Sammendrag
Based on reported concerns from several private individuals, as well as complaints to The Norwegian Water Resources and Energy Directorate (NVE) regarding severe variations in flow, drying of river flows downstream of the hydropower plants and observations of stranded fish, NIVA has investigated the ecological effects of three small hydropower plants in the Nitelva river.
Fish, freshwater pearl mussels, freshwater crayfish and aquatic vegetation were investigated during the summer/autumn of 2019 in the Nitelva river. In addition, the impact of the power plants on the current water flow and hydromorphological status was investigated, as well as the importance of water diverted to the Lake Maridalsvannet. This was done to give an overall assessment of the effects of the hydropower plants, as well as the diverted water, on the ecological processes in the river.
The river stretch from Sagfossen to Rotnes has been prone to frequent and rapid changes in water flow for several years. This has probably have had a negative effect on hydromophology and the habitat suitability of freshwater crayfish and freshwater pearl mussels downstream of Sagfossen. Effects on trout densities are not clear, but it is likely that the effect of the start stop regime has negatively affected trout populations in parts of the river. The build-up of water masses in Sagdammen and Verksdammen acts as sedimentation traps and prevents a natural supply of sediment to the meandering stretch downstream. This also forms the basis for dense populations of Canadian pondweed.
A change in the maneuvering regime, with a halt in the current start stop regime at the lower of the two power plants furthest upstream (as requested by NVE), is expected to have a positive effect on the habitat and the basis for viable populations of freshwater pearl mussels and freshwater crayfish on the river stretch Sagfossen-Rotnes. A return of the water presently diverted to the Maridalsvannet could potentially increase water flow in the Nitelva and could reduce both the length of periods of partial drying just downstream of the dams and generally increase the water level in the river. In addition to a halt in the start stop regime, future monitoring and management of the Nitelva River should include determination of chemical status, continued monitoring of water quality, investigations of the presence of crayfish plague and biotope-adjusting measures, to ensure improved habitats for viable populations of vulnerable and highly endangered species such as freshwater pearl mussels and freshwater crayfish.
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