GeographyBy tradition the Shannon is said to rise in the , a small pool in the townland of on the slopes of in , Republic of Ireland, from where the young river appears as a small trout stream. Surveys have defined a immediate pot catchment area covering the slopes of Cuilcagh. This area includes Garvah Lough, Cavan, to the northeast, drained by ''Pollnaowen''.Note Poll nm1: hole, pit, sink, leak, aperture (''The Pocket Oxford Irish Dictionary – Irish-English'') Further sinks that source the pot include Pollboy and, through Shannon Cave, Pollahune in Cavan and Polltullyard and Tullynakeeragh in County Fermanagh, Northern Ireland. The highest point in the catchment is a spring at Tiltinbane on the western end of the Cuilcagh mountain ridge.Philip Elmer et al. ''Springs and Bottled Waters of the World'' Springer From the Shannon Pot, the river subsumes a number of tributaries before replenishing Lough Allen at its head. The river runs through or between 11 of Ireland's Counties of Ireland, counties, subsuming the tributary rivers Boyle River (Ireland), Boyle, River Inny, Westmeath, Inny, River Suck, Suck, Mulkear River, Mulkear and River Brosna, Brosna, among others, before reaching the Shannon Estuary at . Many different values have been given for the length of the Shannon. A traditional value is . An official Irish source gives a total length of (being fresh and tidal). Some Irish guides now give . Some academic sources give , although most will refuse to give a number. The reason is that there is no particular end to a river that empties into an estuary. The 344 km length relates to the distance between Shannon Pot and a line between Kerry Head and Loop Head, the furthest reaches of the land. (It also assumes the current shipping route via Ardnacrusha, which takes off the distance.) The 280 km distance finishes where the Shannon estuary joins the estuary of the River Fergus, close to Shannon Airport. Longer distances emerged before the use of modern surveying instruments. At a total length of , this means it is the longest river in Ireland. That the Shannon is the longest river in either Ireland or Great Britain was evidently known in the 12th century, although a map of the time showed this river as flowing out of the south of Ireland. There are some tributaries within the which have headwaters that are further in length (from source to mouth) than the source's length of , such as the Owenmore River (County Cavan), Owenmore River, total length in County Cavan and the Boyle River (Ireland), Boyle River, total length with its source in County Mayo, Mayo. The River Shannon is a traditional freshwater river for about 45% of its total length. Excluding the tidal estuary from its total length of , if one also excludes the lakes (Lough Derg (Shannon), L. Derg , Lough Ree, L. Ree , Lough Allen, L. Allen plus Lough Boderg, L. Boderg, Lough Bofin (River Shannon), L. Bofin, Lough Forbes, L. Forbes, Lough Corry, L. Corry) from the Shannon's freshwater flow of , the Shannon, as a freshwater river, is only about long. Apart from being Ireland's longest river, the Shannon is also, by far, Ireland's largest river by streamflow, flow. It has a long term average flow rate of (at ). This is double the flow rate of Ireland's second largest river, the River Corrib (. If the discharges from all of the rivers and streams into the Shannon Estuary (including the rivers River Feale, Feale , River Maigue, Maigue , River Fergus, Fergus , and River Deel, Deel ) are added to the discharge at Limerick, the total discharge of the River Shannon at its mouth at Loop Head reaches . Indeed, the Shannon is a major river by the time it leaves Lough Ree with an average flow rate (at Athlone weir) of , larger than any of the other Irish rivers' total flow (apart from the River Corrib at Galway City, Galway). The Shannon Callows, areas of lowland along the river, are classified as a Special Area of Conservation. Settlements along the river (going upriver) include Kilrush, Tarbert, Kerry, Tarbert, Glin, County Limerick, Glin, Foynes, Askeaton, Shannon, County Clare, Shannon Town, , Castletroy, Castleconnell, O'Briensbridge, Montpelier, County Limerick, Montpelier, Killaloe, County Clare, Killaloe, Ballina, County Tipperary, Ballina, Portumna, Banagher, Athlone, Lanesborough–Ballyleague, Lanesborough, Carrick-on-Shannon, Leitrim, County Leitrim, Leitrim village and .
HistoryThe river began flowing along its present course after the end of the last glacial period. 's ''Geography (Ptolemy), Geography'' (2nd century AD) described a river called Σηνος (''Sēnos'') from PIE *''sai''-/''sei''- ‘to bind’, the root of English ''sinew'' and Irish Wiktionary:sin#Irish, ''sin'' ‘wikt:collar, collar’, referring to the long and sinuous estuary leading up to Limerick. Vikings settled in the region in the 10th century and used the river to raid the rich monasteries deep inland. In 937 the Limerick Vikings clashed with those of Dublin on Lough Ree and were defeated. In the 17th century, the Shannon was of major strategic importance in military campaigns 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 Parliament of England, English Parliamentarian forces. In preparing a Act for the Settlement of Ireland 1652, land settlement, or Plantations of Ireland, plantation after his Cromwellian conquest of Ireland, 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 eastern landholdings for the incoming English settlers. In the Williamite War in Ireland (1689–91), the Jacobitism, 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 (disambiguation), 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.
FolkloreAccording to Irish mythology, the river was named after a woman named Sionann (older spelling: Sínann or Sínand), the granddaughter of Lir. She went to Connla's Well to find wisdom, despite being warned not to approach it. In some sources she, like Fionn mac Cumhaill, caught and ate the Salmon of Knowledge, Salmon of Wisdom who swam there, becoming the wisest being on Earth. However, the well then burst forth, drowning Sionann and carrying her out to sea.Monaghan, Patricia. ''The Encyclopedia of Celtic Mythology and Folklore''. Infobase Publishing, 2004. p.420 A similar tale is told of Boann and the River Boyne. It is believed that Sionann was the goddess of the river. Patricia Monaghan notes that "The drowning of a goddess in a river is common in Irish mythology and typically represents the dissolving of her divine power into the water, which then gives life to the land". A small myth of Sionann tells that the legendary hunter-warrior Fionn mac Cumhaill was attacked by a number of warriors at Ballyleague near north Lough Ree, it is said that when Fionn was near defeated, Sionnan rescued Fionn who arrived with the Stone of Sionann, Fionn threw the stone and the warriors were immediately killed, however, Fionn was afraid of the power of the stone and threw it into the river, where it remains at a low Ford (crossing), ford, and that if a woman named Be Thuinne finds it, that the worlds end is near. The Shannon reputedly hosts a river monster named Cata, first appearing in the medieval Book of Lismore. In this manuscript we are told that Senán mac Geirrcinn, Senán, patron saint of , defeated the monster at Inis Cathaigh. Cata is described as a large monster with a horse's mane, gleaming eyes, thick feet, nails of iron and a whale's tail. Another story also involving a monster has an Oilliphéist panic and attempt to flee Ireland upon hearing Saint Patrick has arrived to remove its kind from the island. It carves out the route that then becomes the Shannon whilst doing so.
NavigationThough the Shannon has always been important for navigation in Ireland, there is a fall of only in the first . Consequently, it has always been shallow with depths in various places. The first serious attempt to improve things came in 1755 when the Commissioners of Inland navigation ordered Thomas Omer, a new, possibly Dutch immigrant from England, to commence work. He tackled four places between Lough Derg (Shannon), Lough Derg and Lough Ree where natural navigation was obstructed, by installing lateral canals and either pound locks or flash locks. He then continued north of Lough Ree and made a number of similar improvements, most notably by creating the first Jamestown Canal which cut out a loop of the river between Jamestown, County Leitrim, Jamestown and Drumsna as well lateral canals at Roosky and Lanesborough, Longford, Lanesborough. The lower Shannon between Killaloe, County Clare, Killaloe and had different topography. Here the river falls by in only . William Ockenden, also from England, was placed in charge of this in 1757 and spent Irish pound, £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 long with six locks was started but the company needed more to complete it. In 1791, William Chapman (engineer), 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 of corn came down to Limerick, as well as slates and Peat, 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 load, often less. With the approaching opening of the Grand Canal (Ireland), Grand Canal, the Grand Canal Company obtained permission from the Directors General and asked John Brownrigg (surveyor), 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 could pass from Athlone to Killaloe, County Clare, 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, County Roscommon, 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 steamboat, steamers on the river which carried passengers and goods. By 1831 14,600 passengers and 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 five 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 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 riverbank 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 the Shannon hydroelectric scheme which established the Ardnacrusha power station on the lower Shannon above Limerick. The old Killaloe to Limerick canal with its five locks was abandoned and the head race constructed from Lough Derg also served for navigation. A double lock was provided at the dam. 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.
DistributariesThe main flow of the river is affected by some distributaries along its course, many of which rejoin it down stream. The Blackwater river in Co. Clare rejoins the main flow below Thomond village. The Abbey River, Limerick, Abbey River flows around the northeastern, eastern, and southern shores of King's Island, Limerick, King's Island, before rejoining the Shannon at ''Hellsgate Island''.
CanalsThere are also many Canals of Ireland, canals connecting with the River Shannon. The Royal Canal and the Grand Canal (Ireland), 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, County Roscommon, Boyle is connected via the Boyle canal, the Boyle River (Ireland), river Boyle and Lough Key. There is also the Ardnacrusha canal connected with the Ardnacrusha dam south of Lough Derg. Near Limerick, a short canal connects Plassey, County Limerick, Plassey with the Abbey River, Limerick, Abbey River, allowing boats to bypass the Curraghower Falls, a major obstacle to navigation. Lecarrow village in County Roscommon is connected to Lough Ree via the Lecarrow canal. Jamestown Canal and the Albert Lock form a link between the River Shannon, from south of Jamestown, County Leitrim, Jamestown, to Lough Nanoge to the south of Drumsna.
EconomicsDespite being long, it rises only Above mean sea level, above sea level, so the river is easily navigable, with only a few locks along its length. There is a Ardnacrusha power plant, hydroelectric generation plant at Ardnacrusha (village), Ardnacrusha belonging to the Electricity Supply Board, ESB. Shipping in Shannon estuary was developed extensively during the 1980s, with over Irish pound, IR£2 billion (€2.5 billion) 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, County Limerick, Aughinish. 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 915 MW coal-fired electricity plant at Moneypoint, fed by regular visits by 150,000 tonne bulk carriers.
Shannon eel management programmeA trap and transport scheme is in force on the Shannon as part of an eel management programme following the discovery of a reduced eel population. This scheme ensures safe passage for young eels between Lough Derg (Shannon), Lough Derg and the Shannon estuary.
FishingThough the Shannon estuary fishing industry is now depleted, at one time it provided employment for hundreds of men along its length. At , fishermen based on Clancy's Strand used the Gandelow to catch Salmon. The Abbey Fishermen used a net and a boat known as a Breacaun to fish between Limerick City and Plassey until 1929. In 1929, the construction of a dam at Ardnacrusha severely impacted salmon breeding and that, and the introduction of quotas, had by the 1950s caused salmon fishing to cease. However, recreational fishing still goes on. Further down the at Kilrush the Currach was used to catch herring as well as drift netting for salmon.
Water extractionDublin City Council published a plan in 2011 to supply up to 350 million litres of water a day from Lough Derg to Dublin city and region. In 2016 the Parteen Basin to the south of lough was chosen as the proposed site of extraction. Water would be pumped to a break pressure tank Knockanacree near Cloughjordan in County Tipperary and gravity fed from there by River Shannon to Dublin pipeline, pipeline to Dublin.
See also* Cromwellian conquest of Ireland * Rivers of Ireland * List of loughs in Ireland