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The middle and upper portions of Cottonwood Creek around Colorado Sp~ings have unde~gone channel imp~ovements consisting of channel widening, riprap channel banks, and levee systems. The National Oceanic and Atmosphe~ic Administration, in coope~ation with communities within EI Paso County, has established a Flash Flood Warning System consisting of observers and automatic ala~m systems (Reference 9). In the EI Paso County Land Development Code, the~e is a zoning ~egulation (IV-Section 32, 1980) rest~icting the construction of structures in the delineated flood plains. The flood plain districts restrict land use to agriculture, grazing, recreation, and parking areas. 3.0 ENGINEERING ~rETHODS For ~he flooding sources studied in detail in El Paso, standard hydrologic and hydraulic study methods were used to determine the flood hazard data required for this study. Flood events of a magnitude which are expected to be equaled or exceeded once on the average during any 10-, 50-, 100-, or 500-year period (recurrence interval) have been selected as having special significance for flood plain management and for flood insurance rates. These events, commonly termed the 10-, 50-, 100-, and SOO-year floods, have a 10, 2, 1, and 0.2 percent chance, respectively, of being equaled or exceeded during any year. Although the recurrence interval represents the long-term average period between floods of a specific m~g~itude, rare floods could occur at short intervals or even within the same year. The risk of experiencing a rare flood increases when periods greater than 1 year are considered. For example, the risk of having a flood which equals or exceeds the 100-year flood (1 percent chance of annual exceedence) in any 50-year period is approximately 40 percent (4 i~ 10). and, for any 90-year period, the risk increases to approximately 6~ percent (6 in 10). The analyses reported herein reflect flooding potentials ~ased on conditions existing in the co~~unity at the ti~~ of co~pletion of this study. Maps and flood elevations will be amended pe~i0dically to reflect future changes. 3.1 Hydrologic Analyses Hydrologic analyses were carried out to establish the peak discharge- frequency relationships for each flooding source studied in detail affecting El Paso County. Fou~ sources of info~mation were used to estimate peak discharge- f~equen~y relationships for the streams studied in detail: (1) hy- drology studies designated by the State of Colorado for local use in =lood plain regulation: (2) material adopted from a 1976 COE hyd~ologic report for El Paso County (Reference 10); (3) new mate- rial gene~ated f~om other existing information and re~orts, or (4) new hya~ologic dat~ developed by CD~. 11