The Cladagh River or Swanlinbar River is a moderately large river which forms from a number of small streams rising on the south-eastern slopes of Cuilcagh Mountain, County Cavan

County Cavan

County Cavan is a county in Ireland. It is part of the Border Region and is also located in the province of Ulster. It is named after the town of Cavan. Cavan County Council is the local authority for the county...

 and flows through the village of Swanlinbar

Swanlinbar

Swanlinbar is a small village on the N87 national secondary road in north-west County Cavan, Ireland, close to the Cladagh river and near the Fermanagh border.The village is in the barony of Tullyhaw....

, before crossing the border into County Fermanagh

County Fermanagh

Fermanagh District Council is the only one of the 26 district councils in Northern Ireland that contains all of the county it is named after. The district council also contains a small section of County Tyrone in the Dromore and Kilskeery road areas....

 and eventually flowing into Upper Lough Erne. It is ultra-oligotroph

Oligotroph

An oligotroph is an organism that can live in an environment that offers very low levels of nutrients. They may be contrasted with copiotrophs, which prefer nutritionally rich environments...

ic upstream before gradually becoming oligotrophic and oligo-mesotrophic through its middle and lower reaches.

The river is a designated Special Area of Conservation

Special Area of Conservation

A Special Area of Conservation is defined in the European Union's Habitats Directive , also known as the Directive on the Conservation of Natural Habitats and of Wild Fauna and Flora...

, containing one of the largest surviving populations in Northern Ireland

Northern Ireland

Northern Ireland is one of the four countries of the United Kingdom. Situated in the north-east of the island of Ireland, it shares a border with the Republic of Ireland to the south and west...

 of the freshwater pearl mussel

Freshwater pearl mussel

The freshwater pearl mussel, scientific name Margaritifera margaritifera, is an endangered species of freshwater mussel, an aquatic bivalve mollusc in the family Margaritiferidae....

. The mussels, estimated to be a minimum of 10,000 in number, are confined to a 6 km (3.7 mi) stretch of undisturbed river in the middle section.