Some polders also have a winter peil as a precaution against the increased fall of water in that season.
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 summer water-level of the pasture polders south of the former Y is about 4 to 8 ft.
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.