It is retained by an earth dam, and it was built between 1820 and 1824 by James Jardine to provide water for the mills of Auchendinny, Milton Bridge and Glencorse, and to supply drinking water to the citizens of Edinburgh.

The dam is 23.5 metres (77 ft) at its highest point, one of the tallest in Britain when it was constructed, and was built at a point where a spur of rock narrowed the channel of the Glencorse Burn, which caused great difficulties in its construction.

The gravel bed on which the burn flowed was up to 16.2 metres (53 ft) deep and when this was removed to create a clay-puddle dyke, the hill on the south side collapsed.

[2] A Water Treatment Works was opened at Glencorse in 2012[3] to replace aged facilities at Alnwickhill and Fairmilehead and treat water from Talla, Fruid and Megget reservoirs.

[4] Beneath the surface of the reservoir are the remains of St Catherine's of the Hopes, a 13th century chapel.