Late Holocene Valley-Bottom Aggradation and Erosion in the South Loup River Valley, Nebraska

Radiocarbon ages of humates, bone collagen, and wood cellulose are used to construct a chronology of valley-bottom aggradation and erosion in the South Loup River Valley in central Nebraska for the past 4800 years. Three episodes of floodplain aggradation are evident from alluvial stratigraphy and radiocarbon ages at four localities. The first episode of aggradation occurred from before 3500 yr B.P. until at least 3000 yr B.P., following deep incision of the valley. The second interval of slow aggradation on the floodplain and soil formation on terraces occurred between 1800 and 1050 yr B.P. A third, rapid episode of valley-bottom aggradation occurred sometime after 1050 yr B.P. Episodes of floodplain erosion during high-magnitude floods occurred before, between, and after these intervals of aggradation. Published radiocarbon ages of peat beds, organic-rich alluvial strata, and buried soils in the North and Middle Loup River Basins reveal that the intervals of aggradation that are documented in the South Loup River Valley are synchronous throughout the Loup River Basin. While the causes for these episodes may be multiple, the effects of climate change on flood regimes and water tables explain most of the fluvial behavior. [Key words: Holocene, alluvial chronology, climate change, Great Plains.]