Megget Reservoir is an impounding reservoir in the Megget valley in Ettrick Forest, in the Scottish Borders. The 259 hectares (640 acres) reservoir is held back by the largest earth dam in Scotland. The reservoir collects water from the Tweedsmuir Hills, which is then conveyed 28 miles (45 km) via underground pipelines and tunnels to Edinburgh. The pipelines are routed through the Manor Valley and the Meldon Hills, to Gladhouse Reservoir and Glencorse Reservoir in the Pentland Hills. These two reservoirs store the water until such times as it is required. Excess water which overflows from the reservoir is returned to the Megget Water, and hence into St. Mary's Loch.

Source: Wikipedia