In Canterbury the plain is what the mountain has lost

From the aircraft descending towards Christchurch, the region reads like a diagram. Snow-covered mountains to the west, a sea of green and yellow fields in the middle, and crossing all of it, from range to coast, grey bands one or two kilometres wide with threads of blue water winding through them. They are rivers, but they look like no river you know from Europe: the water takes up a tiny fraction of the bed. The rest is loose stone.

That stone is the key to everything. It is not there by chance and no farmer brought it: it is the mountain, broken up and carried away. Canterbury is not a plain with mountains in the background. It is a mountain that has changed address and a mountain still standing, with rivers acting as the conveyor belt between the two.

01 Ten millimetres a year

The Southern Alps are not an old, quiet range. They stand on the boundary between two plates, and the Alpine Fault, which runs along the western side of the island, pushes them upward. Displaced river terraces along that fault indicate an average uplift of ten millimetres or more a year over the past ten thousand years. A centimetre a year sounds like little until you multiply it: that is a metre a century.

And yet the mountains do not grow a metre a century. They grow far less, because they are being dismantled at the same time. Moist wind off the Tasman Sea hits the western flank and dumps enormous rainfall there; glaciers grind the rock; the slopes, made of a grey, brittle sandstone called greywacke, shed themselves as scree. Geological sources sum it up in a sentence worth keeping: the range wears away almost as fast as it rises.

That makes the Alps a machine for producing gravel. Aoraki/Mount Cook, the highest point in the country, is only the most visible part of a process that never stops: what is a summit today will be, in a span not measured in human lives, a pebble on a beach on the east coast. In the Hooker Valley, at the foot of the mountain, the beginning of that journey is already visible. What looks like a flat meadow between the slopes is a fill: gravel that has come down off the walls and that the river will keep pushing downhill.

02 Rivers that never settle on a channel

Three large rivers do the heavy work: the Waimakariri, the Rakaia and the Rangitata. They rise in the range, have glaciers at their heads and cross the plain to the Pacific. What is singular is how they do it. A European river has a channel; these have dozens, and they move them around.

They are called braided rivers. They carry so much gravel that they cannot transport all of it, so they keep dropping it, and in doing so they block themselves: the water splits, goes around the deposit, rejoins further down, abandons one channel and opens another. Seen from above, the current looks like a braid of blue threads across a grey band of stone. That band is the real river; the water is only the part that happens to be moving today.

Over time, that back-and-forth builds a fan of gravel at the mouth of each valley. The fans of the different rivers overlap, and the sum of all of them is the Canterbury Plains: about 180 kilometres long, some 750,000 hectares, the largest area of flat land in New Zealand. It is not quite flat: it slopes at an average of one in 132 from the foot of the hills to the coast, which is exactly the gradient you would expect from a fan of sediment. Boreholes on the northern plains pass through layers of gravel and sediment to about 170 metres down, laid mostly during glacial periods, when the rivers came down more heavily loaded.

The mountain sends more than stone. When the north-west wind crosses the range, it leaves its rain on the western flank and comes down to the plains dry and overheated. It is a foehn wind, and in Canterbury it has its own name: the nor'wester. With it, summer maxima regularly pass 30 degrees, and more than 40 has occasionally been recorded. It is the reason a plain that looks green from the aircraft needs irrigation across most of its surface during the growing season: the same wall of mountains that supplies the gravel keeps the water.

03 The island that stopped being one

South-east of Christchurch there is an exception to all of the above: Banks Peninsula. It is not gravel. It is volcanic rock, two adjoining old volcanoes whose craters the sea has flooded into the harbours of Lyttelton and Akaroa. From orbit, its rugged, dark outline stands out clearly from the chequerboard of fields around it, as if someone had pasted in a piece of another map.

In a way, that is what happened. The peninsula was an island. When James Cook passed this way in 1770 he charted it as one, and the mistake is understandable: from the sea, even today, it looks like one. But the plain had long been growing towards it. The gravel coming down from the Alps kept filling the arm of sea that separated it from the coast until it reached its base. According to the sources, it has been joined to the mainland for at least 200,000 years. The peninsula did not move towards Canterbury; Canterbury moved towards the peninsula.

The join left a legible scar. Between the peninsula and the plain lies a large, murky lagoon, Te Waihora or Lake Ellesmere, separated from the Pacific by a long, straight bar of gravel. It is the suture: where island and mainland met, the water was trapped between them.

This explains a feeling anyone has who drives up the Akaroa road from the plain. In a matter of minutes, the landscape changes nature: from kilometre-long straights between hedges to tight bends under green, rounded hillsides. It is not just that there is relief; it is that you have left the gravel and entered the rock. Crossing from one to the other, you are crossing the old shoreline.

04 A city that drinks from a river it never sees

Christchurch sits on the plain, a few kilometres south of the mouth of the Waimakariri. And here the gravel stops being a geological fact and becomes something people notice every day, without knowing it, when they turn on the tap.

The city does not drink from a reservoir or a surface river. It drinks from deep aquifers, and most of that water, around three-quarters, comes from the Waimakariri: it seeps through the river's gravel bed and moves underground towards the city at about 25 metres a day. The gravel acts as a filter. The result is water that for generations has been supplied without chlorine, and whose quality is checked with daily tests. Whether or not it should be chlorinated has been, in recent years, a local argument of an intensity that surprises outsiders. For someone from Christchurch, untreated water is not a luxury: it is part of their idea of the city.

But the gravel that gives the water also gives it back in other ways. In February 1868 the Waimakariri burst its banks and ran into the old channels that still crossed the plain; Victoria Square ended up under a metre of water. An engineer had warned of it years earlier. From then on embankments were built, and in the 1920s work began to confine the river and its branches to a single main channel. Today the Waimakariri runs held by stopbanks along the northern edge of its own fan. That you cannot see it from the city is a decision, not an accident.

And in 2011 it was the ground that remembered what it was made of. During the earthquake sequence, large parts of the city suffered liquefaction: young, loose, water-soaked sediments, laid down by the rivers very recently in geological terms, stopped behaving like solid ground. The damage clustered along the waterways and where the water table was less than one or two metres below the surface. In other words, in the same material that filters the drinking water. Christchurch lives on the youngest and softest end of the gravel that began coming down from the Alps hundreds of thousands of years ago, and what that gravel gives it and what it takes away come from the same place.

05 The dust that never settles

South-west of the plains, beyond the hills that close them, the Mackenzie Basin opens up. It is another landscape made by the same machine, but with a different part: here the agent was not the river but the ice. Sources identify at least four major glacial advances from the moraines they left behind. The three elongated lakes along the northern edge of the basin, Tekapo, Pukaki and Ōhau, formed when the terminal moraines of retreating glaciers dammed their valleys; in Tekapo's case, the moraine that closes it belongs to an advance of between 12,000 and 14,000 years ago.

What everyone remembers about Tekapo is the colour, an opaque turquoise unlike any mountain lake in Europe. It is not an effect of the sky or the lake bed. It is the finest fraction of the same erosion we started with: rock flour, particles so small that when meltwater brings them down from the glaciers at the head of the valley via the Godley River, they stay suspended instead of sinking. They absorb the shortest wavelengths of light, the water absorbs the longest, and what comes back to the eye is that greenish blue. In summer, when the melt carries more sediment, the colour intensifies.

In other words, the same process that fills the plains with coarse gravel fills these lakes with dust. What the river cannot drop along the way, because it is too fine, ends up colouring the water. If in Canterbury the mountain becomes ground, in the Mackenzie it becomes colour.

06 Where the shelf runs out

At the northern end of the region, Kaikōura offers the reverse of the plain. There is no room here for gravel to pile up, because the mountains come almost to the sea and, above all, because the sea is too deep too soon. Off the Kaikōura Peninsula a submarine canyon opens whose head lies at about 30 metres, drops to around 600 and continues to nearly 2,000 where it joins a larger ocean channel. Deep water lies roughly a kilometre from the shore.

That canyon is why Kaikōura is known for whales: sperm whales find abundant food here without travelling far from the coast. But it is worth reading it in terms of sediment too. What the northern rivers pour into the sea does not spread out across a shelf: it falls down the canyon. It is the gravel of the Alps leaving the region.

Nor does the range stay still at this end. In November 2016, a magnitude 7.8 earthquake lifted the seabed by between half a metre and two metres along some twenty kilometres of the Kaikōura coast, with points where the uplift was far greater. Satellite images showed a pale band of newly exposed seabed ringing the peninsula. The same force that pushes the Alps up ten millimetres a year did it here all at once, and left a new terrace that anyone can walk across: dried seaweed, shells and white rock where there had been sea.

07 What stays

There is one place where all of this comes together, and it is Arthur's Pass. At 920 metres, in the saddle between the Ōtira valley, which drops to the west, and the Bealey, which drops to the east, the road crosses the range that manufactures the region. The Otira Viaduct, 440 metres long and completed in 1999, passes over ground so unstable that no road could be built on it; the highway falls 495 metres between the summit and the village of Otira. On the far side, the rain; on this side, the river that starts to braid as soon as the valley opens. It is the factory seen from inside.

But the mark Canterbury leaves is rarely a summit or a lake. It is something stranger: the feeling of standing on a riverbed a kilometre wide, with a thread of water in the distance and grey stones to the horizon, and suddenly understanding that you are not on a plain with mountains behind it. You are on the mountains. The ones that have already fallen. And the water going past is bringing the next ones.