Westworth Reservoir was built by the Guisborough Water Company around 1871, it has in recent years been abandoned.
I don’t know the exact date of closure and remodelling, but a 2002 report talks about the cross-section of the dam being left.
The overflow tower is the most striking reminder, along with a stone spillway which is gradually becoming overgrown.
A small building and plinth remain for an “interpretation panel” that clearly never materialised.
Plan of Spa Wood workings under the reservoir (triggered by discussion in the comments)