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Hydropower generation causes impacts on aquatic ecology, natural scenery, and ecosystems. The possible key ecological impacts of hydropower are described below (based on ICPDR, 2013).

Hydropower dams and weirs cause an interruption of the longitudinal river continuity. Migrating fish species such as the eel and salmon are particularly affected by the fragmentation of their habitats. In addition, when fish pass through hydropower turbines as they move river downstream, a high proportion of them are injured or killed. The impact of acting as migration barriers is common to most types of hydropower plants.

Furthermore, hydropower plants change river hydromorphology. Hydrological processes and sediment transport lose their natural dynamics leading to altered natural structures and habitats.

Hydropower plants change the river flow regime. In rivers which are impounded for hydropower (typical for storage hydropower plants), flow velocity is reduced which can lead to the loss of orientation of fish. Reduced flow velocity results in other negative impacts such as increased deposition of fine sediment in the impoundment.

Another impact from hydropower results from rapidly changing flows called hydropeaking, which is mainly typical for large hydropower plants in combination with reservoirs. Hydropeaking can cause severe morphological and ecological effects on a river and particularly on fish populations.

Often, at run-of-river hydropower plants, a portion of the river water is diverted e.g. through a canal, to produce energy. This leads to large flow reductions immediately downstream of the river diversion as well as changes in flow patterns further downstream.

Water storage and river regulation through hydropower plants often also alter physical and/or chemical conditions downstream, with changes to water temperature, super saturation of oxygen and altered patterns of ice formation in winter.

Hydropower plants and dams are often not standing alone in a river system, but several can be present on the main river as well as on tributaries. The cumulative effects of multiple hydropower plants, in combination with barriers that do not serve electricity generation, need to be considered (Kampa & Berg, 2020). In a chain of impoundments containing several hydropower plants, the sum total of effects can endanger whole fish populations in a river basin.

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