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Lakes

There are some 3,820 lakes in New Zealand with a surface area larger than one hectare. Much of the higher country in the South Island was covered by ice during the glacial periods of the last two million years. Advancing glaciers eroded large steep-sided valleys, and often carried piles of moraine (rocks and soil) that acted as natural dams. When the glaciers retreated, they left basins that are now filled by lakes. The level of most glacial lakes in the upper parts of the Waitaki and Clutha / Mata-Au rivers are controlled for electricity generation. Hydroelectric reservoirs are common in South Canterbury and Central Otago, the largest of which is Lake Benmore, on the Waitaki River.

The South Island has 8 of New Zealand's 10 biggest lakes. They were formed by glaciers and include Lake Wakatipu, Lake Tekapo and Lake Manapouri. The deepest (462 m) is Lake Hauroko, in western Southland. It is the 16th deepest lake in the world. Millions of years ago, Central Otago had a huge lake – Lake Manuherikia. It was slowly filled in with mud, and fossils of fish and crocodiles have been found there.