Lakes are formed when water lies in hollows in the land surface, usually due to faulting, warping or both; to recent volcanic action, or to glaciation. In coastal areas lakes are often found filling hollows in sand dune country or where bay mouths have been blocked by alongshore drift of shingle or sand. Lakes of all these types occur in New Zealand, but by far the largest are those in the volcanic interior of the North Island and in the recently glaciated South Island high country. One of the larger lakes (Waikaremoana) was formed by the blocking of the narrow channel that a river had cut across a high-standing ridge of the eastern scarpland of Hawke's Bay.
Lakes of the North Island
Taupo, in the upper catchment of the Waikato River, is the largest New Zealand lake, set almost in the very heart of the North Island. From it a long fault-bounded depression runs north-easterly and contains another large group of lakes filling hollows in the volcanic plateau. Here is the main hydrothermal district. Lakes of this latter group are Rotorua, Rotoiti, and Rotoehu, in the immediate neighbourhood of Rotorua itself; and Tarawera, Rotokakahi, Tikitapu, Okareka, Rotomahana, Okataina, Rotomakarui, and Rerewhakaaitu somewhat to the south-east of it. The lakes about Rotorua lie mainly within the catchment of the Kaituna River, while those of the south-eastern group overflow to the Tarawera.
All these lakes occupy local depressions associated with local warping and faulting, most, if not all, having also been the scene of violently explosive eruptions that have enlarged and shaped their basins. The most spectacular example of the type is Taupo itself. Its surface area is some 234 square miles; its depth is 350 ft in the southern part and 400 ft in the north where four local depressions are 50 to 100 ft deeper. Motutaiko Island and various hard-rock reefs stand up from the eastern floor while part of the western shore is lined by cliffs nearly 1,000 ft high. All these features seem to be associated with local extensions of rhyolitic lava. Though the water occupies only the lower part of a large tectonic basin, warping and faulting alone seem not enough to account for it. For miles around, the country is mantled with the pumice waste of the Taupo ash showers that bear witness to the violence of many explosive eruptions. The form and distribution of these indicate the site of the lake to be their source.
Recent studies also indicate that eruptions on a vast scale must have occurred within the last 2,000 years or so. The lake level seems to have been raised at least 100 ft, only to be lowered again by the rapid downcutting of the Waikato River outlet. This would be delayed where the outflow stream found the hard shelves of buried ignimbrite that form the Huka Falls and the Aratiatia Rapids.
On or about a line from Ruapehu to White Island, through the sunken lowland that contains all these lakes, most of the hot springs, geysers, and other hydrothermal features occur. With the lakes, they are all directly associated with the volcanic activity that accompanied the raising, warping, and faulting of this central part of the North Island.
In the North Auckland Peninsula are the small Lakes Takapuna and Omapere. The former, near Auckland city, lies in a small volcanic explosion crater, while the latter occupies a basin formed where an old lava flow obstructed the course of the Waitangi River.
Lake Waikaremoana, about 12 miles long, 6 miles in greatest width, and of very irregular outline, was formed by the drowning of a branching river valley system in its own water. The narrow gap cut by a tributary of the Wairoa River across a high ridge in the hills of eastern Hawke's Bay was blocked by a huge landslide falling away from the steep inner scarp face.
The only other notable lakes of the North Island are those in the lower valley of the Waikato River (Waikare and Whangape), together with a few small coastal lakes behind ridges of sand (as near Levin and Kaipara) or shingle (as Palliser Bay). The largest of these is the shallow Wairarapa lake.
South Island Lakes
Most of these are found in the high hard-rock country lately eroded by ice. In almost every major river valley are lakes formed by the ponding of water behind piles of glacial moraine or fans of shingle brought down from tributary valleys in the high bordering ranges. Perched high on the mountain slopes, too, are many minor lakelets or tarns, where water lies in ice-scoured hollows. Another type of little lake is often found in hollows in the moraine unevenly spread over the West Coast lowland. Indeed, the only notable lakes outside the glaciated highland are Waihola on the valley floor of the lower Taieri River, and the shallow coastal Lakes Ellesmere, Forsyth, and Grassmere, enclosed by barrier beaches of shingle swept across the front of former bays.
For convenience of summary, all the high country lakes may be grouped into those lying to the east and to the west of the main Alpine divide, and those in the northern valleys.
Almost every major river valley east of the divide has its glacial lakes, large and small, all of the same general type. Best known for their size and scenic grandeur are, of course, the so-called Southern Lakes of Fiordland, of the upper Clutha Valley, and of the Mackenzie basin in the upper catchment of the Waitaki.
The lakes of Fiordland lie along the eastern margin of the plateau. There are Te Anau, Manapouri, and Monowai draining to the Waiau River, and Hauroko and Poteriteri draining to the south coast by the valleys of lesser streams. All these are a legacy of the scouring, shaping, and deepening of highland valleys by ice and the ponding of the water by piles of moraine. From Te Anau and Manapouri long, narrow trough-shaped arms reach far back into the plateau itself, and similar troughs contain the waters of Monowai, Hauroko, and Poteriteri; all these with their high, steep enclosing walls might aptly be described as freshwater fiords.
In the Clutha Valley are the noble lakes of Wakatipu, Wanaka, and Hawea. Their outlet streams join at Cromwell to make New Zealand's largest river, the Clutha. Similarly, the upper catchment of the Waitaki contains the big lakes of the Mackenzie basin – Ohau, Tekapo, and Pukaki, all destined to contribute to the generation of electric power.
All the valleys of the main Canterbury rivers flowing east from the main divide contain glacial lakes, large and small. Perhaps the more notable of these are Heron and Coleridge, in the Rakaia, and Sumner in the Hurunui. Besides these, however, there is a host of lesser lakes. There are Taylor, Sheppard, Katrine, and Mason in the Hurunui; Pearson and Grasmere in the Waimakariri; Evelyn, Selfe, Catherine, Ida, and Lyndon in the Rakaia; and Clearwater, Howard, and Acland in the Ashburton, to note but a few.
The largest of the West Coast lakes are Brunner in the valley of the Arnold, tributary of the Grey, and Kaniere in the valley of the Hokitika. Farther south, in hollows in the heaps of glacial waste from the bordering ranges, are many picturesque little lakes; best known of these are Mapourika and Ianthe.
Most notable valley lakes of the South Island north are those in the upper catchment of the Buller. There are Rotoroa (8 square miles) and Rotoiti (2¾ square miles). Tennyson, in the upper valley of the Clarence, is a small lake of the same general type.