The occurrence and intensity of flooding in southern Arizona appears to have increased during the last 30 years. While periodic changes in weather affect floods to a degree, human factors have played a greater role in determining flood damage. Floods that occur in wilderness areas are hardly noticed because humans usually are not affected. Floods that occur in urban areas, however, can affect large numbers of people and thus attract attention. The 1983 flood destroyed homes along the Rillito Creek at First Avenue.
Urbanization is an important factor to consider when assessing the intensity of floods in Arizona.
In a natural desert environment only about three percent of the rainfall reaches the washes. In an urbanized area, about 18 percent of the rainfall flows to washes. The additional water in rivers and washes means that downstream flows increase in quantity and velocity. Channels enlarge, becoming deeper and wider, with erosion posing a greater threat. As Tucson urbanized, flooding became more menacing.