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the part of a river channel where the river drops sharply and the current moves swiftly. Rapids usually form where the river surface is broken by rocky ridges or outcrops, rocks not easily erodible, boulder conglomerates, or rockfall. They sometimes form where there once was a waterfall. Hydroelectric power plants are frequently built on rivers that have rapids and a significant steepening in the stream gradient. Two such plants are the Dnieper Hydroelectric Power Plant, built at the rapids of the Dnieper, and Volkhov Hydroelectric Power Plant, built at the rapids of the Volkhov. Rapids usually make navigation and timber flotation very difficult and require the construction of bypass canals.