Main-Stem Seepage and Base-Flow Recession Time Constants in the Niobrara National Scenic River Basin, Nebraska, 2016–18

Abstract

The Niobrara River of northern Nebraska is a valuable water resource that sustains irrigated agriculture and recre-ation, as well as a diverse ecosystem. Large-quantity with-drawals from the source aquifer system have the potential to reduce the flow into the river and to adversely affect the free-flowing condition of the Niobrara National Scenic River (NSR). Therefore, to understand the magnitude and characteristics of those flows, the U.S. Geological Survey (USGS), in cooperation with the National Park Service, began a study to quantify seepage gains/losses along the eastern half of the Niobrara NSR and to create a map characterizing the base-flow recession time constant (tau) in the Niobrara NSR study area. In 2016, a seepage study was completed to quantify seepage gains/losses along the eastern half of the Niobrara NSR. The seepage study results indicated that the main-stem stream-flow on the Niobrara River increases 375 cubic feet per second (ft3/s) in the 39.9-mile study reach (river mile 119.3 to river mile 79.4). Although most of the streamflow increases are attributed to tributary inflows (297 ft3/s, 79 percent), 78 ft3/s are attributed to seepage gains within the reach. Seepage rates in the study reach ranged from 1.41 cubic feet per second per mile ([ft3/s]/mi) to 2.56 (ft3/s)/mi, with a mean seepage rate of 2 (ft3/s)/mi. Tau values were calculated at 10 sites in the Niobrara NSR study area, and kriging geostatistical techniques were used to develop a contour map to estimate tau values at loca-tions where streamflow was not measured. The minimum tau value was 12.1 days at Willow Creek at Atwood Road near Carns, Nebraska (USGS station 06463670), and the maximum value was 45.5 days at Tyler Falls at Fort Niobrara National Wildlife Refuge near Valentine, Nebr. (USGS station 06461150).