Controls on Channel Slopes in Bedrock Landscapes Underlain by Deformed, Heterogeneous Strata

In many landscapes, channel slopes decrease as watershed area increases. However, many bedrock landscapes of the Colorado Plateau include folded, highly anisotropic layers that cause channel slopes to conform to the geometry of deformed stratigraphic boundaries. Here, we examine the role of deformed strata in setting channel slopes of small bedrock channels of the Raplee Ridge Monocline in Mexican Hat, southeastern Utah, USA. Here, a sequence of marine transgressions and regressions have produced a set of alternating shales and limestones, which were subsequently folded into a northwest-striking westward dipping monocline. Recently, incision of the San Juan River has exposed this deformed sequence and initiated a series of small incisional catchments along the northern nose of the fold. Channel slopes conform to the stratal geometry of the fold in low watershed portions of these catchments. Within large catchments, channel slope becomes independent of structural geometry and is well-predicted by Flint's Law. We hypothesize that these transitions in channel slope reflect a threshold that limits incision in small catchments to conform to lithologic boundaries, whereas large channels whose flows are far above this incisional threshold produce channel slopes that are independent of lithologic orientation. To test this hypothesis, we use airborne Lidar data with Structure-from-Motion photogrammetric orthoimages and elevation models to associate channel slopes with different lithologies, orientations, and watershed areas. Detailed lithologic mapping of the monocline documents the areas where resistant versus easily eroded units are exposed in the channels. Preliminary results show that systematic variations in single channel slopes reflect an interplay between bedrock lithology and geologic structure, suggesting that in certain circumstances, underlying lithologic orientation may play a role in determining the geometry of channels and relief structure of landscapes.