Cwm Rheidol is the borderline case — an in-line hydro reservoir on the Afon Rheidol that's part lake, part dammed river, and very much subject to the whims of Statkraft's generation schedule. Twenty-five hectares of slow water below Plynlimon, holding wild brown trout that move in and out with the river, plus the occasional sewin or salmon that's worked up from the estuary. Fly, spin and worm permitted on the Aberystwyth AA ticket. The level fluctuates so the bank-line moves; pick a settled day if you can.
- Reservoir
- Silurian mudstone shale