FORMATION OF SAND RIDGES IN THE EASTERN PLATTE RIVER VALLEY, NEBRASKA

Sand ridges on the floodplain in the eastern Platte River valley are elongate features that range from approximately 150-1600 meters in length and 20-40 meters in width. They are fairly dramatic features as they lie approximately 3-4 meters above the low relief landscape of the surrounding floodplain. The ridges are preferentially found on the southern and eastern banks of abandoned channels of the Platte River. Historically, there has been abundant research that focused on the active Platte River channel bed, its sediment load, and the morphology of the river’s sand bars, however no studies have addressed the formation of these ridges which is the objective of this investigation. Thirteen cores were collected for the project, seven near the town of Silver Creek, and three from Two Rivers State Recreational Area. The ridges were cored using three inch aluminum pipes that are vibrated into the ground by way of a backpack-mounted vibracore apparatus. Cores penetrated to depths of up to 3.8 meters. Particle size analysis was conducted using laser diffraction on samples taken at 10 cm intervals in each core. The ridge fills are predominantly very fine to medium sand with zones of coarse sand. The upper portions of each sand ridge core showed no apparent bedding, but prominent lamination is present in all cores below a meter of the soil surface. OSL dating was used to determine the time of deposition for individual sediment packages within the cores and estimate the depositional ages for all of the sampled sand ridges and adjacent alluvial plain. Nine Preliminary OSL ages suggest the ridges sampled were deposited approximately 500-1,500 years ago. Ongoing investigations are being conducted to investigate whether the sand ridges were deposited by eolian or alluvial processes.