A method for estimating magnitude and frequency of floods in Montana

This report provides methods for estimating flood characteristics at most natural flow sites on rural streams in Montana. It also contains significant flood data and related information for many gaged sites on Montana streams. Frequency curves are provided for 442 gaged sites as defined by log-Pearson Type III analysis. To allow estimates at ungaged sites, mathematical equations relate the 2-, 5-, 10-, 25-, 50-, and 100-year flood magnitudes to basin characteristics. Drainage area, main channel slope, and mean annual precipitation were found to be the most significant estimating variables. Equations presented are limited to use on streams with drainage areas from about 0.1 to 2,600 square miles (0.3 to 6,700 square kilometres), with slope from about 5 to 1,200 feet per mile (1.5 to 366 metres per kilometre), and with precipitation from 10 to 100 inches (250 to 2,500 millimetres).

Nomographs provide a simple graphical means of solving the estimating relations, and illustrative examples are presented.