According to the CD-ROM version of the Encyclopedia Brittanica a polder
is tract of lowland reclaimed from a body of water, often the sea, by the construction of dikes roughly parallel to the shoreline, followed by drainage of the area between the dikes and the natural coastline. Where the land surface is above low-tide level, the water may be drained off through tide gates, which discharge water into the sea at low tide and automatically close to prevent re-entry of seawater at high tide. To reclaim lands that are below low-tide level, the water must be pumped over the dikes. If a sediment-laden stream can be diverted into the polder area, the sediment may serve to build up the polder bottom to a higher level, thus facilitating drainage.
Soil in areas newly reclaimed from the sea contains so much salt that most plants will not grow. Procedures for ridding the soil of salt, therefore, must be used along with diking and draining to develop agriculturally productive land.