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potential flooding condition is that of heavy rains at a time when snow- -melt runoff is highest, in Mayor June. FACTORS AFFECTING FLOODING AND ITS IMPACT Obstructions to floodflows - Several roadways varyinq from county roads to highways to railroads cross the Cache la Poudre River flood plain within the confines of the study reach. The effectiveness of these road- ways in becoming flow obstructions depends upon such factors as the bridne opening size, the height of the roadway above the level of the flood plain, the number of piers supporting the bridge, and the amount of debris lodged in the bridge opening. These roadways can also act to divert floodflows If the roadway is elevated above the flood plain and if the roadway crosses the flood plain at an angle. The severity of this type of floodflow ob- struction and diversion depends upon the magnitude of the particular flood event. Flood damaqe reduction measures - There are no flood control structures on the Cache la Poudre River in the study reach. There are two dams on the North Fork Cache la Poudre River. The Hall igan Dam and Reservoir, located about 24 miles northwest of Fort Collins, was built in 1910 for irrigation storage and is owned by the North Poudre Irrigation Company. The r'li/ton Seaman Dam and Reservoir, built by the city of Greeley in 1947 for municipal water supply, is located about one mile upstream from the North Fork confluence with the CaChe la Poudre River. The flood control effect of both dams Is reflected in the gaging records and was therefore Considered In the analysis. Weld County has enacted regulations governing development on the flood plain of the Cache la Poudre River within that county. Larimer County has not enacted SUCh regulations to date, although they are in the process of doing such. Manmade objects such as bulldinqs, cars, fences, diversion dams, as well as trees, brush, and other veqetation located on the flood plain can cause flow obstructions. These items, while remaining in place, not only tend to create higher stages on the flood piain due to reduced flow area and flow blockage, but may also create higher localized velocities as the floodwaters flow around specific Items. If the floodflows are of such a magnitude so as to dislodqe the obstructions, flood stages may lower in that localized area, but the debris may lodge at locations further downstream thus compounding problems. This floating debris commonly lodges against bridge piers and bridge decks reducing flow area, or if severe enough, complete channel blockage can occur. The blockage can cause severe stage and velocity increases accompanied by bridge damaqe or complete failure. Considerable flOOd discharge attenuation OCCUI s along the Cache la Poudre River. This is especially evident when reviewing the stream gaging data of tables 5 and 6 and the potential peak discharges of table 7. This diSCharge attenuation is a result of stream diversion dams whiCh supply CaChe la Poudre River water to irrigation canals and of the natural water storage which occurs on the flood rlaln. Ice can also cause a flow obstruction problem similar to that described above for debris. This, however, does not appear to be a frequent problem in the study reach. Other factors and their imoacts - The potential exists for flooding along the study reach due to not only the Cache la Poudre River, but also to tributary streams such as Roxelder Creek. The flood hazard presented by the Cache la Poudre River and its tributaries can be reduced by implementing flood warning systems, flood emergency plans, and sound flood plain management concepts. FlOOd warnino and forecastinq - The National Oceanic and Atmospheric Administration (NOAA), National Weather Service provides 15 16