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Abstract

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. Plain Language Summary Human activities are degrading rivers around the world, and one major cause is hydropower dams that divert water away from natural channels. In this study, we investigated losses of river habitat associated with water diversion in Sweden, a country where most rivers are regulated for hydropower. We focused on nearly 1,000 "bypassed reaches," which are stretches of river left largely dry because water is rerouted through hydropower plants. Together, these reaches add up to more than 1,250 km of lost river ecosystem, most of which were once fast-flowing habitats like rapids and waterfalls that support specialized plants and animals. We found that these lost habitats often sit in important positions within river networks, meaning their loss can disrupt how water, species, and nutrients move through entire catchments. Current management struggles to address this problem effectively: most bypassed reaches receive no guaranteed water flow, and where water is released, it is usually much too little to support fish that depend on strong currents. Restoring water to these dry river reaches could be a straight-forward method to improve ecological health and biodiversity in thousands of kilometers of river habitat, not only in Sweden but wherever hydropower diverts water from valuable river habitats.

Keywords

habitat degradation; minimum discharge; hydropower; rheophilic fish; water diversion; environmental flows

Published in

Water Resources Research
2026, volume: 62, number: 2, article number: e2025WR040752
Publisher: AMER GEOPHYSICAL UNION

SLU Authors

UKÄ Subject classification

Environmental Sciences
Oceanography, Hydrology, Water Resources

Publication identifier

  • DOI: https://doi.org/10.1029/2025WR040752

Permanent link to this page (URI)

https://res.slu.se/id/publ/146395