A hydroelectric scheme relies on the immense water pressure created by having a colossal weight of water trapped behind the dam. The water pressure spins the blades of huge turbines that are built into tunnels passing through the dam, and these turbines turn the generators that produce electricity. The water flow can be controlled and even stopped completely by sluice gates in the tunnels. This allows the output of the power plant to be matched to demand and water supply. Stopping the flow also enables engineers to maintain the turbines and other machinery.
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