Several important rivers run through this county that belong only in small part to it. The Shannon rises in the northwest extremity. The source is a pool called Lugnashinna, near the western base of Tiltinbane Mountain, on the north side of Glengavlin: from this the river flows for 7 miles till it touches Leitrim; next it runs for a mile and a half on the boundary between Cavan and Leitrim; then it enters Leitrim; and after another mile and a half falls into Lough Allen.

The Owenmore flows west through the valley of Glengavlin, and joins the Shannon about 2 miles below Lugnashinna. This is, properly speaking, the real head water or main stream of the Shannon, though it is not called by the name.

The Owenayle, running south on the western boundary line between Cavan and Leitrim, joins the Shannon just before the latter enters Lough Allen.

The Claddagh rises, on the southeast slopes of Cuilcagh Mountain, and, flowing through Swanlinbar, enters Fermanagh for Lough Erne; it is joined at Swanlinbar by the Blackwater, called in the early part of its course the Owensallagh.

The Woodford River runs for the greater part through Cavan; issuing from Garadice Lough (in Leitrim), and flowing by Ballyconnell, it forms for the rest of its course, to Upper Lough Erne, the boundary between Cavan and Fermanagh.

The Erne, from its source in Lough Gowna, to near where it enters Upper Lough Erne, belongs to this county.

The Annalee flows west into Lough Oughter, passing by the villages of Ballyhaise and Butlersbridge; in the early part of its course it is called the Annagh, flowing from Lough Sillan and through Lough Tacker, near Shercock.

The Annalee is joined by the Dromore River, which rises in Dromore Lough, on the boundary of the county near Cootehill, and a little further on by the Bunnoe stream from the north.

The Blackwater rises on the eastern slope of Benbrack, and flows southeast near the boundary with Leitrim till it enters Garadice Lough.

The Inny, flowing through Lough Sheelin and Lough Kinale, forms for some distance the boundaries between this county and those of Meath and Westmeath.

The Meath Blackwater flows for 2 to 3 miles through Cavan from its source in Lough Ramor.

The Moynalty River, flowing southeast from its source near Bailieborough, forms, for 5 to 6 miles, the boundary between Cavan and Meath, entering Meath 2 miles above Moynalty.

The center of the county, especially that portion occupied by the two baronies of Upper and Lower Loughtee, is broken up by innumerable small lakes, the intervening portions of land being thickly populated and well cultivated, and in many parts, especially along the lake shores, beautifully wooded.

Lough Oughter is an extraordinary complication of water: a large lake broken up into a number of small sheets by promontories, peninsulas, and islands, of all shapes and sizes, wooded, verdant, and cultivated.

It contains among others the islands of Eonish, Trinity (in which are the ruins of Trinity Abbey), and Inch: and on a rock in the midst of the lake stands Clogh-Oughter Castle in ruins.

On the southern boundary is Lough Sheelin, more than half of which belongs to Cavan, a beautiful lake, nearly 5 miles long by about 2 miles broad. Near this is the smaller Lough Kinale, of which less than half is in Cavan.

Lough Gowna, which is very much broken up, something like Lough Oughter, lies on the southwestern boundary, and belongs in part to this county.

Lough Ramor, near the southeast border, is about 4 miles long, with an average width of 1 mile, and is diversified with a number of lovely little wooded islands.

In the east, near Shercock, is the pretty Lough Sillan, and the two smaller Loughs, Tacker and Barnagrow. Brackley Lough, nearly a square mile in extent, lies in the northwest, near the village of Bawnboy.