Aquatic primary production is the foundation of many river food webs. Dams and associated flow regulation change the physical template of rivers, often driving food webs toward greater reliance on aquatic primary production. Nonetheless, the effects of regulated flow regimes on primary production are poorly understood. Hydropeaking is a common dam flow management strategy that involves sub-daily changes in water releases proportional to fluctuations in electrical power demand. This flow regime causes an artificial tide, wetting and drying channel margins and altering river depth and water clarity, all processes that are likely to affect primary production. In collaboration with dam operators, we designed an experimental flow regime whose goal was to mitigate negative effects of hydropeaking on ecosystem processes. The experimental flow contrasted steady-low flows on weekends with routine hydropeaking flows on weekdays. Here, we quantify the benefit of this experimental flow on springtime gross primary production (GPP) 90-to-400 km downstream of Glen Canyon Dam on the Colorado River, AZ, USA. Across all reaches, GPP during steady-low flows was 38% higher than during hydropeaking flows, mostly due to reduced turbidity during steady-low flow days. At the weekly scale, the experimental flow increased GPP even after controlling for variation in weekly mean discharge, demonstrating a negative effect of hydropeaking on GPP. We estimate that the environmental flow increased peak springtime carbon fixation by 0.34 g C m-2 d-1, which is ecologically meaningful considering median C fixation in 365 U.S. rivers of 0.44 g C m-2 d-1 and the fact that native fish populations in this river are food-limited.
