Middelzee

It ran from as far south as Sneek northward to the Wadden Sea and marked the border between main Frisian regions of Westergoa (Westergo) and Eastergoa (Oostergo).

It flowed from Saalien glacial till plateau in a southwest direction, and met the sea west of Het Bildt.

The connection to the Wadden Sea became blocked by sand dunes in the Weichselian time period, and the mouth of the river was forced more and more easterly, until it was heading in a north-northwest direction from Akkrum.

The 12th century also saw land reclamation along the north west shore of Oostergo, shrinking the funnel shaped part of the Middelzee.

The border between Eastergoa and Westergoa in the former Middelzee is now drained by the River Zwette (West Frisian: Swette), that runs from Sneek to Leeuwarden, but which once reached the southern edge of Het Bildt.

[1] By the fifteenth century the Middelzee was reduced to a funnel shape along Frisia's north coast and further silting of the remaining part rendered it unusable.

From the early sixteenth century the polder of Het Bildt was formed in that funnel, and is now a Frisian municipality that fills the mouth of what once was the Middelzee.