BASELEVEL VERSUS BEDROCK CONTROLS ON INCISION AND STEEPNESS OF ALPINE CANYON OF THE SNAKE RIVER, WY

The Yellowstone region is recognized as the most dynamically deforming and evolving terrain in the Interior West. Ideas about landscape evolution related to the Yellowstone hotspot are well developed; however, measurements of Quaternary incision rates are lacking and patterns of uplift and subsidence are untested. To address these research needs, this study: 1) establishes a chronostratigraphy of fluvial terraces from optically stimulated luminescence (OSL) ages, and 2) conducts topographic analyses to investigate signatures of bedrock and baselevel controls on incision along the Alpine Canyon of the Snake River, WY.

Results indicate three morphologically distinct reaches through the study area; an upper and lower bedrock canyon separated by a wide alluvial reach. Landslide deposits are predominant in places where the underlying bedrock is relatively weak Jurassic and Cretaceous sandstone and shale. There are four primary terrace deposits along the canyon often preserved beneath tributary fans. At least two terrace deposits are timed with glacial epochs and one is Holocene in age. Considering the relative heights of terrace straths and preliminary ages, incision rates are indeed relatively rapid for the Interior West.

A major knickzone along the Snake River above the mouth of Alpine Canyon plus patterns of tributary steepness signify active baselevel fall, consistent with incision propagating upstream and encountering resistant bedrock. The T2 terrace tread is highest in the upstream bedrock canyon and at the canyon mouth, which suggests possible activity of a previously unpublished fault in Astoria and a recent pulse of incision. High incision rates and increasing unit stream power at the canyon mouth also support this concept. Both k_sn and chi plots of tributaries show higher steepness in downstream reaches approaching the main stem Snake River; and tributaries are generally steeper to the west near the mouth of the canyon. Varying bedrock strength in tributaries influences steepness, yet underlying lithology has a lesser impact on the main stem. We propose that active baselevel fall from either subsidence in the Snake River Plain or unexpected activity along the Grand Valley fault at the mouth of the canyon is driving incision along the Snake River instead of upstream uplift related to the Yellowstone hotspot.