The River Lagan is a historic and ecologically significant waterway in Northern Ireland, rising on Slieve Croob in County Down and flowing through the city of Belfast before entering the Irish Sea. The river drains approximately 609 square kilometres of predominantly agricultural land in its upper reaches, transitioning to urban Belfast and Lisburn in its lower sections. The catchment includes one significant tributary, the Ravernet River, along with several minor tributaries including the Carryduff River, the River Farset, and the Blackstaff River. The Lagan has a rich angling heritage and has undergone remarkable ecological recovery over recent decades, transforming from a heavily industrialised and polluted waterway into a fishery capable of supporting brown trout, salmon, and a range of coarse species. The river flows to the Stranmillis Weir, from which point it becomes estuarine.