landscape evolution model to investigate the process of fluvial network organization and the evolution of regular ridge-and-valley patterns on simulated mountain ranges. In particular, we investigate the behavior of such patterns when subjected to a perturbation in landscape processes from a previous steady state, resulting from a sudden variation in the pattern of bedrock erodibility, from homogeneous to a gradient. We analyze the time evolution of the mean ratio λ' between the linear spacing of adjacent valleys and the half width of the mountain range. We show how a valley spacing ratio of~0.5 is first achieved at steady state under uniform bedrock erodibility. After applying the gradient of bedrock erodibility across the landscape, we observe that λ' first increases and then decreases to a new steady-state value that is smaller than the original value. A detailed analysis of the simulations, through observations of surface 'snapshots' at repeated time intervals, allows to gain some insight into the mechanisms governing this fluvial network reorganization process, driven by the migration of the main divide toward the side characterized by lower bedrock erodibility. On both sides of the range the new steady-state valley spacing is obtained through mechanisms of catchment reorganization and competition between adjacent fluvial networks. In particular, catchment reorganization is characterized by the growth of smaller catchments between shrinking larger catchments on the side with lower erodibility, and the growth of larger catchments on the side with higher erodibility.
The Ethiopian Highlands, with up to 1,500‐m‐deep canyons surrounded by low relief plateau surfaces, are one of the most spectacular examples of transient fluvial landscapes on Earth. We analyze river profiles extracted from a 90 m digital elevation model of the upper Blue Nile catchment and identify 116 major knickpoints on 137 river profiles. We use 1‐D river profile models to simulate three potential mechanisms for knickpoint formation: plateau uplift, capture of large lakes or internal drainage on the plateau surface, and sediment‐flux‐dependent river incision with a low sediment flux from the plateau surface. We define a normalized upstream knickpoint propagation distance, χkpj, and demonstrate that the erosion models predict different distributions of this metric in transient profiles following plateau uplift. Model knickpoints resulting from the scenario of plateau uplift or common base‐level fall using the stream‐power model display similar upstream propagation distance in χ space. The results of the same scenario modeled with the sediment‐flux‐dependent incision model show upstream knickpoint propagation distance proportional to catchment area. Perturbations to these trends result from drainage capture. Comparing the model results with observed χkpj values of knickpoints and field observations, we recognize effects characteristic of the sediment‐flux‐dependent incision model. However, most profiles are best explained by the systematic transfer of drainage area from the plateau to the surrounding rivers. We propose a new model of landscape evolution for the upper Blue Nile catchment dominated by discrete events of capture of drainage area from the plateau.
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