Summary of Significant Floods in the United States, Puerto Rico, and the Virgin Islands, 1970 Through 1989

Wyoming is located in the Western Interior of the United States. Although most of the State is arid or semiarid, the mountainous areas can average more than 60 in. of precipitation per year. The topography controls the amount and distribution of the precipitation. The mountain ranges in the State generally trend north-south, and they create barriers to the moist Pacific Ocean airmasses moving east across the State. The moisture condenses into precipitation as the mountains create orographic lifting and cooling of the airmasses. The mountain ranges also create barriers to upslope systems from the Gulf of Mexico. The Laramie Range and Bighorn Mountains keep most spring snows and rains from reaching the interior basins. As a result, the mountainous areas of the State receive the majority of the precipitation. Summer thunderstorms deliver most of the precipitation in eastern Wyoming and along the mountain ranges. Wet spring snowstorms that bring moisture from the Gulf also contribute substantially to annual totals.

Flooding in Wyoming generally is caused by rapid melting of snow, intense summer thunderstorms producing flash floods, and ice jams. Some of the most severe floods are the result of spring rainstorms on a melting snowpack.

On the basis of regionalization procedures that relate flood characteristics to watershed and climatic characteristics, the factors affecting the magnitude of maximum discharges for streams in Wyoming are contributing-drainage area, mean basin elevation, mean annual precipitation, and basin slope. In the plains and high desert regions, a geographic factor is used (Jennings and others, 1994).

Rainfall combined with snowmelt to cause flooding in south-central Wyoming during May 12-17, 1984. The North Platte River and its tributaries and Little Snake River were most affected by the flood. The flooding on the Little Snake River was compounded when a dam was breached.

Jennings, M.E., Thomas, W.O., Jr., and Riggs, H.C., 1994, Nationwide summary of U.S. Geological Survey regional regression equations for estimating magnitude and frequency of floods for ungaged sites: U.S. Geological Survey Water-Resources Investigations Report 94-4002, 196 p.

Paulson, R.W., Chase, E.B., Roberts, R.S., and Moody, D.W., compilers, 1991, National water summary, 1988-89-Hydrologic events and floods and droughts: U.S. Geological Survey Water-Supply Paper 2375, 591 p.