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This paper launches concepts instrumental to environmental impact assessment (EIA) studies on hydropower schemes and lake regulations. Norwegian hydro-electric lakes (hydrolakes) and their environmental features are described, and evaluated against non-manipulated waters. A tentative classification of hydrolakes vs. natural waters is proposed. The need for a multiple approach to habitat classification is emphasized. Recommendations for future biological impact assessment approaches are suggested.Hydrolakes differ broadly from natural lakes by combining physical features not ordinarily co-occurring in non-manipulated water bodies. Storage type hydrolakes (reservoirs) feature winter draw-downs and enhancement of yearly level fluctuations; whereas other types of hydro-electric lakes have elevated water levels throughout winter. Hydrochemistry and optics of the studied hydrolakes exhibited no clear differences to non-impacted Norwegian inland waters. All lakes had signs of sublacustrine erosional activity related to internal waves and thermocline movements.

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