An assessment of low flows in streams in northeastern Wyoming

Low flows were assessed and summarized in the following basins in northeastern Wyoming: Little Bighorn, Tongue, Powder, Little Missouri, Belle Fourche, Cheyenne, and Niobrara River, and about 200 river miles of the North Platte River and its tributaries. Only existing data from streamflow stations and miscellaneous observation sites during the period, 1930-80, were used. Data for a few stations in Montana and South Dakota were used in the analysis. Data were available for 56 perennial streams, 38 intermittent streams, and 34 ephemeral streams. The distribution of minimum observed flows of record at all stations and sites and the 7-day, 10-year low flows at mountain stations and main-stem plains stations are shown on a map. Seven day low flows were determined by fitting the log Pearsons Type III distribution to the data; results are tabulated only for the stations with at least 10 years of record that included at least one major drought. Most streams that originate in the foothills and plains have no flow during part of every year, and are typical of much of the study area. For stations on these streams , the frequency of the annual maximum number of consecutive days of no flow was determined, as an indicator of the likelihood of extended periods of no flow or drought. For estimates at ungaged sites on streams in the Bighorn Mountains only, a simple regression of 7-day, 10-year low flow on drainage area has a standard error of 64%, based on 19 stations with drainage areas of 2 to 200 sq mi. The 7-day, 10-year low flow in main-stem streams can be interpolated from graphs of 7-day, 10-year low flow versus distance along the main channel. Additional studies of low flow are needed. The data base, particularly synoptic baseflow information, needs considerable expansion. Also, the use of storage-analysis procedures should be considered as a means of assessing the availability of water in streams that otherwise are fully appropriated or that are ephemeral. (Author 's abstract)