A measuring weir was constructed on the River Irfon in the late 1890s. It spanned the river a just below Cefn-llys-gwynne. The weir was of substantial construction, concrete, steel and timber. The most visible surviving features are the concrete `abutments? on each bank of the river. On the (accessible) east bank the abutment measures about 3m high, above the river bed, and 1.6m thick. Between the abutments steel uprights were spaced at intervals across the river, driven into the bedrock. Some can still be seen. Although the weir has largely been dismantled/eroded it is thought that timber members were bolted to the uprights and a measuring gauge secured to it. The abutments were thus intended to prevent local river bank erosion to ensure that all water flowed over the top of the weir crest, rather than around its sides, allowing flow rates to be measured.
The site of the weir was initially intended for the construction of a dam for a proposed Irfon Reservoir.

The weir was one of several constructed as advance works in preparation for the London Water (Welsh Supply) Scheme (see also NPRNs 422070 & 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