A moderate August 1984 rainstorm produced substantial debris flows from tributaries of Beaver Creek, a small Missouri River tributary located near Helena, Montana. The debris flows occurred only in the parts of the drainage that had been burned by an extensive forest fire just prior to the rainstorm. Peak debris discharges were determined at 31 sites by using a variation of the slope-area, superelevation, and critical-flow methods and were determined to be close to or larger than maximum known water floods in Montana.

Unit peak discharges ranged from 0.4 to 34,000 cubic feet per second per square mile, with the largest value and most spectacular debris flow occurring in Bear Gulch Creek. The Bear Gulch Creek debris flow apparently was triggered by mass erosion of burned topsoil near the drainage divide. The flow increased by bulking until it reached an estimated peak of 29,900 cubic feet per second at a measurement site 0.7 mile upstream from the mouth. The discharge rapidly attenuated to 3,100 cubic feet per second about 0.4 mile downstream. The large peak and rapid attenuation are presumed to result from debris damming and subsequent release near the upstream measurement site.

The peak debris discharge at the upstream measurement site on Bear Gulch Creek was determined by the superelevation method. The computed discharge was then used in the slope-area equation to solve for Manning's n value, and it was found that the /? value had to be doubled to account for the additional energy loss caused by the debris load. The doubling of n values was presumed to be applicable to all other discharge-measurement sites where the slope-area method was used.

The solids-to-water ratio of the Bear Gulch Creek debris flow was estimated by mixing water with a dried sample of deposited debris. The reconstituted debris slurry was 87 percent solids by weight and 72 percent solids by volume. Thus, the peak water discharge at the measurement sites can be estimated as 28 percent of the peak debris discharge. Making this adjustment to the data still results in two peak discharges occurring above the envelope curve for maximum floods in Montana.