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A 'needle dam' is a weir designed to maintain the level or flow of a river through the use of thin "needles" of wood. The needles are leaned against a solid frame and are not intended to be water-tight. Individual needles can be added or removed by hand to constrict the flow of the river, forming a sluice.
One early needle dam maintains the level of Lake Lucerne in Lucerne, Switzerland by restricting the flow of the Reuss River, several are still in operation on the Meuse River, France and others were built in the United States in the 19th century.