Silenced Rapids and Waterfalls: Habitat Loss and Management of Bypassed Reaches in the Regulated Rivers of Sweden

Habitat destruction is a global driver of biodiversity loss. In rivers, damming and river regulation for hydropower have caused extensive loss of high gradient, riffle, rapid and waterfall habitats. Restoring these habitats, which support unique biodiversity, should be an urgent priority, but inadequate documentation hampers evaluation of different management strategies. Focusing on Sweden, where river regulation affects most catchments, we mapped and quantified characteristics (e.g., slope, length, discharge) of 968 bypassed reaches (BRs), that is, river sections that are dewatered due to diversion of discharge for hydropower production. The extent of habitat loss associated with BRs is substantial, summing to a total length of 1,256 km, 94% of which is predominantly comprised of former riffles, rapids and waterfalls. BRs are typically located in larger rivers at central river network positions, highlighting their potential importance for hydrological and ecological connectivity. These habitat losses are poorly addressed by current management: three quarters of Swedish BRs have no mandated minimum discharge. Of the remainder, 88% have a discharge <2 m³/s, a flow threshold below which, on average, the proportion of rheophilic fish decline rapidly in Swedish BRs. Based on these findings for Sweden, and given the ubiquity of hydropower elsewhere, we suggest that the thousands of kilometers of mostly dry high-gradient habitat linked to diversion hydropower worldwide likely represent high value restoration targets. Increased, ecologically meaningful, flow releases in these reaches has great potential to rehabilitate essential habitat for threatened rheophilic organisms and ecosystem functioning of regulated rivers.