Gaddings Dam near Lumbutts up the hill from the Shepherds Rest Public House near Todmorden. The Dam is a popular training venue for open water competitors, triathletes andchannel swimmers, and for as long as the most ancient locals can remember, has been a favourite place for the people of Todmorden and Walsden to take a swim. High and exposed, this disused reservoir is often wild and windy, sometimes tranquil, and occasionally a baking sun trap. But whatever the conditions, it usually manages to reward those who make the hike to the top of the moor - not a long hike but sufficient to ensure the place is never crowded. See the following excellent website: http://abelian.org/gaddings/
This is a crowd-sourced map. Abilities vary, and conditions change. Always assess risks for yourself before getting in.
Avoid diving in until you've surveyed the stones that lie beneath some of the more broken parts of the dam walls.