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" J;.<" ',,, ~ SOURCES OF MOISTURE Rainfall and melting snow in the mountains feed four major river systems with headwaters in Colorado. These are the Missouri, Arkansas, Rio Grande, and Colorado River basins as shown in Figure 1. Precipitation in each basin is related to the seasons and to two major sources of moisture, Summer showers and thunderstorms, which occur primarily from June through September, are caused by moisture from the Gulf of Mexico or the Pacific Ocean, During the fall, occasional general rainstorms and thun- derstorms occur from wet and warm cyclonic air masses which move in from the southern Pacific Ocean. Winter and spring rain and snow storms are generally a result of moist air masses which originate in the cooler northern Pacific Ocean and move inland across the Pacific Northwest. FLOOD TYPES Floods in Colorado may be caused by: I} Rain in a general storm system. 2} Rain in a localized intense thunderstorm. 3} Melting snow. 4} Rain on melting snow. 5} Ice jams, 6} Failure of man-made structures (dams and levees). Each of these causes results in floods which have distinct characteristics relative to rate of rise, duration, volume, and flood season, General rain floods result from moderate to heavy rainfall lasting several days over a wide geographic area. They are characterized by a slow steady rise in sFream depth and a peak flood of long duration. As various minor streams empty into larger and larger channels, the peak discharge on the mainstem channel may progress upstream or downstream (or remain stationary) over a considerable length of river. General rain floods can result in con- siderably large volumes of water. The general rain flood season is histori- cally from the beginning of May through October. Because the rate of rise is slow and the time available for warning is great, the possibility of saving lives is increased but millions of dollars in valuable public and private pro- perty are put at risk, The October 5, 1911, floods in Pagosa Springs and Durango were a result of a general rain system over tributaries of the San Juan River basin. The June 3, 1921, flood in Pueblo was a result of a general rain system in the Upper Arkansas River basin, The damaging floods of June 1965 in the Denver metropolitan area were a result of heavy to torrential rainfall over large portions of the South Platte River basin which lasted several days. Damaging thunderstorm floods are caused by intense rain over basins of relatively small aerial extent, They are characterized by a sudden rise in stream level, short duration, and relatively small volume of runoff. Because there is little or no warning time, the term "flash flood" is often used to 2