Estimates of monthly streamflow characteristics and dominant-discharge hydrographs for selected sites in the lower Missouri and Little Missouri River basins in Montana

Various streamflow characteristics were estimated for water-reservation purposes for 17 sites in the lower Missouri River Basin and four sites in the Little Missouri River Basin in Montana. The characteristics were mean monthly and annual streamflow and monthly mean streamflow that is exceeded 90, 80, 50, and 20 percent of the time. In addition, dominant-discharge hydrographs were estimated for 10 of the 17 sites in the lower Missouri River Basin and for four sites in the Little Missouri River Basin. Dominant discharge was considered to be equal to the peak discharge having a recurrence interval of two years. Monthly streamflow characteristics generally were based on a common 1937-86 base period. A mixed-station record-extension program was used to estimate missing flow data for streamflow-gaging stations. Two methods were used to estimate characteristics at ungaged sites. One method was based on corre- lating discharge measurements at the estimating site with concurrent discharges at a nearby gaged site. The second method was based on using a drainage-area ratio to transfer characteristics at a gaged site to the estimating site. Dominant discharges for gaged sites were obtained from a previous flood-frequency report or by fitting a log-Pearson Type 3 probability distribution to recorded peak-flow data. A drainage-area-ratio adjustment was used to transfer dominant dis- charges from gaged sites to ungaged sites. Dominant-discharge hydrographs were determined from visual examination of recorded hydrographs having maximum daily discharges that were relatively close to the estimated dominant discharges.