The River Tween originates in the southern uplands of Scotland, near the town of Moffat in Dumfries and Galloway. It flows eastward through the Scottish Borders and forms a natural boundary between Scotland and England.
The prophet, Lailoken, fell to his death into the river Tweed, where he was impaled through the heart.
Source
The Romance of Merlin | Peter H. Goodrich, 1990