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). Several researchers who worked on this study say most of the gas was stored underground in sediment, and sand-dwelling microbes created the rest when the water reached them. The riverbed normally releases greenhouse gases gradually as part of the typical carbon cycle, but the Delta released a significant amount in a matter of just eight weeks during the pulse flow, though the researchers aren’t yet sure exactly how much.
The consequences of that are still tough to quantify, says Karl Flessa, a co-author of the study and co-chief scientist of Minute 319, but he doesn’t think the risks of emitting greenhouse gases outweigh the benefits of watering a parched ecosystem and growing new plant life. Since the pulse flow event, vegetation has thrived in the riparian zone where the land meets the river in the Colorado River Delta – cottonwoods and willows have turned the space greener than it had been in years.
The U.S. and Mexico are currently in negotiations about more restoration efforts when this one expires in 2017. And now, the researchers plan to look into how the duration of floods like this one affects water chemistry, how controlled flooding could support coastal stability, and how the consequences of flood pulses compare to a steady, minimum water flow in rivers like the Colorado.
This study may actually strengthen the case for consistent flow of the Colorado River. Keeping the Delta wet is something advocates have long been fighting for because it could help with wildlife protection, water access for those who live near the Delta in Mexico, and irrigation and water rights throughout the region. If a significant release of greenhouse gases happened because the riverbed was dry for so many years, “the costs of drying out rivers are greater than we knew,” says Jennifer Pitt, director of the Colorado River Project at the Audubon Society. She also said a more holistic view of the carbon in river ecosystems — specifically, how much carbon is sequestered and restored by new plant life — is necessary to quantify the full impact.
Butman and some U.S. Geological Survey scientists are already running similar tests in the Columbia and Mississippi Rivers using new tools to map methane and carbon dioxide (similar to how methane plumes from oil wells are mapped on land). In some parts of the riverbed of the Columbia River, they’re finding methane at three to five times the concentration in the atmosphere; in others, it’s up to almost 1,000 times.
What they’re finding could end up having big impacts on water management: As humans manipulate their water sources in times of drought, the impacts those tweaks have on the carbon cycle could become a part of planning, too. “What’s really interesting,” Flessa says, “is getting people to start thinking about rivers and having a carbon budget. It’s a whole new way of thinking about rivers.”
