The River Lagan (; 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. The River Lagan forms much of the border between County Antrim and County Down in the east of Ulster. It rises as a tiny, fast-moving stream near to the summit of Slieve Croob; Transmitter Road runs nearby. From here it continues on its journey to Belfast through Dromara, Donaghcloney and Dromore. On the lower slopes of the mountain, it is joined by another branch from Legananny (Cratlieve) Mountain, just opposite Slieve Croob. At Dromara, about four miles from its source, its height above the sea is 390 ft (119m). As the river continues on its journey to Belfast, it turns east to Magheralin into a broad plain between the plateaus of Antrim and Down.