About 150 miles east of San Diego, Morelos Dam stops the Colorado River in its tracks right at the US-Mexico border. Here, the last stretch of the once-mighty river is diverted from its natural path into an irrigation canal, bound for Mexican farms.

It’s been this way for most of the last half century. But then, for a few weeks this spring, it suddenly wasn’t.

What happened this spring was a “magical experience,” said river activist Yamilett Carrillo, “because we got to see a river coming back to life.”

By the time it reaches the dam, most of the Colorado north of the border has already been diverted to feed cities and farms in the US. And below the dam, most of the river’s delta, stretching to the Gulf of California 100 miles away and once one of the largest and most vibrant wetlands in North America, is usually dry as a bone.

Carrillo, who heads a coalition of Mexican and US environmental groups called the Colorado River Delta Water Trust, helped convince the governments of the two countries to try a one-time experiment — open up the Morelos Dam for two months to mimic the floods that used to flow into the Delta every spring, and see what would happen.

“We haven't had a river for decades,” Carrillo said, “so you can imagine, we are learning a lot about the river — how it behaves, where it moves, how the ecosystem responds.”

The flood is over now, and researchers have moved in to start gauging its impact and assess what species of trees, birds and other native species may have been drawn back by the flood.

“We look for the seeds of cottonwoods and willows” said Alejandra Calvo, working at a site about 10 miles south of the dam. She was checking out seed traps — pieces of plywood covered in a sticky substance and mounted in the sand where the water rushed through.

The water-loving tree species used to thrive here. Most have died off, but scientists are hoping the flood will help bring them back. The release was timed to match the season when the few trees still in the area disperse their seeds. If some settle here, new trees could take root in the damp soil.

On this day, in this spot, Calvo didn’t find cottonwood or willow seeds. But some might yet be found, and they have been found elsewhere.

In any case, scientists didn’t expect the experiment to be perfectly successful. Ecosystem restoration is a slow and uncertain business, and researchers will be studying the effects of this spring’s flood for years.

But they won’t just be watching. They’ll also be lending a hand.

Farther south in the delta, members of a work crew splashed down a channel one recent day carrying pallets of slender, foot-high willow trees while Guadalupe Fonseca of the Arizona-based Sonoran Institute dug holes in the channel’s bank for the saplings.

The crew has planted thousands of trees in recent weeks. They hope the trees will survive, Fonseca said, because even after the big flood, a trickle of water will continue to flow into parts of the delta. The deal that made this spring’s flood possible also called for conservation groups to acquire water rights from farmers in the area and send that water back into the river through an irrigation canal over the next four years.

It all adds up to a very small step in the very big task of restoring at least a few pockets of the delta to something like their former selves, a task that’s only getting harder as water stresses in the region get worse.

But conservation groups here say they have already begun to show what’s possible.

A few years ago, the Mexican conservation group Pronatura planted a stand of cottonwood trees. Today the trees are 30 feet tall and full of birds.

Birds are among the best indicators of ecosystem health, said Pronatura’s Yuliana Dimas. “We hear lots of them here.”

And Yamilett Carrillo said conservation groups already scored one unexpected victory in their quest. In May, as the experimental flood was ending, water from the Colorado reached the Gulf of California for the first time in decades.

“That was a major celebration for everybody working on this,” Carrillo said. “It's something that we wanted but we knew that maybe the flows aren't going to be enough.”

The scientists working here have the rest of the summer to come up with a first report on whether the flood experiment is working. The results will help policymakers decide whether or not it’s worth repeating in the future.

But Carrillo said she already knows the answer.

“We know it’s working,” Carillo said. “When you begin walking down the river and you see the little seedlings coming out of the soil, you know that it is working.”

The Colorado River Delta, now the largely brown and speckled area in the lower left of this NASA photo, was once one of the largest and most biologically rich wetlands in North America, stretching from the borders of California, Arizona and the Mexican state of Baja California to the Sea of Cortez roughly 100 miles to the south. It's been largely destroyed by water diversions from a series of dams built in the mid-20th century.


NASA/Jacques Descloitres, MODIS Land Rapid Response Team

After being largely emptied by other upriver dams in the US, the last drops of the Colorado River are diverted to Mexican farms by the Morelos Dam on the US-Mexico border. For two months this spring, the dam was opened to create a flood through parts of the river's former delta below the dam, meant to mimic the seasonal floods that washed through the area every spring before the dams were built.


NASA Earth Observatory image by Jesse Allen and Robert Simmon

Water flowing through a habitat restoration site in the Colorado River Delta on May 30, 2014.


Jill Replogle

Alejandra Calvo and Juan Butrón, from the Mexican environmental organization, Pronatura, examine a seed trap.


Jill Replogle

The pod of screwbean mesquite, a tree native to the Colorado River Delta. May 29, 2014.


Jill Replogle

1875 Employees of the Sonoran Institute, an Arizona-based conservation group, plant cottonwood and willow trees in the Mexicali Valley.


Jill Replogle

Yuliana Dimas from the Mexican conservation group Pronatura at the organization's native tree nursery near San Luis Rio Colorado.


Jill Replogle

A pair of burrowing owls in the Mexicali Valley. May 30, 2014. (Photo by Jill Replogle)


Jill Replogle

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