Bedrock and Sediment Controls on Channel Morphology in the Henry Mountains, Utah

Mixed bedrock-alluvial channels on and around the Henry Mountains and Navajo Mountain, southeast Utah, are ideal for unraveling the interconnected controls of sediment flux and bedrock properties on fluvial incision into bedrock because (1) channels incise through a range of lithologies with widely varying physical properties, and (2) sediment types vary systematically and predictably in different channels, with extreme differences in the durability and size distribution of sediment. Tertiary intrusive rocks (diorite) form the cores of the Henry Mountains and sedimentary units surround the central peaks, primarily alternating between sandstones and shales. Channels that tap into diorite at the peaks of the Henry Mountains tend to have smoothly concave profiles, and sediment in these channels has a strongly bimodal distribution, consisting primarily of quartz sand and large diorite clasts. The observed size distribution is caused by the dramatic difference in durability between the igneous intrusive rocks and the surrounding sedimentary units. In contrast, Navajo Mountain exposes the same sedimentary sequence but has no exposed intrusive rock, and therefore no source for durable diorite sediment. Channels on both the Henry Mountains and Navajo Mountain that do not source igneous intrusives but only cut into sedimentary units have more variable channel profiles, with steps and slope changes that often correlate with mapped geologic units, and these channels transport sand and sandstone clasts. DEM and remote sensing analysis and qualitative field observations suggest that systematic variations in slope-area scaling and channel morphology (slope, width, percent rock exposure, erosional bed forms, roughness) occur as functions of substrate lithology and diorite sediment supply. The tools and cover theory of sediment supply and bedrock erosion, first hypothesized by Grove Karl Gilbert in his report Geology of the Henry Mountains and recently explored experimentally and theoretically by other researchers, may be the channel-scale mechanism that causes the observed rougher and smoother longitudinal profiles. In some places, high channel slopes over resistant bedrock lithologies in channels with little diorite sediment may represent less efficient local erosion due to a lack of large and durable diorite clasts (one aspect of the tools effect). Conversely, the cover effect is seen where channel slopes appear to be set by the need to transport coarse diorite sediment, as slopes are often significantly steeper over diorite clast-laden reaches than over bare bedrock or sand-laden reaches. Further research may allow the evaluation and refinement of specific erosion models, in particular focusing on ways that variable bedrock lithology affects sedimentation, channel incision and landscape response.