The 1944 U.S.-Mexico Water Treaty remains the single most important bilateral agreement on water resources between the two countries. The “Treaty” has not only been durable but has proven to be applicable to environmental challenges not foreseen by its drafters, a function of both its context, treaty design, and recent diplomacy. Its recent extension and further application to binational watershed ecology is affected by its intersection with other binational agreements affecting wildlife and ecological resources and sanitation. This paper examines the application of the 1944 Treaty to the environmental functions of U.S.-Mexico rivers and its relationship with other shared environmental agreements for their potential to strengthen binational environmental management of these transboundary watercourses.