Back 2014, an unprecedented transnational experiment attempted to restore, temporarily, the flow of the Colorado River to the Gulf of California. As part of a landmark agreement between the United States and Mexico, the International Boundary Water Commission unleashed an eight-week “pulse flow” of some 105,000 acre feet of water from a small dam on the US-Mexico border to help restore the Colorado River delta.
Photo by Karl W. Flessa/University of Arizona
Conservationists hoped the water would revitalize the delta — which has been bone dry for nearly 60 years as a result of upstream dams and diversions on the Colorado — and bring back trees, animals, and aquatic life that were once abundant in the region when it was flush with water. (The transnational agreement authorized environmental flows of water into the Colorado River Delta from 2013 to 2017.)
Two growing seasons after that engineered release, it appears that birds, plants and groundwater in the delta, which lies south of the US-Mexico border, have indeed been benefitting from it.
Native willows and cottonwoods have sprung up wherever the pulse flow inundated bare soil and in response to this post-flood vegetation, birds have begun flocking to the area, according to the latest monitoring report prepared for the International Boundary and Water Commission by a bi-national University of Arizona-led team.
The interim report, released on Wednesday, documents the effects of the environmental flows in the delta from the initial pulse in March 2014 plus subsequent supplemental deliveries of water through December 2015.
"Some of the cottonwoods that germinated during the initial pulse flow are now more than 10 feet tall," Karl W. Flessa, UA professor of geosciences and co-chief scientist of the team that’s monitoring the impact of the pulse, said in a statement.
Migratory waterbirds, nesting waterbirds, and nesting riparian birds have all increased in abundance, the report says. The monitoring team found that the abundance of 19 bird species of conservation concern, including vermillion flycatchers, hooded orioles, and yellow-breasted chats, was 43 percent higher at the restoration sites than at other sites in the floodplain.
Photo by Sarah Murray
Some of the water from the pulse flow and subsequent smaller environmental flows recharged the groundwater, which had both ecological and social benefits, said Eloise Kendy, a senior freshwater scientist with The Nature Conservancy's North America Water Program who helped compile the report. The vegetation greened up in areas that received surface water and also in some areas that did not. "The farmers [whose irrigation canals were used for some of the water deliveries] were happy because it recharged the aquifer they use for groundwater irrigation," she said. "And plants that were outside the inundation zone got a big drink of water.
Dams and river diversions built in the twentieth century have for decades prevented the river — that once flowed freely from the Rocky Mountains all the way to the Sea of Cortez in Mexico — from completing its journey to the sea. These days it dies after it crosses the US-Mexico border. The southernmost dam on the river — Mexico’s Morelos Dam, near Yuma, AZ — diverts nearly all of the river water into an aqueduct that serves agriculture and homes in Tijuana. South of the dam, the river channel travels about 75 miles to the Gulf of California. With the exception of a few wet years, the river has not reached the Gulf of California since 1960.
Before 1960, spring snowmelts regularly sent water gushing down the Colorado River into the delta, scouring the river bottom and overtopping the bank and creating the ideal conditions cottonwood and willow trees to germinate and establish. But since then, salt cedar or tamarisk, an invasive plant, has taken over the riverbanks. Since cottonwoods and willows need bare ground and sunlight to germinate, they cannot establish themselves on tamarisk-covered riverbanks.
The March 2014 pulse flow delivered a fraction of the water the pre-1960 spring floods used to bring to the delta. Staff from the Sonoran Institute and Pronatura Noroeste, a Mexican conservation group, cleared some areas of non-native vegetation beforehand. The researchers hoped that reducing competition would allow native plants such as willows and cottonwoods to germinate and grow after the pulse flow.
"We reconnected the meanders to the main river channels so when the pulse flow came there were these nice backwater areas where the conditions were good for the establishment of native trees," said Karen Schlatter, a restoration ecologist of the Sonoran Institute's Colorado River Delta Program, who was part of the monitoring team. In those restoration areas, cottonwood and willow seeds that germinated after the pulse flow have become 10 to 13 foot trees, and bird diversity and abundance has increased.
"Now we have diverse habitat types, including lagoons, cottonwoods-willow forest, mesquite bosque and marshes," Schlatter said. "We are seeing a much higher diversity of riparian bird species in the restoration sites compared to other areas along the river."
The pulse flow has also reduced soil salinity in some areas that had been targeted for restoration. "We didn't expect that — it is a huge bonus," Schlatter said. Reducing the soil salinity makes conditions more favorable for native plant species. If there's another pulse flow, she suggests clearing tamarisk and other non-native vegetation from the river's bank ahead of it would be helpful.
The pulse was the only water release planned so far. Once this pilot project ends in 2018, US and Mexican officials will review findings and discuss whether other discharges should be made.
Part of the impetus for the pulse experiment was to determine whether a healthy delta system can be maintained without a lot of water. Of course, the delta can’t be restored to what it was a say, a century ago, given the cities and towns that need Colorado’s water aren't going anywhere, as well as the fact that much of the delta land has since been converted to farmland. But, as Flessa says, this short-term experiment “really demonstrates that a little bit of water does a lot of environmental good."