The Pontcysyllte Aqueduct is an aqueduct which carries the Llangollen Canal over the valley of the River Dee, east of Llangollen in north Wales.
The aqueduct is 1007 feet long, 11 feet wide, and 5 feet 3 inches deep. It is constructed of cast iron troughs mounted on 19 masonry arches, and carries the canal 126 feet above the river (up to the ironwork).
Mortar used in the construction comprised lime, water and oxen blood. The iron castings were produced at the Plaskynaston Foundry, and each casting dovetails into the next. To caulk the joints, Welsh flannel was dipped in boiling sugar, after which the joints were sealed with lead.
Part of what was originally called the Ellesmere Canal, it was one of the first major feats of civil engineering undertaken by leading civil engineer Thomas Telford (supervised by the more experienced canal engineer William Jessop). The iron was supplied by William Hazeldine from his foundries at Shrewsbury and nearby Cefn Mawr. It was opened on 26 November 1805, having taken around ten years to design and construct at a total cost of £47,000.
The footpath is cantilevered over the trough, which is the full width of the aqueduct, so that narrowboats are able to move more freely through the water. There are no railings on the canal side, just the edge of the trough and a sheer drop, although there are holes for railings which were never fitted.
There used to be an accessible small handle in a recess on the footpath in the middle of the centre span, the pulling of which would cause a cascade of water to tumble into the River Dee below.
In 2005 the aqueduct was entered for World Heritage status in its 200th anniversary year.