The River Shannon (, , ) is the major river on the island of Ireland, and at in length, is the longest river in the British Isles. It drains the Shannon River Basin, which has an area of , – approximately one fifth of the area of Ireland.
Known as an important waterway since antiquity, the Shannon first appeared in maps by the Graeco-Egyptian geographer Ptolemy ( 100 – 170 AD). The river flows generally southwards from the Shannon Pot in County Cavan before turning west and emptying into the Atlantic Ocean through the long Shannon Estuary. Limerick city stands at the point where the river water meets the sea water of the estuary. The Shannon is tidal east of Limerick as far as the base of the Ardnacrusha dam. The Shannon divides the west 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 thirty-five crossing points between the village of Dowra in the north and Limerick city in the south.
Course
By tradition the Shannon is said to rise in the Shannon Pot, a small pool in the townland of Derrylahan on the slopes of Cuilcagh Mountain in County Cavan, 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. 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.
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, subsuming the tributary rivers Boyle, Inny, Suck, Mulkear and Brosna, among others, before reaching the Shannon Estuary at Limerick.
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 claimed lengths emerged before the use of modern surveying instruments.
At a total length of , it is the longest river in Ireland. That the Shannon is the longest river in the British Isles 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 Shannon River Basin which have headwaters that are further in length (from source to mouth) than the Shannon Pot source's length of , such as the Owenmore River, total length in County Cavan and the Boyle River, total length with its source in County 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 (L. Derg , L. Ree , L. Allen plus L. Boderg, L. Bofin, L. Forbes, 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 flow. It has a long-term average flow rate of (at Limerick). This is double the flow rate of Ireland's second highest-volume river, the short River Corrib (. If the discharges from all of the rivers and streams into the Shannon Estuary (including the rivers Feale , Maigue , Fergus , and 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).