New Zealand being a narrow and mountainous country has short, swiftly flowing rivers of little use for navigation; most of those that traverse regions of hard-rock highland carry coarse shingle right to the sea. Stream systems adjust themselves both to slope and to structure of the country off which the water runs, as is very well shown by the pattern of channels that drain the western salient of Taranaki. Here is a simple pattern of radial streams all rising at or near the summit of Mt. Egmont. Yet another distinctive pattern is that of the freely branching streams of the Wanganui – Taranaki inland district where the country is made mainly of the weak mudstones locally known as papa. In sharp contrast again is the trellised pattern of those Hawke's Bay streams that cross parallel ridges of harder rock to reach the sea.
An almost ubiquitous feature of the valleys of New Zealand rivers is the occurrence of series of terraces left as the streams have cut their valleys downward. The history of these is rather complex and is related to such factors as intermittent uplift of the land and changing level of the sea in glacial and interglacial periods. Most of the minor features of the land have been made by the rivers that have dissected it.
North Island Rivers
(The approximate length of rivers in miles is given in parentheses.)
Deep in the interior of the North Island is the large dome-like highland complex of the volcanic mass of Ruapehu, and the Kaimanawa, Kaweka, and Ahimanawa Ranges of the hard-rock eastern axis. From this high and rugged region comes the run-off water that feeds the larger master streams, Waikato (220) and Rangitaiki flowing north, the Rangitikei (115) and Whangaehu (85) flowing south, and the Ngaruroro (85), Tutaekuri and Mohaka (80) flowing into Hawke Bay. To this wide bay, too, come the Tukituki (65) from the Ruahine highland, and the Wairoa (50) from the Huiarau Range.
From the Huiarau and Raukumara Ranges of the north-east emerge the Waipaoa (50), the Waiapu (55), the Motu, the Waioeka and the Whakatane (60). Of these, the Waipaoa is of special interest for its wide, open delta plain on which Gisborne is based. As a result of the deforestation of the steep slopes of the bordering mudstone hills, flood control for the protection of the plain has become a problem of first concern.
The Waikato, stabilised in flow by Lake Taupo, the site of hydroelectric plants, and containing in its lower valley (and that of its tributary, the Waipa) much rich farming land, is New Zealand's largest and, perhaps, most important river. Just east of and parallel to it, the Waihou (90) and Piako (60) traverse the former swampland of the Hauraki Plain to reach the Firth of Thames.
The largest river of the North Auckland Peninsula is the (Northern) Wairoa (95) which, rising in the hills of the north east, flows into Kaipara Harbour. Its valley gave access to the great kauri forests now cleared away. The western rivers are established in deep, branching trenches cut in the mudstone plateau of the Wanganui-Taranaki inland; the largest of these is the Wanganui itself (140) which rises in the high interior of the Central Plateau about Ruapehu. Similar in type are many other streams of which the Mokau (75), Waitara (65), and Patea (65) are the largest.
East of the main divide is an important group of three streams: the Tukituki (65) flowing to Hawke Bay, the Ruamahanga (70) to Palliser Bay, and the Manawatu (100) that crosses the divide by the Manawatu Gorge to reach the west coast near Foxton. These rivers together occupy the Hawke Bay – Palliser Bay lowland, one of the most significant features of the North Island landscape.
As the mountain axis nears Wellington, west coast streams draining from it are short and steep of gradient; such are the Ohau, Otaki, and Waikanae. The Hutt (35) flows in a fault-angle valley into Wellington Harbour.
South Island Rivers
All the northern part of the South Island is a complex of huge crust blocks bounded by faults. These fracture lines guide the main rivers; the only one flowing across the grain of the country by a series of narrow gorges is the Buller (105). So the pattern of rivers is very simple. Into Golden Bay flow the Aorere (45) and the Takaka (45); to Tasman Bay the Motueka (75) and Waimea; and to the north-east coast the Wairau (105), Awatere (70), and Clarence (125). Of special significance are the valleys of the Wairau and the Buller; together they provide the one easy routeway across this mountainous country from Blenheim to Westport.
Of the West Coast rivers much the largest is the Grey (75); the others are short swift torrents descending from the Alpine axis to cross the West Coast lowland in wide terraced trenches. Such are the Taramakau (45), Hokitika (40), Wanganui (35), Whataroa (35), Waiho, Karangarua (30), Haast (60), and Arawata (45). Making and maintaining the roadway across these to South Westland is a continuing problem. By contrast, the big North Canterbury rivers, the Waiau (110) and the Hurunui (90), cross more open range and basin country, having cut narrow gorges across the hard-rock ranges that enclose the basins.
The Waimakariri (93), Rakaia (95), Ashburton (67), and Rangitata (75) still have minor glaciers at or near their sources, these being puny remnants of the great tongues of ice that must have filled their valleys but a few thousand years ago. These are the main rivers that built the Canterbury Plain of shingle brought from the high country. In the plain itself they flow in wide terraced channels along courses subject to capricious change. Of special interest to the nearby city of Christchurch is the control of the lower Waimakariri. There are smaller rivers in mid and South Canterbury such as the Waipara, Ashley, Selwyn, Orari, Opihi, Pareora, and Waihao, but these are rather “front country” rivers fed to flood mainly by rains from an easterly quarter in contrast to the nor-westerly storms that flood those of the back country.
The great rivers of the south are the Waitaki (135), draining the glacial Lakes Tekapo, Pukaki, and Ohau of the Mackenzie basin; the Clutha (210) with its Lakes Wanaka, Hawea, and Wakatipu; and the Waiau (115) draining Te Anau and Manapouri. The catchments of all these three major rivers reach back to that highest part of the Alpine divide that extends from Mt. Cook to Mt. Aspiring. From the upper catchment of the Clutha, the Lindis Pass leads to the Waitaki and the Haast Pass to South Westland.
Other important southern rivers that do not reach as far back as the main divide are the Taieri (125), Mataura (120), Oreti (105), and Aparima (65). Waste brought down by the last three has made the extensive Southland Plain.