Geomorphic Parameters for Developing a Hydrologic Model to Infer Holocene Climate Variability, Middle Snake River near Bliss, Idaho

Abstract:

The geomorphology and stratigraphy preserved in a canyon reach of the Middle Snake River provide model parameter constraints for estimating Holocene paleohydrology. Channel constrictions, which acted as hydraulic weirs throughout the Holocene, were created in this reach by the Bonneville Flood (~17.5 ka) that left very large (>10 m) slabs of basalt and 2–3 m diameter boulder deposits near the canyon floor. Post-Bonneville Flood landforms and deposits that formed during the Holocene are situated less than ~30 m above river level (arl) in this reach and include fluvial and boulder terraces, alluvial fans, and incised tributary alluvial units. Relative topographic position of these geomorphic features, cross-cutting relations, multiple buried soils, depositional and erosional contacts, and radiocarbon dates from terraces (Qt) and alluvial fans provide a geomorphic and stratigraphic framework and a Holocene chronology for this area. The relative stratigraphic position of a massive silty sand that overlies Bonneville Flood gravel in Qt5 (~20 m arl) and Qt4 (~10 m arl) deposits and comprises all of Qt3 (~5 m arl) deposits indicates changes in Holocene discharge; longitudinal profiles of fluvial terraces graded to hydraulic constrictions provide reasonable estimates of paleo-stage. Fifteen radiocarbon dates yielded ages of ~8670 and ~3500 cal yr BP for Qt4 deposits and ~1100 and ~100 cal yr BP for Qt3 deposits and help define periods of episodic cutting and filling. Timing of Qt4 and Qt3 cut-and-fill episodes and alluvial fan formation correlates well with Holocene global and regional paleoclimate events inferred from Great Basin lake histories including wet periods from ~9.0 to 8.0 ka and ~4.2 to 2.5 ka, the Medieval Climatic Anomaly (~1.2 to 0.8 ka), and the Little Ice Age (~0.3 to 0.6 ka). The fluvial geomorphology documented in this study will be used to develop a watershed-scale hydrologic model to infer paleoprecipitation in the region during the Holocene.