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<br />Mountain and Plains Flooding: The Physical Context <br /> <br />(. <br /> <br />by <br />John E. Costa <br /> <br />Department of Geography <br />University of Denver <br /> <br />Denver, Colorado 80208 <br /> <br />Despite the general aridity of the region, an area of the western Great Plains <br />extending about 50 miles east of the Rocky Mountains from Wyominq to New Mexico is <br />subject to very intense rainstorms which locally can generate very large volumes of <br />runoff in a very short time. The usual source of these cloudburst rainstorms are <br />warm, moist air masses moving northward frornthe Gulf of Mexico, coupled with cool, <br />moist air in the northern Great Plains. When moist air is uplifted rapidly into an <br />unstable atmosphere, condensation is rapid and rainfall volumes are tremendous: 12 <br />,. to 24 inches of rain in 4 to 6 hours. This is a greater rain intensity than most <br />people have seen, or ever will see, and its magnitude is difficult to comprehend. <br />Two documented incidents may help to appreciate the intensity of these cloudbursts. <br />The first occurred on July 25, 1896: "The daughter of a rancher was riding on Green <br /> <br />nountain looking after the stock when the storm started. By the time she reached <br />the barn she was practically unconscious on her horse, and had to be revived by means <br />used for resuscitating victims of drowning, as the intensity of the rain made it <br />impossible for her to breathe. The other incident occurred during the series of <br />cloudbursts that caused the Arkansas River flood of June 3, 1921; a horse was drowned <br />in an open field" (Follansbee and Sawyer, 1948, p. 22). <br />Conditions are most favorable for cloudburst storms where there is a large <br />range in temperature within a relatively small area. This condition occurs spatially <br />in the foothills of the Rocky Mountains to an elevation of approximately 7500 feet, <br />ie <br />