The River Shannon (Sionainn or Sionna in Irish) is, at 386 km (240 miles), the longest river in Ireland. It divides the west of the island of Ireland (principally the province of Connacht) from the east and south (Leinster and most of Munster). County Clare, being west of the Shannon but part of the province of Munster, is the major exception. The river represents a major physical barrier between east and west, with fewer than twenty crossing-points between Limerick city in the south and the town of Carrick on Shannon in the north.
The Shannon has been an important waterway since antiquity, having first been mapped by the Graeco-Egyptian geographer Ptolemy. The river flows generally southward from the Shannon Pot in County Cavan before turning west and emptying into the Atlantic Ocean through the 113 km (70 mi) long Shannon Estuary. Limerick city stands at the point where the river water meets the sea water of the estuary. The Shannon is unaffected by sea tides east of Limerick.
By tradition the Shannon is said to rise in the Shannon Pot, a small pool on the slopes of Cuilcagh Mountain in County Cavan, from where the young river appears as a small trout stream. Surveys have defined a 12.8 km² immediate pot catchment area covering the slopes of Cuilcagh. This area includes Garvah Lough, Cavan, 2.2 km to the northeast drained by Pollnaowen. Further sinks that source the pot include Pollboy and, through Shannon Cave, Pollahune in Cavan and Polltulyard and Tullyrrakeeragh in Fermanagh, Northern Ireland. The highest point in the catchment is a spring at Tiltinbane on the western end of the Cuilcagh mountain ridge.
From the Shannon Pot, the river has a number of tributaries before replenishing Lough Allen at its head.
In the seventeenth century, the Shannon was of major strategic importance in military campaign in Ireland, as it formed a physical boundary between the east and west of the country. In the Irish Confederate Wars of 1641-53, the Irish retreated behind the Shannon in 1650 and held out for two further years against English Parliamentarian forces. In preparing a land settlement, or plantation after his conquest of Ireland Oliver Cromwell reputedly said the remaining Irish landowners would go to "Hell or Connacht", referring to their choice of forced migration west across the river Shannon, or death. Thus freeing up the landholdings for the incoming English settlers.
In the Williamite war in Ireland (1689-91), the Jacobites also retreated behind the Shannon after their defeat at the Battle of the Boyne in 1690. Athlone and Limerick, cities commanding bridges over the river, saw bloody sieges. (See Sieges of Limerick and Siege of Athlone).
As late as 1916, the leaders of the Easter Rising planned to have their forces in the west "hold the line of the Shannon". However, in the event, the rebels were neither well enough armed nor equipped to attempt such an ambitious policy.
The Shannon river is closely bound up with Ireland's social, cultural, military, economic and political history.
The lower Shannon between Killaloe and Limerick was a different story. Here the river falls by 30m (100ft) in only 20km (13m). William Ockenden, also from England, was placed in charge of this in 1757 and spent £12,000 over the next four years without fully completing the task. In 1771 parliament handed over responsibility to the Limerick Navigation Company with a grant of £6,000 to add to their subscriptions of £10,000. A lateral canal 5 miles long with 6 locks was started but the company needed more to complete it. In 1791, William Chapman was brought in to advise and discovered a sorry state of affairs. All the locks had been built to different dimensions and he spent the next three years rebuilding most of them. The navigation was finally opened in 1799, when over 1,000 tons of corn came down to Limerick as well as slates and turf. But even then, there were no tow paths in the river sections and there were still shoals in the summer months, no harbour facilities at Limerick and boats were limited to 15-20 tons load, often less.
With the approaching opening of the Grand Canal, the Grand Canal Company obtained permission from the Directors General and asked John Brownrigg to do a survey which found that much of Omer's work had deteriorated badly, so they started repairs. After protracted negotiations on costs and conditions, the work was completed by 1810 so that boats drawing 5'9" could pass from Athlone to Killaloe. Improvements on the lower levels were also undertaken, being completed by 1814.
When the Royal Canal was completed in 1817 there was pressure to improve the navigation above Lough Ree. The Jamestown Canal was repaired, harbours built and John Killaly designed a canal alongside the river from Battlebridge to Lough Allen which was opened in 1820.
In the latter part of the 1820s, trade increased dramatically with the arrival of paddle-wheeled steamers on the river which carried passengers and goods. By 1831 14,600 passengers and 36,000 tons of freight were being carried. This put new pressure on the navigation and a commission was set up resulting in the Shannon Navigation Act of 1835 appointing fiver Commissioners for the improvement of navigation and drainage who took possession of the whole navigation. Over the next 15 years many improvements were made but in 1849 a railway was opened from Dublin to Limerick and the number of passengers fell dramatically. Freight, which had risen to over 100,000 tons per year was also halved.
But the work the commissioners carried out failed to solve the problems of flooding and there were disastrous floods in the early 1860s. Given the flat nature of most of the river this was not easily addressed and nothing much was done till the twentieth century.
One of the first projects of the Irish Free State in the 1920s was to build the Ardnacrusha power station on the lower Shannon near Killaloe. The old navigations were abandoned and the head race constructed from Lough Derg which also served for navigation. A double lock has been provided for navigation.
In the 1950s traffic began to fall and low fixed bridges would have replaced opening bridges but for the actions of the Inland Waterways Association of Ireland which persuaded the Tánaiste to encourage passenger launches, which kept the bridges high enough for navigation. Since then the leisure trade has steadily increased, becoming a great success story.
There are also many canals connecting with the river Shannon. The Royal Canal and the Grand Canal connect the Shannon to Dublin and the Irish Sea. It is linked to the River Erne and Lough Erne by the Shannon-Erne Waterway. Ballinasloe is linked to the Shannon via the River Suck and canal, while Boyle is connected via the Boyle canal, the river Boyle and Lough Kee. There is also the Ardnacrusha canal connected with the Ardnacrusha dam south of Lough Derg. Lecarrow village in County Roscommon is connected to Lough Ree via the Lecarrow canal. Jamestown Canal forms a link between the River Shannon, from south of Jamestown, to Lough Nanoge to the south of Drumsna.
Despite being more than 300 km (200 miles) long, it rises only 76 m (250 feet) above sea level, so the river is easily navigable, with only a few locks along its length. There is a hydroelectric generation plant at Ardnacrusha belonging to the ESB.
Shipping in Shannon estuary was developed extensively during the 1980s, with over IR£2 billion (€2.5Billion) investment. A tanker terminal at Foynes and an oil jetty at Shannon Airport were built. In 1982 a large scale alumina extraction plant was built at Aughinish Island. 60,000 tonne cargo vessels now carry raw bauxite from West African mines to the plant, where it is refined to alumina. This is then exported to Canada where it is further refined to aluminium. 1985 saw the opening of a huge coal-fired electricity plant at Moneypoint, fed by regular visits by 150,000 tonne bulk carriers.