Gravel bars are sites of increased CO₂ outgassing in stream corridors

Streams are significant sources of CO₂ to the atmosphere. Estimates of CO₂ evasion fluxes (f _CO2) 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 CO₂ to the atmosphere, with on average more than twice as high f _CO2 as those from the streamwater, affecting f _CO2 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 _CO2 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 CO₂ from external sources (e.g. downwelling streamwater, groundwater) within GBs. Our study shows that GB f _CO2 increased f _CO2 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^rd, 4^th and 5^th order streams, respectively. These findings shed new light on regional estimates of f _CO2 from streams, and are relevant given that streamwater thermal regimes change owing to global warming and human alteration of stream corridors.