ALLUVIAL STRATIGRAPHIC EVIDENCE FOR CHANNEL INCISION DURING THE MEDIEVAL WARM PERIOD ON THE CENTRAL GREAT PLAINS

Alluvial valley fills from tributaries to the upper Republican River, southwest Nebraska, USA, provide soil- and morpho-stratigraphic evidence for an episode of channel incision between c. 1100 and 800 14C yr BP. Eleven new radiocarbon age estimates from these valley fills provide absolute age control. This local episode of channel incision correlates with other alluvial stratigraphic studies from the central Great Plains and demonstrates regionally synchronous fluvial behavior. Proxy records of palaeo-hydrologic conditions in and around the Great Plains (including lacustrine, aeolian and geomorphological sources) indicate that channel incision correlates with a multi-centennial episode of common, widespread drought. Temporally, this drought episode corresponds to the period recognized in many regions as the Medieval Warm Period (MWP). Therefore, this research demonstrates: 1) a causal relationship in the semiarid central Great Plains between drought and increased fluvial erosion; and 2) a local/regional geomorphic response to a particular episode of global climate change.