The Zuiderzee was a shallow bay of the North Sea in the northwest of the Netherlands, extending about 100 km inland and at most 50 km wide, with an overall depth of about 4 to 5 metres (13–16 feet) and a coastline of about 300 km. It covered 5,000 km2 (1,900 sq mi). Its name means "southern sea" in Dutch, indicating that the name originates in Friesland, to the north of the Zuiderzee. In the 20th century the majority of the Zuiderzee was closed off from the North Sea by the construction of the Afsluitdijk, leaving the mouth of the inlet to become part of the Wadden Sea. The salt water inlet changed into a fresh water lake now called the IJsselmeer after the river that drains into it, and by means of drainage and polders, an area of some 1,500 km2 (580 sq mi) was reclaimed as land. This land eventually became the province of Flevoland, with a population of nearly 400,000 (2011).