Water-covered area and meso-habitats of the river Vassbygdelvi

Abstract

The purpose of this study was to assess how the water-covered area and meso-habitats change with discharge in the river Vassbygdelvi in Aurland, Sogn and Fjordane county, with focus on low discharges. The river stretch from Vassbygdvatn to Jørve was divided into two reaches. In the lower reach water lines were measured and meso-habitats were mapped at three different discharges. In the upper reach water lines were measured at one discharge. In addition to the measurements by the use of differential GPS, aerial images were analysed to calculate the water-covered area at discharges given by the images. The measurements show that the water-covered area increases rapidly up to a discharge of 0.5 m3/s. At higher discharges the area rises less strongly at the same increase in discharge. Especially in the lower reach the increase in area is relatively small when the discharge rises from 0.7 m3/s to around 7 m3/s. This is an effect of the weir and pool constructions in the lower reach. It is recommended to increase the minimum flow in Vassbygdelvi from 0.3 m3/s to between 0.5 m3/s and 0.6 m3/s. This discharge range leads to app. 10 % larger water-covered area and maintains a more homogeneous distribution of habitats compared to a discharge of 0.3 m3/s. Furthermore, a minimum flow of 0.5 m3/s during winter time would avoid longer continuous periods of very low discharge as they occur under the current flow regime and might contribute to greater winter survival of juvenile fish.

Water-covered area and meso-habitats of the river Vassbygdelvi