The Afon Tryweryn is a dam-controlled whitewater river in North Wales, flowing through Snowdonia National Park in Gwynedd. Approximately 13 miles (21 km) long, it originates in the Cwm Tryweryn valley and drains into Bala Lake (Llyn Tegid), where it meets the River Dee. Water releases from Llyn Celyn Reservoir are controlled by Natural Resources Wales (NRW), meaning the river runs reliably when scheduled releases occur. The National White Water Centre (NWWC) receives release schedules from NRW typically a few days in advance and publishes updates via their website and Facebook page. The reservoir was created in the 1960s, submerging parts of the Tryweryn valley and generating significant protests from local communities.

The river is divided into two main paddling sections. The Upper River runs roughly 2 km alongside the National White Water Centre and carries grade 3–4 rapids. The Lower River begins just below the centre at the raft take-out steps and continues approximately 6 km down to Bala car park, featuring grade 2–3 rapids throughout with one grade 4 rapid near the end — Bala Mill Falls — which can be portaged via the leat. Both sections are fast-flowing with long continuous rapid stretches. The river is not suitable for novice kayakers or canoeists, though it is considered appropriate for novice whitewater rafters with guidance.

Access to the Upper River (referred to as the "Top Site") runs from the Welsh Water gate below the stilling basin to just above the raft put-in steps. There is no access above the Welsh Water gate. Right-bank access is via a stile on the downstream side of Irish Bridge. A car park is available near the centre for paddlers bringing their own craft, with a shuttle bus service operating for the lower section run to Bala. A kayak wash station is available opposite the Arenig building to help prevent the spread of invasive non-native species.

Safety requirements for independent paddlers include a whitewater-suitable craft, a buoyancy aid meeting ISO12402-5 or EN393, and a helmet meeting EN1385. All paddlers must be capable of self-rescue. The river is not supervised by safety teams, and paddling with a buddy is strongly recommended. Miss Davies' Bridge is structurally weak — paddlers should avoid passing beneath it when a vehicle is crossing. The lower river below the NWWC is not regularly inspected for hazards, so paddlers should remain alert to new obstructions. Rafts share the water and may be unable to take evasive action, so paddlers should ensure their line is clear before committing to any rapid.