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Summary: 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. Recent improvements to dissolved oxygen sensors placed at many national USGS gaging stations can be used together with statistical modeling to better understand controls on riverine primary production.  Here we describe primary production dynamics during a flow experiment designed by a USGS adaptive management program.  The experiment compared hydropeaking flows to low and steady flows below a large dam.  Dissolved oxygen and water quality monitoring during the experiment showed that steady-low flows led to clearer water and higher rates of primary production than hydropeaking flows over both short- and long-time scales and up to 400 km downstream from the dam.