Hjartdøla and Heddøla rivers are home to the red-listed species freshwater pearl mussel and eel, as well as prioritised freshwater organisms such as lake trout and European whitefish. Reduced flow will change the growth and living conditions for fish and freshwater pearl mussels in Hjartdøla. The negative impact is expected to be greatest during the winter due to low flow and freezing of shallow areas. The assessment found that this would have a medium negative impact for fish and benthos in Hjartdøla river. The effects of the measures taken in the Skogsåa watercourse are related to a reduction in the spawning area for fish and benthos, and are assessed as having a low negative impact.
The side streams Grovaråa, Vesleåa, Kvitåa, Uppstigåa and Skorva are of less importance to the fish population, and the degree of impact has been assessed as low negative. In Vesleåa/Kjempa, a slight deterioration in living conditions for trout and brook lamprey is expected, and the degree of impact is therefore assessed as low to medium. The impact in Heddøla is linked to changes in the water temperature downstream of the power plant outlet. The impact of this is assessed as medium negative.