Residual flow in regulated rivers are important, but often viewed as a bottleneck. Increasing the residual flow in regulated rivers will lead to a loss in hydropower production, and in high-head hydropower schemes this is costly. Finding the right balance between hydropower production and environmental interests are thus vital. Surprisingly often they are also based on a kind of educated guess. Effects of habitat quality are usually ignored or underestimated, and wetted area becomes a proxy for the productivity of a river. 

 Our study is based on a field experiment and a theoretical approach based on hydraulic modelling. Two stretches in a regulated river (Aurland) where the design was to optimize wetted area, were modified to improve habitat quality for Atlantic salmon (Salmo salar) and anadromous brown trout (Salmo trutta). Groundsills, small-head weirs and bank protections were removed. Sediment was cleaned and loosened. Large woody debris was added. The residual flow discharge was not changed. The modification resulted in a significant decline of wetted area (- 25 %) but in an increase of spawning habitats (+ 400 %) and more shelter at juvenile habitats (+ 320 %). Natural spawning increased and the total number of juveniles estimated by electrofishing increased significantly after the measures (380 %).

Our analysis indicates that restoration and maintenance of high quality fish habitats may compensate the loss of wetted area until a certain degree. This is important when trying to balance hydropower production and environmental mitigation.