The Undammed project studies the impact of dams removal on river networks and ecosystems. These structures can be a source of methane emissions with a significant climate impact. In the last years, Europe has initiated a river restoration strategy by removing such dams, especially those no higher than 10 metres. But little is yet known about the effects on the carbon cycle of the river network once the dam has been removed.
Cross mill Weir, a small concrete block and sandstone dam in Barrhead, Scotland, which was removed in 2023.
The Undammed project is coordinated by Rafael Marcé, who was a researcher at the Catalan Institute for Water Research (ICRA) at the beginning of the project and now is a researcher at the Centre for Advanced Studies in Blanes (CEAB-CSIC). ‘Nowadays, we know that dams are a source of methane emissions, a greenhouse gas that has a much greater global warming potential than carbon dioxide’ explains Marcé. The project, which also involves teams from the Universitat de les Illes Balears and the Universitat de Lleida, aims to determine the climatic impact of the different dams.
A carbon sink, but a methane emitter
As Marcé explains, ‘when a dam is built, a new ecosystem is created and sediment accumulates at the bottom, composed mostly by organic matter that has been washed away by the flow’.
This has two effects, he says. On the one hand, the accumulation of organic matter is still retained carbon, a CO2 sink. However, the other effect is that microorganisms that degrade organic matter proliferate in these sediments, which in turn generate methane, a gas with a much greater greenhouse effect than CO2.
CEAB-CSIC researcher and Undammed Coordinator Rafael Marcé.
The latest IPCC (Intergovernmental Panel on Climate Change) reporting guidelines for emission inventories are beginning to include these processes in their reports and recommend starting to calculate emissions from these dams, because they may be having a greater climate impact than previously thought.
The latest IPCC recommend starting to calculate emissions from these dams.
¿In what cases would it be better to remove dams? That’s what researchers are questioning: what happens when a dam or weir is removed, what hydrological, morphological and biogeochemical effects it has on the river and surrounding areas, and whether the reduction in methane emissions is significant and offsets the loss of other potential benefits of the dam.
A case study in Monistrol de Calders
The researchers, in collaboration with the Catalan Water Agency (ACA), sought a dam that could serve as a case study. They found one in Monistrol de Calders (Barcelona), where the ACA planned to remove the lock of the former Rio textile colony, which was linked to an old mill. The researchers monitored biogeochemical and morphological indicators before, during, and after the dam's removal.
Preliminary figures reveal that carbon emissions from the dam were 1 mole of CO2 equivalent per square metre per day, which after dam removal dropped to 0.1 mole per square metre per day.
At Monistrol de Calders, preliminary figures show that removing the dam reduced methane emissions by up to 10 times
These are not definitive results, warns Rafael Marcé, but they suggest that dam removal could significantly aid decarbonisation. 'For example, if we take the Monistrol de Calders dam, we calculate that it emitted 125 tonnes of CO2 equivalent per year. Then, considering that there are around 170,000 barriers in Spain's watercourses, 'if only 5% of them were similar to the one in Monistrol, we would be talking about 1 million tonnes of annual carbon equivalent emissions that could be reduced.'
The Catalan Water Agency has a plan to remove these structures. However, each case should be evaluated. 'We want to know where to focus the effort on landscape decarbonisation, to understand which dams should be prioritised for removal – because they are hotspots for methane and carbon emissions – and which ones should not,' Marcé explains. 'There are other cultural, social, ecosystem services, and even legal considerations.
Monistrol de Calders dam before its removal (left) and the state of the river course afterwards (right) Image: Undammed project and ACA.
For instance, while removing the dam is generally beneficial for biodiversity, sometimes the barrier in the watercourse prevents the spread of invasive species. Other times, the dam may be one of the few swimming areas in the region.
Finding out whether removal is a priority or not
Therefore, the longterm goal of the research is to characterise the dams to determine in which cases their removal would be a priority and in which cases it would not. Additionally, many of these dams are linked to hydroelectric production, an energy source previously considered clean, as methane emissions had not been taken into account.
The long-term goal of the research is to characterise the dams to determine in which cases their removal would be a priority and in which cases it would not.
Other questions that Undammed seeks to answer include what happens to the sedi-ments when the dam is removed, how the removal of the barrier affects morphology, and whether the removal of dams will impact the infiltration and recharge of groundwater, among many others.
'In the current context of the climate emergency, the modification of carbon sinks and methane emission hotspots should be added to the usual environmental considerations included in dam removal projects,' the project explains.
However, 'it is necessary to anticipate the trade-offs between the recovery of hydrolog-ical and sedimentary balance, the elimination of methane emission hotspots in flooded areas, and the possible atmospheric remobilisation of organic carbon buried in sediments.'
Undammed project includes three coordinated subprojects that bring together a unique collection of cutting-edge methods from various disciplines (morphodynamics, carbon biogeochemistry, numerical modelling). In this way, it aims to offer a new perspective on how dam removal aligns with various environmental policies at the national level (e.g., Recovery, Transformation, and Resilience Plan), European level (e.g., EU Green Deal), and global level (e.g., COP26 agreements). Undammed will help to take in-formed decisions about the fate of small dams that will be dismantled over the next decade.
To learn more:
Dam Removal Europe, iniciativa europea de restauración de cursos fluviales