Reservoir dam overflow spillage. At times of high rainfall water is spilled over the top of the dam wall of Pen-y-Garreg, one of the Elan valley group of reservoirs in Wales, UK. Built in the late 19th century to supply drinking water to the industrial city of Birmingham it is still an important component of water supplies to Wales and some adjoining English counties. Dwr Cymru Welsh Water, are a not-for-profit organisation with 90 dams and reservoirs in Wales.