PRELIMINARY INVESTIGATION OF PRACTICAL METHODS FOR ESTIMATING LOW-FLOW FREQUENCIES OF STREAMS IN SOUTHWESTERN MONTANA
A report to Yellowstone Tongue Areawide Planning Organization, Broadus, Montana
From Earth Science Services, Inc., Bozeman, Montana
1976
PURPOSE OF INVESTIGATION
The purpose of this investigation was twofold. The main purpose was to examine the accuracy of certain practical methods for estimating low-flow frequencies of (1) ungaged streams and (2) streams with short periods of streamflow records. A secondary purpose was to produce low-flow frequency estimates for the Little Powder River near Broadus, Montana.
METHOD FOR UNGAGED STREAMS
A method for estimating the flow-duration curves for ungaged streams was described by Wisler and Brater (1959). In this method, two gaged streams are used to estimate the flow-duration curve for an ungaged stream. The two gaged streams selected are ones that have drainage basin characteristics most like the characteristics of the ungaged stream. Flow-duration curves are plotted for the gaged streams, and the flow-duration curve of the ungaged stream is estimated by interpolating a curve between the two known curves. However, in the present study, it was necessary to extrapolate (instead of interpolate) from the two known curves, because the ungaged stream (simulated by the Little Powder River) was expected to have low-flow characteristics that were not intermediate to the characteristics of the two gaged streams. This unfortunate circumstance existed because no appropriate gaged streams were found that were expected to have flow-duration curves that would bracket the curve of the Little Powder River.
The two gaged streams selected for comparison streams in the present investigation were Little Beaver Creek and Box Elder Creek. These were the only streams in the area which had long periods of streamflow record and which could be expected to have a flow regimen similar to the Little Power River. Other streams with long periods of record have tributaries in high mountain areas, whereas the Little Powder River does not. Melting of the mountain snowpack was expected to produce a significantly different flow-regimen. The drainage basin characteristics of the Little Powder River and the two comparison streams are shown in Table 1.
Table 1. Drainage basin characteristics.