Zuider ZeeThe Zuider Zee (Dutch: Zuiderzee) was a former shallow inlet 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 meters and a coastline of about 300 km. Its name means "southern sea" in Dutch, indicating that the origin of the name can be found in Friesland to the north of the Zuiderzee (also see North Sea). In the 20th century the majority of the Zuiderzee was closed off from the North Sea (leaving the mouth of the inlet to become part of the Wadden Sea) and the salt water inlet changed into a fresh water lake called the IJsselmeer (IJssel-lake) after the river that drains into it. The river IJssel is an estuary branch of the river Rhine.
In classical times there was already a body of water in this location, called the Flevo Lake by Roman authors. It was, however, much smaller in size than its later forms and its connection to the main sea was much narrower. During the early Middle Ages this began to change as increasing sea levels and storms started to eat away at the coastal areas which consisted mainly of peatlands. In this period the inlet was referred to as the Almere indicating it was still more of a lake, but when the mouth and size of the inlet were significantly widened in the 12th and 13th century by several major storms, the name Zuiderzee came into general usage. The size of this inland sea remained largly stable from the 15th century onwards due to improvements in dikes, but when a storm would push North Sea water into the inlet, the Zuiderzee would become a volatile cauldron of water, frequently resulting in flooding and the loss of ships.
Around the Zuiderzee many fishing villages grew up and several developed into walled towns with extensive trade connections, at first with the Baltic Sea, England and the Hanseatic League, but later also with the rest of the world when the Netherlands established its colonial connections, in particular towns in Holland such as Amsterdam, Hoorn and Enkhuizen. When that lucrative trade diminished, most of the towns were left with fishery and some industry until the 20th century when tourism became the major source of income. Contained within the Zuiderzee were four small islands, the remains of what were once larger islands or peninsulas connected to the mainland. These were Wieringen, Urk, Schokland and Marken. The inhabitants of these islands also subsisted mainly on fishery and related industries and still do in the case of Urk and Wieringen. All islands are now part of the mainland or connected to it.
The 20th century would see the taming of the Zuiderzee as a large closing dam (the Afsluitdijk) was constructed. Completed in 1932, the Zuiderzee was now the IJsselmeer and large areas of water could be reclaimed for farming and housing. This enormous project, called the Zuiderzeeworks, ran from 1919 to 1986, culminating in the creation of the new province of Flevoland.