"These are big political steps for Mexico to take," said Jeffrey Kightlinger, general manager of the Metropolitan Water District of Southern California, which will buy some of Mexico's water. "Chances are we won't have a surplus and we won't have a shortage but, if we do, we'll have the guidelines in place on how we're going to handle it."
In 2007, facing an eight-year drought, California, Arizona and Nevada agreed on how much each state should sacrifice during shortages on the river. That same year, the U.S. and Mexico promised to work on ways to jointly address shortages.
The negotiations gained a sense of urgency for Mexico in 2010 after a magnitude-7.2 earthquake damaged canals and other infrastructure, forcing it to store water temporarily in Lake Mead.
"No matter how sacred the treaty is, and it is, the evidence is overwhelming that you have to adjust it," said Carlos de la Parra, who advised the Mexican government on the agreement.
Los Angeles-based Metropolitan and two other agencies — the Southern Nevada Water Authority and the Central Arizona Water Conservation District — will buy water from Mexico as part of $21 million in payments from the U.S. to Mexico that also call for wetland preservation and other environmental measures south of the border.
California's largely agricultural Imperial Irrigation District, the largest single recipient of Colorado River water, refused to sign the agreement because it felt it should have been allowed to buy some of the water from Mexico. U.S. officials said they hoped to address those concerns.
"This is such a historic arrangement that I think, at the end of the day, it's going to be difficult for Imperial not to sign on," Salazar said.
The river is also a major source of water for Colorado, New Mexico, Utah and Wyoming.
Associated Press writer Ken Ritter in Las Vegas contributed to this report.