The River Lagan (from Irish Abhainn an Lagáin 'river of the low-lying district'; Ulster Scots: Lagan Wattèr)[1] is a major river in Northern Ireland which runs 53.5 mi (86.1 km)[2] from the Slieve Croob mountain in County Down to Belfast where it enters Belfast Lough, an inlet of the Irish Sea.

[3] The river drains approximately 609 square km of agricultural land and flows to the Stranmillis Weir, from which point on it is estuarine.

[4] The catchment consists mainly of enriched agricultural grassland in the upper parts, with the lower section draining urban Belfast and Lisburn.

[6] The name Belfast originates from the Irish Béal Feirste, or the mouth of the Farset, the river on which the city was built and which flows into the Lagan.

In 1989 the Laganside Corporation was established by the British government to redevelop the areas surrounding the Lagan in Belfast.

One of the main functions of the weir was to reduce unsightly mud flats at low tide.

The weir is a series of large steel barriers which are raised as the tide retreats to keep the river at an artificially constant level.

A section of National Cycle Route 9, which will eventually link Belfast with Dublin, follows this towpath.

Brown trout and several other species remained present in the upper reaches of the river throughout the worst of the downstream urban problems.