Polder

A polder is a low-lying tract of land enclosed by dikes that forms an artificial hydrological unit, kept dry only through continuous management rather than natural drainage. Three kinds are recognized: land reclaimed from a lake bed or the seabed, a floodplain cut off from a river or the sea by an embankment, and a former marsh, called a koog in Germany, drained after being similarly enclosed. Because the enclosed ground continues to settle and, where it contains peat, to shrink as the material dries and decomposes, a polder's surface typically ends up lower than the surrounding water level, so water that seeps in through the dikes, the ground or rainfall has to be pumped out or released through sluices at low tide rather than draining away on its own. The Netherlands is the country most associated with the technique, with roughly a fifth of its land area reclaimed this way, but polders are also built in Bangladesh, Belgium, Canada, China and other countries with suitable low-lying coasts, deltas or former wetlands.