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.

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There is a global consensus around meeting the United Nations Sustainable Development Goals (SDGs), one of which is to "protect and restore water-related ecosystems" under SDGs (target 6.6). Rising energy demand has increased the number of dams being constructed worldwide for a seemingly flexible and low-carbon emitting power source. Hydropower provides the grid flexibility needed to achieve high penetration of wind and solar energy, compensating for the intermittency of these sources (Clack et al., 2017;Denholm & Hand, 2011). There are more than 3,700 hydropower dams (>1 MW) currently planned or under construction worldwide (Zarfl et al., 2015). According to the 2018 hydropower status report by the international hydropower association (International Hydropower Association, 2018), growth in hydroelectric generating capacity was fastest in East Asia and the Pacific, with 9.8 GW of capacity added in 2017. China, Brazil, and India together added around 16 GW of the installed capacity in 2017. Ecologically sensitive regions like Amazon and the Himalayas are also observing an increasing trend of significant hydropower construction (Winemiller et al., 2016), and currently, only 1/3rd of large global rivers as classified as free-flowing (Grill et al., 2019).Streamflow regulation, intended for hydropower generation purposes, causes river flow alteration from the natural flow state by changing monthly, seasonal, and sub-daily flow patterns of RRs (

A negative effect of hydropower on river environment includes rapid changes in flow and habitat conditions. Any sudden flow change could force fish to move towards a refuge area in a short period of time, causing serious disturbances in the life cycle of the fish. A probability-based model was developed to quantify the impact of hydropeaking on habitat suitability for two fish species, brown trout (Salamo trutta) and Grayling (Thymallus thymallus). The model used habitat preference curves, river velocity and depth to develop the suitability maps. The suitability maps reveal that habitat suitability deteriorates as flow increases in the studied part of the river. The probability model showed that, on average, suitability indices are higher for adult grayling than juvenile trout in hydropeaking events in this part of the river. The method developed shows the potential to be used in river management and the evaluation of hydropeaking impacts in river systems affected by hydropower.

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