In time of drought the water in the canals and boezems is allowed to run back into the polders, and so serve a double purpose as water-reservoirs.
As late as the 12th and 13th centuries each of these islands consisted of several smaller islands, many of whose names are still preserved in the fertile polders which have taken their place.
The area south of this would be divided into four polders, with reservation, however, of a lake, Yselmeer, in the centre, whence branches would run to Ysel and the Zwolsche Diep, to Amsterdam, and, by sluices near Wieringen, to the northern part of the sea.
The four polders with their areas of fertile soil would be: (I) North-west polder, area 53,599 acres; fertile soil, 46,189 acres.
Some polders also have a winter peil as a precaution against the increased fall of water in that season.
Nearly all the polders of Zeeland and South Holland are able to discharge naturally into the sea at average low water, self-regulating sluices being used.
In some deep polders and drained lands where the water cannot be brought to the required height at once, windmills are found at two or even three different levels.
The whole of the lakes to the north of the former Y, including the famous Purmer and Beemster lakes, and the Wieringerwaard and Zype sea-polders, were drained in the beginning of the 17th century; but the Waard-en-Groet, the Anna Paulowna and the Koegras sea-polders to the north of these, were only added to the mainland in the first half of the 19th century.
The final removal of polder water, however, is only truly effected upon its discharge into the " outer waters " of the country, that is, the sea itself or the large rivers freely communicating with it; and this happens with but a small proportion of Dutch polders, such as those of Zeeland, the Holland Ysel and the Noorderkwartier.
Boezems, like polders, have a standard water-level which may not be exceeded, and as in the polder this level may vary in the different parts of an extended boezem.
Polders shown in the table.