The IJsselmeer lake is the large lake on the East side of Amsterdam. It has existed since 1932, when the dyke called the 'Afsluitdijk' was completed.
Before that time, this was not a lake, but open sea water, called Zuiderzee. The Afsluitdijk was created because of the danger presented by the sea, which sometimes caused floodings.

When the Zuiderzee became the IJsselmeer lake, the water changed from salt to fresh water. This meant the end of the fishery industry in towns like Marken, Volendam, Edam and other places. These historic towns have been beuatifully preserved, and are important tourist attractions nowadays.