We drank the river for six days straight.

During the day, thirsty from paddling the rafts, we dipped red enamel mugs into the river and gulped down the contents. In the late afternoon, we collected water in buckets from the river's edge for the cooks to use. Some of it was allocated for hand-washing, dish-washing and teeth-cleaning.

Through each of the six days of our Clarence River raft trip, the water was glassy clear and highly potable - a resounding endorsement for Tourism New Zealand's "100 per cent Pure" slogan, which is an absurdity from many other environmental perspectives.

When the sun shone, as it did most days, the river took on a sheen of aquamarine blue that seemed to reflect its glacial origins, but thankfully not the icy temperatures of the headwaters above Lake Tennyson.

Head guide Bridget Jessep had told us at the safety briefing that mishaps such as capsizing and falling overboard were unlikely as the Clarence was a Grade 2 river.

What we should be wary of was not a ducking but - believe it or not - dehydration.

Apparently it is the most common hazard for rafters. You could be paddling for a few hours, mesmerised by the diverse, slowly passing landforms, and completely forget to drink.

Each raft had an enamel mug attached to a rope by a karabiner. "Drink often," said Bridget. "Even if you think you don't need to."

So we drank the river. Would you go down to your local river or stream and do that, day after day? Contaminants, farmland run-off, giardia and other nasties are rife these days, but the lack of stock or people along the Clarence appear to keep it drinkable.

In the autumn, when the river is often low, it still flows fresh and powerful. By the sixth day, with no complaints of a sore tummy from anyone and only minor muscular discomfort despite a heck of a lot of paddling, we began to feel that the water really did possess a life force. This is no ordinary river. Besides its exceptional water quality, its geological setting and isolation are standouts.

The distance from the raft put-in at the Acheron River confluence, in Molesworth Station country, to the Clarence's mouth north of Kaikoura, is about 180km to 200km.

For urban dwellers with no previous rafting experience, there are undoubtedly bragging rights at the end, for this is a journey into wilderness, a physical challenge and a superlative outdoor learning experience. The rewards are immense.

In addition to the legendary Molesworth, the river passes through three other back-country farms: Muzzle, Bluff and Waiau-Toa stations.

Our guides tell us how the farms operate and how far they lie from tarseal. Only one family lives close to the Clarence, the Nimmos, who farm Muzzle Station. It has been going for 150 years, long before the first geological survey pronounced the Clarence something of a natural wonder.

The river occupies a trench - the trace of an old fault line - between the Seaward and Inland Kaikoura Ranges, which reach an arresting height of 2884m at the fabled peak of Tapuae-o-Uenuku. For much of its journey to the sea, the Clarence is wetting the fault's whistle and its taniwha-like writhing makes a raft trip a lot longer than the flightpath of a shelduck or shag down the valley.

Around just about every corner is something new - a different rock pattern, perhaps, or a new plant to try to identify.

Hundreds of rapids and riffles arrive in quick succession, but often the next rapid can't come soon enough for aching arms if the winds are against you.

After two hours, we come to the first riverbank camp on a terrace above Dillon Creek and, in less time than it takes for the autumn dusk to curtain the camp, the tents are up and sleeping bags and mattresses unrolled, the campfire is going for it, the wine is waiting for our botanists to return from a recce, and the kitchen is preparing the first gourmet meal of the trip.

Between the blue cod/risotto main course and the chocolate brownie dessert with icecream, there is a cabaret act. Three possums pop up at the top of the low cliff above us, their red eyes twinkling in the firelight. They watch us silently before scuttling off. Who said possums need trees?

Next morning, which is sunlit with a hint of frost, the first gorge awaits with a rapid known as the Chute. We don helmets for our first burst of white-water excitement and exit backwards into a run of smooth water.

Soon afterwards, we get our first look at rock walls decorated with wavy lines of greywacke and sedimentary rock, the product of underground pressure-cooking aeons ago, plus uplift and tilt in more recent times.

The ever-changing landforms are as surprising as the gourmet tucker. In the days ahead, we come upon large spherical concretions lying at the river's edge, the Clarence's own version of the Moeraki Boulders - sandstone cliffs, a zone of creamy chalk and limestone, and a river-diverting natural quarry of gravel that has been dislodged from the cliffs above by earthquakes.

The further we go, the more diverse the birdlife. Black shag, paradise shelduck, pipit, harrier, welcome swallow and Canada geese (the latter too numerous for the farmers, who blitz their flocks from time to time) are soon joined by the forest birds - bellbird, grey warbler, silvereye and, at a manuka campsite on the second-to-last night, the South Island robin.

The river emerges from a richly forested gorge that has taken us two days to negotiate and suddenly we are among coastal plains facing a salty wind from the east.

Under the highway and railway bridges, the river rushes. There is a short breather before it meets its destiny, the Pacific Ocean.

Half an hour's drive north of Kaikoura on State Highway 1, motorists speed across the frothing Clarence River and probably think it's like their river back home, a bit polluted. But this is no ordinary river. You can drink it.

FURTHER INFORMATION

In summer and the shoulder months, the Clarence River hosts numerous rafting and kayaking parties.

The largest commercial rafting operation is Clarence River Rafting, which was set up by Ben Judge and his wife, Sandie, 12 years ago at the little settlement of Clarence, north of Kaikoura. Ben was a furniture-maker before he thought of rafting adventures on the river. Sandie produces some of the gourmet food, including bacon-and-egg pie and to-die-for chocolate brownies. Trip options range from half a day to six days.

Neville Peat paid his own way down the Clarence River.