CONTRASTS BETWEEN THE LATE QUATERNARY GEOLOGIC RECORDS OF THE NIOBRARA NATIONAL SCENIC RIVER (NIOB) AND THE MISSOURI NATIONAL RECREATIONAL RIVER (MNRR) OF THE CENTRAL GREAT PLAINS: RELATIONS TO LATE PLEISTOCENE GLACIATION OF THE LAURENTIDE ICE SHEET

Two National Park Service units of the National Wild and Scenic River System, NIOB and MNRR, have markedly contrasting late Quaternary geologic and geomorphic records, which are both hydro-geologically related to the late Pleistocene record of the James Lobe of the Laurentide Ice Sheet. The Niobrara, one of several rivers that dissect the eastward slope of the north-central Great Plains, is one of many tributaries of the Missouri River. The course of the Missouri River through the Dakotas largely coincides with the limit of late Wisconsin glaciation in the Dakotas, and thus reflects the strong influence of glaciation. The Niobrara River watershed includes large areas of eolian sand and was not glaciated except along a few kilometers near its confluence with the Missouri River. The gradient of the Niobrara River is about 10 times higher than that of the Missouri River. During the Holocene, the Niobrara has incised more than 60 m through high terrace sediments that were deposited as valley floor fluvial sediment of latest Pleistocene age (ca 14-10 ka). In contrast, the Missouri had at least 10 m of net valley floor aggradation during postglacial time as the late Pleistocene valley train outwash system was transformed to a postglacial system with a geologic record dominated by meander belts in response to a high, nonglacial sediment supply. We hypothesize that the late Pleistocene high terrace of the Niobrara was graded to a proglacial lake in the area of the present river mouth where it was dammed by the James Lobe at its latest Pleistocene extent. Thus the large amount of postglacial incision by the Niobrara is partly a response to a base-level decrease from deglaciation at its downstream end.