Gravel bars are sites of increased CO outgassing in stream corridors.

Streams are significant sources of CO to the atmosphere. Estimates of CO evasion fluxes (f ) from streams typically relate to the free flowing water but exclude geomorphological structures within the stream corridor. We found that gravel bars (GBs) are important sources of CO to the atmosphere, with on average more than twice as high f as those from the streamwater, affecting f at the level of entire headwater networks. Vertical temperature gradients resulting from the interplay between advective heat transfer and mixing with groundwater within GBs explained the observed variation in f from the GBs reasonably well. We propose that increased temperatures and their gradients within GBs exposed to solar radiation stimulate heterotrophic metabolism therein and facilitate the venting of CO from external sources (e.g. downwelling streamwater, groundwater) within GBs. Our study shows that GB f increased f from stream corridors by [median, (95% confidence interval)] 16.69%, (15.85-18.49%); 30.44%, (30.40-34.68%) and 2.92%, (2.90-3.0%), for 3, 4 and 5 order streams, respectively. These findings shed new light on regional estimates of f from streams, and are relevant given that streamwater thermal regimes change owing to global warming and human alteration of stream corridors.