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Several irrigation ditches cross the study basins, notably the South Canal (fed by the Gunnison Tunnel from the Gunnison River) and the Loutsenhizer Canal (which draws from the Uncompahgre River). The South Canal typically carries from 900 to 1,000 cfs of flow, while the Loutsenhizer conveys approximately 200 cfs. Other irriga- tion ditches in the basin include the Vernal Mesa Ditch, the Hairpin Ditch, the Garrett Ditch, the AM Lateral, and the AB Lateral. These ditches carry significantly less water than either the South Canal or the Loutsenhizer Canal (i.e., 50 cfs or less). 3.0 ENGINEERING METHODS For the flooding sources studied in detail in the community, 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 premium rates. These events, commonly termed the 10-, 50-, 100-, and 500-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 magnitude, 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 occurrence) in any 50-year period is approxi- mately 40 percent (4 in 10), and, for any 90-year period, the risk in- creases to approximately 60 percent (6 in 10). The analyses reported herein reflect flooding potentials based on conditions existing in the community at the time of completion of this study. Maps and flood eleva- tions will be amended periodically 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 the community. Peak discharges for floods of the 10-, 50-, 100-, and 500-year recurrence intervals for the Uncompahgre River were computed in April 1979 by the U.S. Army Corps of Engineers (Reference 1). In the U.S. Army Corp of Engineers study, frequency curves for rainfall and snowmelt events on the Uncompahgre River at the Delta and Colona stream gages were developed. The Delta gage is located approximately 30 miles downstream of Montrose, and the Colona gage measures flow on the Uncompahgre River approximately 10 miles up- 9