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A recent estimate of CO2 outgassing from Amazonian wetlandssuggests that an order of magnitude more CO2 leaves rivers through gasexchange with the atmosphere than is exported to the ocean as organic plusinorganic carbon. However, the contribution of smaller rivers is still poorlyunderstood, mainly because of limitations in mapping their spatial extent.Considering that the largest extension of the Amazon River network is composed of small rivers, the authors\' objective was to elucidate their role inair-water CO2 exchange by developing a geographic information system(GIS)-based model to calculate the surface area covered by rivers with channelsless than 100 m wide, combined with estimated CO2 outgassing rates at theJi-Paraná River basin, in the western Amazon. Estimated CO2 outgassing wasthe main carbon export pathway for this river basin, totaling 289 Gg C yr-1,about 2.4 times the amount of carbon exported as dissolved inorganic carbon(121 Gg C yr-1) and 1.6 times the dissolved organic carbon export (185 Gg Cyr-1). The relationships established here between drainage area and channelwidth provide a new model for determining small river surface area, allowingregional extrapolations of air-water gas exchange. Applying this model to theentire Amazon River network of channels less than 100 m wide (third to fifthorder), the authors calculate that the surface area of small rivers is 0.3 ± 0.05million km2, and it is potentially evading to the atmosphere 170 ± 42 Tg C yr-1as CO2. Therefore, these ecosystems play an important role in the regionalcarbon balance.

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