polder

thumb|Aerial view of Flevopolder, the [[Netherlands]] alt=|thumb|Satellite imagery|Satellite image of [[Noordoostpolder, the Netherlands (595.41 km2)]] A polder () 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: Land reclaimed from a body of water, such as a lake or the seabed Flood plains separated from the sea or river by a dike Marshes separated from the surrounding water by a dike and subsequently drained; these are also known as koogs, especially in Germany

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.