The lake Almere was an inland lake in the place of today's IJsselmeer in the center of the Netherlands.
Almere (lake) Wikipedia
The texts of ancient Romans called it the Lake Flevo. Lake Almere is mentioned among others in a life of saints written by Bishop Anglo-Saxon Saint Boniface in 753, or a deed of gift from the town of Urk.
Its etymology may be eels, in Dutch aal or ael, so: "ael mere" = "eel lake"
Presumably, the water of Almere at that time was freshwater or slightly brackish.
A number of factors have led during the Middle Ages to the transformation of the lake to an inland sea that will be called Zuiderzee, these are:the rising sea levels due to global warming known as the Medieval Warm Period.
excavation of peat by the Frisians in West Friesland, near the Vlie a river that connects Almere to the North Sea.
the floods such as the All Saints' Flood (1170) or St. Lucia's flood 1287.
The name of the new town of Almere in Flevoland was given in 1984 in memory of this body of water.