A measuring weir was constructed on the Afon Ysgir in the late 1890s. It spanned the river a short distance downstream from the Tyrbach Bridge. The only surviving trace of the weir is a roughly constructed `abutment? wall on the east bank of the river. Built of drystone rounded boulders it measures about 1m high and 1.5m wide.

The weir was one of several constructed as advance works in preparation for the London Water (Welsh Supply) Scheme (see also NPRNs 422071 & 422072). This was a proposal to supply London with water from reservoirs built in mid-Wales. Some 22 reservoirs were planned within a development area of more than 420 square miles across Radnorshire, Breconshire and Carmarthenshire. The weirs were intended to gauge water flow volumes in advance of reservoir construction. The proposal passed through parliament but eventually failed following a Royal Commission report into the scheme. Had it succeeded the scheme would have resulted in massive landscape change and the destruction of entire villages and farms.

Ref:
Richard R. Rees (2015), 'Everybody Can Have Their Own Bathwater: the astonishing story of London's bid to capture a Welsh river' (Llanwrda, privately published)

David Leighton, RCAHMW, 18 May 2017