In reply to SmokinGun:
The Prysor flows into one end of the Trawsfynnydd reservior and out below the dam at the other.
The gorge in the video is Ceunant Llennyrch and the waterfall is Rhaeadr Du below
the dam. Yes it is the cooling water for the Magnox powerstation no it is not radio active
Parking can be difficult in the lane above the gorge please don't block the gates.