The Upper Shannon catchment, above Carrick-on-Shannon (area: 1,301 km2) has two distinct reaches: The River Shannon (basin area: 576 km2) which rises in Cavan and the Boyle River (basin area: 725 km2), with its source in Mayo which have their confluence at Lough Eidin/Drumharlow approximately 4.7 km upstream of Carrick-On-Shannon.
The Boyle River has its origins in the rivers Lung and Breedoge which flow into Lough Gara. The 29 km (18 mi) Lung River is fed by Urlaur Lough and Errit Lough on the Mayo/Roscommon border.
On the bridge at Knockvicar
The river flow from the furthest reaches of the Boyle catchment to Limerick city has a measurement of 290 km (180ml). When added to the Shannon's 102.1 km (63.5ml) estuary this gives a total river flow of 392.1 km (243.5ml), which makes it the longest river within the River Shannon basin (from source to mouth), 31.6 km (19.5ml) longer than the Shannon Pot source.
Lung River at Crunnaun Bridge near Ballaghadereen Co. Roscommon