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

Colorado is located in the west-central United States. Approximately 50 percent of the State is covered by the Rocky Mountains and Colorado Plateau, and the other 50 percent is Great Plains. Colorado's location and the high-elevation mountains combine to create a complex and diverse climate statewide. Mean annual precipitation ranges from 7 in. in south-central Colorado to 60 in. in the mountains (Doesken and others, 1984). The major moisture sources for Colorado are the Pacific Ocean and the Gulf of Mexico (Paulson and others, 1991). Seasonal, large-scale atmospheric circulation interacts with the mountainous topography to produce three major precipitation patterns.

The most important precpitation pattern, the mid-winter pattern, is created by frontal systems from the Pacific Ocean that are directed by the polar jetstream into Colorado from the northwest, west, and southwest. These airmasses must rise over the mountains causing a major orographic component to be added to the winter precipitation pattern. Therefore, during the winter season, most of the precipitation occurs in the higher mountains and in parts of the western valleys.

The second precipitation pattern affects the eastern half of the State during the spring and summer. In the Great Plains and the foothills of the Rocky Mountains, a substantial increase in precipitation begins in early March and continues through June. As temperatures rise, the supply of moisture from the Gulf of Mexico steadily increases over the plains east and southeast of Colorado. This moisture and increased convective activity result in periodic, widespread rainfall and occasionally severe thunderstorms east of the mountains. May and June tend to be the wettest months in the northeastern quarter of Colorado.

The third precipitation pattern is the summer monsoon, which occurs over the southern half of the State in July and August. Rainfall from thunderstorms over the mountains can become deadly flash floods as runoff is concentrated into the steep and narrow mountain valleys.

Colorado is divided into three basic flood regions: (1) the mountainous region, running north-south through the central part of the State; (2) the plateau region to the west of the mountainous region; and (3) the plains region to the east of the mountainous region. Each region has different factors affecting runoff.

On the basis of regionalization procedures that relate flood characteristics to watershed and climatic characteristics, the magnitude of maximum discharges for streams in the mountainous region of Colorado is dependent upon drainage area, mean basin slope, and mean annual precipitation. In the plateau region, maximum discharges for streams are dependent on drainage area, mean annual precipitation, and, to a small extent, mean basin elevation. The important variables for the plains region are effective drainage area (total area minus controlled or dammed area), the 100-year, 24-hour rainfall, and channel slope (Jennings and others, 1994).

The flash floods that occurred in the Big Thompson and Cache la Poudre River Basins during the night of July 31 and morning of August 1, 1976, resulted in 144 deaths and $39 million in damages (Paulson and others, 1991). The floods were a result of slow-moving monsoonal thunderstorms that produced as much as 12 in. of rain in 3 hours over the downstream part of the Big Thompson River Basin.

Doesken, N.J., McKee, T.B., and Richter, D.B., 1984, Analysis of Colorado average annual precipitation for the 1951-1980 period: Fort Collins, Colorado State University, Colorado Climate Center Climatology Report 84-4, 53 p.

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.