Rivers are a defining feature of the New Zealand landscape. There are many, and in such a small country they are never far away – for example, as you circle the near-perfect cone of volcanic Mt Taranaki (Mt Egmont), you will cross, on average, one river every kilometre.
The South Island has 40 major river catchments, and the North Island has 30. (A catchment is the land area drained by a river.) Over 180,000 kilometres of rivers have been mapped, and this excludes thousands of small streams.
Mountains and rainfall
Mountains and weather are two major factors affecting the formation of rivers. Much of New Zealand is raised high above the sea, with mountain ranges running along the middle of the North and South islands. Buffeted by the winds of the roaring forties, New Zealand experiences intense rainfall where westerly airflows are driven across the mountains.
The annual 8–10-metre rainfall of Fiordland produces rivers of fearsome force and volatility, while similar rainfalls occur throughout the Southern Alps and feed the short, flash-flooding rivers of Westland.
Some moisture falls as snow, especially in the mountains, and is temporarily stored there. Much water from rain and snow seeps into the ground, to be gradually released back to the surface as it flows downhill in streams and rivers. Unlike many countries, which lose 70% of their precipitation through evaporation, New Zealand loses on average just 30%, ensuring that its river volumes in their natural state are relatively large.
Seasonal variations in rainfall mean that river levels change during the year. Most rivers on the east coast of both islands have lower flows during the dry summers. But in the South Island’s eastern snow-fed rivers, lowest flows occur during winter and spring, before the snow melts.
The headwaters of most of the major rivers in the South Island were once glaciers. During ice ages, glaciers moved down the mountains, carving out river valleys, and carrying down glacial gravels (called outwash). The Canterbury Plains were formed from outwash gravels, which also created the wide beds of braided rivers such as the Rakaia.
A river runs through it
The North Island’s Manawatū River flows through the central mountain range, rather than down it, which is the usual pattern. This is because the river was flowing there before the mountains were formed, and it kept to its course as they rose around it. The South Island’s Clutha and Buller rivers also flow through gorges formed by rising mountain ranges.
Earthquakes and volcanic activity can affect the course of rivers. Wherever faulting occurs and there is enough water flow, rivers will find the faults and cut down along these planes of least resistance. Maruia Falls, on the Maruia River in the Buller area, is a dramatic example of the effects of the Murchison earthquake of 1929. The river was diverted by a landslide during the earthquake, and cut a new channel that causes the river to cascade over a cliff edge.
Rocks and rivers
Rivers are influenced by rock type, which varies greatly throughout the country. For example, the Whanganui River cuts through relatively soft mudstones and sandstones, and its waters slide gently between steep, bush-clad cliffs of papa rock. In contrast, the Clutha and Kawarau rivers in Otago, which have their source in alpine snow and ice, tumble among hard schist boulders and river-worn gravel, through arid gorges.