shallow inlet of the North Sea between the West Frisian Islands and the northern Netherlands mainland. The inlet extends from Noord-Holland to the northeast, where the islands gradually curve toward the mainland and the channel narrows to a few miles. Until the completion of the IJsselmeer dam (Afsluitdijk), Waddenzee formed the northern part of the former Zuiderzee. A saltwater tidal delta, the Waddenzee consists of sand flats, mostly uncovered at low tide, intersected by deep channels. It connects with the North Sea through inlets between the West Frisian Islands, with depths to 150 feet (50 m). There is some fishing, and it is a refuge for waterfowl. The chief ports are Den Helder and Harlingen.