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 224 miles (360 km) made up of 63.5 miles (102.2 km) tidal water flow and 160.5 miles (258.3 km) freshwater flow.^[1]

The Shannon Basin is Ireland's largest basin with an area of 11,700 km² (4,500 sq mi). Including the estuary and the River Feale, the total catchment drains a total of 16,865 km² (6,512 sq mi).^[2]

It has a Long Term Average Flow of 208.1 m³/s (7,350 cu ft/s) (at Limerick City).^[3] This is double the flow rate of Ireland's second largest river, the River Corrib (104.8 m³/s [3,700 cu ft/s]).^[3] If the discharges from all of the rivers and streams into the Shannon Estuary (including the rivers Feale 34.6 m³/s, Maigue 15.6 m³/s, Fergus 25.7 m³/s, and Deel 7.4 m³/s)^[4][5] are added to the discharge at Limerick, the total discharge of the River Shannon at its mouth at Loop Head reaches 300 m³/s (11,000 cu ft/s).

The River Shannon is a traditional freshwater river for just 45% of its total length. Excluding the 63.5-mile (102.2 km) tidal estuary from its total length of 224 miles (360 km), if one also excludes the lakes (L. Derg 24 mi, L. Ree 18 mi, L. Allen 7 mi^[6] plus L. Boderg, L. Bofin, L. Forbes, L. Corry) from the Shannon's freshwater flow of 160.5 miles (258.3 km), the Shannon as a freshwater river is only about 100 miles (160 km) long.

The Shannon River Basin is part of the Shannon International River Basin District (SHIRBD)^[7] administrative area which has an area of 17,963 km² (6,936 sq mi) 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 7,666 km (4,763 mi) of rivers, 1,220 km (760 mi) of coastline including estuaries, and 113 lakes, including 53 over 50 hectares (120 acres) 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 34 inhabitants per square kilometre (88/sq mi) (Census data 2002).^[8]

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 14.5 km (9.0 mi)^[9] through the valley of Glangevlin before joining the Shannon about 3 km (2 mi) below the Shannon Pot at Lugnashinna,^[10] thus adding 11 km (7 mi) to the Shannon's overall length, bringing it to 372 km (231 mi).

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 290 km (180 mi).^[11] When added to the Shannon's 102.2 km (63.5 mi) estuary this gives a total river flow of 392.1 km (243.6 mi), which makes it the longest river within the River Shannon basin (from source to mouth)—31.6 km (19.6 mi) longer than the Shannon Pot source. Thus the Boyle-Shannon river can be regarded as having the longest natural river flow in Ireland.^[12]

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.^[13]

Freshwater Catchments (With Areas - km²) Going downstream

Left Bank:

• L Allen (upstream from L. Allen outlet) 415 km^2[2]
• Eslin River 73 km^2[16]
• River Rinn 311 km^2[16]
• Camlin River 352 km^2[16]
• River Inny (Leinster) 1,254 km^2[16]
• River Brosna 1,248 km^2[16]
• Little Brosna River 662 km^2[16]
• Ballyfinboy River 182 km^2[16]
• Nenagh River 321 km^2[16]
• Mulkear River 660 km^2[2]

Right Bank:

• Boyle River 725 km^2[16]
• Hind River 78 km^2[16]
• River Suck 1,600 km^2[16]
• Cappagh/Kilcrow 414 km^2[16]
• River Graney 295 km^2[16]

Estuarine Catchments Areas

Left Bank:

• River Maigue 1,000^2[2]
• River Deel 426 km^2[2]
• River Feale 1,170 km^2[2]

Right Bank:

• Bunratty River 233 km^2[2]
• River Fergus 1,043 km^2[2]

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:

• River Shannon
• Shannon Callows
• Rivers of Ireland
• List of rivers in Ireland
• List of loughs of Ireland