Horncastle stands at the confluence of the Rivers Bain & Waring.

The River Bain rises west of Ludford and flows southwards through Horncastle to the River Witham near Tattershall.  The name Bain is derived from the Celtic word for bright or clear.

The River Waring rises from springs near Belchford, five miles northeast of Horncastle.  The Waring is joined by the Scrafield Beck just north of Horncastle. The name Waring is derived from the Celtic word for rough.

Whilst both rivers normally have a low flow, owing to the nature of the catchment area, heavy rains quickly increase the river flow and occasionally lead to flooding in the Town.