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The current resurgence of hydropower expansion toward tropical areas has been largely based on run-of-the-river (ROR) dams, which are claimed to have lower environmental impacts due to their smaller reservoirs. The Belo Monte dam was built in Eastern Amazonia and holds the largest installed capacity among ROR power plants worldwide. Here, we show that postdamming greenhouse gas (GHG) emissions in the Belo Monte area are up to three times higher than preimpoundment fluxes and equivalent to about 15 to 55 kg CO 2 eq MWh −1 . Since per-area emissions in Amazonian reservoirs are significantly higher than global averages, reducing flooded areas and prioritizing the power density of hydropower plants seem to effectively reduce their carbon footprints. Nevertheless, total GHG emissions are substantial even from this leading-edge ROR power plant. This argues in favor of avoiding hydropower expansion in Amazonia regardless of the reservoir type. 

Abstract. The Belo Monte hydropower complex located in the Xingu River is the largestrun-of-the-river (ROR) hydroelectric system in the world and has one of thehighest energy production capacities among dams. Its construction receivedsignificant media attention due to its potential social and environmentalimpacts. It is composed of two ROR reservoirs: the Xingu Reservoir (XR) inthe Xingu's main branch and the Intermediate Reservoir (IR), an artificialreservoir fed by waters diverted from the Xingu River with longer waterresidence time compared to XR. We aimed to evaluate spatiotemporalvariations in CO2 partial pressure (pCO2) and CO2 fluxes(FCO2) during the first 2 years after the Xingu River impoundmentunder the hypothesis that each reservoir has contrasting FCO2 andpCO2 as vegetation clearing reduces flooded area emissions. Time ofthe year had a significant influence on pCO2 with the highest averagevalues observed during the high-water season. Spatial heterogeneitythroughout the entire study area was observed for pCO2 during both low-and high-water seasons. FCO2, on the other hand, only showed significantspatial heterogeneity during the high-water period. FCO2 (0.90±0.47 and 1.08±0.62 µmol m2 d−1 for XR and IR,respectively) and pCO2 (1647±698 and 1676±323 µatm for XR and IR, respectively) measured during the high-water season wereon the same order of magnitude as previous observations in other Amazonianclearwater rivers unaffected by impoundment during the same season. Incontrast, during the low-water season FCO2 (0.69±0.28 and 7.32±4.07 µmol m2 d−1 for XR and IR, respectively) andpCO2 (839±646 and 1797±354 µatm for XR and IR,respectively) in IR were an order of magnitude higher than literatureFCO2 observations in clearwater rivers with naturally flowing waters.When CO2 emissions are compared between reservoirs, IR emissions were90 % higher than values from the XR during low-water season, reinforcingthe clear influence of reservoir characteristics on CO2 emissions.Based on our observations in the Belo Monte hydropower complex, CO2emissions from ROR reservoirs to the atmosphere are in the range of naturalAmazonian rivers. However, the associated reservoir (IR) may exceed naturalriver emission rates due to the preimpounding vegetation influence. Sincemany reservoirs are still planned to be constructed in the Amazon andthroughout the world, it is critical to evaluate the implications ofreservoir traits on FCO2 over their entire life cycle in order toimprove estimates of CO2 emissions per kilowatt for hydropower projectsplanned for tropical rivers.