The River Blackwater or Ulster Blackwater is a river mainly in County Armagh and County Tyrone, Northern Ireland. It also forms part of the border between the United Kingdom and the Republic of Ireland, flowing between Counties Tyrone and Monaghan, intersecting into County Monaghan briefly. Its source is to the north of Fivemiletown, County Tyrone. The river divides County Armagh from County Tyrone and also divides County Tyrone from County Monaghan. Route The Blackwater's length is 91.3 km (56.75 mi). If the Blackwater's flow is measured through its path through the 30 km (19 mi) Lough Neagh and onwards to the sea via the 64.4 km (40ml) Lower Bann, the total length is 186.3 km (115.75 mi). This makes the Blackwater–Neagh–Bann the longest natural stream flow in Ulster and is longer than the Munster Blackwater. Names The River Blackwater was originally known in Irish as Cluain-Dabhail meaning "meadow of Dabhal". This was anglicised as Clanaul, the former name of the parish of Eglish, as well as Glenaul, which was used as the name of the former local district electoral division of Armagh Union. It has also been known as An Abhainn Mhór (The Great River). Part of the rivers course forms the border between Counties Tyrone and Monaghan. Inland navigation history The inland waterways network of Ireland had developed in the later part of the eighteenth century, and in 1814, the Directors General of Inland Navigation were keen to join the various parts together. This required a link between Lough Neagh and Lough Erne, and another between Lough Erne and the River Shannon. Accordingly, they asked John Killaly, their engineer, to look at the first of these routes, "in general terms". Although the Directors did not proceed immediately, due to lack of funds, the scheme was revived in 1822 and the canal was eventually completed in 1841. Below the first lock at Charlemont, the canal joined the river, and used the river channel to reach Lough Neagh. The venture was not a success, as significant volumes of traffic failed to develop, and the canal was taken over by the Board of Works in 1851. Meanwhile, The Office of Public Works (The O.P.W., often known as 'the Board of Works') had obtained powers in 1842 to carry out various projects around the Lough Neagh basin, with the intent of improving drainage, navigation and mill-power. Following a survey by their engineer, John