Numerous streams, including the Vecht, Eem, and Ysel, discharged their waters into this lake and issued thence as the Vlie (Latin Flevus), which reached the North Sea by the Vliegat between the islands of Vlieland and Terschelling.
In the Lex Frisonum the Vlie (Fli, or Flehi) is accepted as the boundary between the territory of the East and West Frisians.
In time, however, and especially during the 12th century, high tides and north-west storms swept away the western banks of the Vlie and submerged great tracts of land.
The principal sea-inlets in the north are the Texel Gat or Marsdiep and the Vlie, which lead past the chain of the Frisian Islands into the large inland sea or gulf called the Zuider Zee, and the Wadden or " shallows," which extend along the shores of Friesland and Groningen as far as the Dollart and the mouth of the Ems. The inland sea-board thus formed consists of low coasts of sea-clay protected by dikes, and of some high diluvial strata which rise far enough above the level of the sea to make dikes unnecessary, as in the case of the Gooi hills between Naarden and the Eem, the Veluwe hills between Nykerk and Elburg, and the steep cliffs of the Gaasterland between Oude Mirdum and Stavoren.
West of the Vlie and around the mouths of the Rhine.