Hundreds of lakes ranging in size from small ponds to bodies of water tens of miles long are scattered throughout the South Island, over an area which was ice covered during the later part of the Pleistocene era, about 20,000 years ago. This area extends from the main divide in South Nelson near Lakes Rotoiti and Rotoroa to the south coast of Fiordland west of the Waiau River mouth. Its eastern margin follows the eastern foothills of the Southern Alps through Canterbury to the Mackenzie basin and the upper Clutha Valley to northern Southland. Its western margin is close to the west coast from Greymouth south to Fiordland. Most of these lakes owe their permanence and great depth to over-deepening of valley floors by glaciers which are able to excavate depressions far below the level of the bedrock at the outlet end of their valleys.
The following large inland lakes occupy over-deepened valleys excavated by valley glaciers: Brunner, Coleridge, Hawea, Manapouri, Monowai, Ohau, Pukaki, Te Anau, Tekapo, Wakatipu, and Wanaka (qq.v.). Other large inland lakes in this category are Lakes Rotoroa and Rotoiti in Nelson, Lakes Alabaster and Wilmot in western Otago, and Lakes Hauroko and Poteriteri in west Southland. Most of these lakes have nearly flat beds of sand and mud and probably reach maximum depths of about 500 ft.
Of the great southern lakes of Canterbury, Otago, and Southland, those of Pukaki, Tekapo, Ohau, Hawea, Wanaka, Wakatipu, and South Mavora all have prominent ridges of terminal moraine encircling their southern extremities. These indicate that the lakes occupy the sites of late Pleistocene valley glaciers. During the early stages of melting back of the valley glaciers, the early lakes, impounded behind the terminal moraine barriers, probably stood at higher levels than at present and supported the floating ice tongues of the retreating glaciers. Clear evidence of older high lake levels can be seen in the strand lines and benches around Lake Wakatipu. Outlet streams cut progressively deeper valleys through the unconsolidated glacial drift, thus lowering the lakes to their present levels. In the case of Lakes Wakatipu and Hawea, this lowering process has been greatly retarded by the outlet streams exposing in their beds comparatively hard bedrock which has formed resistant sills at the lake outlets. Lake McKerrow, at sea level at the mouth of the Hollyford River in north-west Otago, is connected to the sea by a sinuous tidal river cutting through beach deposits and sand dunes, through which small ocean-going fishing boats can reach the lake. Lake McKerrow is very similar in origin to the glacially excavated fiords further south. Sutherland Sound is similarly connected to the sea by only a tidal river. Lake Hakapoua in southern Fiordland was also once reached by boat from the sea, but in recent years has been raised a little above sea level by a landslip in the outlet valley.
A very large number of smaller lakes and ponds dot the glaciated areas of the South Island, ranging in height from a few hundred feet above sea level almost to the summits of peaks 6,000–7,000 ft. Most of the higher ponds occupy the floors of cirques, that is, amphitheatre-shaped depressions on mountain slopes abandoned by ice comparatively recently in geological time. The water is retained behind iceworn sills of rock at the outer edge of the cirques across which outlet streams usually run in a shallow notch. Some of these small bodies of water have been named as lakes. They are particularly plentiful in the massive granitic rocks of Fiordland and in the highly metamorphosed schists of West Otago and South Westland. Lake Quill, west of Mackinnon Pass on the Milford Track and the source of Sutherland Falls(q.v.), is perhaps the best known New Zealand lake of this type.
The Maoris had many interesting legends to account for the origin of these glacial lakes, one of the most fanciful being the tale of the exploits of Raikaihaitu, the great digger of lakes. According to the legend, when the chief and his followers arrived in the Uruao canoe (c. 850 A.D.), they explored the interior of the South Island, naming the various lakes they met. Raikaihaitu had brought with him from his former home in the tropics as long ko or wooden spade, and with this he dug “the wells of Raikaihaitu”. The names of the lakes are Takapo (Tekapo), Pukaki, Ohou (Ohau), Hawea, Wanaka, Whakatipu-wai-maori (Wakatipu), and Whakatipu-wai-tai (McKerrow). Legend affirms that Wakatipu was the most difficult one to dig because of its great depth, its rocky surroundings, and the height of the adjacent mountains. Only after strenuous exertions and many invocations (karakia) was Raikaihaitu able to complete the task. He next went on to Te Anau and down the Waiau River till he reached the southern limits of Murihiku. He then returned along the east coast as far as Banks Peninsula.
by lan Charles McKellar, M.SC., Geologist, New Zealand Geological Survey, Dunedin.