Beaver impoundments trap large amounts of organic matter, and remineralization of the organic matter to CO2 could make them a significant source of atmospheric CO2. Measurements in two beaver impoundments in the Adirondack Park of New York State indicated that: (i) during the ice-free season CO2 emission averaged 197 mmol m-2 d-1; (ii) concentrations of dissolved inorganic carbon (DIC) in the water column were always >100 &mgr;M, reaching >1000 &mgr;M beneath ice cover and midsummer; and (iii) the rate of CO2 production by sediment samples from beneath the water column could exceed 300 mmol m-2 d-1 in midsummer. The CO2 emitted from the impoundments to the atmosphere is supplied both by the sediments beneath the impoundment and by transport into the water column from the surrounding uplands. If beaver impoundments cover 3% of the landscape in north temperate forests then the magnitude of the presumed CO2 sink in this landscape would be reduced by 7% compared with estimates based only on the terrestrial component. ¿ American Geophysical Union 1994