The Lagan Weir, completed in 1994, at a cost of £14m, is located across the Lagan between the Queen Elizabeth Bridge and the M3 bridge in Belfast, Northern Ireland. The Lagan Weir was jointly funded by the Corporation and the European Commission. It was constructed by Charles Brand Ltd and designed by Ferguson and McIlveen.
The weir is a series of massive steel barriers which are raised as the tide retreats to keep the river at an artificially constant level. This improvement to the sewerage system combined with massive dredging of the river by mechanical excavators, and installation of an underwater aeration system, has led to a marked improvement in water quality and the environment around the river. Lagan Weir, dredging and aeration have increased water quality in the river and salmon is returning. An otter and seals have followed the fish that now move up river to spawn in what was once an aquatic death trap.
The objective of the structure is to keep the level of the river artificially constant, as it is a tidal river the level of the water varied by up to three metres between high and low tide. This exposed mudflats which were unsightly and emitted a strong odour, particularly in the summer months.
The transformation of the riverside by the construction of the weir has been a catalyst for development along the riverside. Another part of the project is the "Lagan Lookout" centre which explains the history and function of the weir as well as the history of the Lagan itself.