EVOLUTION OF RIVER VALLEYS IN CENTRAL AND SOUTHERN NEBRASKA: MIDDLE WISCONSIN TO PRESENT

Incision and aggradation of the Loup, Platte, and Republican River valleys has been reconstructed from stratigraphic relationships exposed in cutbanks, roadcuts, and excavations for canals and dams. The chronology of fluvial events has been dated by radiocarbon.

The greatest depths of Late Quaternary incision are within the Loup River Basin in central Nebraska and in tributaries to the Republican River in southwestern Nebraska and northeastern Colorado. Uplift probably explains much of the spatial variability of incision depths, but the thickness of Late Quaternary eolian deposits is a contributing factor to at least Late Wisconsin and Holocene depths of valley incision. Finally, climate changes, dune blockages of rivers, dune-dam failures, groundwater levels, and drainage captures often explain local, within-basin depths of incision.

Late Quaternary evolution of the Loup, Platte, and Republican River valleys involves multiple episodes of incision and valley aggradation. These valleys were deeply incised about 30,000 yr B.P. and were stable (soils formed). The Middle Wisconsin was generally a time of valley aggradation with alluvium, colluvium, and loess of the Gilman Canyon Formation (Severence Fm in eastern Nebraska). The details of aggradation vary from valley to valley, but radiocarbon ages of humates in buried soils cluster at approximately 28,000, 24,000 and 21,000 yr B.P., suggesting multiple episodes of aggradation (and intervening episodes of incision). A major episode of incision and drainage capture occurred within the Loup and lower Platte River basins by about 21,000 radiocarbon yrs B.P. There is evidence of deep incision sometime after 20,000 yr B.P. and before 14,000 yr B.P. in the Loup and Republican River valleys. The thickness of Peoria Loess that overlies Late Wisconsin alluvium (Todd Valley Member of Peoria Formation) generally supports incision shortly before 14,000 yr B.P. There may have been some drainage captures within the Loup River Basin at this time, but most probably occurred between 24,000 and 21,000 yr B.P. Deep incision occurred throughout the basins between 14,000 and 10,500 yr B.P. Valleys apparently stabilized sooner in the western regions where eolian sediment inputs may have been more meager. Valleys filled episodically from 10,500 to 4,000 yr B.P. Shortly after 4,000 yr B.P. incision, interrupted by episodic aggradation, formed the deep, modern valleys.