Despite the lack of surface flows, the Colorado River riparian corridor in Mexico has proven to be ecologically resilient. Floods in the 1980s and 90s in the region brought back large swaths of native riparian habitat, which still persist today in some areas along the river. Because the historic floodplain is extremely important for agricultural production and therefore the local economy, habitat maintenance must be integrated with continued utilization of lands and water for this purpose.The riparian, marsh, and open-water areas found in Reach 4 provide critical habitat for both migratory and resident riparian bird species. Due to its ecological importance, the riparian corridor and Reach 4 in particular have been designated as conservation priorities by a binational team of U.S. and Mexican government officials, conservation organizations, and scientists. This project will use surface (HEC-RAS) and groundwater (MOD FLOW) models for Reach 4 of the Colorado River in Mexico to determine the extent of the riparian corridor that will maintain aquatic habitat and support native riparian trees (primarily Fremont cottonwood, Goodding’s willow, and coyote willow). Models will help determine how depth to groundwater varies due to: I) environmental flows (base and pulse flows), 2) contributions of agricultural return flows, and 3) increase in groundwater utilization for irrigation.