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199B. Colorado F10utI Hazard Mitigation Plan (409) Floods in June 1983, along the Cache Ia Poudre River in Fort Collins and Greeley; along Clear Creek and ~s tributaries in Silver Plume and Georgetown; and along the Arkansas River in Fremont and Chaffee counties were principally due to melting snow. The 1984 flood~ on the western slope were primarily snowme~ flood- ing. 2.2.1.4 Rain on Snow Floods Rain on snowme~ flooding occurs most often in Col- orado during the month of May. It is at this lime 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 it begins to affect western 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 Iasting'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 metting snow. Usually such rain is over a small part of a basin, and the resuiting 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 me~ing 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 fiooding from rain on me~ing snow. The effect of rain on me~ing snow in the Colorado River basin in 1983 was fe~ as far downstream as Mexico. In 1984, rain or melting snow caus6d severe flooding condi- tions at Paonia. On May 28, 1993, rain on snowme~ 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 agriculture land experienced severe bank erosion in unincorporated Delta County. The stone riprap at Paonia's wastewater treatment plant experienced severe damage as did the bank armoring at Huff Addition 31 (a subdivision). A photo FIIe:FHMP-Chapter-2.pub ;.-;;,-,-'-,,--.....,..;....,~~~..... ."-' -..' ...,-"..-------' Chapter 2 - 7 1/27118 6:14 AI8