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<br /> Recent Presidentially Declared Major Disasters <br /> River <br />Year ' Location Basin Cause <br /> ,>" '.,-:~" ,..,.; <br />1965 Front Range._ . S, Platte Sustained Rainfall <br /> 33 counties; Arkansas <br />1969 Front"Range S, Platte Sustained Rainfall - <br /> 15 counties' <br />1970 S9uthwest Colorado Sustained Rainfall <br />1\)73 (1) Kersey S, Platte Dam Failure <br /> (2) Front Range S, Platte Sustained Rainfall <br /> 13 Counties <br /> (3) Southwest Colorado Sustained Rainfall <br /> 13 Counties <br />1976 Big Thompson Flash flooding, heavy <br /> Front Range rainfall over short <br /> 2 counties S. Platte duration <br />1982 lawn Lake S, Platte Dam Failure <br /> Front Range <br /> 1 county <br /> <br />Causes of Floods- <br />Floods in Colorado occur on "riverine" systems which consist of a <br />basin or watershed and channels ranging from small streams to <br />major rivers, which convey the normal flow of water through the <br />watershed. The area adjacent to the channel is the floodplain. A flood <br />is aJlow of water greater than the normal carrying capacity of the <br />stream channel. The rate of rise, magnitude, duration. and frequency <br />of floods are a function of specific physiographic characteristics of <br />the basin. For example, the rise in water surface elevation is quite <br />rapid on small and steep gradient streams and slow in large and flat <br />sloped streams. <br />The causes of floods are the accumulation of water from pre- <br />cipitation in a variety of forms or the failure of man-made structures <br />such as dams or levees. There are some who feel that a heavy snow- <br />pack is the chief cause of serious flooding and that once the majority <br />of the snowpack has melted, the state is safe from flooding. Others <br />feel flooding is only a result of isolated thunderstorms in mountain <br />canyons. These notions do not, however, reflect the full variety of <br />causes of flooding related to precipitation which are: <br />1) rain in a general storm system, <br />2) rain in a localized intense thunderstorm, <br />3) meiting snow, <br />4) rain on melting snow, <br />5)' ice jams. <br /> <br />Precipitation in each of Colorado's river basins is related to the <br />seasons and to two major sources of moisture. Summer showers and <br />thunderstorms that occur from June through September are caused <br />by moisture from the Gulf of Mexico or the Pacific Ocean. During the <br />fall, occasional general rainstorms and thunderstorms occur from wet <br />and warm cyclonic air masses which move in from the southern Pacif- <br />ic Ocean. Winter and spring rain and snow storms are generally a <br />result of moist air masses which originate in the cooler northern <br />Pacific Ocean and move inland across the Pacific Northwest. <br /> <br /> <br /> <br /> <br />Polar Combination <br />AirMass <br /> <br />Weather Cycle <br /> <br /> <br />\' <br />11\' <br /> <br /> <br /> <br /> <br />2 <br />