Air–water oxygen exchange in a large whitewater river

Abstract

Lay AbstractAir–water gas exchange governs the rate at which atmospheric gases flow into and out of aquatic ecosystems. Knowing this rate is necessary to calculate river photosynthesis and respiration, but there are few data from large rivers, and there are no data that include whitewater rapids. We studied the Colorado River, Grand Canyon; this river has flat reaches separated by extremely large, steep, whitewater rapids. We measured gas transfer velocity (the height of the water column that can exchange all of its gas per hour) by measuring how quickly the river gained oxygen as it flowed over the first 7 major rapids. The Colorado River has low oxygen concentration as it flows out of Glen Canyon Dam, located 25 kilometers upriver from Lees Ferry. We found that gas transfer velocity increased as river slope increased. Gas transfer velocity was low in flat reaches but was up to 800 times higher in rapids, which were the highest rates ever measured in a river. Based on the rate of change of oxygen concentration per meter of river drop, we estimated gas transfer velocity for the remainder of the Colorado River in Grand Canyon. Gas exchange varied 5‐fold depending on the slope of the immediate reach. Gas transfer velocity was higher for the Colorado River than for other aquatic ecosystems because of its large rapids. Our approach of scaling gas transfer velocity to the entire river will allow comparing gas transfer velocity across rivers that have variable river slopes, such as the Colorado River, Grand Canyon.