The Shannon River Basin consists of the area containing Ireland's longest river, the River Shannon, and all of its tributaries and lakes. The official Ordnance Survey Ireland length of the Shannon from its Shannon Pot source is made up of tidal water flow and freshwater flow.
The Shannon Basin is Ireland's largest basin with an area of . Including the estuary and the River Feale, the total catchment drains a total of .
It has a Long Term Average Flow of (at Limerick City). 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 Feale 34.6 m<sup>3</sup>/s, Maigue 15.6 m<sup>3</sup>/s, Fergus 25.7 m<sup>3</sup>/s, and Deel 7.4 m<sup>3</sup>/s) are added to the discharge at Limerick, the total discharge of the River Shannon at its mouth at Loop Head reaches .
The River Shannon is a traditional freshwater river for just 45% of its total length. Excluding the tidal estuary from its total length of , if one also excludes the lakes (L. Derg 24 mi, L. Ree 18 mi, L. Allen 7 mi 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.
The Shannon River Basin is part of the Shannon International River Basin District (SHIRBD) administrative area which has an area of in area. In addition to the Shannon Basin, the district also covers coastal parts of counties Kerry and Clare which drain to the sea. The SHIRBD contains of rivers, of coastline including estuaries, and 113 lakes, including 53 over in size. The main land use throughout the SHIRBD area is agriculture (70.7%). Peatlands (11.1%) and forestry (3.2%) are also important. The SHIRBD's population is 618,884 at (Census data 2002).
There are some tributaries within the River Shannon system which have headwaters that are further in length (from source to mouth) than the Shannon Pot source, such as the Owenmore River (County Cavan) in County Cavan, which flows west for through the valley of Glangevlin before joining the Shannon about below the Shannon Pot at Lugnashinna, thus adding to the Shannon's overall length, bringing it to .
Also the Boyle River has a similar claim. The river flow from the furthest reaches of the Boyle catchment to Limerick city has a measurement of . When added to the Shannon's estuary this gives a total river flow of , which makes it the longest river within the River Shannon basin (from source to mouth)â longer than the Shannon Pot source. Thus the Boyle-Shannon river can be regarded as having the longest natural river flow in Ireland.
The River Shannon Basin touches more than half (17) of Ireland's counties:- Limerick, Clare, Tipperary, Offaly, Westmeath, Longford, Roscommon, Kerry, Galway, Leitrim, Cavan, Sligo, Mayo, Cork, Laois, Meath and Fermanagh.
Towns and Villages (with 2011 populations) within the Shannon River Basin and the rivers and riverbanks on which they stand:
Shannon River: (going downstream)
Freshwater Catchments (With Areas â km<sup>2</sup>) Going downstream
Left Bank:
Right Bank:
Estuarine Catchments Areas
Left Bank:
Right Bank:
There are many other smaller tributaries which join the Shannon along its journey.
There are a multitude of lakes within the Shannon River Basin, both on the main river and throughout the sub-catchments.
Here is a table showing the major lakes: