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COLORADO Colorado is divided into three basic/general flood regions: the mountainous region, running north and south through the central part of the state, the pla- teau region to the west of the mountainous region, and the plains region to the east of the mountainous region (fig. I). The mountainous region has two distinct sub- regions, the southern subregion (essentially defined by the Rio Grande basin) named the Rio Grande Region, and the northern subregion named the Mountain Region. The plateau region has two distinct subregions, the southern subregion named the Southwest Region, and the northern subregion named the Northwest Region, The plains region has two distinct subregions, the sandhi lis subregion and the non-sandhills sub- region. There are insufficient data to describe the indi- vidual subregions, so the entire region is named the Plains Region, A complicating factor in estimating the magni- tude and frequency of floods in Colorado is mixed- population flood hydrology. A mixed-population flood- hydrology site is one where floods are caused by two or more distinct meteorological processes and can be located in a plains or plateau region and have part of its drainage area in an adjacent mountain region. The mixed-population sites are considered as being in the foothills or transition regions located between the mountain regions and the plains or plateau regions, An overall foothills region cannot be shown on a map, because within the drainage area of a site, that qualifies as being in a foothills region, there can be subregions that are entirely within a plains or plateau region and by themselves cannot be part of a foothills region. At a foothills region site, annual peak discharges are derived from snowmelt at higher elevations in the mountain regions, from rainfall at lower elevations in the plateau or plains regions, or from a combination of rain falling on snow. No one report describes the complicated flood hydrology of Colorado. In this summary, several reports are used, WESTERN COLORADO Summary Western Colorado is divided into four hydrologic subregions (fig. I), The regression equations developed for these hydrologic regions are for estimating peak discharge (QT) having recurrence intervals T that range from 2 to 500 years. The explanatory basin variables used in the equations are drainage area (A), in square miles; mean annual precipitation (P), in inches, minus 10; mean basin elevation minus 5,000 feet, per 1,000 feet (EB); and mean basin slope (SB), in feet per foot. The variables P and EB are transformed by the con- stants in the computer application of the equations. The user should enter the actual value of P and EB. These variables, with the exception of P, can be measured from topographic maps and P can be determined from the mean annual precipitation map in U.S. Weather Bureau (1967), The regression equations were devel- oped from peak-discharge records available through 1981 at 247 stations, The regression equations are applicable to streams whose flows are not significantly affected by the works of man, Standard errors of esti- mate of the regression equations range from 39 to 71 percent. Procedure Topographic maps, the map of hydrologic regions (fig. I), the mean annual precipitation map in V,S, Weather Bureau (1967) and the following equa- tions are used to estimate the needed peak discharges QT, in cubic feet per second, having selected recur- rence intervals T. Mountain Region Q2 = 74.3AO,693 SBO,894 Q5 = 81.5AO,698 SBO,719 QIO = 86.IA 0,699 5BO,635 Q25 = 91.5A 0,699 SB0.550 Q50 = 94.9AO,699 SB0.497 QlOO= 98.5AO,698 SB0.452 Q200= 102AO,6975B0.412 Q500= I06AO,696 SBO,364 COLORADO 39