The city of Belfast is built on the River Lagan and it can be truly said that the city owes much to the river and its deep basins that provided the infrastructure for one of the largest shipyards in the world at one time, Harland and Wolff, and numerous other smaller associated engineering works.
The River Lagan is the major river in Northern Ireland which runs 60 km from the Slieve Croob Mountain in County Down to Belfast where it enters Belfast Lough, the main inlet of the Irish Sea. The River Lagan forms the border between County Antrim and County Down. It rises as a tiny fast moving stream off the Transmitter Road near to the summit of Slieve Croob. From here it continues on its journey to Belfast through Dromara 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 390ft (119m). As the river continues on its journey to Belfast it turns east to Magherlin into a broad plain between the Antrim plateau and the plateau of Down.
The river drains approximately 609 square km of agricultural land and flows over 70 km from the Mourne Mountains to the Stranmillis Weir. The catchment consists mainly of enriched agricultural grassland in the upper parts, with a lower section draining urban Belfast and Lisburn. There is one significant tributary, the Ravernet River, and there are several minor tributaries. Water quality is generally fair though there are localised problems and occasional pollution incidents, mainly due to effluents from farms.
Because of industrial use at Belfast, fish stocks died out on the River Lagan and despite extensive re-stocking 20 years ago, the river remains less than bountiful in that regard. What is there is mainly salmon and brown trout.
The Lagan is navigable from close to the Lisburn area into Belfast Lough. A tow path has been constructed from Belfast to Lisburn, about 10 kilometres.