If you look at a map of England, you'll see on the southwestern side of the island, between Cornwall and Wales, a large waterway reaching inland from the Atlantic. This is the Estuary (in Witch, it would be “Firth”) of the River Severn.
The Severn, Britain's longest river, is traditionally considered a “female” river, its patron deity a goddess.
In its valley and throughout its watershed there dwelt, some 1300 years ago, the Anglo-Saxon tribe known as the Hwicce, from whom, some would say, derive the witches of today. And indeed, plenty of witches still live along the Lady Severn, though most of us now live elsewhere.
In any given landscape, the names of the largest rivers will always give access to the oldest reachable underlying linguistic substratum. (Think of the Mississippi, Ojibwe for “Big River.”) And so it is for the Severn.