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Tucson, Ariz.

On the Colorado River Delta, some 250 miles west southwest of Tucson as the crow flies, sits Cienega de Santa Clara.

It’s a 63-square-mile patch of wetland – a key stop for migrating birds along an arid stretch of the Pacific flyway. It’s the largest remaining wetland on the Colorado River Delta and part of an internationally recognized conservation area.

It also sits squarely in the center of a debate over a multimillion-dollar water desalting plant just west of Yuma, Ariz., which is slated for a trial run later this year. Salty wastewater from the plant would flow down a drainage system that currently feeds Cienega de Santa Clara, essentially dooming the wetland, environmentalists say.

Proponents of the trial argue that the freshwater the plant would provide is sorely needed for the region’s cities and farms.

The debate highlights the challenge of slaking the thirst of a growing population that draws on the already-oversubscribed Colorado River, while protecting the region’s ecological gems. The challenge is expected to grow more acute if global warming dries out the region further, as many climate models project.

If, however, other sources of water for the wetlands can be found, particularly if the water comes from sources on both sides of the US-Mexican border, it would represent “a tremendous breakthrough,” says Karl Flessa, a paleobiologist at the University of Arizona who studies the river’s historical ecosystems.

Sitting in his office beneath a pair of enlarged satellite photos of the delta, Dr. Flessa explains that such an outcome would represent the first time US water managers had allocated Colorado River water specifically for environmental purposes outside the Grand Canyon. It would also represent the first time water had been delivered across the border for environmental uses.

Why the wetlands expanded

Getting there, however, is the hard part.

The wetland’s water is brackish residue from farmland east of Yuma. Once, that water was channeled back into the Colorado and into Mexico. But in the 1970s, the US agreed to improve the quality of Colorado River water Mexico receives. The quickest way to attack that problem was to divert the water – and it ended up in Cienega de Santa Clara. The influx of water stimulated the wetland’s expansion from a few hundred acres in 1977 to more than 40,000 acres of marshland – a patchwork of open water and expanses of cattails.

Meanwhile, the US Bureau of Reclamation was pursuing a high-tech approach – building the Yuma Desalting Plant. The idea: Run the brackish farm water through the plant and return it to the river. But for technical and budget reasons, the plant is not on line full time.

Throughout this period, the diverted farm water has not counted toward Mexico’s treaty-specified allotment of Colorado River water, explains Lorrie Gray-Lee, who heads the Bureau of Reclamation office responsible for the agency’s activities along the last 700 miles of the Colorado River. So the US has made up the difference by releasing additional water from Lake Mead to meet its treaty obligations.

But a prolonged drought has triggered a clamor on the US side of the border for that additional release. Water managers say operating the desalting plant is their ticket to getting that water. “The difficulty we have is that operating the plant requires a lot of money. It’s about as expensive as taking bottles of Evian and pouring them into the river,” Ms. Gray-Lee says.

A plan to save wetland, supply water for US

Four years ago, as pressure to restart the plant was building, a group of Arizona water managers, as well as environmentalists, farmers, and scientists, quietly hammered out a set of principles and options for meeting water needs north of the border while saving the Cienega de Santa Clara. It included the establishment of a monitoring program in the Cienega – a project the University of Arizona’s Flessa is designing with Mexican colleagues. It backed the notion of running the plant on a pilot basis for 90 days. It explored, too, the idea of using excess groundwater in the Yuma area to help meet US treaty obligations to Mexico.

Since then, however, water-management districts in southern California and Nevada have offered to help pay the cost of running the plant on a pilot basis – in exchange for water. And Arizona has insisted that by law, its water cannot be used outside of Arizona.

That leaves the desalting plant the main option left standing – at least for now. The Bureau of Reclamation’s goal is to run it for 365 days during an 18-month period to make sure the plant can function and to get a better sense of its annual operating costs. The project would still need to go through an additional permitting process, including environmental impact assessments, before it could operate around the clock.

But to many environmentalists, the inertia of Western water politics is likely to cement the plant – once it is operating – as the lead option for freeing up Lake Mead water now bound for Mexico.

Plenty of alternatives exist, they argue, though some of them need further evaluation. Mexico itself is looking for potential sources of farm wastewater that could help maintain the flow into the Cienega. In addition, runoff from farms in the San Luis and Mexicali Valleys could be tapped. If the desalting plant does eventually run full time, its saline effluent could be pumped to the Cienega’s southernmost reaches, where evaporation already has left the water on what essentially are salt flats suitable for little more than brine shrimp.

“Turning to desalination in some cases may be appropriate” as the Southwest deals with rising populations and a drier future, acknowledges Jennifer Pitt, who heads the Environmental Defense Fund’s conservation efforts along the Colorado River.

But before such energy-intensive approaches are switched on, she says, far more needs to be done to reduce water demand in cities, irrigate crops more efficiently, and establish laws that allow greater flexibility and recognize ecosystems as among a river’s legitimate users.

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