In a drought-hit Mexican border region at the center of growing competition with the United States for water, conservationists are working to bring a once-dying river delta back to life. On a stretch of the Colorado River, which on the Mexican side of the frontier is mostly a dry riverbed, native cottonwood and willow trees have been planted in place of invasive shrubs. It is the fruit of two decades of work by environmentalists along the lower part of the river from the US-Mexican border to the upper estuary of the Gulf of California, also known as the Sea of Cortez. IMAGES AND SOUNDBITES