SEDIMENTS AND THEIR HYDRAULIC CONDUCTIVITIES IN THE PLATTE RIVER FROM SOUTH-CENTRAL TO EASTERN NEBRASKA

The Platte River in Nebraska receives baseflow from the surrounding aquifers in some segments and recharges the aquifers in other segments. The drought events in this area in the past decade have increased the vulnerability of the river ecosystem. The streambed connects the stream water and the subsurface flow; thus, investigation of the streambed hydrogeological properties is a necessary step for understanding the hydrologic connection between the river and the aquifer in this heavily groundwater-irrigation river valley. This presentation will discuss the spatial variation of the streambed sediments, hydrostratigraphy and hydraulic conductivity from study sites dotted along the Platte River from Lexington in south-central Nebraska to Ashland in eastern Nebraska. We used Geoprobe to take sediment cores and produce electrical conductivity logs of the channel sediments. We combined in-situ and in-lab permeameter tests to determine the streambed hydraulic conductivities. Generally, the top layer streambed consists mainly of sand and gravel, contains a very small amount of silt and clay, and shows good sorting. This sediment texture leads to high permeability for the top-layer streambed in the Platte River. We believe that the hyporheic processes lead to the permeable sediments in the channel surface. However, fine-grained sediments can appear below the top-layer streambed and reduce the streambed permeability. Silt-clay thin layers occur below the channel surface at some sites but do not exist at other sites. When existing, these sediment layers reduce the vertical hydraulic conductivity of the streambed. The grain size of the top streambeds decreases downstream, primarily due to the input of the finer sediments from the Loup River near Columbus and the Elkhorn River in eastern Nebraska. Accordingly, the hydraulic conductivity of the streambed also decreases in eastern Nebraska.