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The swift, powerful tides that can make oodles of electricity are surprisingly rare. In the United States, only a few places—including the Gulf of Maine, Washington's Puget Sound, Manhattan's East River, and the waters under the Golden Gate Bridge—create a muscular flow near cities with sizable power needs. Tide-harvesting devices take advantage of tidal waters impressive power density: Tides can move at twice the speed of waves and can generate eight times the potential energy. But they also carry a vast supply of sediment, larvae, and plankton. As new generator designs reach the ocean floor, they must avoid disturbing marine life.