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t99a. Colorado Rood Hazard Mitigation Plan (40B) F'loods in June 1983, along the Cache la Poudre River in Fort Collins and Greeley; along Clear Creek and its tributaries in Silver Plume and Georgetown; and along the Arkansas River in Fremont and Chaffee- counties, were principally due to mefiing snow_ The 1984 floods on the westem slope were primarily snowmen flood- ing_ . 2.2.1.4 Rain on Snow Floods Rain on snowmen flooding occurs most often in Col- orado during the month of May_ It is at this time of year that large general rainstorms occur over westem Col- orado_ These rainstorms are most often caused when warm moist air from the Gulf of Mexico begins pushing far enough north that tt begins to affect westem weather. In combination with this movement of air mass is the continued possibility of cold fronts moving into Colorado from the Pacific Northwest When these weather phenomena collide, long lasting-general rain- storms can often occur. Rain on snowmelt exacer- bates an already tenuous situation as snowmelt wa- ters rush down heavily incised stream channels_ Any abnormal increase in flow from other sources usually causes streams to leave their banks_ During the spring months of May and June when rivers are running high, there is a potential for flooding due to- rain falling on melting snow_ Usually such rain is over a small part of a basin, and the resufiing flood is of short duration and may often go unnoticed in the lower reaches of a large drainage basin_ To some extent, the cloud cover associated with the rain system can slow the melting cycle and offset the compound effect_ in some cases, however, rainfall may be heavy and widespread enough to noticeably affect peak flows throughout the basin_ Flooding along the Colorado River in Grand Junction in July of 1884, along Clear Creek at Georgetown In June of 1965 and along the Gunnison and Colorado rivers at Grand Junction in June of 1983, are examples of flooding from rain on melting snow_ The effect of rain on mening snow in the Colorado River basin in 1983 was fen as far downstream as Mexico_ In 1984, rain or melting snow caused severe flooding condi- tions at Paonia_ On May 28, 1993, rain on snowmelt flooding occurred at Paonia on the North Fork of the Gunnison River The rainfall occurred over a 5-hour period during the evening_ This caused _the North Fork of the Gunnison _ River to reach its highest level since the 1984 flood season_ Many miles of agricunure land experienced severe bank erosion in unincorporated Delta County_ The stone riprap at Paonia's wastewater treatment plant experienCed severe damage as did the ban k armoring at Huff Addttion 31 (a subdivision)_ A photo File:FfIMP.Qlapte<.2.pub Chapter 2. 7 llZ1/986.14 AM