Part of the Elan Aqueduct (nprn 95469), itself part of the Elan Valley Water Scheme (nprn 96459), the Caethon Syphon carries the pipeline over Nant Caethon. It consists of the syphon inlet and washout chamber (nprn 309607), the Nant Caethon aqueduct (nprn 309608) and adjacent washout chamber (nprn 309609), an air valve chamber (nprn 309610) and the syphon outlet (nprn 309611) and covers a distance of some 1.49km.

B.A.Malaws, 05 December 2003.