Rates of ebullition and composition of bubbles were measured along a nutrient-enriched segment of the South Platte River below Denver, Colorado. Ebullition was widespread in the South Platte up to 81 km downstream from Denver. Ebullitive fluxes of 0.44 and 0.29 g N m⁻² d⁻¹ were recorded at two sites downstream of Denver and represented 6–16% of the diffusive N₂ efflux from this region. These data indicate that not accounting for ebullitive N₂ losses can, at some locations, lead to a considerable underestimation of dentrification rates using the open-channel (gas exchange) method. Gas bubbles often were >98% N₂; methane dominated in a few organic-rich areas. Rates of ebullition related significantly to variations in temperature and dissolved organic carbon. Ebullition was not observed in four tributaries of the South Platte River, despite their moderate to high concentrations of nitrate and dissolved organic carbon. The data demonstrate that ebullition can contribute significantly to N₂ effluxes in running waters exhibiting high rates of denitrification.