River Incision and Knickpoints on the Flank of the Yellowstone Hotspot — Alpine Canyon of the Snake River, Wyoming

Abstract:

Understanding patterns of deformation and testing geophysical models in the dynamic region of the Yellowstone Hotspot requires Quaternary-scale records of incision and uplift, which are currently absent. This study examines fluvial terraces and longitudinal-profile metrics along Alpine Canyon of the Snake River, WY. Because the Snake is the only regional river crossing from the uplifting Yellowstone Plateau and flowing into the subsiding Eastern Snake River Plain, it provides an opportunity to investigate both ends of the phenomenon. Field observations through Alpine Canyon indicate that Pleistocene incision rates in this region are relatively high for the interior western U.S., that the river switches between bedrock and alluvial forms, and that incision/uplift is not uniform. Two endmembers of regional deformation may be tested: 1) the arch of high topography surrounding Yellowstone is uplifting and terraces converge downstream as incision rates decrease towards the Snake River Plain, or 2) baselevel fall originates at the subsiding Snake River Plain and terraces diverge as incision rates increase downstream. Datasets include surficial mapping, rock strength measurements, surveying of the longitudinal profile and terraces using RTK-GPS, optically stimulated luminescence dating of fluvial-terrace deposits, and investigation of drainages through k_sn and χ analyses. Initial results indicate that there are four primary terrace deposits along the canyon, three of which are timed with glacial epochs. Considering the relative heights of terrace straths and preliminary ages, incision rates are indeed relatively high. There is a major knickzone covering the last 15 km of the canyon that is also reflected in tributary profiles and is consistent with a wave of incision propagating upstream, favoring the second endmember of active baselevel fall downstream.