Weirs as a Mitigation Measure in Regulated Rivers - The Norwegian Experience
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Hydropower has been extensively developed in Norway and mitigation measures afe one of the major elements in the planning and licensing procedure. Measures include flow management, weirs, substrate improvement, fish ladders and fish stocking. Many of these measures have been developed and evaluated in two major R&D programmes, "The Weir Project" and "The Biotope Adjustment Programme" spanning the,period 1973-97. Weirs have been widely used in Norwegian rivers. They lessen the effects of river regulation, can improve fish recruitment, stabilise groundwater levels and concentrate flows. They can also function as sediment traps in rivers with high sediment loading. Many weirs have been in place for over a decade, providing valuable experience regarding ecological function as well as maintenance. Weirs may be constructed of concrete, wood or moraine material (rocks and boulders). Their forms may vary, both in relation to the material, purpose and environmental context. Stream gradient must also be taken into account. Weirs are usually positive with regard to landscape and aesthetic considerations, fish survival during winter and increased biodiversity. Negative effects may include increased sedimentation, fish community changes, migration barriers and excess macrophyte growth. However, these detrimental effects can be reduced by active planning and management.