Which river system contains XXX?
River Lagan
The River Lagan (from Irish Abhainn an Lagáin, meaning ``river of the low-lying district''; Ulster Scots: Lagan Wattèr) is a major river in Northern Ireland which runs 53.5 miles (86 km) from the Slieve Croob mountain in County Down to Belfast where it enters Belfast Lough, an inlet of the Irish Sea.