IJsselmeer is the name of the large lake created when the Dutch cut the Zuiderzee off from the North Sea with a closure dike. This step was taken to put a stop to the sea flooding towns on the Zuiderzee. Once the dike was in place (1932), the sea had become a lake. Fresh water from the River IJssel gradually replaced the seawater. Because there was a shortage of arable land, it was decided to create four polders in the south eastern part of IJsselmeer. The third polder was finished in 1968 but the fourth, in Lake Markermeer, was never drained after protests from the environmental movement and water sports enthusiasts.