 It is not known precisely in what year(s) the canal was constructed, though it already appears on the very first reliable maps of Friesland, which date back to the 1500s.Before the canal was constructed, there were two small naturally formed rivers, now called the Zuider Ee, ending in the sea at Leeuwarden; and the Noorder Ee, connecting Dokkum to the sea.At the time, both of these cities were still located on the coast and important local sea harbors.