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GU0885 Niobrara River Basin Description. The Niobrara River begins as a small stream in eastern Wyoming entering Nebraska in the northwestern corner of the state. It then flows eastward roughly parallel to the northern boundary of the state until it discharges into the Missouri River above Lewis and Clark Reservoir in the northeastern corner. Ponca Creek enters Nebraska from South Dakota in Boyd County where it then parallels the Niobrara River and empties into the Missouri River up- stream from the Niobrara's mouth. The western or upper part of the basin is characterized by flat tablelands bordered on the north by the Pine Ridge. The Sandhills region extends into this area and the central part of the basin. In places in the basin. the river has formed a narrow valley with steep walls rising hundreds of feet to meet the uplands. The entire area provides many scenic vistas with coniferous trees and rock outcroppings in the western reaches and the remainder covered with a mixed torest of eastern hardwoods and western pines. Major tributaries of the Niobrara River include the Keya Paha River, the Snake River, Pine Creek, Plum Creek, Long Pine Creek, Eagle Creek and Verdigre Creek. The steady flows of the Snake River, Plum and Long Pine Creeks and other Sandhills streams contribute to the con- stancy at the Niobrara's flow. The flow in the western section is variable. Of approximately 7,582,000 total acres? almost 2.2 million acres,8 mostly in the western part of the basin, are suitable tor irrigation. How- ever, only about 347,540 acres had been devel- oped for irrigation in 1975.9 WATER USE IN 1975'0 NIOBRARA- PONCA CREEK SURFACE WATER GROUNDWATER Average Ave. Total Amount of Total Amt. of Acres Amount of Water Used Area Amt, of GW Used Irrigated Water Used Per Acre Irrigated GW Used Per Acre ACRES ACRE-FEET ACRE-FEET/ACRE ACRES ACRE-FEET ACRE-FEET/ACRE 54,600 115,100 2.11 292,940 389,600 1.33 The average annual precipitation varies from approximately 15 inches in the west to 24 inches in the east. 11 There are two major reservoirs in the basin which measure over 500 acres in surface area: Box Butte Reservoir in Dawes County, operated by the U.S. Bureau of Reclam- ation with a surface area of 1,060 acres and a storage capacity ot 3 \ ,060 acre-teet, and Merritt Reservoir in Cherry County, also operated by the Bureau with a surface area of 2,906 acres and storage capacity of 74,500 acre-feet.'2 The po- tential O'Neill Unit which would include the Norden Dam and Reservoir, is located in the central part of the basin. The mean annual inflow for the Niobrara River at the state line is 2,936 acre-feet compared with a mean outflow at its mouth of 1.124,103 acre- feet.'3 A number at streams ariSing in the Sandhills contribute to the Niobrara's flow. There are also plentiful supplies of groundwater throughout most of the basin. The quality of groundwater in the sandstone aqUifers in the west and in the Sandhills area has been described as excellent. Soils, for the most part, range from coarse sands in the Sandhills region to silts and clays on the tablelands. More productive soils can be found on the Box Butte tablelands and in the western Mirage Flats area. Interstate Arrangements. Two interstate compacts have been negotiated on the Niobrara River but only one has been finalized. The Upper Niobrara River Compact between Wyoming and Nebraska was ratified by both Legislatures in 1963 and the proposed Lower Niobrara River and Ponca Creek Compact between Nebraska and South Dakota was ratified by the Nebraska and South Dakota Legislatures in 1961, but has never been approved by Congress. The South Dakota Legislature subsequently repealed the compact. The Upper Niobrara River Compact includes the area in Nebraska and Wyoming which is naturaily drained by the Niobrara River west at Range 55 West 01 the 6th P.M. (west 01 Agate, Nebraska). It essentially gives Wyoming unre- stricted use of the surface flow of the river with the exception of those restrictions imposed by Wyoming law and a few minor requirements relating to the size of storage reservoirs with priority dales after August 1, 1957. and to when water may be stored during the year tor storage 1-5