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<br />2.2.4 General Rain Floods <br /> <br />General rain floods ca,1 result from moderate to heavy <br />rainfall occurring over a wide geographic area lasting several <br />days. They are characterized by a slow steady rise in stream <br />stage and a peak flood of long duration" As various minor <br />streams empty into larger and larger channels, the peak discharge <br />on the mainstem channel may progress upstrE!am or downstream (or <br />remain stationary) over a considerable length of river. <br /> <br />General rain floods can result in considerably large volumes <br />of water. The general rain flood season is historically from the <br />beginning of May through October. <br /> <br />Because the rate of rise is slow and the time available for <br />warning is greatl few lives are usually lost, but millions of <br />dollars in valuable public and prilrate property are at risk. <br /> <br />The October 5, 1911 floods in pagosa Spr ing sand Durango <br />were a result of a general rain system ove,r tributaries of the <br />San ,Juan River basin in southliestern Colorado, The June 3,1921 <br />flood in Pueblo was a result of a general rain system in the <br />Upper Arkansas River Basin. '~he damaging floods of June 1965 in <br />the Denver-Metro area were a result of heavy to torrential <br />rainfall over large portions of the South Platte River basin <br />which lasted several days. <br /> <br />2,2,5 Thunderstorm Floods <br /> <br />Damaging thunderstorm floods are caused by intense rain over <br />basins of relatively small aerial extent. 1~hey are characterized <br />by a sudden rise in stream ll~vel, short duration, and a <br />relatively small volume of runoff, Because there is little or no <br />warning time, the term "flash flood" is often used to describe <br />thunderstorm type floods. <br /> <br />The thunderstorm flood season in Colorado is from the middle <br />of July through October. <br /> <br />The widely pUblicized Bi9 Thompson Canyon flood disaster of <br />July 31, 1976, was a result of an intense thunderstorm cell which <br />dropped up to 10 inches of rain in a few hours over the bas in, <br /> <br />2,2,6 Snowmelt Floods <br /> <br />Snowmelt floods are typically characterized by smooth <br />diurnal fluctuation in dischal:ge rate with each successive daily <br />peak discharge slightly greater than in the previous day. The <br />cumulative hydrograph exhibitH a ,;tair-step shape rising over a <br />period of several days. Snowllelt floods generally yield a large <br />volume of runoff water in (~omparison to thunderstorm floods, <br /> <br />-37.- <br />