SHANNON, the principal river of Ireland. It flows with a bow-shaped course from north to south and south-west, from the north-west part of the island to its mouth in the Atlantic on the south-west coast, with a length of about 240 m. and a drainage area of 4,544 sq.m. Rising in co. Cavan in some small pools at the foot of Cuilcagh Mountain, the Shannon crosses co. Leitrim, traversing Lough Allen (9 m. in length), the first of a series of large lakes. It then separates co. Roscommon on the right (W.) bank from counties Leitrim, Longford, Westmeath and Offaly County on the left. In this part of its course it forms Loughs Boderg (7 m. long), Forbes (3 m.) and Ree (18 m.), and receives from the west the river Boyle and from the east the Inny, while in co. Longford it is joined by the Royal Canal.
It now separates co. Galway on the right from Offaly County and co. Tipperary; receiving the Suck from the west and the Brosna from the east, and forming Lough Derg (23 m.). Dividing co. Clare from counties Tipperary and Limeri'ck, the Shannon reaches the city of Limerick and debouches upon an estuary 6o m. in length with a direction nearly east and west. This divides co. Clare on the right from counties Limerick and Kerry on the left. A wide branch estuary, that of the Fergus, joins from the north, and the rivers Mulkear, Maigne and Deel enter from the south. From Lough Allen to Limerick, where the Shannon becomes tidal, its fall is 144 ft. With the assistance of short canals the river is navigable for light vessels to Lough Allen, and for small steam ers to Athlone; while Limerick is accessible for large vessels. The salmon-fishing is famous ; trout are also taken in the loughs and tributary streams. Carrick-on-Shannon, Athlone, Killaloe and Castleconnel are favourite stations for sportsmen. The islands of the loughs are in several cases sites of early religious settlements, while of those on the river-banks the most noteworthy is that of the seven churches of Clonmacnoise.
One of the first matters to engage the attention of the Gov ernment of the Irish Free State was the provision of cheap elec trical power, and it was decided that power could be most eco nomically produced by the construction of a central generating station capable of utilising the immense resources of the Shannon.
Outline of the scheme provided for the devel opment of hydro-electric power from the Shannon in three stages —called the partial development, the further development and the full development respectively. In the partial development Lough Derg alone was to be used for storage, but the winter level would not be raised. In the further development the storage in Lough Ree and Lough Allen would be added; and in the full development the storage in Lough Derg would be increased by raising the level of the lake above winter high water level. The head race constructed for the partial development was to be the size necessary for the full development ; but the tail race, which is mostly in rock, would be enlarged for the further development.
In the Shannon the period of maximum flow coincides with that of maximum demand, thereby reducing storage requirements. On the other hand, as the fall from Lough Derg to the sea is gradual, it was necessary to construct a long head race in order to obtain an adequate fall. The Siemens-Schtickert project (constructional work began in 1925) utilises the total fall of ioo ft. between Lough Derg and Limerick in a single large step, by leading the water out of the river bed into an inlet canal, or head race, to leave the Shannon at Parteen, above O'Brien's Bridge, and to extend a distance of about 61 m. to Ardnacrusha. Here, as an initial installation, are three vertical Francis turbines each of 38,500 h.p. The water is conducted to the turbines by large steel pipes 20 ft. in diameter and 140 ft. long and carried back to the Shannon in a tail race i m. in length. The electric energy produced in the power station is to be transmitted by means of a high tension network to most of the towns and villages of the Free State with a population of over zoo. By means of a network radiating in a closed ring formation from the power-station, Dub lin, Cork and Maryborough, all stations will be fed from two directions. A semi-public Board, known as the Electricity Supply Board, has been formed to control the distribution of current. The Board has extensive powers of acquiring existing undertakings and has already acquired the majority of such undertakings.