Llyn Brianne is a man-made lake

Lake

A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...

 or reservoir in the headwaters of the River Tywi

River Tywi

With a total length of the River Towy is the longest river flowing entirely within Wales, and is noted for its sea trout and salmon fishing.The river rises within of the Teifi on the lower slopes of Crug Gynan in the Cambrian Mountains and, flowing through the Towy Forest, forms the border...

 in central Wales.
The reservoir was constructed by Wimpey Construction

George Wimpey

George Wimpey was formed in 1880 and, based in Hammersmith, operated largely as a road surfacing contractor. The business was acquired by Godfrey Mitchell in 1919 and he developed it into the UK’s pre-eminent construction and housebuilding firm. In 2007, Wimpey merged with Taylor Woodrow to create...

 in the late 1960s and early 1970s in order to regulate the flow in the River Tywi to support large potable water abstraction at Nantgaredig

Nantgaredig

Nantgaredig is a village in Carmarthenshire.The village is most notable for being the home of actor Julian Lewis Jones. The winner of the 1990 Cheltenham Gold Cup, Norton's Coin, was trained by Sirrel Griffiths at his farm at Nantgaredig...

 in the lower reaches of the river near Carmarthen

Carmarthen

Carmarthen is a community in, and the county town of, Carmarthenshire, Wales. It is sited on the River Towy north of its mouth at Carmarthen Bay. In 2001, the population was 14,648....

, providing water to the Felindre water treatment works; the treated water is piped to a large area of South Wales

South Wales

South Wales is an area of Wales bordered by England and the Bristol Channel to the east and south, and Mid Wales and West Wales to the north and west. The most densely populated region in the south-west of the United Kingdom, it is home to around 2.1 million people and includes the capital city of...

 up to the borders of Cardiff

Cardiff

Cardiff is the capital, largest city and most populous county of Wales and the 10th largest city in the United Kingdom. The city is Wales' chief commercial centre, the base for most national cultural and sporting institutions, the Welsh national media, and the seat of the National Assembly for...

.

Under the reservoir there are a couple of houses, which were flooded to make way for the water reservoir.