Producing around 97% of the electricity by hydropower production and progressively adopting the Water Framework Directive, Norway has a potential challenge to determine environmental flows. A recent study also stated that around 50 regulated watercourses have high priority in the revision of their regulation permit, which in many cases will require implementation or revision of environmental flows. Comparing the hydrological data before and after the impact from a number of regulated salmon rivers distributed geographically around Norway provides knowledge on the effect of various configuration of the regulatory system, effects of mitigation measures and shows the historical evolution of regulated flow regimes. Quantitative changes to flow regimes are identified, and specific changes are linked to special characteristics of the regulation system. Further, types of flow alterations are linked to specific ecological responses in Atlantic salmon based on literature data. The IHA method is used to describe flow changes. Additional parameters will be included for Norwegian Rivers. A challenge is the lack of pre-regulation data for several of the older systems, and a quantile regression model will be used to simulate unregulated runoff. Preliminary results show major changes after regulation in minimum and maximum flow duration, decrease of annual low pulses and increase of high pulses, both with lower duration, magnitude of monthly discharge with higher mean flows in winter.Timing, frequency, and rate of change are also altered. The knowledge from this analysis will be the basis for further studies of environmental flows and the evaluation of the current best practice.