At the end of the Colorado, a new rift opens over the river's last drops

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Under a longstanding treaty, the Colorado River irrigates 3 million acres of farmland and supplies water to 30 million people in the United States and Mexico. Between population growth and a decade long drought, the Colorado is under such stress that Western states ? desperate to maintain water supplies ? want to purify agricultural runoff currently diverted into Mexico. But as The World's Lorne Matalon reports, Mexico covets that water, because it has given birth to a productive wetland.

MARCO WERMAN: Now in the American west the climate problem is drought. The U.S. government says the region is facing potentially serious drought conditions again this year; bad news for the millions of people and the millions of acres of farmland that depend on water from the Colorado River. Pressure on the river's water supply has been building for years. It's especially acute where the Colorado crosses the U.S. border into Mexico. And as Lorne Matalon reports, the fate of one crucial wetland is now at stake.

LORNE MATALON: Columns of moist air hover above still water in the Cienega de Santa Clara mirroring the desert sky. The wetland is an oasis in dry northern Mexico, a haven for birds and fish, some endangered.

FRANCISCO ZAMORA ARROYO: The Cienega is the most important wetland in the Colorado River delta.

MATALON: Francisco Zamora Arroyo spends his workdays on he Cienega navigating sliver thin channels that snake through islands of yellow cattails. He's a biologist from Arizona who monitors the status of water and wildlife across the wetland's 63 square miles.

ARROYO: It's the perfect habitat for many species and birds. It's unique and it's located along a migratory corridor, so it's even more important that way.

MATALON: But the Cienega is also an accidental oasis. It was created 33 years ago by the diversion of polluted agricultural runoff from farms across the border in the U.S. Now though, with water growing ever more scarce in this part of the world, the U.S. wants to keep that runoff and recycle it. That's put the Cienega's future in question and made it a symbol of the growing water conflicts in the border region. Allocation of water from the Colorado River and Rio Grande is regulated under 1944 U.S.-Mexico treaty. The two countries have agreed to renegotiate parts of the pact. And under a trial run this year, a flow of wastewater from Arizona's farms in to the Cienega will stop. Instead it will go to a desalting plant in Yuma, Arizona.

KARL FLESSA: That would free up 100,000 acre feed in the United States and that could be used for cities and farms in the U.S.

MATALON: That's University of Arizona geoscientist Karl Flessa. A hundred thousand acre feed is enough water to supply 400,000 households in the U.S. southwest. The Yuma plant will remove the salts that have leached into the water from local farmland and funnel it back into the water supplies of the growing cities of Phoenix, Las Vegas and San Diego.

FLESSA: And we would still be able to meet the treaty obligations to Mexico.

MATALON: That's because the U.S. says it will replace the water that now flows to the Cienega. But it hasn't said how. That worries people concerned about the Cienega. It also worries Mexican farmers. Juan Butron Mendez irrigates seven acres of wheat with the last traces of Colorado water as the river dries up in Mexico. He's not happy with it's quality, but he says it's better than no water at all. Butron is also a naturalist working to save the wetland. He says the plan to keep that water is the latest example of the U.S. giving Mexico short shrift on Colorado water. The U.S. counters that Mexico violates it's water treaty obligations as well. Butron says it's time for the countries to cooperate as the treaty mandates. He says competing needs can be met by leaving fields fallow on a rotating basis on both sides of the border. Butron also wants the U.S. to use it's own groundwater to bolster urban water supplies, not the wastewater currently sent to Mexico. But that ground water is already spoken for as well. It's used to irrigate vast fields across the southwest and U.S. farmers have been assured that their water supply won't change under a new treaty. Those major questions, of which side will give what, remain unresolved. So for now, supporters of the Cienega Santa Clara are pinning their hopes on smaller, local efforts to save water. In Tucson, Arizona, for example, developers will soon have to supply half of the water used in landscaping from harvested rainwater. Across the border in Nogales, Mexico, watering lawns is only allowed on certain days. And a growing number of people on both sides are recycling gray water. For example, using water from dishwashers and laundry machines to water their lawns or wash their cars. It's a start, but Francisco Zamora says saving places like the Cienega will take more than goodwill and local efforts. It will take a comprehensive bi-national agreement. Zamora serves as an advisor to the international commission that's renegotiating the treaty. He says the process might be the last chance.

ARROYO: And to me, this is it. If we cannot reach an agreement and secure water for the different areas, including the Cienega, I'm not sure we're going to have another opportunity in the future. We are at a key moment in time where Mexico and the U.S. can make a difference, not only in the short term, but for the long term.

MATALON: Both sides say they hope to strike a new agreement within two years. Neither side will agree to any specifics in a reworked treaty before analyzing the results of the Yuma desalting plant's trial run in May. With nowhere else to go, the fate of the wildlife that depends on the Cienega de Santa Clara hangs on the outcome of the negotiations. For The World, I'm Lorne Matalon, San Luis Rio Colorado, Mexico.