A7 highway crosses the Afsluitdijk

The A7 highway runs straight for 32 kilometers, with the Wadden Sea on one side and the IJsselmeer, a freshwater lake, on the other. It travels along the top of the Afsluitdijk, a massive dike that connects North Holland to Friesland.

Afsluitdijk’s history and construction

The Afsluitdijk was completed in 1932 after construction began in 1927. It was built to protect the Netherlands from flooding and reduce the vulnerable coastline from over 300 km to 32 km. The dike was largely constructed by hand, with thousands of workers placing stones along a route designed by Cornelis Lely.

Zuiderzee becomes IJsselmeer

The closure of the last tidal channel, the Vlieter, in 1932 cut off the Zuiderzee from the North Sea. This transformed the saltwater sea into the IJsselmeer, a freshwater lake named after the river that flows into it. The transition from salt to freshwater happened in just a few years.

Impact on fish migration

The dike disrupted migration routes for hundreds of fish species, including the now-extinct Zuiderzee herring. Some fish species rely on moving between the salty Wadden Sea and the freshwater IJsselmeer to spawn and raise their young. The closure of the dike broke this cycle, leading to significant declines in fish populations.

Modern upgrades and fish passage

After nearly 90 years of service, the Afsluitdijk was reinforced to meet modern standards and address climate change. The dike was raised by 2 meters on the Wadden Sea side, with heavy concrete blocks installed to protect against waves. New sluices and a solar-powered pumping station were also added to improve water drainage and fish passage.

Fish-friendly improvements

A 4 km migration river was built on the Friesland side to allow fish to move freely between saltwater and freshwater. At Den Oever, a pass helps fish navigate to an acclimatization basin before entering the IJsselmeer. Species like eels, salmon, and shads are already using these new fish-friendly structures.