Cardiff Bay Barrage lies across the mouth of Cardiff Bay, Wales between Queen Alexandra Dock and Penarth Head. It was one of the largest civil engineering projects in Europe during construction in the 1990s.

Cardiff Bay is an area and freshwater lake in Cardiff, Wales. The site of a former tidal bay and estuary, it is the river mouth of the River Taff and Ely. The body of water was converted into a 500-acre (2.0 km2) lake as part of a UK Government redevelopment project, involving the damming of the rivers by the Cardiff Bay Barrage in 1999. The barrage impounds the rivers from the Severn Estuary, providing flood defence and the creation of a permanent non-tidal high water lake with limited access to the sea, serving as a core feature of the redevelopment of the area in the 1990s.