Sub-daily and weekly flow cycles termed ‘hydropeaking’ are common features in regulated rivers worldwide. Weekly flow periodicity arises from fluctuating hydropower demand and production tied to socioeconomic activity, typically with higher consumption during weekdays followed by reductions on weekends. Here, we propose a novel weekly hydropeaking index to quantify the 1920-2019 intensity and prevalence of weekly hydropeaking cycles at 400 sites across the United States of America and Canada. A robust weekly hydropeaking signal exists at 1.1% of sites starting in 1920, peaking at 17.0% in 1963, and diminishing to 3.2% in 2019, marking a 21st century decline in hydropeaking intensity. We propose this decline may be tied to recent, above-average precipitation, socioeconomic shifts, alternative energy production, and legislative and policy changes impacting water management in regulated systems. Vanishing weekly hydropeaking cycles may offset some of the prior deleterious ecohydrological impacts from hydropeaking in highly regulated rivers.