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Headwater and Piedmont Streams are Hotspots of CO2 and CH4 Emission

According to recent scientific research conducted by the University of Liège, rivers in the Andean mountains contribute 35% and 72% of riverine emissions of carbon dioxide (CO2) and methane (CH4) in the Amazon basin, the world’s largest river. The findings of this study have been published in the journal Communications Earth & Environment.

Image Credit: University of Liège

The Amazon River, the world’s largest river, contributes significantly to greenhouse gas (GHG) emissions.

It is the largest river on the planet in terms of freshwater flow. We are talking about a discharge of 6,600 cubic kilometers of water per year. It is also the largest hydrographic basin with a surface area of 6,300,000 km2, which is comparable to the size of the United States of 9,834,000 km².

Alberto Borges, FNRS Research Director, FOCUS Research Unit (Faculty of Science), University of Liège

Alberto Borges adds, “In addition, the Amazon River drains the largest rainforest on the planet, which provides rivers with large amounts of organic carbon that is transformed by microbes into CO2 and CH4, and then emitted across surface waters into the atmosphere.

The Amazon River flows from (headwaters) the Andes Mountains to the Atlantic Ocean, passing through Peru, Colombia, Ecuador, and Brazil. The erosion of rocks at the river’s headwaters in the Andes is the primary source of mineral particles that travel about 3,000 km across South America to the river’s mouth in Belem, Brazil, where they flow into the Atlantic Ocean.

All the studies on CO2 and CH4 emissions to the atmosphere by Amazon rivers have been carried out so far in the plains of the central Amazon, at least 1,000 km from the Andes,” notes Alberto Borges, “whereas mountain rivers show very different rates of CO2 and CH4 emissions from those of lowland rivers.

Lowland Rivers and Mountain Streams

Three river systems run through the mountains and across the plains. The first is a small mountain stream that flows quickly over steep, rocky terrain. This encourages a rapid physical exchange of gases with the atmosphere. Conversely, steep terrain does not permit the accumulation of soils that support CO2 and CH4 production.

The second system, the lowland river, is broad and winding, spanning flat terrain. The slower flow of water does not encourage as strenuously the physical exchange of gases with the atmosphere as mountain rivers.

But the higher temperature (lower altitude) promotes the growth of more vegetation (forests), and the flat terrain promotes the accumulation of thicker soils than in the mountains. This should favor CO2 and CH4 production and transport to lowland watercourses.

Eventually, the flat terrain encourages the development of floodplains linked to lowland rivers, which also provide CO2 and CH4.

There is a third type of river system. Located in the plains at the foot of mountain ranges, it is called a 'piedmont river'. From a physical point of view, these rivers resemble lowland rivers, but they receive massive amounts of particles from mountain rivers located upstream. These particles are temporarily deposited, then resuspended and transported further downstream until they reach the ocean.

Gonzalo Chiriboga, Study First Author and Doctoral Student, Chemical Oceanography Unit, University of Liège

When the particles settle as sediment, they stimulate the formation of CH4 via fermentation. 

CO2 and CH4 emissions have so far only been measured in lowland rivers in the central Amazon, so we were missing potentially important pieces of the puzzle, which is crucial for the largest river on the planet.

Gonzalo Chiriboga, Study First Author and Doctoral Student, Chemical Oceanography Unit, University of Liège

One concern has been explained in an article published in Communications Earth & Environment, which presents data on Ecuador’s mountain and piedmont rivers along a transect ranging in elevation from 175 m to 3990 m.

We found that mountain rivers in the Andes have higher emissions than piedmont rivers, and are hot spots for CO2 and CH4 emissions, with significantly higher flux intensities than in the lowland rivers of the central Amazon,” Gonzalo Chiriboga states.

This is a major study that demonstrates that streams and rivers in the Andean mountains’ headwaters and piedmont account for 35% of CO2 emissions and 72% of CH4 emissions at the basin scale.

The study was supported by the Academy of Research and Higher Education (ARES) in the framework of cooperation with the Universidad Central del Ecuador.

Journal Reference

Chiriboga, G. & Borges, A. V. (2023). Andean headwater and piedmont streams are hot spots of carbon dioxide and methane emissions in the Amazon basin. Communications Earth & Environment.

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