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In the Rio Grande transboundary basin, environmental flows have not been considered as an integral part of the water management. This research focuses on the Big Bend, a reach located along the Rio Grande mainstem. Important natural regions of the Chihuahuan Desert are threatened due to the lack of environmental flows. In this paper is estimated the maximum volume of water available for environmental flows without affecting human and international water requirements, and without increasing the flood risk in Presidio-Ojinaga. Environmental flows are proposed based on an analysis of the prior reservoir alteration hydrology of the river. A planning model was built to simulate the water allocation system and evaluate alternative policies. A reservoir reoperation policy for Luis L. Leon reservoir is proposed to supply environmental flows without violating the system constraints. The policy that supplies the maximum water to the environment is two-thirds (66%) of the prior reservoir alteration conditions; it also improves human water supply, treaty obligations, and decreases flood risk.