HOLOCENE INCISION INTERRUPTED BY AGGRADATIONAL EPISODES ALONG THE MIDDLE NIOBRARA RIVER, NORTHERN NEBRASKA: COMPLEX LANDSCAPE RESPONSE TO POSTGLACIAL CLIMATE HISTORY

Stratigraphic and geomorphic relations in central northern Nebraska demonstrate that 60 m of net incision by the Niobrara River since latest Pleistocene time was interrupted after mid-Holocene by 8 m of valley aggradation (from a grade of a few meters above the present grade) before resumption of ongoing incision. These new findings result from recently initiated geologic mapping of the valley corridor of the Niobrara National Scenic River of the National Park Service and the Middle Niobrara Preserve of The Nature Conservancy. Exposure of stratigraphy underlying fluvial terraces was provided by the unplanned runaway drainage of a stock pond in ~2000, which exposed 8 m of fluvial sand that had buried Cedar trees (Juniperus virginiana) in growth position beneath a fluvial terrace defining a former river grade about 10 m above the present river. At a location ~100 m downwash in the same gully, similar fluvial sand underlying a younger inset terrace had buried an organic horizon, which may be the lateral equivalent of the soil in which the Cedar trees were rooted. The higher 10 m-terrace in this area is contiguous with a surface forming the infill of a broad incised meander, and indicates the incised meander was cutoff during the more recent incision cycle. The most recent incision cycle has left a record of more than 8 inset terraces with bounding scarps during progressive incision, punctuated by terrace formation. The high terraces of late Pleistocene age and underlying Cenozoic record are based on Voorhies (1987) and on extensive previous work by scientists of the Nebraska Geological Survey, Nebraska State Museum, and American Museum of Natural History. The late Pleistocene and higher terraces are commonly mantled by young dunes of relatively simple morphology and low amplitude, which change southward into higher amplitude and more complex and varying forms of the Sand Hills. Factors that drive fluvial incision, aggradation, and terrace formation relate to a complex variety of climatic controls on sediment supply, fluvial discharge and stream power, vegetation cover, the eolian system, the groundwater system, seasonal river ice conditions, and base level control such as late Pleistocene proglacial lakes, or dune-dammed lakes.