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Colorado River Delta water release worked, scientists say
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Colorado River Delta water release worked, scientists say

These two photos taken by Dale Turner of The Nature Conservancy show a portion of the Sonoran Institute's restoration site before and after the pulse flow. The photo showing new vegetation was taken in early October.

A portion of the restoration site before (left) and after the pulse flow. By September, new vegetation was growing.

A burst of new water last spring transformed the Colorado River Delta from a mud flat into a budding greenbelt.

Photos, satellite images and monitoring equipment show more and greener plants, and a higher water table in the delta, due to the artificial spring 2014 release of water into the river at the Mexican border south of Yuma. The images and preliminary statistical results of what’s known as the delta “pulse flow” were released Wednesday at a briefing featuring a University of Arizona scientist and two Tucson-based federal scientists.

“Here’s a success story on the border and with water in the West,” said Karl Flessa, a UA geosciences professor and one of two chief scientists involved in the water release. “Who can believe it?”

About 105,000 acre-feet of water was released into the delta from March 23 to May 18 from Morelos Dam at the border and two other points farther south. The release was triggered by a groundbreaking agreement reached in 2012 by the U.S. and Mexican governments.

Details of the findings include:

  • In areas where released water flowed, the amount of vegetation increased 43 percent between August 2013 and August 2014.
  • In areas just outside where the river flowed, vegetation levels rose 23 percent over the same period.
  • The entire stretch of river affected by the released water got 36 percent greener in June 2014 compared to June 2013.
  • Bird life increased along the river, although changes in bird populations generally will take longer to observe. Migratory waterbird populations rose in the open water areas from 2013 to 2014. Bird diversity increased in the entire floodplain in that period.
  • More than 90 percent of the water released from Morelos and the first release site downstream of that recharged the aquifer within the first 37 miles of the international border.

At the same time, the water table managed to rise in response to the pulse flow “in all reaches,” said a report on the release effort from the International Boundary and Water Commission.

Near the international border, the water table near the river channel rose about 27 feet just after the pulse flow, Flessa said. Downstream of the border, the water table rose between 3 and 12 feet.

Overall, the pulse flow reversed 13 years of declines in vegetation along the river since the last natural flood reached the delta in 2000, said Pam Nagler, a U.S. Geological Survey physical scientist based in Tucson.

“We are very happy and satisfied with the results,” said Osvel Hinojosa, a program director for Pronatura Noroeste, a conservation group in Mexico active in planning the delta restoration project.

More flooding than expected occurred in the region between Morelos Dam and the international border, but the flow was slower and less intense downstream, Hinojosa added. “We were not sure if the pulse flow was going to reach the sea, and it did,” he said. Regeneration of cottonwood and willow trees was very strong in some sites but was less widespread than expected, he said.

Not only was the water delivered, the operators of the river system did an amazing job of delivering it “pretty spot on” to where it was requested, said Jennifer Pitt, of the Environmental Defense Fund, another group involved in planning the delta water release.

“That kind of delivery is quite unprecedented for them. Typically they deliver year-round, steady flows to irrigators, not like this one with one day low and the next day quite high,” Pitt said.

At the same time, it’s worth noting that the pulse flow brought the delta less than 1 percent of what used to flow down the river in the era before dams were built on the Colorado, she added.

In 1922, renowned naturalist Aldo Leopold went to the delta and saw green lagoons. “That is not what we were expecting to see” after this year’s pulse flow, Pitt noted. “We’re expecting to be able to create and support a ribbon (of life) that helps to re-establish connectivity on the river corridor.”

This effort wasn’t just about birds, trees and water, the UA’s Flessa added.

“Let’s not lose sight of the fact that this habitat consists of people. The celebrations, the fishermen taking fish, the parties, the barbecues, the kids splashing in the water last spring, it was wonderful to see,” Flessa said.

Contact reporter Tony Davis at or 806-7746. Follow Davis on Twitter @tonydavis987

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