A number of rivers flow down the glens and valleys of the Slieve Bloom Mountains. Those which follow a course to the west of the mountains make their way to the basin of the Shannon, while those rivers flowing to the east journey to the basin of the Barrow.

Co. Laois is drained by two of the sister rivers, the Barrow (north) and the Nore (south). The River Suir is the third of the well-known sister rivers. The Barrow rises in Glenbarrow, a small valley that runs between the Ridge of Capard and Knockanadtumba Mountain. It forms a natural border with Offaly near Portarlington for roughly 6 miles. Further along its course, it forms the boundary at short intervals between Laois and Kildare, and then Carlow.

Some tributaries of the Barrow also flow through Laois. These include:

Glenlahan River
Owenass River
Fushoge River

The River Nore also has a number of tributaries which follow courses through Co. Laois:

Mountain River
Owenbeg/Owenveg (boundary between Kilkenny & Laois for short span)
Clough River (later flows into Kilkenny and forms into the Dinin)


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