Tuesday, July 26, 2016

The Colorado River’s unexpected carbon footprint

by Lyndsey Gilpin

When water rushed over the dry riverbed of the Colorado River Delta for the first time in two decades, thousands of bubbles popped up in the sand. Alongside the bank, a group of scientists stood in awe, theorizing that oxygen and nitrogen trapped in the sediment were the cause. But nearly two years later, in early 2016, the team discovered those bubbles were actually composed of  greenhouse gases – methane and carbon dioxide – that dissolved into the water, traveled downstream, and eventually made their way into the air.

The Colorado River supplies water to 40 million people. It is used so heavily by farms and communities in the West that it rarely reaches the ocean, so where the river should meet the Gulf of California, only a dry delta exists. In 2012, Mexico and the U.S. hashed out the Minute 319 pact to allow for a one-time pulse flow to restore water in the Delta so scientists could study the regenerative capability of the floodplain ecosystem. So in 2014, the U.S. released over 100,000 acre-feet of water at Morelos Dam near Yuma, Arizona, to restore wildlife and native plant habitats in the Delta downstream. But a new study by University of Florida, University of Arizona, Yale University and University of Washington researchers shows the water also caused the ground to rapidly emit carbon stored for years beneath the riverbeds, which could have an impact on the global carbon cycle and affect future river restoration.

“It’s still a big unknown on the true magnitude of these fluxes, but these large river(beds) are turning out to have really high concentrations of carbon dioxide and methane,” says David Butman, an environmental science and engineering professor at the University of Washington who worked on the study. “Looking at the exchanges of carbon gasses between landscapes, the atmosphere, and water as we look to restore these disturbed ecosystems may be important.”

The study, funded in part by the National Science Foundation, is a step toward understanding carbon balance in water systems and the impact it could have on carbon levels on land and in the ocean. It’s still unclear why carbon was released, but the study documented that 30 percent more greenhouse gases came out of the riverbed and dissolved into the water at one site during the Minute 319 flow than before it (they’re still working to determine how much was released into the atmosphere)...

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