The Ystalyfera Aqueduct is the largest on the Swansea Canal (NPRN 34376) and spans the Afon Twrch at Gurnos on a roughly east-west axis. Originally the river lay to the west but was diverted as part of a building scheme which included the 4m high weir on the newly diverted course of the river. When in spate the Afon Twrch carried river boulders, and the weir was designed to protect the piers of the bridge from under-cutting. It also served a feeder to the west. Two other feeders from the Afon Twrch on the upstream side of the aqueduct have been destroyed. A large circular culvert through the northern end of the aqueduct carried the tailrace water from a fulling mill at Gurnos.

The aqueduct was built by Thomas Sheasby in 1794-98 using hydraulic mortar from Aberafan - the Swansea Canal aqueducts were probably the first in Britain to use such mortar as the waterproofing agent instead of puddling clay. The structure consists of three segmental arches built on top of a feeder weir, the crest of the weir being paved to prevent any scouring of the foundations.

RCAHMW, February 2011