How the Dutch turned a dangerous saltwater bay into a massive freshwater reservoir

Countries & Cities · Published 29 September 2026 Swipe more like this →

In short: The IJsselmeer is not a natural lake but a feat of hydraulic engineering. Created by sealing off the Zuiderzee with a 19-mile dam in 1932, this shallow body of water transformed a volatile sea inlet into a vital freshwater source, protecting the Netherlands from floods while creating entirely new land.

For centuries, the Zuiderzee was a perilous bay of the North Sea that threatened the Dutch coast with storm surges and rising tides. Following a catastrophic flood in 1916, the government initiated the Zuiderzee Works, a monumental engineering project designed to tame the waters. The centerpiece was the Afsluitdijk, a 19-mile-long dam completed in 1932 that physically separated the bay from the North Sea and the Waddenzee. By blocking the salt water and allowing the inflow of fresh water from the IJssel River—a branch of the Rhine—the basin was gradually flushed, turning the brackish inlet into a stable freshwater lake.

The transformation had profound economic and environmental consequences. The new freshwater environment significantly reduced the rusting of steel boat hulls and minimized the growth of marine organisms like barnacles, reducing the need for toxic antifouling coatings. Furthermore, the project enabled the reclamation of vast tracts of land. By constructing encircling dikes and pumping out the water, the Netherlands added 626 square miles of fertile polders to its national territory. These reclaimed areas eventually formed the province of Flevoland in 1986, showcasing the country's long-standing mastery over its aquatic landscape.

Today, the IJsselmeer serves as a critical reservoir for drinking water and agricultural irrigation, particularly during summer droughts. Its water levels are meticulously regulated by sluices in the Afsluitdijk, which discharge excess water into the Wadden Sea. However, the system faces ongoing challenges, including rising sea levels and the need for advanced pumping infrastructure to manage flood risks. While the lake has been further subdivided—most notably by the Houtribdijk, which created the hydrologically separate Markermeer—it remains a cornerstone of Dutch water management, supporting everything from freshwater fisheries to renewable energy through offshore wind farms.

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