The River Duff, known in Irish as An Dubh ("the black"), runs 14 miles (23 km) through County Leitrim and County Sligo in northwest Ireland, draining the Dartry Mountains and Ben Bulben before reaching the sea. It is also known locally as the Black River in County Leitrim, and historically formed part of the ancient border between Ulster and Connacht.

For paddlers, the run consists of approximately 500 metres of class 2/3 rapids — shallow but manageable — leading into a 12-foot (roughly 3-metre) vertical waterfall with a deep plunge pool. The hole at the base of the fall is considered harmless, and the drop can be run in surprisingly low water. The waterfall is accessible to a wide range of craft, from small playboats to larger beginner boats, and is suitable for paddlers wanting to attempt tricks such as freewheels or kickflips once the straightforward line is dialled in. A more challenging alternative channel runs river right above the main fall; it is distinct and unlikely to be entered accidentally, but is considerably more demanding.

The put-in is at Bunduff Bridge on the northern border between County Sligo and County Donegal, well signposted on the road north from Sligo toward Donegal. Parking is available on the roadside. Both banks can be walked to inspect the rapids and the final fall before committing to the run. At the take-out, paddlers can either walk back up to the car or continue out into the surf.

As a short mountain river, water levels on the Duff can change very rapidly following heavy rain, and the river can rise to high volume quickly. Paddlers should assess conditions carefully before putting on.

The Duff is a salmon fishing river, with the season running from 1 February to 30 September. During the fishing season, a steel cage or similar structure is sometimes placed submerged beneath the final waterfall, reportedly to deter poaching. This poses a serious hazard to paddlers. It is strongly advisable to avoid the river during the fishing season, and if paddling during that period, the last fall must be inspected thoroughly before running.