CONTROLS ON THE GEOMORPHIC SIGNATURE OF HOLOCENE CLIMATE ON RIVERS IN NORTHERN YELLOWSTONE NATIONAL PARK

Fluvial terrace sequences may reflect climatic forcing, but in complex and locally variable ways. In northern Yellowstone, Holocene terraces have limited downstream extent and exist mainly where local aggradation of latest Pinedale outwash provided vertical space for their development. Lower Soda Butte Creek displays mostly fill-cut terraces; ¹⁴C dates on overbank sediments show that late Holocene treads formed during colder, wetter periods ~3400-2800 (T2), 2000-1300 (T3), and 800-100 cal yr BP (T4), generally times of Neoglacial advances in western North America. We infer that these treads formed when moderate stream power promoted vertical stability and lateral migration of channels, with sediment supplied from lateral erosion into tributary fans. However, significant gravel aggradation preceded T2 tread formation, which may reflect coarse sediment input from major mid-Holocene growth of tributary fans ~6000-4000 cal yr BP, partly from fire-related sedimentation. Holocene terrace preservation is minimal on Slough Creek, probably because greater stream power and lower tributary sediment input allowed rapid incision down to granitic bedrock knickpoints, followed by vertical stability and lateral migration that erased most terraces. Nonetheless, the timing of heightened lateral erosion into tributary fans is similar to Soda Butte Creek. Late Holocene terraces of the Gardner River in upper Gardners Hole also show similar ages. Thick, fine-grained sediments of middle Holocene fill terraces on the Gardner suggest reduced discharges, consistent with regional paleoclimatic evidence for aridity.

Overbank gravel deposits imply that incision between terrace levels is related to extreme floods. The largest historic floods were generated by anomalous spring warmth with rapid snowmelt and rain on snow, consistent with a warmer climate, as in the Medieval Climatic Anomaly, when Soda Butte Creek incised. At this same time, however, the upper Gallatin River proximal to steep mountain slopes aggraded with coarse angular gravel in a main channel fan, probably associated with fire-related sedimentation, as it did in a similar climate ~2000 cal yr BP. In sum, rivers of northern Yellowstone show substantial impact from Holocene climatic change, but effects are strongly dependent on local geomorphic and hydrologic factors.