The IJsselmeer was created by closing off the Zuiderzee by means of the Afsluitdijk.
The Afsluitdijk was completed on May 28, 1932. At the last point where the dike is closed, there is a monument on the dike itself. Since then, the name of the closed part of the Zuiderzee has been changed to the IJsselmeer. Named after the river IJssel that flows into it. The part that was not closed off was given the name Waddenzee. As a result, the more than 300 km of coastline of the Zuiderzee was shortened to just 32 km, the length of the Afsluitdijk. And the IJsselmeer changed from salt to freshwater. Contained by a fine example of Dutch ingenuity of the Dutch Delta Design.