Simulated Flood Injected Life Into Thirsty Colorado River Delta, New Data Shows

Oct 25, 2017

In the spring of 2014, a rare moment happened on the lowest stretches of the Colorado River. Instead of its last few drops being diverted at a dam near the U.S.-Mexico border, it flowed through its banks.

“There was a remarkable amount of excitement and energy in the air,” says James Leenhouts, a U.S. Geological Survey hydrologist who runs the agency’s Arizona Water Science Center and was present for the moment. “One of the most remarkable impacts of the flow was the human impact.”

In communities where the river hadn’t flowed for nearly two decades, impromptu festivals broke out, children waded into the water and mariachi bands set up to entertain the crowds. As the water settled into the valley it was clear the “experimental pulse flow” made a profound cultural imprint on the region the Colorado River once regularly flooded.

Before water managers opened the flood gates to simulate a flood, the lower stretches of the Colorado River were dry.
Credit Courtesy U.S. Geological Survey

A group of scientists, including those from the U.S. Geological Survey, are also gaining insight into how the flow was felt by plants, animals and the overall delta ecosystem.  

Using a mix of groundwater monitoring and satellite imagery, scientists say even the pulse’s modest flow of water — approximately one-twentieth the amount that spilled into the delta before humans built the river’s massive upstream dams — recharged aquifers, greened plant life and spurred the return of bird species.

Years before the flow, Mexico and the United States agreed to the experiment, and to the idea that the water was not just for human use, but can and should be used to revive ecosystems. The agreement -- an update to a 1944 treaty between the two countries -- gave Mexico the ability to store more water in American reservoirs and, just once, flood the final miles of the dry river bed to see what happened.

A satellite photo shows the lower Colorado River basin before the pulse flow in 2013, with limited green vegetation.
Credit Courtesy U.S. Geological Survey

From above you can use the naked eye to see the water’s effect. Before and after photos show plant life greening not just in the river’s bed where water actually flowed, but beyond the banks, which Leenhouts says is a sign of recharged groundwater.

After the 2014 pulse flow, scientists were able to measure an increase in greening plant life in the restored river ecosystem, as this satellite photo shows.

“In the two years following the flow it was possible to measure increased green up,” using satellite images, he says.

During that same period both the number of and varied species of birds increased, he says, a side effect of the revived vegetation.

Even though the pulse flow only lasted a few months in 2014, its effects lasted for years. The greenness of vegetation wasn’t as vibrant the following year, but it was still greener compared to 2013. That indicates the single sustained flow provided enough water to keep plants alive for at least a year.

Because the amount of water was significantly lower than a natural flood, the water’s power was diminished. In an unencumbered flood, the water’s power would rip out non-native plants, move sediment and reshape its banks. During the pulse the water was more effective in areas where scientists and volunteers had cleared out invasive plants or moved sediment piles by hand.

“What we were able to ask ourselves is, ‘what’s the best [way] to do this in the future?’” Leenhouts says.

Scientists’ chance to study and test the effect of simulated floods isn’t over.  A new update to the 1944 treaty signed this year allows for more pulse flows when Mexico stores surplus water. 

This story is part of a project covering the Colorado River, produced by KUNC and supported through a Walton Family Foundation grant.