The texts of ancient Romans called it Lake Flevo. Lake Almere is mentioned among others in a life of saints written by Anglo-Saxon Bishop Saint Boniface in 753, and 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 Lake Almere at that time was fresh water or slightly brackish.
A number of occurrences during the Middle Ages led to the transformation of the lake to an inland sea that would be called the Zuiderzee, which are: