Relations for estimating the flood magnitudes for ungaged sites in Montana have been updated. The State was divided into eight regions and separate multiple-regression equations for each region were developed that relate logarithms of annual flood magnitude to logarithms of basin characteristics for exceedance probabilities of 50, 20, 10, 4, 2, and 1 percent. The standard errors of estimate for an exceedance probability of 1 percent ranged from 39 to 58 percent in the western and central parts of the State and from 47 to 83 percent in the eastern part. The standard errors of estimate indicate a substantial improvement over previous studies. Techniques for transferring annual flood-frequency information at gaged sites to ungaged sites on the same stream have been updated. Included are curves relating flood-frequency information to drainage area for eight major streams in the State. Maximum known flood peaks in Montana are compared with estimated 1-percent-chance flood peaks and with national maximum known flood peaks. Values of flood discharges for selected exceedance probabilities and values of significant basin characteristics for all gaging stations used in the analysis are tabulated. Included are data for 339 stations in Montana and 34 nearby stations in Canada and adjoining States. The purpose of this report is to present updated techniques for estimating flood magnitude for exceedance probabilities of 50, 20, 10, 4, 2, and 1 percent for unregulated streams in Montana. The relations presented herein provide more reliable predictions than those in previous studies because of more extensive streamflow-gaging records and improved analytical procedures. The report is based on gaging data from unregulated streams having at least 10 years of streamflow record. Included in the analysis are 339 streamflow-gaging sites in Montana, 8 in Canada, 14 in North Dakota, 4 in South Dakota, and 8 in Wyoming. Locations and station numbers of all gages used in the analysis are shown in figure 1. Some streamflow-gaging sites having more than 10 years record were excluded from the analysis because the data were considered to be unreliable or unrepresentative of the region.

have described techniques for estimating flood-peak magnitude and frequency in Montana. At the time of most of those analyses little information was available from small streams and the techniques were limited to use on relatively large streams. Because the relations given in this report are based upon a larger data base and were defined by a more technically rigorous analysis, they are believed to be more reliable than relations previously suggested.

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Peak-streamflow data, collected since July 1, 1955, are presented in this report. Originally, 45 crest-stage gaging stations were established. Since 1955, the program of data collection has been expanded and reevaluated. As of the end of water year 1989, data were being collected at 158 stations and had been collected at 328 total stations. The stations record peak stages of flow from upstream drainage areas that range in size from less than 1 square mile to several hundred square miles. The purpose of the program is to collect sufficient peakflow data to permit development of methods for estimating the magnitude and frequency of floods at any point in Montana. For each station, this report briefly describes the station location, drainage area, period of record available, and type of gage including altitude of gage. The tabular data consist of annual maximum gage height and discharge listed by water year and month or month and day. PURPOSE OF THE PROGRAM The purpose of the program is to collect development of methods for estimating the magnitude and frequency of floods at any point in Montana. The ultimate objective is sufficient peak-flow data to permit to determine methods for estimating the 1-percent-chance flood (one having a 100-year recurrence interval) with the reliability needed for road design. Reliability depends, in part, on having a relatively dense network of crest-stage gaging stations. REFERENCES CITED