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 uses. An accidental 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 led water managers to rethink the possibilities for the wastewater, placing the preservation of the wetland, the Ciénega 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, purify 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.