Thomas Telford's aqueduct carries a cast-iron trough only 3.4 metres wide, and the water inside it has never been drained for structural failure in more than two centuries of service. Completed in 1805 after ten years of work, Pontcysyllte was built to move coal and limestone across the Dee valley. It now moves narrowboats, walkers and paddlers instead, and a llangollen aqueduct canoe tour follows a route that engineers laid out for freight. The design was radical for its moment. Nineteen hollow masonry piers rise from the valley floor, tapering as they climb to reduce dead load. Above them sit cast-iron arches and flanged plates bedded in Welsh flannel soaked in boiling sugar and lead — a jointing method that still holds. William Jessop shared the engineering credit, though Telford's name attached itself to the structure. At 307 metres long and 38 metres above the river, it remained the tallest navigable aqueduct in the world when it opened, and it holds that title today. UNESCO inscribed the aqueduct and eleven miles of the Llangollen Canal as a World Heritage Site in 2009, citing the corridor as a masterpiece of the Industrial Revolution. That listing changed the character of the place. The Canal & River Trust now manages Trevor Basin as a visitor site rather than a working wharf, and the towpath crossing — free to walk, open 10:00–16:00 daily — draws hundreds of thousands of people each year. The Horseshoe Falls weir upstream, the Chirk Aqueduct to the south and the Froncysyllte embankment all belong to the same continuous feat of water engineering. What distinguishes the crossing from a canoe is the asymmetry of the parapet. On the towpath side, a rail protects walkers. On the opposite side, the iron trough ends flush with open air, so a paddler sits level with the drop. Boats appear to float unsupported when seen from the valley below, an effect noted by Victorian travel writers who called it the stream in the sky. Guided operators launch from the basin and work the calm, lock-free pound in both directions. The canal itself remains a functioning water supply, drawing from the Dee at Llantysilio to feed reservoirs downstream. Conservation work is continuous: iron plates are monitored, masonry repointed, water levels held constant. Anyone joining a llangollen aqueduct canoe tour is passing over a piece of live infrastructure rather than a monument, which is precisely why the Trust regulates paddling access and why the structure still earns its heritage designation.