Will a changed water flow regime in River Kvina affect salinity and risk of remobilization of toxic aluminum in the Feda fjord?

Summary

Sira-Kvina Kraftselskap is planning to transfer water from two sub-catchments (51.5 km2) in the uppermost part of River Kvina, to the neighboring River Sira. By increasing and distributing residual water flow better over the year, while at the same time carry out habitat measures, the intention is to maintain good conditions for Atlantic salmon despite the reduction in annual flow. Changes in the water flow regime in River Kvina can, however, affect the propagation and mixing of freshwater in the Feda fjord. It has been shown previously that aluminum from the river can be toxic to fish in the mixing zone between fresh water and brackish water, in particular when the salinity is in the range of 1-7 PSU. Continuous monitoring of temperature and salinity in the period 15 April - 17 September, as well as measurements of vertical salinity profiles 4 times spread over the same period, showed that a variation in water flow in Kvina of 20-60 m3/s gives a variation in the prevalence of the potentially toxic brackish layer (<7 PSU) from 1.5 to 6.5 km outward the fjord, if the wind blows in an inwards direction. Based on this, one would assume that a river flow up to 60 m3/s will be more critical for migrating smolt than flows down to 20 m3/s. Similarly, an increased river flow from 5 m³/s to 20 m³/s, will increase the prevalence of the brackish zone considerably, from just a few hundred meters outside the river mouth to 1.5 km outwards. The results show that a slightly reduced flow in River Kvina, given constant aluminum concentrations, will reduce the prevalence of the potentially toxic brackish layer in the Feda fjord. Hence, when dimensioning “artificial” floods to facilitate smolt migration, one should avoid flow levels that will significantly expand the prevalence of the potentially toxic brackish layer in the Feda fjord.