The Upper Santa Cruz and the Upper San Pedro Rivers are binational watercourses that cross the international border between the State of Arizona and Sonora, Mexico. The basins that support these rivers are adjacent, with the San Pedro lying to the east of the Santa Cruz. Both the Santa Cruz and San Pedro Rivers flow north to join with the Gila River, which in turn forms part of the lower Colorado River Basin (Figure 1). Because of the semiarid climate with low rainfall and high evaporation, both the Santa Cruz and San Pedro Rivers are ephemeral in certain reaches. Human populations within each basin have depended on these rivers for thousands of years (Logan, 2002 Ferguson and Colwell-Chanthaphonh, 2006). The San Pedro River Basin is one of the most important flyways for migrating birds in North America with more than 400 species identified. In 2004, Congress recognized the importance of the river at the continental scale in passing Section 321 of Public Law 108-136, which requires the restoration and maintenance of sustainable yield in the regional aquifer with the goal of maintaining flow in the San Pedro River (Upper San Pedro Partnership, 2004).
