 One of the most important riparian areas in the United States, the San Pedro River runs through the Chihuahuan and Sonoran Desert transition zone in southeastern Arizona.The river's stretch in the southern San Pedro Valley is home to 84 species of mammals, 14 species of fish, 41 species of reptiles and amphibians, and 100 species of breeding birds.It also provides invaluable habitat for 250 species of migrant and wintering birds and contains archaeological sites representing the remains of human occupation from 13,000 years ago.