Middelzee

thumb|right|An 1834 map of the former Middelzee The Middelzee (Dutch for "middle sea"; ), also called Bordine, was the estuary mouth of the River Boorne (West Frisian: Boarn) now in the Dutch province of Friesland. 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). Other historical names for the Middelzee include Bordaa, Borndiep, Boerdiep, and Bordena. The names like Bordine, mean "border".

==Pre-history== Back in the Pleistocene the Boorne was a river that had a drainage basin in Friesland, Drenthe, and Groningen. It flowed from Saalien glacial till plateau in a southwest direction, and met the sea west of Het Bildt. The Boorne passed the current location of Akkrum and Rauwerd. 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. In the Holocene, the sea rose flooding the valley to form an estuary. Before 7000 years ago, the sea was rising at a rate of 0.75 meters per century, and the rising sea was faster than the sedimentation There are still estuary sediments west of Jorwerd, that were deposited during 6400-5300 before present (Mid-Atlantic). By 5500 years ago, sea level rise slowed down to a rate of 0.15 meters per century. This allowed sedimentation to catch up and fill in the estuary. Tidal inlets further to the west were blocked by sediment by 3300 years ago. However the Boorne remained open and gradually eroded further and further south and then branched west south of what is now Westergo. People started living adjacent to the channels about 600 BC. The tidal channel moved to the east to what would become the Middelzee. Before the year 900, the new tidal channel scoured deeper and deeper and flooding extended even further inland. After 900 the flooding reached its maximum extent, and it became known as the Middelzee. The southwestern extremity joined up with the Marne Estuary near Bolsward.