Richey et al (2002), as reviewed by Grace and Malhi (2002), provide evidence that Amazonian rivers that become loaded with biomass in upland and flooded forests can become sources of carbon dioxide before the river water reaches the ocean. They assert that estimates of carbon fluxes from the Amazon region should include the effect of decaying vegetation being transported by Amazonian rivers. They estimate that 0.5 Gt/yr is emitted to the atmosphere by this method.
References
Grace, J., and Y. Malhi, 2002: Carbon dioxide goes with the flow. Nature, 416 594-595.
Richey, J. E., J. M. Melack, A. K. Aufdenkampe, V. M. Ballester, and L. L. Hess, 2002: Outgassing from Amazonian rivers and wetlands as a large tropical source of atmospheric CO2. Nature, 416, 617-620.