New growth in delta could offset CO2 released from riverbed
Human management of natural ecosystems always has unintended consequences, and the Colorado River is no exception. After decades of intense dam building and diversions, the mighty river is a mere shadow of it former self, reduced to a trickle in some places and polluted by return flows in others. Along its entire length, ecosystems, including riparian zones and native fish, have suffered, with some of the biggest impacts in the Colorado River delta.
In an effort to restore at least some key reaches of the river, scientists and water managers have teamed up to try mimic some of the Colorado’s natural functions, with controlled releases of water to build up beaches. Those efforts culminated in early 2014 during an eight-week experiment that unleashed a mighty torrent of water from Morelos dam (on the border with Mexico and the USA).
The huge surge (130 million cubic metres) of water raised river levels down to the delta, which has been starved of water for decades. Scientists closely monitored how the release — and potential future releases — affect agricultural crops and natural plant and animal life of the lower delta.
But the pulse flow had another side effect. As the water washed over earth and rocks that had been dried out for many years, it dissolved carbon and sent a surge of greenhouse gases into the atmosphere.
“We saw a rapid release of greenhouse gases (CH4 and CO2) from the riverbed sediments to the floodwaters. These gases were largely derived from carbon which had been stored in the dry riverbed, perhaps for decades,” Dr. Thomas Bianchi said at the recent Goldschmidt conference in Yokohama, Japan. “This shows that more work is needed to better understand the more unpredictable consequences of floods and droughts on aquatic ecosystems, particularly in the face of global climate change,” said Bianchi, a geologist at the University of Florida.
The findings were based in part on radiocarbon studies showing that some of the dissolved inorganic carbon was more than 800 years old, suggesting that trapped CO2 and inorganic carbon was dissolved and released rapidly into the river water when flooded.
The findings suggest the need for taking a long-term view, with an eye to climate change, when considering the impacts of human-induced changes in river flows. Carbon stored in riverbeds is more likely to be released in a variable climate with more floods and drought than under more stable conditions. The drying and rewetting of once natural river deltas may fundamentally alter the processing and storage of carbon, Bianchi said.
Open questions include how the duration of the wet and dry periods might affect the release of greenhouse gases, or whether maintaining minimum water flow levels might help. It’s also possible that renewed plant growth could compensate for the releases by sucking more CO2 out of the air. Marsh communities that frequently form in deltas have the potential to store such greenhouse gases in their soils for long periods of time.
Restoration of the Colorado River delta and other areas similarly impacted could benefit climate adaptation in other ways, for example by limiting delta erosion and making coastlines more stable.
Resolving these uncertainties is critical for assessing the role of inland waterways on global carbon budgets, identifying potential feedback loops under a changing climate, and planning future flow restoration events.
Understanding those complex interactions requires a long-term bi-national commitment from the U.S. and Mexico to assess and monitor the effects of flow regimes, but Bianchi said scientists agree that “aiming for restoration is clearly the right thing to do.”
Commenting, Professor Elizabeth A. Canuel (Virginia Institute of Marine Science), said:
“This presentation reports an unexpected finding that a short-term controlled flooding event on the Colorado River resulted in the release of greenhouse gases from the newly wetted riverbed sediments,” said researcher Elizabeth A. Canuel, from the Virginia Institute of Marine Science.
That contradicts, to some degree, the conventional wisdom that the production of greenhouse gases generated from aerobic and anaerobic respiration of organic matter is higher in dry soils, rather than wet soils.
“However, as this preliminary study shows, dry river sections can become “hot spots” of biogeochemical transfer and transformation when organic matter and nutrients accumulated in the sediments are activated during rewetting phases and first-pulse events such as this controlled flooding event,” Canuel said.