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(ī`səlmār'), shallow freshwater lake, NW Netherlands, bordering on the provinces of North Holland, Flevoland, and Friesland. It was formed from the old Zuider ZeeZuider Zee
, former shallow inlet of the North Sea, c.80 mi (130 km) long, indenting NE Netherlands. In ancient times Lake Flevo, it was joined to the North Sea by a great flood in the 13th cent.
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 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 FrieslandFriesland
or Frisia
, province (1994 pop. 607,000), c.1,325 sq mi (3,430 sq km), N Netherlands. Leeuwarden is the capital. The province includes several of the West Frisian Islands along the North Sea coast and borders on the IJsselmeer in the southwest.
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. The IJsselIJssel
, river, 72 mi (116 km) long, branching from the Neder Rijn (Lower Rhine) River near Arnhem, E Netherlands, and flowing N into the Ketelmeer (an arm of the IJsselmeer), near Kampen. The IJssel is connected to the Rhine and carries about one tenth of the Rhine's water.
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 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 FlevolandFlevoland,
province (1994 pop. 253,700), 931 sq mi (2,412 sq km), central Netherlands, on the E shore of the Markermeer and IJsselmeer. Lelystad is the capital; other cities include Almere and Emmeloord.
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 (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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