Ijsselmeer, or Ĳsselmeer ( [ɛi̯səlˈmeːr]; Lake IJssel, alternative spelling: Lake Yssel; Frisian: Iselmar) is a shallow artificial lake of 1100 km² in the central Netherlands bordering the provinces of Flevoland, North Holland and Friesland, with an average depth of 5 to 6 m. The Ĳsselmeer is the largest lake in Western Europe.
The Ĳsselmeer is a freshwater lake fed through the Ketelmeer by the river IJssel, which gives it its name. It is mainly fed by water from the Rhine since the IJssel is a branch of it.
The first two letters of the name are capitalized because IJ is a digraph in Dutch, possibly a ligature, so it is treated as a single letter for most purposes (see IJ (digraph)).
The Ĳsselmeer was created in 1932 when a large shallow inland bay, the Zuiderzee, was closed by a 32 km dam, the Afsluitdijk. This was part of a major hydraulic engineering project known as the Zuiderzee Works, that years later led to the reclaiming of land from the Ĳsselmeer, thereby diminishing the size of the lake.
In 1975 the Ĳsselmeer was further split in two by the completion of the Houtribdijk, now also called Markerwaarddijk because it was originally designed to border the Markerwaard; this dike runs from Enkhuizen southeast to Lelystad. This former southern part of the Ĳsselmeer is now the hydrologically separate Markermeer.
The Ĳsselmeer functions as a major fresh water reserve, serving as a source for agriculture and drinking water. It also offers a number of opportunities for recreational activities.
The province of Flevoland was established in 1986 and consists of three polders reclaimed from the Ĳsselmeer.
The narrow lake east of IJsselmeer is Lake Ketelmeer. The lake receives water from rivers that carry industrial pollutants from factories upstream. Those polluted sediments have settled to the bottom in a thick layer of contaminated sludge. To restore a normal aquatic environment, this material needs to be removed from the lakebed.
In 2010 an artificial island called the IJsseloog was installed in Lake Ketelmeer. It is a repository for contaminated material dredged from the bottom of the lake. Once the repository is full, it will be capped and turned into a nature reserve.