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A polder (Dutch pronunciation: [ˈpɔldər] (Loudspeaker.svglisten)) is a low-lying tract of land that forms an artificial hydrological entity, enclosed by embankments known as dikes. The three types of polder are:

  1. Land reclaimed from a body of water, such as a lake or the seabed
  2. Flood plains separated from the sea or river by a dike
  3. Marshes separated from the surrounding water by a dike and subsequently drained; these are also known as koogs, especially in Germany
Aerial view of Flevopolder, the Netherlands
Satellite image of Noordoostpolder, the Netherlands (595.41 km2)

The ground level in drained marshes subsides over time. All polders will eventually be below the surrounding water level some or all of the time. Water enters the low-lying polder through infiltration and water pressure of groundwater, or rainfall, or transport of water by rivers and canals. This usually means that the polder has an excess of water, which is pumped out or drained by opening sluices at low tide. Care must be taken not to set the internal water level too low. Polder land made up of peat (former marshland) will sink in relation to its previous level, because of peat decomposing when exposed to oxygen from the air.

Polders are at risk from flooding at all times, and care must be taken to protect the surrounding dikes. Dikes are typically built with locally available materials, and each material has its own risks: sand is prone to collapse owing to saturation by water; dry peat is lighter than water and potentially unable to retain water in very dry seasons. Some animals dig tunnels in the barrier, allowing water to infiltrate the structure; the muskrat is known for this activity and hunted in certain European countries because of it. Polders are most commonly, though not exclusively, found in river deltas, former fenlands, and coastal areas.

Flooding of polders has also been used as a military tactic in the past. One example is the flooding of the polders along the Yser River during World War I. Opening the sluices at high tide and closing them at low tide turned the polders into an inaccessible swamp, which allowed the Allied armies to stop the German army.

Netherlands has a large area of polders: as much as 20% of the land area has at some point in the past been reclaimed from the sea, thus contributing to the development of the country. IJsselmeer is the most famous polder project of the Netherlands. Some other countries which have polders are Bangladesh, Belgium, Canada and China. Some examples of Dutch polder projects are Beemster, Schermer, Flevopolder and Noordoostpolder.