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carried west by Government Highline Canal and spilled into Indian Wash. Thus the canal provides protection for the irrigated lands immediately below the canal. Of the total watershed area above Government Highline Canal, app.oximately 9.6 square miles (90 percent) are affected by the IW-l structure and canal. The Indian Wash channel has recently been channelized by the city, from just below North Avenue to Texas Avenue. This project, which also included the replacement of several stream crossings, increased the capacity of the Indian Wash Channel. The design discharge for the project was 1,000 cfs. Figure 6 shows a typical section of the project. The Horizon the city. flows up to Dri ve Channel has been channel i zed throughout much of Long condui t s convey minor flows around Mesa Mall and the 500-year flood between 25t and 26 Roads. 3.0 ENGINEERING METHODS For the flooding sources studied by detailed methods ln the community, standard hydrologic and hydraulic study methods were used to determine the flood hazard data required for this study. Flood events of a magni tude 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 floodplain management and for flood insurance 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 laO-year flood (1 percent chance of annual exceedence) in any 50-year period is approximately 40 percent (4 in 10); for any 90-year period, the risk increases 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 elevations will be amended periodically to reflect future changes. 3.1 Hydrologic Analpes Hydrologic analyses were carried out to est.ahlish peak discharge-frequency relationships for each flooding source studied by detailed methods affecting the community. In 1985 the COE performed a hydrologic study of the Colorado River (Reference 10). A hydrology report on the Colorado prepared by J. F. Sato and Associates, River rnc. dated May 1989, was (Reference 11) In thi s 13