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Start at the free Glasfynydd Forest car park beside the Trecastle to Llanddeusant road, near Pont ar Wysg. Follow the waymarked reservoir route clockwise. Cross the road carefully, follow the woodland track north, then keep to the established track around the western end and along the north-western shore. Continue along the lane above the northern shore, turn onto the dam crossing, and return south on the local lane before turning right at the road junction for the car park.
Allow about three hours and fifteen minutes for this roughly nine-kilometre circuit, including short stops. The route combines forest tracks, reservoir views and the dam, with around 100 metres of climbing. Much of the surface is compacted stone, but it can be muddy after rain. There are gentle undulations and a short climb near the start. The road return has no continuous pavement, so stay alert to vehicles and use the available verge where appropriate.
The tracks are shared with cyclists. Keep clear of the water, spillway and working reservoir structures, close gates behind you and follow any current signs. Carry sturdy footwear, waterproofs, water and food. The forest and reservoir sit in a remote part of Bannau Brycheiniog, with open stretches exposed to the weather. Picnic tables are provided, but do not rely on refreshments or toilets at this car park.
Parking at Glasfynydd is free and overnight parking is not permitted. Follow the brown signs from Trecastle towards Usk Reservoir; the forest car park is farther along the road beyond the turning for the dam. The nearby Welsh Water dam car park is an alternative if Glasfynydd is full. Check the Natural Resources Wales Glasfynydd Forest page and signs at the trailhead for changes before setting out.
Route information: Natural Resources Wales and Bannau Brycheiniog National Park.
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