Application of multiscale environmental flow methodologies as tools for optimized management of a Norwegian regulated national salmon watercourse

The river Surna has been regulated for hydropower production since the late 1960s, with a minimum flow requirement of ca. 30% of the mean annual flow downstream of the power plant outlet, leaving a large by‐pass section with residual flow. This river has recently been selected as a national salmon watercourse by the Norwegian Parliament. Mitigations to protect the Atlantic salmon stock will be given priority in these rivers, and hence an environmental flow assessment (EFA) has been started to optimize hydropower production in relation to salmon habitat. A suite of methodologies for EFA like habitat‐hydraulic models, indicators of hydrological alteration (IHA), mesohabitat analysis, temperature simulations and optimalization of habitat improvements, has been applied across scales and seasons in this catchment. The EFA has been related to various hydropower schemes, to suggest more salmon‐friendly hydropower operation. A temperature‐adjusted running of the hydropower plant was found to have a major influence on the production of salmonids. Downstream of the hydropower station, rapid ramping has often occurred. This is possibly harmful for juvenile fish inhabiting the shallow parts of the river due to stranding. Based on our habitat modelling toolkit, we have suggested ramping guidelines to drastically reduce stranding of juvenile salmonids. Our EFA has indicated a great potential with mitigations to increase salmon production in the river Surna. For maximum gain, combinations of more gentle flow ramping, habitat improvements and/or environmental flow requirement (EFR) in the by‐pass section and temperature alteration by new intake arrangements and/or temperature‐adjusted run of the power plant are needed. Copyright © 2007 John Wiley & Sons, Ltd.