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Numerous irrigation canals cross the Dry Creek channel and directly intercept drainage flows, as mentioned previously. Several of these canals, including the Larimer and Weld Canal (Eaton Ditch), the Larimer County Canal, the Terry Inlet, the Poudre Valley Canal, and the North Poudre Ditch, ~ave large enough flow capacity to significantly affect flood magnitudes on Dry Creek, For the more frequent flood events, the Larimer and Weld Canal (Eaton Ditch), the Larimer County Canal, and the Terry Inlet were assumed to intercept all Dry Creek flows reaching their channels (Reference 1). As development within the basin continues, these canals will intercept greater flows on a more frequent basis. . This will increase the probability of sporadic overflows from the canals, which can cause severe damage, particularly if an embankment fails. For this reason, separation of drainage flows and irrigation flows should be considered for at least the lower frequency events (Reference 1). There are no flood-control structures on the Cache La Poudre River in the study area, although there are two irrigation dams on the North Fork Cache La Poudre River, The effect of the two dams is considered negligible in the study area. There are no flood- control structures that reduce discharges on Buckhorn Creek, Redstone Creek, Fish Creek, Black Canyon Creek, or the Fall River. 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 SOO-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 avera2e 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 exceedence) in any 50-year period is approximately 40 percent (4 in 10), and, 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. 11