LATE HOLOCENE ALLUVIAL RESPONSE TO HYDRO-CLIMATIC CHANGE IN THE UPPER REPUBLICAN RIVER BASIN, GREAT PLAINS, USA

Late Holocene alluvial records from the central Great Plains, USA, demonstrate a strong relationship between rates of past fluvial processes and century- to millennial-scale hydro-climatologic change. Field-based research from the upper Republican River basin, southwestern Nebraska, provides evidence of late Holocene fluvial processes in the form of soil-stratigraphic and morpho-stratigraphic relationships, alluvial sedimentology, and radiocarbon age determinations. Aggradation of semiarid floodplains occurred sometime after 3500 yr B.P. until the onset of widespread regional drought during the Medieval Warm Period (MWP) (1200 to 800 yr B.P.). This aggradation episode was punctuated by multiple periods of reduced aggradation rates and consequent formation of cumulic floodplain soils. Subsequently, MWP drought caused rapid, widespread channel incision across the central Great Plains and resulted in the formation of ubiquitous late Holocene terraces. MWP channel incision likely resulted from drought-induced reduction in the density of prairie grasses and the associated increase in surface runoff rates and sediment transport capacities of ephemeral streams. Following the MWP incision episode, channels in the region aggraded rapidly, generating floodplain stratigraphy with well-preserved sedimentary structures and an absence of pedogenic features. Paleoclimatic conditions responsible for such widespread and synchronous episodes of aggradation and incision are inferred by analysis of proxy records and contemporary hydro-climatic processes. Proxy records indicate that droughts were more common, more intense and longer lasting during the MWP on the Great Plains than during the period of instrumental record (including the severe droughts of the 1930s and 1950s). Nevertheless, contemporary synoptic-scale conditions responsible for drought provide insight into the mechanisms responsible for widespread late Holocene geomorphic adjustment.