Hydropower production has a range of effects in regulated river systems. Including transforming water reaches from lentic environments to lotic (a; Ätraforsdammen, Sweden), bypassed river reaches with reduced or no flow (b; Lilla Piteälven, Sweden), water level fluctuations in reservoirs (c; Umeälven, Sweden), hydropeaking with resulting erosion of the river banks (d; Oreälven, Sweden), increased winter temperatures and changed ice dynamics downstream bottom spills (e; Klarälven, Sweden), supersaturated water in tailraces or downstream spillgates (f; Stølselva, Norway),barriers to upstream fish movements (g; Parana river, Brazil), barriers to downstream fish movement (h; Klarälven, Sweden). Photos: Duncan Philpott (a), Olle Calles (b,g), Johan Östergren (c), Emil Nordström (d), NORCE LFI (f), Daniel Nyqvist (e,h).