The San Pedro River flows north out of Mexico until it hits the Gila River, then heads west dumping into the Colorado River, ultimately draining into the Pacific Ocean.
Via the Bureau of Land Management:
The San Pedro Riparian National Conservation Area (NCA) contains almost 57,000 acres of public land in Cochise County, Arizona, between the international border and St. David, Arizona. Congress designated the riparian area as a Riparian National Conservation Area on November 18, 1988. Some 40 miles of the upper San Pedro River meanders through the area.
The primary purpose for the special designation is to protect and enhance the desert riparian ecosystem, a rare remnant of what was once an extensive network of similar riparian systems throughout the American Southwest.
The San Pedro River runs through the Chihuahuan Desert and Sonoran Desert in southeastern Arizona. It contains one of the most important riparian areas in the United States.
More than 80 species of mammals, two native species and several introduced species of fish, more than 40 species of amphibians and reptiles, and 100 species of breeding birds call the river home. It also provides invaluable habitat for 250 species of migrant and wintering birds and contains archaeological sites showing remnants of human occupation from 13,000 years ago.