Kim Gavigan (right), chief of the water resources planning section, briefs Col. Kirk Gibbs, commander of the U.S. Army Corps of Engineers Los Angeles District, on the flood risk management study for the Lower Santa Cruz River Sept. 7. Much of the flood plain is located between two major municipalities, Phoenix and Tucson, and is in Pinal County which, according to the study, was the second fastest growing county in the United States during the past decade.