Interesting that the source of both the Lancashire and West Yorkshire Calders is at the same place - the top of the pass / watershed between Burnley and Todmorden. They start as small unnamed streams, but if you look on the OS map, both are identified as "River Calder" within a mile or so.

I wonder if at one time they were thought of as one river. It is after all in the Cliviger Gorge, which before the Industrial Revolution was considered to be a wild and lonely place, haunted by boggarts and hobgoblins, where travellers used to hurry through as quickly as possible.