STREAMBED HYDRAULIC CONDUCTIVITY IN CENTRAL AND WESTERN NEBRASKA

Streambed hydraulic conductivity (K_s) is a fundamental parameter controlling the exchange of water across the stream-aquifer interface. However, in central and western Nebraska, few data on this parameter are available. The U.S. Geological Survey, in cooperation with the Nebraska Platte River Cooperative Hydrology Study (COHYST) and the University of Nebraska-Lincoln, performed successive studies to better define K_s in streams of central and western Nebraska. The results of a pilot study to determine the most appropriate instream technique for measuring K_s were that in the sandy streambeds of seven sites falling-head permeameter tests analyzed using the Hvorslev solution were the most suitable for characterizing the K_s of the top foot of the sediments. However, in cases where ground-water/surface-water interactions were restricted by relatively fine-textured sediments present below the top foot, alternative methods were utilized to determine K_s. Following the pilot study, streambed samples were collected at 58 sites to determine sediment texture to a depth of 4 feet. At 15 sites where sediments with the finest texture were deeper than 1 foot, vertical profiles of K_s were determined using slug-tests. At 22 sites without a low-permeability layer deeper than 1 foot, falling-head permeameter tests were performed. Results from these studies indicate that in central and western Nebraska K_s is generally higher in the sandy streambeds of main-stem rivers, whereas tributary streams located outside the floodplains of those main-stem rivers are characterized by lower K_s values.