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. Along their urban reaches, the Rillito Creek and Santa Cruz River have become deeply incised, with their channel bottoms as much as 40 feet below their banks. Waters no longer overflow the banks as they did when the rivers were shallow streams. Meanwhile homes and businesses have been constructed along river banks in the belief they are safe from floods. In reality, however, many areas become subject to bank erosion that can threaten structures.