Waddenzee, shallow inlet of the North Sea in the Netherlands. It was formerly part of the Zuider Zee, and covers an area of about 10,000 sq km (3,900 sq mi). It lies between the Dutch mainland and the West Frisian Islands, which form a barrier that partly shields it from the North Sea. The completion of the IJsselmeer Dam (the Afsluitdijk) in 1932 cut the Waddenzee off from the rest of the Zuider Zee, which became the IJsselmeer, an inland freshwater lake. The Waddenzee is so shallow that parts of its sandy bottom are uncovered at low tide. In between these sand flats are deep channels running in gaps between the islands. The Waddenzee serves as a breeding ground for fish and shellfish. There are ports at each end: Harlingen to the east and the naval base of Den Helder to the west.