INDICATIONS OF RAPID GEOMORPHIC RESPONSE OF THE SOUTH PLATTE RIVER SYSTEM IN EASTERN COLORADO DURING THE LATE PLEISTOCENE–HOLOCENE TRANSITION

The South Platte River has headwaters in the Colorado Rocky Mountains and flows across the semiarid, short-grass prairie of eastern Colorado, a region highly susceptible to drought. Part of the High Plains Ecoregion, the area has a mean annual precipitation of ~30–50 cm and a mesic temperature regime. In this type of semiarid, drought-prone environment, geomorphic systems tend to be highly sensitive to climate change, and surficial deposits can provide a record of past system response to that change. Past droughts have been severe enough to mobilize dune sand and generate wind-blown silt. In addition, alpine glaciation has affected the headwaters region of the river. Surficial geologic mapping is in progress along the South Platte River corridor in eastern Colorado between the towns of Masters and Fort Morgan to better understand the river’s history and its relation to eolian deposits in the region. Along this stretch of river, late Pleistocene sidestream alluvium of Kiowa and Bijou Creeks overlies and interfingers with late Pleistocene mainstream alluvium of the South Platte River. Previous workers have correlated the late Pleistocene surface to the Kersey (or Broadway) terrace, and the next lower surface in the area to the Kuner terrace. Our OSL and 14C dates for sidestream alluvium from Kiowa, Bijou, and an unnamed creek (sampled at or near the South Platte River) yield information about timing of river aggradation and incision that generally is consistent with what previous workers have found for Kersey and Kuner terraces upstream. These dates, combined with our mapping research, imply that the South Platte River in eastern Colorado either was highly sensitive to rapidly changing environmental conditions during the late Pleistocene–Holocene transition, or crossed threshold conditions during that time that triggered rapid response of the geomorphic system. Our results indicate that the late Pleistocene surface was abandoned during this time of transition, and that river incision could have been brief in duration but impressive in scale, with ~15 m of downcutting possibly occurring within the span of several hundred years. Cessation of this episode of downcutting was followed by a period of renewed aggradation that formed the next lower terrace fill.