YUMA, Ariz.— The Southwest drought has reached the point where even drain water is coveted.

Beginning nearly 40 years ago, the briny runoff from the “;salad bowl”; of southern Arizona, some of the most productive farmland in the nation, has been channeled into an arid plain of the Sonoran desert in Mexico.

It is an engineered solution to the vexing problem of keeping the nearby Colorado River free of agricultural wastewater too heavy in salt compounds for drinking water and other use. A result south of the border has been a thriving man-made wetland, the largest in the river's delta, a key stopover for migratory birds and home to a bounty of endangered and threatened species.

But now the protracted drought in the Southwest has water managers rethinking the possibilities for the wastewater, placing the preservation of the wetland, the Cienega de Santa Clara, at the center of a delicate balancing act between the growing thirst of California, Nevada and Arizona and the delta's ecology.

The biggest challenge involves a plan to take some of the wastewater, treat it at a desalination plant, and direct it to other uses under a treaty that proportions the Colorado River among the Western states and Mexico.

The desalination plant, here in Yuma, about 30 miles north of the Cienega, is a $256 million federal plant that, although completed 17 years ago, has never run beyond two brief trial periods. It has been long maligned as among the federal government's biggest white elephants, a plant essentially mothballed from the start by budget and technical difficulties and the vagaries of the Colorado River supply.

But so strapped is the region for water that the plant is getting another look. The water the plant could produce would allow Western states to preserve that quantity upriver for their use rather than send it to Mexico, as is done now to fulfill treaty requirements.

Like a family tired of fighting at reunions, the effort to revive the plant has brought a concerted effort by the United States, Mexico and some environmentalists on both sides of the border—normally acrimonious over big water decisions—to get along.

A pact among the governments, environmental groups, and water agencies calls for a test run of the plant at one-third of its capacity while substituting the lost water flows to the Cienega with well water and other means.

“;This is huge,”; said Jennifer Pitt of Environmental Defense, a group helping to negotiate the agreement and not often complimentary toward big water interests. “;This is the first time anything like this has ever happened.”;

But other environmental groups are wary, even quietly exploring the possibility of bringing the test to a halt. The government is expected to make a final decision on the test in the coming weeks, with a start date projected early next year.

Critics voice skepticism that the water agencies would make long-term commitments to protect the Cienega and bristle at the fact that the United States has completed an environmental review of the desalting project, without a detailed study of the environmental consequences on the Cienega.

The United States, through the Bureau of Reclamation, which operates the plant and oversees the distribution of Colorado River water in the West, maintains there is no legal obligation to do so because the Cienega is in Mexico.

That environmental review, released Aug. 26, concluded the plant would pose no significant harm to the environment around it.

“;The failure of that plant to run for many years has allowed this tremendous wetlands to survive,”; said Kieran Suckling, executive director of the Center for Biological Diversity, a Tucson-based environmental group. “;To put that water at risk is of tremendous concern.”;

Although they decline to make any formal long-term commitments to protect the wetlands without the results of the test, U.S. water managers said in interviews that they would like to maintain them.

“;This is starting to put pressure on everybody to resolve the issue of the Cienega and keep the Cienega in water,”; said Pat Mulroy, who heads the agency that supplies Las Vegas' water. “;There is a little pull and tug to see how much water the Cienega needs. There is an assumption it needs all the water currently going there and an assumption we have to replace it in its entirety. Somewhere in there is probably a middle road.”;

The plant would draw briny water from an agricultural drain that runs next to it on its way to Mexico, providing the wetland's lifeblood. The drain was built in the early 1970s as a temporary solution to a problem that severely strained relations between the United States and Mexico.

On its 1,450-mile journey from the Rockies to the Gulf of California, the Colorado River decreases in might as Western states tap it, while increasing in salt compounds, partly because of runoff from rocks and soil but also from the waste of some of the largest farming operations in the world.

Mexican farms downriver began suffering and, after heated negotiations, the United States agreed to improve the quality of the water delivered to Mexico by desalinating some of it at the border.

While the plant was being planned and built, the federal government attacked one of the main culprits of the salt problem, by diverting farm wastewater near Yuma south onto sparsely populated land in Mexico.

End result: the Cienega.

At first blush, the Cienega seems run of the mill: tule grass the height of a person or taller emerging from a labyrinth of streams and ponds under a brick-oven sun.

But listen closely, Juan Butron Mendez, a Mexican naturalist advised one recent morning. A trilling sound like a referee not blowing hard enough on his whistle. A willow flycatcher, one of the endangered birds that call this wetland home, was beckoning from the weeds.

“;That is something you may not hear anymore if the United States is not careful,”; Mendez said, standing at the edge of the wetlands, whose other endangered residents include the Yuma clapper rail and desert pupfish.

Environmental groups, with researchers from the University of Arizona, have begun a more detailed study of the Cienega, to understand better its water needs and ecology. Every month, researchers from Arizona and Mexico head out in boats to plant and check monitors and other equipment among the thick grass and turbid water.

“;If we lose this, we don't know what will happen to the birds,”; said Francisco Zamora Arroyo, a geographer with the Sonoran Institute, a Tucson-based nonprofit, who studies the Colorado River Delta. “;Some will be able to reach other marshes, some won't.”;

But Zamora, on a rocking skiff helping colleagues plunge measurements into the swamp, said he sensed a shift in attitude on both sides of the border.

“;This comes at a time when we have a lot of hope,”; he said. “;We have been doing a lot of research in the last 10 years here. At least this time there is a dialogue.”;