IJsselmeer, the largest lake in the Netherlands. It serves as a freshwater reservoir and is part of a project, begun in 1920, to change much of the Zuider Zee, an arm of the North Sea, into productive new tracts of land called polders. The polders are used for farming and as sites for industry and planned communities. The southern part of the Zuider Zee became the IJsselmeer and the polders; the northern portion, edged by the West Frisian Islands, is now called the Waddenzee.

The IJsselmeer was created in 1932 by the completion of a 19-mile (31-km) dam across the Zuider Zee, blocking out the North Sea's salty water. The IJssel (a branch of the Rhine) and other rivers flowing into the IJsselmeer have made it a freshwater lake. The dam has navigational locks, sluices to control the lake's level, and a highway on top. The IJsselmeer is shallow and covers an area of about 470 square miles (1,220 km2). The area has decreased over the decades as polders have been added.

The first polder, Wieringermeer, was completed in 1930. It was followed by Noordoost Polder (1945), Oostelijk Flevoland (1957), and Zuidelijk Flevoland (1968). Markerwaard, the largest polder, was begun in 1963. There is commercial fishing on the IJsselmeer. Amsterdam, on the IJ, an arm of the IJsselmeer, is the chief city on the lake.