Sea, with a wall in front of it
How Does a Lagoon Form?
A lagoon is a piece of sea that got fenced off, and the still water behind the fence becomes something entirely different.
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Step 1 of 5
Currents carry sand along a coast.
Waves that arrive at an angle push sand sideways along the shore, a little with every wave. Over years that adds up to a river of sand moving down the coast.
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Step 2 of 5
Where the coast turns, the sand keeps going.
At a bay the shoreline bends but the sand does not. It carries straight on, building a bar out across the opening until the bay is nearly closed off.
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Step 3 of 5
Behind the bar, everything changes.
A hundred metres apart, and two completely different places to live. Cut off from waves, the water goes still, shallow and warm, and the salt level swings with rain and evaporation. It is nothing like the sea a hundred metres away.
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Step 4 of 5
Still shallow water is enormously productive.
Sunlight reaches the bottom, sediment settles, and plants root in it. Lagoons are nurseries — a large share of sea fish spend their first months in one.
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Step 5 of 5
Coral builds the same thing out of animals.
Around a tropical island, coral grows fastest where the waves are strongest, building a reef offshore. What it encloses is a lagoon made by living things.
Lagoons are fragile. One storm can breach the bar, and the still warm water behind it becomes ordinary sea again within days.
The short version
A lagoon is water fenced off from the sea by a bar or reef, which makes it warm, still and unusually full of life.
Try it yourself
Look at a puddle behind a ridge of sand at the beach. That is a lagoon, in miniature and for one tide.