The Zuiderzeewerken (Zuiderzee Works) are a system of dams, land reclamation and water drainage works in the Netherlands during the twentieth century. Plans for the works date back to the 1600s, but it was not until a very bad flood in 1916, that the Dutch parliament finally agreed to pay for the works. Now it is one of the Seven Wonders of the Modern World.
First some dams or dykes were built to separate the South Sea (Dutch: Zuiderzee) from the North Sea. The biggest was the Afsluitdijk ('closure dyke'). It is 32 kilometres (19.9 mi) long. When it was finished in 1932, the South Sea was completely cut off and from then on would become a lake called the IJsselmeer.
Land reclamation meant building more dams and pumping out the water inside. The new land is called a polder. Polders make new land for agriculture and make flood protection better, because the lake is smaller.
There are gates and sluices at the ends of the Afsluitdijk. The gates, called locks, can be opened to let ships through. The sluices let water out of the IJsselmeer. If there were no sluices the lake would fill up and flood the countryside. This is because it is always being filled with water from rivers and the polders which are still being drained.
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