Colorado River Delta still benefiting from flood experiment

Colorado River delta
The Colorado River Delta captured in a 2004 image from the International Space Station. Via NASA Earth Observatory.

Monitoring report documents renewed life

Staff Report

In just two years following a man-made flood in the Colorado River Delta, cottonwoods and willows have grown 10 feet tall, rebuilding habitat for other native plants and animals, according to a new monitoring report on the international experiment to re-water the long-arched region.

“This short-term event has had lasting consequences. This really demonstrates that a little bit of water does a lot of environmental good,” said Karl W. Flessa,  professor of geosciences at the University of Arizona.

The new growth helped boost bird life, and  bird diversity is still higher than before the experimental flood.. According to the report, migratory waterbirds, nesting waterbirds and nesting riparian birds all increased in abundance. 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, the monitoring team found.

Upstream dams and water diversions for farms and cities, mostly in the U.S., have dried up most of the river south of the border. With the exception of a few wet years, the river has not reached the Gulf of California since 1960.

Before 1960, spring floods regularly roared down the Colorado River, scouring the river bottom and overtopping the bank, thereby creating the conditions necessary for cottonwood and willow trees to germinate and establish.

Some of the water from the pulse flow and subsequent smaller environmental flows recharged the groundwater, which had both ecological and social benefits. The vegetation greened up in areas that received surface water and also in some areas that did not.

The March 2014 pulse flow delivered a fraction of the water the pre-1960 spring floods delivered. People from Sonoran Institute and Pronatura Noroeste 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.

In addition, the pulse flow reduced soil salinity in some areas that had been targeted for restoration, which  makes conditions more favorable for native plant species.

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