Situated in mid-Canterbury, 60 miles west of Christchurch, Lake Coleridge is 11 miles long, up to 3 miles wide, and has an area of 18 square miles. Its maximum depth is 680 ft. It lies in the eastern foothills of the Southern Alps, but no major river enters or leaves it. This is because it lies on a depression formed by an excavation of rock and morainic deposits by a minor tongue of ice from the Wilberforce Valley during the latest major ice advance of the Ice Age, whereas the main ice and subsequent main drainage joined the Rakaia Valley. The lake is 1,667 ft above sea level, and about 450 ft above the Rakaia Valley 2 miles to the south. Advantage was taken of this difference of level in designing the first major hydroelectric power scheme in New Zealand which began operation in 1915. Subsequently the capacity of the station was increased to 34,500 kW. Water is diverted from the Harper River, a tributary of the Wilberforce, into the head of Lake Coleridge to provide sufficient water for the power station.
by Richard Patrick Suggate, M.A.(OXON.), D.SC.(N.Z.), F.R.S.N.Z., New Zealand Geological Survey, Christchurch.