USING CHEMICAL ANALYSES OF POINT-BAR SAND ON A SHARED FLOODPLAIN OF THE MISSISSIPPI AND MISSOURI RIVERS TO IDENTIFY THE SOURCE

Geomorphology of a shared 6.5 km-wide floodplain near the confluence of the Mississippi and Missouri rivers suggests that the Missouri River with an order of magnitude greater sediment load but an order of magnidude less discharge than the Mississippi River dominates the floodplain. The larger straight Mississippi River channel seems to have stayed in close proximity to its current location along the limestone bluff at the northern floodplain margin. In contrast, the smaller sinuous Missouri River has numerous abandoned channels that occupy the southern floodplain which is 7 m higher in altitude than the Mississippi side of the floodplain. A 0.3 km-wide medial area between the rivers with thick surfical overbank sediment cannot be equivocably identified as belonging to either stream based on geomorphology.

Samples for chemical analysis were taken from the modern channels of both rivers and along a transect of five cores perpendicular to the floodplain orientation where the present rivers are within 3.8 km of each other. Two of the cores are from the questionable medial area. Chemical analysis of the fine sand fraction from point-bar sand shows that the sediment deposited by each river can indeed be distinguished. Of the 47 elements analyzed, 16 elements have distinctly different concentrations and the bivariate distribution of these elements is statistically significant. This allows identification of the Mississippi River as the sediment source in the medial floodplain where the source is not clearly defined. Thus the Mississippi and Missouri channel deposits actually have approximately the same aerial extent across the floodplain, but the relatively high altitude along the Missouri River may reflect the order of magnitude higher sediment load of the Missouri River compared to that of the Mississippi River.

Optically stimulated luminescence (OSL) dates show that this floodplain is very young, with the basal channel fill of the oldest abandoned Missouri River channel (based on cross -cutting relationships) in the study area dated 3.19 ± 0.28 ka. The Mississipi River point-bar sand in the nondescript medial area is cross cut by the Missouri River channels and must be somewhat older than 3.2 ky.