In reply to silvermonkey:
Plenty to go at. The Dolgarrog Gorge next to the Afon Ddu sometimes gets used (or it certainly used to) - I've never ventured into it because there's some sort of tipper system that can flash flood the gorge apparently. Never sounded worth the risk to me.
Geirionydd Gorge in Gwydyr Forest is interesting and quite straightforward. Start by traversing some slabs on the right side, and further up you find the 'washing machine'. A short adit the river flows out of; you wade in then climb out the top.
https://www.flickr.com/photos/mikepeacock/2324593142/in/album-7215760408566...
https://www.flickr.com/photos/mikepeacock/5951330503/
The Afon Prysor at Maentwrog is another big one that groups use. I've ascended it for a while from the bottom to meet a deep pool and slab that turned me back. I think the groups start higher up above Rhaeadr Du and descend it. The gorge runs for a long way up to Trawsfynydd Dam, though I've no idea how much you can access. I'm not sure if the dam ever releases water.
Another surprisingly good one is the Afon Mawddach in the Coed y Brenin. You need low water levels, but it gives a very long trip traversing deep pools, and ends at Rhaeadr Mawddach itself.
https://www.flickr.com/photos/mikepeacock/9284292543/in/album-7215762645728...
https://www.flickr.com/photos/mikepeacock/9284292827/in/album-7215762645728...
There are loads more to be discovered, and plenty that no one ever visits.