Late Quaternary Evolution of an Iconic Braided Stream: Dating the Alluvial Fill of the Platte River, Nebraska, USA

Despite the prominence of the Platte River in the literature on fluvial sedimentology, its alluvial history has not been well documented. We have studied the Platte’s alluvial fill to understand how the river system responded to climate changes over the last glacial-interglacial cycle. Geoprobe cores from multiple locations along the river’s valley in Nebraska demonstrate that the fill is dominated by sand and gravelly sand, and that deposits of silty clay through silty sand are minor components. We also generated an alluvial chronology using luminescence dating methods on sand samples collected from depths of up to 15 m below the present floodplain. Our results demonstrate that sediments dating to Oxygen Isotope Stages 2 and 3 lie at depths of 2–15 m below the present floodplain, and that portions of the upper 5–8 m of the alluvial fill dates to the Holocene. We hypothesize that: (1) the Platte River aggraded during the late Pleistocene, and (2) the upper portions of these Pleistocene sediments were reworked as the river locally entrenched into those older sediments during the Holocene. Late Pleistocene valley aggradation was likely related to increases in sediment availability in the Rocky Mountain headwaters, as well as sediment influx from local landscapes in Nebraska.