IJsselmeer īˈsəlmārˌ [key], shallow freshwater lake, NW Netherlands, bordering on the provinces of North Holland, Flevoland, and Friesland. It was formed from the old Zuider Zee by the construction of a dam (completed 1932). The dam, 19 mi (31 km) long, has navigation locks and drainage sluices (which control the lake's level) and carries a roadway connecting North Holland with Friesland. The IJssel River, from which the lake takes its name, is the chief feeder of the IJsselmeer.

Considerable areas have been reclaimed from the former Zuider Zee since 1930, when the reclamation of Wieringermeer (part of North Holland) was completed. The largest of the reclaimed areas is the Northeast Polder (185 sq mi/479 sq km), which with Eastern and Southern Flevoland (all reclaimed after 1937) now is part of the province of Flevoland (est. 1986). Parts of the polders were flooded in World War II but have since been salvaged.

The shallow southwestern end of the IJsselmeer, now called the Markermeer, was separated from the lake by the construction of a dam and was originally intended to be reclaimed as well. The IJsselmeer and Markermeer are important freshwater fishing grounds, but silting in the Markermeer decreased water quality. Beginning in 2016, several artificial islands with wetlands were constructed in the Markermeer near the dam to improve water quality and increase wildlife habitat.

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