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The global development of hydropower dams has rapidly expanded over the last several decades and has spread to historically non-impounded systems such as the Amazon River’s main low land tributaries in Brazil. Despite the recognized significance of reservoirs to the global methane (CH 4 ) emission, the processes controlling this emission remain poorly understood, especially in Tropical reservoirs. Here we evaluate CH 4 dynamics in the main channel and downstream of the Santo Antônio hydroelectric reservoir, a large tropical run-of-the-river (ROR) reservoir in Amazonia. This study is intended to give a snapshot of the CH 4 dynamics during the falling water season at the initial stage after the start of operations. Our results show substantial and higher CH 4 production in reservoirs’ littoral sediment than in the naturally flooded areas downstream of the dam. Despite the large production in the reservoir or naturally flooded areas, high CH 4 oxidation in the main channel keep the concentration and fluxes of CH 4 in the main channel low. Similar CH 4 concentrations in the reservoir and downstream close to the dam suggest negligible degassing at the dam, but stable isotopic evidence indicates the presence of a less oxidized pool of CH 4 after the dam. ROR reservoirs are designed to disturb the natural river flow dynamics less than traditional reservoirs. If enough mixing and oxygenation remain throughout the reservoir’s water column, naturally high CH 4 oxidation rates can also remain and limit the diffusive CH 4 emissions from the main channel. Nevertheless, it is important to highlight that our results focused on emissions in the deep and oxygenated main channel. High emissions, mainly through ebullition, may occur in the vast and shallow areas represented by bays and tributaries. However, detailed assessments are still required to understand the impacts of this reservoir on the annual emissions of CH 4 . 

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.