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--- -- -- The State Geologist has undertaken an analysis of Colorado's most dangerous canyons in which flash flooding can occur. Major canyons which are threatened by flash floods are shown in Annex "A". While the bulk of these canyons exist along the Front Range, their dispersion over the State is general; many communities with significant population are closely associ- ated with these danger areas. Flash flooding occurs annually throughout Colorado on streams and floodplains and the danger is greatest to settlements located close to the major streams identified in "Riverine Flooding". Riverine Flooding Large scale floods can develop from sustained or heavy rainfall from storm systems in the spring, summer and fall months in Colorado. But the most dangerous flood potential is in the spring when rivers are high during the snowmelt run off. Usually, rainfall in addition to snow melt run off is necessary before flood ing occurs. These floods differ from flash floods in that the speed of onset is slower and time available for warning is greater; fewer lives are lost, but millions of dollars worth of valuable farmlands, roads, bridges, and other valuable assets, are at stake. It should be noted, however, that although riverine floods may occur over most of a river system, flash floods may simultaneously occur in headwater areas where steeper gradients exist. The Rio Grande, South Platte, Arkansas, and the Repub- lican Rivers have a long history of flooding onto the plains areas. In 1965, as a result of heavy rains concentrated around the Castle Rock area, widespread flooding occurred in the Denver Metro area, loss of life and very high damage was also suffered along the Front Range and on the eastern plains. As more development takes place in the Metro area, run off potential increases and extensive damage can be expected despite new flood control dams. On the Western Slope the Colorado, Yampa, San Juan, Gunnison (N. and S. Fork), Uncompahgre, Animas, White and many other streams can be expected to flood in any given year, The most likely periods for disastrous flooding are during the spring snowmelt when rainfall occurs during peak run off periods in May and June, and when relatively heavy rainfall occurs on the Western Slope in September and October. On the Western Blope in Colorado, water volumes in the spring are normally large and gradients are relatively steep. Riverine flooding can threaten property but the distinction between flash and riverine flooding is blurred. Lives are also at stake. 5