Boulders-induced Geomorphic Adjustment at a Range of Spatial Scales in Experimental Landscapes

The portion of boulders considered to be rarely mobile is thought to impact the geomorphic response of landscapes to tectonic activity and climate. Recent studies demonstrated that boulders modify hillslope and river processes, implying that their long-lasting presence is consequential for landscape form. Understanding the relationship between geomorphic features and boulders thus poses vital information for extracting valuable geomorphic parameters. However, due to the scarcity of data encompassing both boulder parameters and associated hillslope and river morphologies, the impact of boulders on landscape form remains unclear. We have investigated the roles of boulders in landscape modification by conducting a laboratory experiment at a landscape scale. The experiment included an initial phase, where a fluvial topography evolved under uniform uplift and rainfall by incision into a substrate crafted of saturated silica powder. The second phase simulated landscape response to the emplacement of boulders along main trunk channels. Spheres with uniform sizes of 1.5 mm were placed along seven main trunks belonging to basins that drain the eastern experimental mountain flank, and the experiment was allowed to continue. 3D models of the evolving topography were generated every 20\30 minutes. Our analysis reveals that following the emplacement of boulders, the landscape changed its form at a range of spatial scales. At the cross-section scale, the hydraulic geometry scaling relationships were altered where boulders were present in the channel. At the catchment scale, topographic inversion was observed, with tributaries transforming into ridges. At the channel profile scale, perched reaches bounded by knickzones developed. Concurrently, the main drainage divide migrated towards the boulder-covered flank. This multitude of modifications in landscape features at a wide range of scales is interpreted to result from extensive and persistent boulder cover inhibiting local fluvial vertical erosion. The altered morphologies presumably promote a new erosional steady-state under the influence of boulder cover.