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.”