From The Island Press (John Fleck):
The Colorado River is often described as being shared among seven states, but the number is really nine—seven in the United States and two in Mexico. U.S. farms and cities use most of the river’s water, and what little is left when it arrives at the U.S.-Mexico border near the towns of Algodones and San Luis is diverted for use by Mexican farms and cities. The last hundred miles of river channel between the border and the Sea of Cortez is usually dry.
The agreement includes provisions for the two nations to share shortages if (when?) drought and climate change shrink the river. The deal gives Mexican water users the ability to store their water in Lake Mead, the massive storage reservoir behind Hoover Dam on the Arizona-Nevada border, near the city of Las Vegas. Storage is critical to give Mexico flexibility in managing its water. U.S. water agencies will contribute under the deal to water efficiency improvements to Mexican infrastructure, with some of the saved water available for use in the United States.
Crucially, the agreement also sets aside water for habitat restoration in the dry river channel of Mexico.
The agreement was negotiated over a more than two-year period, but it is really rooted in more than a decade of increasingly deep collaboration between a community of water managers on both sides of the border. When the Trump administration took over in January, there was fear that the carefully crafted deal, so beneficial and important to communities on both sides of the border, would be sidelined by the heated rhetoric over free trade and immigration, over NAFTA and walls. But Wolf was right. Even as conflict raged over other issues, the trust and reciprocity built around the Colorado River proved remarkably resilient. The old saw that “water is for fighting over” was proven wrong again.