Manmade Flood Gives Life to Colorado River Delta

A battered ecosystem begins to recover.

By Brett Walton, Circle of Blue

On March 23, 2014, the gates were opened wide at Morelos Dam on the U.S.-Mexico border near Yuma, Arizona. For the next eight weeks, water pitched into the dry bed of the Colorado River, wetting its delta like the spring floods that coursed through braided channels before the river was dammed.

Authorities called it a pulse flow. River communities in Mexico, some with teenagers who had never seen water between the banks, called it a blessing.

Scientists can now back the sentiment with data. A scientific assessment of the pulse flow was made public on October 19 by an international team of researchers who are studying the delta ecosystem’s response to the surge of water. Researchers found that the change in the delta was rapid.

Cottonwoods and willows that germinated during the pulse flow now shade the scientists who are studying them. The pulse flow flushed salts from the soil, which aided the growth of native trees. Birds — Gila woodpecker, brown-headed cowbird, ash-throated flycatcher, yellow-breasted chat and others — flocked to the new habitat. The vegetation helped boost the abundance of 19 key bird species by 49 percent.

Hydrologic connections occurred as well. Nearly 95 percent of the water soaked into the ground and recharged the aquifer. The fraction that traversed the entire course of the river channel completed a symbolic journey. Water reached the Gulf of California, some 77 miles from Morelos Dam, for the first time since 1997.

There are social and political benefits, too. The source of the pulse flow water is an agreement between the governments of the United States and Mexico. A new agreement is now being discussed, and another round of environmental flows is likely to be included. Researchers are hopeful that the success of the pulse flow experiment will prompt negotiators to allocate more water to the delta.

The study, the second of three scientific reports that will be released, assessed the condition of the delta after two growing seasons, through December 2015. The findings will guide the long-term restoration of an area that is an internationally significant bird habitat and home to several endangered species.

“The key lesson here is that even though it was a short-term event, it is actually having lasting effects on vegetation and birds,” Karl Flessa, a University of Arizona professor and chief scientist monitoring the pulse flow, told Circle of Blue.

After the Flood

The pulse flow may have mimicked a pre-dam spring flood, but it was a faint resemblance. At its peak, the river in April 2014 was less than three percent of the median flood discharge before 1935, when Hoover Dam walled off Boulder Canyon hundreds of miles upstream. The pulse was a faucet drip, not a firehose.

That fact leads Flessa to another conclusion. Because a limited amount of water is available, large-scale restoration projects like this need to be managed, not left to nature, he said.

For the pulse flow to reach the targeted restoration areas, project managers had to be directors. They pulled out invasive plants such as tamarisk. They connected meanders to the main channel by excavating and grading the soil. On the bare ground they seeded native willow and cottonwood. Instead of sending water through porous stretches of the river channel they redirected the flow through irrigation canals, to avoid seepage.

“It’s not enough anymore to dump water into the riverbed,” Flessa said. “Active management is key to success.”

Even though it focused on science, the report notes that the pulse flow experiment had deep social importance too. Karen Schlatter, an ecologist at the Sonoran Institute and a co-manager of the monitoring team, recalls families in the Mexicali Valley kayaking and swimming in the river, some for the first time.

“People in this area have for a long time gone without knowing that they have a river nearby,” Schlatter told Circle of Blue. “To have a river suddenly come through the community was something special and people want to see more of that.”

More Delta Water in Next Agreement?

The water for the pulse flow came from an agreement called Minute 319. Every few years, the United States and Mexican governments update a 1944 treaty over shared rivers. Those updates are called minutes.

Parties on both sides called Minute 319 “historic,” for several reasons. One, the agreement brought Mexico into the shortage agreements that were worked out in 2007 between Arizona, California, and Nevada. Mexico now takes a cut in its annual allocation if Lake Mead drops below certain thresholds. Two, it allowed those three U.S. states to pay for irrigation improvements in Mexico and gain the extra water. And third, it allocated 158,000 acre-feet of water to the Colorado River delta.

The water came in two forms. Two-thirds of the total was designated as the pulse flow, which was released over eight weeks in 2014. This amount was deducted from surplus water Mexico had stored in Lake Mead after a 2010 earthquake in the Mexicali Valley damaged irrigation canals. The remaining one-third, called base flow, has been doled out periodically to the two main restoration areas, Miguel Aleman and Laguna Grande.

The base flows support the vegetation that was established during the pulse flow, explained Schlatter.

The base flow water did not come from the governments. It came instead from environmental groups such as the Sonoran Institute that have leased or purchased water rights in the lower Colorado River, often from Mexicali Valley farmers. The program, called the Delta Water Trust, began in 2008.

A similar arrangement for the future is being discussed. The U.S. and Mexican governments are negotiating a new minute, to replace Minute 319, which expires at the end of 2017. Schlatter said that a goal of the research team was to complete this second assessment report in time to inform the negotiation. It worked. Negotiators asked the team earlier this year to develop delta restoration scenarios for a range of flows. Schlatter would not mention specific volumes, but said they were similar to the flows included in Minute 319.

“We’re hopeful that, given the type of information we were asked to provide, that the next minute will set aside water for environmental flows,” Schlatter said. “Both the United States and Mexican governments want this to continue.”

The federal government appears to be leaning in that direction.

“We anticipate including an environmental component in the next Colorado River agreement between the two countries, along with elements to address water conservation, water operations and drought response actions,” Patricia Aaron, a Bureau of Reclamation spokeswoman wrote to Circle of Blue in an email.