THE SAGA OF THE MISSOURI RIVER FROM LGM TO TODAY, AND WHAT THE VALLEY FILL TELLS OF THE RIVER’S RESPONSE TO ICE, RAIN, AND GIA

The modern Missouri River is continental-scale, and long interpreted as a glacial river in a non-glacial time. Its place in the Quaternary history of North America is clearly prominent, but its assumed glacial past is based indirectly on valley position and flanking loess. The details of its Quaternary role are written in the valley fill. These valley fill strata have thusfar escaped close scrutiny, leaving a notable void in the glacial anthology of western North America. This study summarizes the work of over 50 students in 61 floodplain maps over the last 20 years, and is supplemented by water-well and OSL data to collectively generate a chronology of Missouri Valley stratigraphy. These data reveal five surfaces atop five alloformations recording incision and aggradation of the river since the LGM. The elevated Malta Bend surface formed in the LGM when glacial sediment load was high compared to meltwater discharge. Breakdown of the Laurentide and Cordilleran ice sheets at the end of the LGM caused up to 18 m of incision to generate the Carrolton braided outwash surface. By 13.5 ka the outwash phase had ended, the river aggraded from lower Carrolton levels to within 4 m of the Malta Bend surface, and the meandering Salix surface formed. With the Younger Dryas at about 12 ka the river re-incised briefly to Carrollton levels to form the short-lived Vermillion surface. By 8 ka, the river had aggraded back to Malta Bend elevations to form the Holocene and current Omaha meandering-to-braided surface. Some conclusions are apparent from these data. First, the Carrolton incision coincides with incision in all the major rivers draining the LGM ice sheets. It confirms the “Big Wash,” a worldwide valley incision phase in response to the breakdown and draining of the LGM cryosphere. Second, incision cycles do not match models of GIA and appear to be climate driven. Third, the Holocene channels record continental tilting because of glacial rebound and forebulge relaxation, and argue for a forebulge axis at least 200 km farther south than models predict. Fourth, the position of the Missouri Valley appears to reflect longterm pinning of the river between the ice sheet and the forebulge axis. Lastly, the valley was carved by a pre-LGM glacial tongue in the reach extending ~100 km up dip of Sioux City, IA.