Air-water oxygen exchange in a large whitewater river

Air–water gas exchange governs fluxes of gas into and out of aquatic ecosystems. Knowing this flux is necessary to calculate gas budgets (i.e., O₂) to estimate whole‐ecosystem metabolism and basin‐scale carbon budgets. Empirical data on rates of gas exchange for streams, estuaries, and oceans are readily available. However, there are few data from large rivers and no data from whitewater rapids. We measured gas transfer velocity in the Colorado River, Grand Canyon, as decline in O₂ saturation deficit, 7 times in a 28‐km segment spanning 7 rapids. The O₂ saturation deficit exists because of hypolimnetic discharge from Glen Canyon Dam, located 25 km upriver from Lees Ferry. Gas transfer velocity (k₆₀₀) increased with slope of the immediate reach. k₆₀₀ was < 10 cm h^− 1 in flat reaches, while k₆₀₀ for the steepest rapid ranged 3600–7700 cm h^− 1, an extremely high value of k₆₀₀. Using the rate of gas exchange per unit length of water surface elevation (K_drop, m^− 1), segment‐integrated k₆₀₀ varied between 74 and 101 cm h^− 1. Using K_drop we scaled k₆₀₀ to the remainder of the Colorado River in Grand Canyon. At the scale corresponding to the segment length where 80% of the O₂ exchanged with the atmosphere (mean length = 26.1 km), k₆₀₀ varied 4.5‐fold between 56 and 272 cm h^− 1 with a mean of 113 cm h^− 1. Gas transfer velocity for the Colorado River was higher than those from other aquatic ecosystems because of large rapids. Our approach of scaling k₆₀₀ based on K_drop allows comparing gas transfer velocity across rivers with spatially heterogeneous morphology.