Also known locally as the Blessington Lakes, the reservoir was created when the waterfall at Poulaphouca on the River Liffey (which flows from the Wicklow Mountains to Dublin) was dammed by the ESB for a hydroelectric plant which is still in use today. The valley was flooded and the resulting lakes extend over approximately . A small village was submerged by the damming of the waterfall, and the remains of roads can still be seen leading down into the lake.