The acqua alta, or high tide, floods the Piazza San Marco, Venice, Italy. The aqua alta is usually most pronounced between autumn and spring. Like all tides, the aqua alta is governed by the gravitational pull of the moon. In addition, local wind patterns (called the sirocco and the bora) augment the tide.

Photograph by Jodi Cobb

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A flood happens when water overflows or soaks land that is normally dry. There are few places on Earth where people don’t need to be concerned about flooding.

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A lagoon is a shallow body of water that may have an opening to a larger body of water, but is also protected from it by a sandbar or coral reef

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Encyclopedic entry. The regular rise and fall of the ocean’s waters are known as tides. Along coasts, the water slowly rises up over the shore and then slowly falls back again.

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Photograph by Jodi Cobb

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