ZUIDER ZEE or ZUYDER ZEE AT WIKI AND ENCYCLOPEDIA
A land-locked inlet on the coast of Holland bounded N. by the chain of the Frisian Islands, and W., S., and E. by the provinces of North Holland, Utrecht, Gelderland, Overysel, and Friesland. It is about 85 m. long N. to S., and from lo to 45 M. broad, with an area of 2027 sq. m., and contains the islands of Marken, Schokland, Urk, Wieringen, and Griend. In the early centuries of the Christian era the Zuider Zee was a small inland lake situated in the southern part of the present gulf, and called Flevo by Tacitus, Pliny, and other early writers . It was separated from the sea by a belt of marsh and fen uniting Friesland and North Holland, the original coast line being still indicated by the line of the Frisian Islands. Read more at Online Encyclopedia
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. Read more at Wikipedia