New tools are needed to evaluate the impacts of short-term hydropower regulation practices on downstream river systems and to progress towards sustainable river-flow management. As hydropower is increasingly being used to balance the energy load deficit caused by other less flexible sources, sub-daily flow conditions across many regulated river (RR) systems are changing. To address this, we used wavelet analyses to quantify the discharge variability in RRs and categorized the level of variability based on the conditions in natural free-flowing rivers. The presented framework used the definition of fluvial connectivity (Grill et al., 2019) to identify free-flowing rivers used in the study. We tested the developed framework in 12 different RRs in Finland and found higher overall averaged sub-daily variations, with up to 20 times larger variability than natural conditions. A large, highly regulated Finnish river system was found to have the highest sub-daily variations in winter, while smaller RRs with lower levels of regulation the highest variations in summer. The proposed framework offers a novel tool for sustainable river management and can be easily applied to various rivers and regions globally. It had flexibility to analyze sub-daily variations in desired seasonal or other ecologically sensitive periods.