For 56 days last spring, a unique pulse of water drawn from Hoover Dam's Lake Mead coursed into Mexico to the Colorado River's parched delta – once an ecological emerald set in the tawny expanse of the Sonoran Desert.
The release, an experiment in restoring habitats at a landscape scale, marked the first time the United States has delivered Colorado River water to the delta for environmental reasons. And, researchers say, the experiment has yielded encouraging results.
In addition to greening up vegetation already established along the river, "the pulse flow was quite effective" in stimulating the emergence and initial growth of new cottonwood trees, willows, and mesquite, said Karl Flessa, a conservation biologist at the University of Arizona at Tucson.
Dr. Flessa is one of two chief scientists on the project. Along with two colleagues, he presented initial results from the experiment during a briefing Wednesday at the fall meeting of the American Geophysical Union in San Francisco.
The US and Mexican governments authorized the one-off release in November 2012 as part of a broader addendum to the US-Mexico Water Treaty, a 1944 pact divvying up rights to water from the Colorado, Rio Grande, and Tijuana Rivers among the two countries.
The experiment involved more than 21 researchers from universities, government agencies, and environmental groups interested in seeing how effective this relatively small pulse of water might be in reversing a browning along the river that has occurred during the past 13 years.
In essence, the scientists were trying to recreate the kind of pulse, if not of the same the magnitude, that once came with spring snow melt in the Colorado River Basin. Those seasonal pulses helped lace the delta with side channels, dotted with lagoons and teeming with wildlife. In addition to birds native to the area, the delta sits beneath the Pacific Flyway, a key route for migratory birds.
In advance of the water's release, researchers gathered baseline data on the river. In addition, researchers cleared a combined 400 acres of vegetation that wasn't native to the area at two sites along the river. They planted cottonwood and willow seedlings on roughly 100 acres and hydroseeded other portions to produce native plants.
The objective was to see how the coming "spring flood" would affect a range of restoration treatments, Dr. Flessa writes in an e-mail. These ranged from areas not treated at all or merely cleared of non-native plants to areas where tree seedlings were lined up in tidy rows.
Operators at the Morelos Dam, which spans the river at the US-Mexican border, opened the flood gates on March 23 to release what would ultimately amount to 130 million cubic meters of water into the dry riverbed below. Some water also entered the river in Mexico via irrigation canals.
Within the first 37 miles downstream of Morelos Dam, some 90 percent of the surface water vanished beneath the dry, sandy riverbed. .
But it still continued to migrate toward the Gulf of California. Evidence of the groundwater's migration came in part via gravity meters researchers had set up along the river's course.
By May 15, surface water in the river reached all the way to the Gulf, although the amount was small, only 1 percent of the amount released at Morelos Dam.
Evidence for migration of subsurface water came from the Landsat 8 satellite data as well as from data gathered by NASA's Terra satellite, noted Pamela Nagler, a researcher at the US Geological Survey's Sonoran Desert Research Station in Tucson and another member of the experiment's science team.
"Vegetation became greener downstream, strongly suggesting that groundwater derived from the pulse flowed into the lower reaches and supported vegetation further south," she said.
Within the wet zone, where water flowed in the river channel, the satellite data revealed a 43 percent increase in greening between August 2013 and August 2014. The broader zone to either side of the river, known as the riparian zone, greened up by 23 percent.
Given the good start the pulse flow gave to the areas planted with seeds, which germinated and are growing, the new plants have "a good shot" at maturing and thriving for 20 to 40 years, Flessa said, noting that this will be a long-term effect from the experiment.
For plants already established, it remains to be seen if the greening trend will continue, he added.
The team will continue to monitor the effects of the pulse release through 2017.
It's unclear if the experiment can be replicated or if the pulse can be repeated on a regular basis, Flessa noted. Much of the western US is in the grip of a long-term drought. And the river's water already is fully spoken for to slake cities' thirst and irrigate farms.
"Any future allocations for nature would have to be the result of negotiations to modify those allocations," he said.
But based on these initial results, "we certainly hope that environmental flows will be included" in any renewal of the broader treaty addendum that made this year's experiment possible, he said.