Quantifying short-term changes in river flow is important in understanding the environmental impacts of hydropower generation. Energy markets can change rapidly and energy demand fluctuates at sub-daily scales, which may cause corresponding changes in regulated river flow (hydropeaking). Due to increasing use of renewable energy, in future hydropower will play a greater role as a load balancing power source. This may increase current hydropeaking levels in Nordic river systems, creating challenges in maintaining a healthy ecological status. This study examined driving forces for hydropeaking in Nordic rivers using extensive datasets from 150 sites with hourly time step river discharge data. It also investigated the influence of increased wind power production on hydropeaking. The data revealed that hydropeaking is at high levels in the Nordic rivers and have seen an increase over the last decade and especially over the past few years. These results indicate that increased building for renewable energy may increase hydropeaking in Nordic rivers.

Water temperatures control life histories and diversity of aquatic species. Hydropower regulation, particularly in high head systems, alters natural water temperature regimes, which may have profound and long‐term impacts on aquatic environments. Temperatures in by‐pass sections and reaches affected by residual/environmental minimum flows fluctuate more than in natural flow regimes, driven more by influence of air temperatures. Reaches downstream of power plant outlets tend to become warmer in winter and colder in summer, driven by stratification behind the reservoir dam. In hydro‐peaked systems high‐low temperature effects may thus be aggravated. We review alterations of hydropower to natural thermal regimes, impacts on key organisms in terms of survival, development and behavioral thresholds, and potential mitigation measures, with focus on Atlantic salmon and brown trout in high northern latitude stream systems. Previous syntheses have focused mainly on flow changes and ecological impacts. Temperature effects may not always be correlated with flow changes, although there are some unique challenges with temperature changes in far northern latitudes, for example, related to the seasonal and colder climates. To help knowledge‐based management and identify potential knowledge gaps, we review how hydropower regulation may impact seasonal water temperatures, what impacts changes to stream system temperature regimes may have to key organisms, for example, Atlantic salmon and brown trout, and what adaptations and behavioral variations they may exhibit to respond to changed temperature regimes, and finally what good practices can be recommended for mitigating temperature impacts. This synthesis indicates that there are impacts to the fish and their supporting food webs, in particular related to growth and development, and the potential for negative impacts seems higher, and better studied, than positive impacts in northern river systems. Some of these impacts may be modified by directed hydropower regulation practices, but here effect studies and knowledge are limited.