Equations are presented for estimating flood magnitudes for ungaged sites in Montana based on data through 1983. The State was divided into eight regions based on hydrologic conditions, and separate multiple-regression equations were developed for each region. These equations relate annual flood magnitudes and frequencies to basin characteristics and are applicable only to natural-flow streams. In three of the regions, equations also were developed relating flood magnitudes and frequencies to basin characteristics and channel-geometry measurements. The standard errors of estimate for an exceedance probability of 1 percent ranged from 39 to 87 percent. Techniques are described for estimating annual flood-magnitude and flood-frequency information at ungaged sites based on data from gaged sites on the same stream. Included are curves relating flood-frequency information to drainage area for eight major streams in the State. Maximum known flood magnitudes in Montana are compared with estimated 1-percent-chance flood magnitudes and with maximum known floods in the United States.
Values of flood magnitudes for selected exceedance probabilities and values of significant basin characteristics and channel-geometry measurements for all gaging stations used in the analysis are tabulated. Included are 375 stations in Montana and 28 nearby stations in Canada and adjoining States.
