There is a clear need for identifying simple relations between changes in flow and ecosystem response for the improved management of rivers and the implementation of the EU WFD. Despite this, it appears very difficult to identify widely applicable relations between hydrological/hydraulic parameters and ecosystem response, and one specific water flow target/methodology could hardly be found nor recommended. The review of management practices of setting environmental/minimum flows in selected European countries, rather revealed that a number of approaches are used, most of them, however, ending up in minimum flow/environmental flows in the range of 5-10% of mean annual flow. The authors would propose to use the building block methodology (BBM) as a conceptual framework for setting flow targets in regulated rivers. This would support the overall idea of the EU WFD of introducing ecosystem-based management with stakeholder involvement. This is in line with recommendations given to authorities in e.g. the UK where trials are currently undertaken. The authors believe the use of hydrological/hydraulic analysis still have an unleashed potential in the environmental management of regulated rivers in general and related to the EU WFD in particular, possibly supporting rapid assessment ecological status in rivers in the future.
Hydropeaking consists of variations in discharge and water level due to releases of water retained in storage basin to generate electricity according to the market demand. These unnatural flow fluctuations create frequent and rapid variations in terms of flow magnitude, flow velocity, water depth, water temperature, wetted area and sediment transport which also can affect channel morphology. Such changes may lead to degradation of physical conditions and habitats in local ecosystems which directly affect biological communities in rivers. Mitigation measures can enhance the ecological state of rivers and lakes altered by hydropeaking. They are classified into 3 different types: Operational measures place constraints on the hydropower plant regime itself, fixing threshold values for amount of water released; construction measures involves construction of hydraulic structures like retention basins; and in-stream measures are renovation or maintenance works carried out inside the river. Mitigation measures are site-specific and thus local investigations must be carried out to ensure successful implementation of measures. In addition, long-term monitoring and systematic evaluation should be conducted during and after the completion of rehabilitation projects to assess the benefits of measures on local ecosystems. The literature review gathers examples of abatement measures implemented in several countries to mitigate negative impacts of hydropeaking. Examples are classified in a table and sorted by the aim of the measures.