Three-Dimensional Character of the Holocene Platte River Alluvium in Nebraska

John D. Horn, Christopher R. Fielding, and Robert M. Joeckel
Geosciences, University of Nebraska, Lincoln, Lincoln, NE

This paper represents the first investigation into the three-dimensional character of the Holocene Platte River alluvium in Nebraska. During the Late Pleistocene-Holocene, the central Platte River incised a valley with ~50m thick deposits of complex channel forms near Grand Island, Nebraska. Both active and inactive channel components of this system reflect secular changes in bar morphology arising from flow variations. The channel bodies have been quantified using 10-m resolution Digital Elevation Models, Soil Survey data, 7 ½ minute topographic maps and aerial photography. Ground-Penetrating Radar was used to determine stratigraphic architecture of the fluvial features on the two surfaces present in the study area. Qap2 is a middle-late Holocene surface that comprises several inactive braided channels. The currently active surface, Qap1, consists of an inactive and an active system divided into several anabranches. Channels in both surfaces have barform stratigraphy similar to other braided stream systems. The effects of anthropogenic modification, such as reservoir dams and diversion canals, have been studied using sequential aerial photographs. These effects have caused a decrease in sediment load and discharge by ~30%; and have given the active system a more anastomosing character with incised channels and wooded islands. These flow regime changes are artificial, but resemble those expected in a change to dry climate conditions. The abandoned channels are between 0.6 and 1.2 km wide with typically transverse bars. These data can be used towards a model for similar deposits in the subsurface to determine the nature of a potential reservoir.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas