The aqueduct viewed from the valley below

The aqueduct was designed by civil engineers Thomas Telford and William Jessop for a location near an 18th-century road crossing, Pont Cysyllte. After the westerly high-ground route was approved, the original plan was to create a series of locks down both sides of the valley to an embankment that would carry the Ellesmere Canal over the River Dee. After Telford was hired the plan was changed to an aqueduct that would create an uninterrupted waterway straight across the valley. Despite considerable public scepticism, Telford was confident his construction method would work because he had previously built a cast-iron trough aqueduct – the Longdon-on-Tern Aqueduct on the Shrewsbury Canal.[1]

The aqueduct was one of the first major feats of civil engineering undertaken by Telford, who was becoming one of Britain's leading industrial civil engineers; although his work was supervised by Jessop, the more experienced canal engineer. Ironwork was supplied by William Hazledine from his foundries at Shrewsbury and nearby Cefn Mawr. The work, which took around ten years from design to construction, cost around of £47,000. Adjusted for inflation this is equivalent to no more than £4,060,000 in 2021[2], but represented a major investment against the contemporary GDP of some £400 million.[citation needed]

The Pontcysyllte aqueduct officially opened to traffic on 26 November 1805. A plaque commemorating its inauguration reads:

The nobility and gentry, the adjacent Counties having united their efforts with the great commercial interests of this country. In creating an intercourse and union between England and North Wales by a navigable communication of the three Rivers, Severne [sic] Dee and Mersey for the mutual benefit of agriculture and trades, caused the first stone of this aqueduct of Pontcysyllty [sic], to be laid on the 25th day of July MDCCXCV [1795]. When Richard Myddelton of Chirk, Esq, M.P. one of the original patrons of the Ellesmere Canal was Lord of this manor, and in the reign of our Sovereign George the Third. When the equity of the laws, and the security of property, promoted the general welfare of the nation. While the arts and sciences flourished by his patronage and the conduct of civil life was improved by his example.

The bridge is 336 yd (307 m) long, 12 ft (3.7 m) wide and 5 ft 3 in (1.60 m) deep.[3] It consists of a cast iron trough supported 126 ft (38 m) above the river on iron arched ribs carried on eighteen hollow masonry piers (pillars). Each of the 18 spans is 53 ft (16 m) wide. With the completion of the aqueduct, the next phase of the canal should have been the continuation of the line to Moss Valley, Wrexham where Telford had constructed a feeder reservoir lake in 1796. This would provide the water for the length of canal between Trevor Basin and Chester. The plan to build this section was cancelled in 1798, and the isolated feeder and a stretch of navigation between Ffrwd and a basin in Summerhill was abandoned. Remnants of the feeder channel are visible in Gwersyllt. A street in the village is still named Heol Camlas (Welsh: Canal Way).[4] The physical construction was undertaken by John Simpson (d.1815) of Shrewsbury.[5]

With the project incomplete, Trevor Basin just over the Pontcysyllte aqueduct would become the canal's northern terminus. In 1808 a feeder channel to bring water from the River Dee near Llangollen was completed. In order to maintain a continual supply, Telford built an artificial weir known as the Horseshoe Falls near Llantysilio to maintain water height.

Subsequently, the Plas Kynaston Canal was built to serve industry in the Cefn Mawr and Rhosymedre areas in the 1820s. There might have been another canal extension ("Ward's") but detailed records do not survive.[6] Goods traffic was brought down to the canal by the Ruabon Brook Tramway which climbed towards Acrefair and Plas Bennion. This railway was eventually upgraded to steam operation and extended towards Rhosllannerchrugog and Wrexham.[6]

In 1844, the Ellesmere and Chester Canal Company, which owned the broad canals from Ellesmere Port to Chester and from Chester to Nantwich, with a branch to Middlewich, began discussions with the narrow Birmingham and Liverpool Junction Canal, which ran from Nantwich to Autherley, where it joined the Staffordshire and Worcestershire Canal. The two companies had always worked together, in a bid to maintain their profits against competition from the railways, and amalgamation seemed to be a logical step. An agreement was worked out by August, and the two companies then sought a Private Act of Parliament to authorise the takeover. This was granted on 8 May 1845, when the larger Ellesmere and Chester Canal Company was formed.[7]

 Plaque commemorating the construction of the Pontcysyllte Aqueduct between 1795 and 1805 by Thomas Telford

In 1846, the canal and the aqueduct became part of the Shropshire Union Railways and Canal Company. But the intent of the merger was to build railways at a reduced cost, by using the existing routes of the canals they owned.[8] By 1849, the plan to turn canals into railways had been dropped.[8] As the aqueduct was largely in an area that was served by railways owned by the Great Western Railway, the London and North Western Railway was more than happy for the canal to remain open as long as it remained profitable.[citation needed][clarification needed] With the start of the First World War in 1914, the Shropshire Union – of which the Pontcysyllte aqueduct was a part – served the war effort with its fleet of more than 450 narrow boats.[8]

Commercial traffic on the canal greatly declined after a waterway breach near Newtown, Powys (now part of the Montgomery Canal) in 1936. By 1939 boat movements across the aqueduct to Llangollen had ceased. The canal was formally closed to navigation under the London Midland and Scottish Railway Company Act of 1944. On 6 September 1945, due to inadequate maintenance, the canal breached its banks east of Llangollen near Sun Bank Halt. The flow of hundreds of tons of water washed away the embankment of the railway further down the hill, tearing a 40-yard (37 m) crater 50 feet (15 m) deep.[9] This caused the first traffic of the morning, a mail and goods train composed of 16 carriages and two vans, to crash into the breach, killing one and injuring two engine crew.[10]

The aqueduct was saved (despite its official closure to waterway traffic) because it was still required as a water feeder for the remainder of the Shropshire Union Canal. The aqueduct also supplied drinking water to a reservoir at Hurleston. In 1955 the Mid & South East Cheshire Water Board agreed to maintain the canal securing its future.[11]

In the latter half of the 20th century, leisure boating traffic began to rise. In a rebranding exercise by British Waterways in the 1980s, the former industrial waterway was renamed the Llangollen Canal. It has since become one of the most popular canals for holidaymakers in Britain because of its aqueducts and scenery. The Pontcysyllte Aqueduct is now maintained and managed by the Canal & River Trust (branded Glandŵr Cymru in Wales). Otters have been seen in the area.[12]