Geograph - photograph every grid square

On the left - the dam. On the right, the overflow spillway, dry because the reservoir is well below full and has been for many months.

This data has been adapted from the "enchantedtowy" website: The dam is 299 feet (91 metres) high and 951 feet (290 metres) long, it is of crushed and larger rock clay filled construction. In 1996 the height of the spillway was increased by 3 feet and a hydro-electric plant built at the base of the dam, seen to the left of the spillway and in the picture above. The capacity of the dam was increased from 13,400 million gallons to 14,200 million gallons. The normal flow is 1.5 million gallons a day during the summer driving a single turbine. During the winter and spring there are three turbines operating, producing 4.3 Megawatts of electricity. When the water in the dam is below the spillway, a plume of water comes from the turbines, as in this image. Link (Archive Link )

The cilycwm.com website gives this information: Llyn Brianne reservoir was originally built (1968-1972) to satisfy the growing demands of heavy industry in and around Swansea for water, notably the tin plate works at Felindre and a smelting plant at Llansamlet. Ironically, both these have since closed down and water from Llyn Brianne which is surplus to the Swansea area's needs now goes into the general supply, quenching thirsts all the way from Llanelli, across West and South Wales and as far east as Cardiff.

Wikipedia has: The Llyn Brianne reservoir was built in 1972 by damming a section of the river Towy at SN791484 to supplement flows in the river during low flows, and as compensation for extracting water supplies some forty miles downstream at Nantgaredig. Link

SN7948 : Quarry face by Llyn Brianne Dam, Carmarthenshire