Feature

Our Underwater Amazon

New Zealand's longest river flows under the sea, from Kaikoura to the Kermadec Trench.

By K. B. Lewis

More than a century after the Waikato, the Amazon and the Nile appeared in school atlases, New Zealand marine scientists are still discovering major segments of the planet's drainage system.

East of New Zealand, they have found a submarine "river", with torrents, canyons, gorges, flats, meanders, levees and delta fans, that dwarfs anything in on-shore New Zealand. It is over 1,700 kilometres long -- four times longer than New Zealand's longest river -- and yet only two years ago, most of it was unknown.

Every poet knows that mountains will ultimately wash to the sea. News items show this happening during every big flood. Rivers that on summer days are just a series of placid pools, turn into torrents that tumble boulders along their beds. When the load of pebbles, sand and mud reaches the sea, most people think that is the end of it. The sea seems to be just one big settling pond, but it isn't. Mountains to sea is very much less than half the story.

The rivers that cross the Canterbury Plains have fallen 2,000 metres in the 200-300 kilometres from their headwaters in the Southern Alps. Their loads must still fall another 5,000 m and travel up to 1,700 km to a resting place in the great Southwest Pacific Ocean Basin.

What happens when the debris from the Southern Alps reaches the Canterbury coast? Firstly, southerly storms and ocean currents move it north towards Cook Strait. It has been estimated that nearly 10 million tonnes per year are moving along the inner continental shelf, but none of it ever reaches Cook Strait. The route is blocked by a canyon, nearly a kilometre deep, that cuts across the continental shelf almost to the beach just south of Kaikoura. This canyon is the famous feeding ground of whales. The longshore-moving pebbles, sand and mud pour into it and, we believe, build out as a rubbish tip or scree of debris.

Landslide!

The debris is stable only so long as its foundations are stable. Kaikoura is not a good place for stable foundations. It is in the middle of the zone of major faults that separates two of the world's major crustal plates. This is the place where the Pacific Plate, which goes all the way to Los Angeles, grinds into and past the Australian Plate, which goes almost to India. Periodically the mounting pressure is released violently, the foundations shake, and the heap of pebbles, sand and mud collapses catastrophically.

What happens when this occurs is illustrated by the records of breaking submarine cables after earthquakes off North America and Fiji. At least part of the loose mass is shaken into a great turbid slurry. The slurry is very much denser than seawater, so it roars down the submarine slope like water from a burst dam. Submarine cables breaking in succession suggest that the slurry can travel at up to 70 kilometres per hour, eroding the seabed as it goes and growing as it pulls cold water and eroded seabed into itself.

At many places in the world, such slurries pour from their canyons onto the nearest flat ocean floor. They slow and deposit their load over a vast area in a sheet only a few centimeters thick. At a very few places, the slurry is confined in a deep ocean-floor channel for hundreds or even thousands of kilometres. These channels are remarkable in that they show most of the features of major river systems on land. They are a major headache for submarine cable engineers.

While still in the Kaikoura Canyon, the turbid mass cascades 2,000 m downwards over a distance of only 60 km. Here, it gains much of the momentum it requires for its long journey. At the end of the steep canyon, it turns northwards into the Hikurangi Trough between the Chatham Rise and the North Island. This trough marks the line where the Pacific Plate dives down under the edge of the North Island. A tributary, the Cook Strait Canyon, adds sediments that settle from the strong tides near Cape Terewhiti.

Wider than a Mile...

As the Hikurangi Trough widens out, the main turbid flow is confined in a channel that has many of the characteristics of a great river channel crossing a flood plain. The classic example is the Mississippi.

However, this is even bigger, being 5-10 km wide and dropping at least 200 m below the surrounding plain. It meanders in great loops, with leveed stop banks on each side and wide overbank flats. The levees form naturally as the turbid flow overtops the channel, dumping its load where the flow suddenly slackens.

In the Hikurangi Trough, the left bank levee (looking downstream) is always higher than the right one, opposite to that of the Mississippi. The reason, according to physics theory, is the effect of the Coriolis force on the flow, which always pushes water to the left in the southern hemisphere.

For 750 km after leaving the Kaikoura Canyon, the Hikurangi Channel meanders between long-extinct volcanic seamounts, along the broad valley of the Hikurangi Trough. This valley is filled with over three kilometres of flat-lying mud and sand, derived mainly from turbid water that has overtopped the channel, just like the Mississippi Valley.

The channel gradually fades to less than three kilometres wide and less than 90 m deep as it flows northwards. Two years ago, we thought that the channel must either end as it merged into a submarine plain off Mahia or struggle on towards the depths of the Kermadec Trench.

Travelling On

Then, a survey to resolve this produced a surprise result. The channel doesn't stop, nor does it continue northwards. Off Mahia, it turns due east between two seamounts and becomes incised over 400 m into a wide submarine plateau. [Undersea River Found, Sept 1990]

Why it turns east is not clear. One theory is that a major collapse of the continental slope off East Cape blocked an old route to the north. Another idea is that the eastward trending part is the old part and the southern segment has been realigned by plate movements.

Late last year we tracked the channel for a further 600 km to the edge of the plateau. Near the outer edge of the plateau, it is cut through over half a kilometer but still leveed, which means that it is overtopped in flood. At the foot of the plateau-edge scarp, it empties into the flat Southwest Pacific Ocean Basin as an enormous delta-like fan. But even that isn't the end of it.

Flowing along the toe of the scarp is a deep ocean current of cold, nutrient-rich water from Antarctica that exceeds the flow of all onshore rivers put together. It is a major factor in controlling ocean circulation, climate and life on this planet.

The turbid flows from the Hikurangi Channel, now over 1,400 km from Kaikoura and little more than slightly-muddy water, are picked up by the Western Boundary Current and swept north again. A leveed channel running for a further 300 km along the toe of the scarp is a product of both waning turbid flows and the deepsea current. Finally, over 1,700 km after leaving Kaikoura, the diluted remnants of the once catastrophic slurries, wafted along by the deepsea boundary current, almost reach the edge of the Kermadec Trench.

The Hikurangi Channel is, as far as we know, the longest submarine channel in the southern hemisphere and one of fewer than half a dozen such features worldwide that are still active today. The deposits from ancient deepsea channels, when scraped off the conveyor-belt ocean floor as happens today off the Wairarapa coast, are the building blocks of new land and of mountain ranges throughout the world.

Finding the end of deepsea drainage systems may be thought of as being asmuch an academic exercise as finding the source of the Nile was a century ago. It will probably remain just part of the understanding of our planet until the burgeoning use of computer links, fax machines and video telephones forces a telecommunications cable link on the direct, great circle route to the Americas. At that time the engineers will have to take into account the rivers and mountains of the undersea world.

Keith Lewis is a marine geologist with the New Zealand Ocean-ographic Institute.