Several weeks ago I was fishing a river in the South Island of New Zealand with Melbourne friends Ian and James. I’ve been asked not to name the river in question, but I can describe it. Flowing down from glaciers not far from Mt Cook, it is simultaneously an outrageous blue colour, and see-every-pebble-two-rod-lengths-down clear. The mountains on either side are so steep and so high that you actually have to tilt your head back to see their tops, and yet the valley floor is a kilometre wide and almost dead flat, having been neatly filled by a few million years worth of glacial debris.

Like so many South Island rivers, this one is braided, with several distinct channels weaving a complicated path across the gravel and rubble. Some channels are little more than creeks, others are almost too powerful to cross. And in common with many glacial rivers, this one is classified as ‘unstable’, with anabranches, riffles and pools being destroyed and re-created with every flood. Unstable rivers are generally regarded as poor trout habitat and indeed, all the relevant guidebooks claim that the river I’m talking about holds few trout. But I wonder how many of the said guidebook authors have actually fished this river, because while theory would suggest a low trout population, in practice it holds good numbers of rainbows, many of which are very big.

We visited the river on the last day of a trip that had turned out so well, we could afford to gamble the final session on a water that the books said was bad but a quiet tip said was good. Ian and James hooked into good trout almost straight away. In my haste to catch a fish of my own, I raced off to a downstream anabranch. In hindsight, I should have stopped for a minute or two and carefully observed exactly where and how my successful companions were fishing. This mistake cost me two hours of fishlessness, and an almost terminal loss of confidence. But that’s another story.

By mid afternoon I had finally worked out that (a) the big rainbows were holding surprisingly, in the quietest sections of what was generally a boisterous stream; and that (b) repeated presentations with perfect drift were much more important than fly choice. Which brings me to the first of the three trout that deserve special mention.

I was working the current seam of a deep blue chute with two tungsten nymphs, 1.5 metres below a snow-white shuttlecock indicator. Just as the flies drifted by, a red-sashed rainbow, side-lit by the April sun, rose up from the bottom and very deliberately ate the indicator. Somehow I managed to avoid the urge to strike, and after some puzzled chewing, the fish ejected the indicator and sank back into the depths.

Now, all fly fishers know the hard truth about trout that eat indicators: no matter how quick the change to a dry fly with a hook in it, no matter how inspired the fly choice, and no matter how perfect the subsequent presentation; never, in all fly fishing history (well, at least my history) has an indicator-eater subsequently been caught on a dry fly. In fact you are lucky if you ever see them again.

Yet for some reason, perhaps I had been drugged into a state of absurd optimism by the splendour of the valley, I sat down on the gravel, removed the nymphs and indicator, and replaced them with a single Shaving Brush dry. I then cast the solitary Shaver up along the seam, and watched as it floated down, gently rocked by dissipating wavelets from the rapid further upstream. Then in exactly the same place as before, the big rainbow glided up through the blue water, stuck its nose out, and ate the fly. I lifted, and after the obligatory minutes of pandemonium that you get from 5 pound rainbows in 9°C water flowing at several knots, I beached it. The day was certainly turning around!

The remainder of the pool failed to produce, but above the next rapid, the best looking stretch of the river came into view. Here, the channel I was fishing had angled all the way across the shingle, to literally slam into the slope of mountains on my right. Therefore, unlike most of the river, this section was bounded on one side by a steep, rocky bank, complete with grass and shrubs. And while the main force of the current had plainly tried to bully its way along this bank, this very action had undermined a series of boulders, which had toppled into the current every few metres. Thus the right side of the pool, which I rock-hopped towards in the late afternoon light, conformed perfectly with the traits the local rainbows apparently liked. Not only did the right-hand side of the pool feature a perfect feeding lane curling along about a metre from the bank, but the full force of the current had been deflected away to the middle of the river. Any fish in this strip near the bank could utilise the cover of numerous submerged and semi-submerged boulders, while also sheltering from the main current and enjoying a constant drift of food.

Guessing at the erratic depth, I changed set-up again, this time suspending a single tiny tungsten nymph beneath the Shaving Brush. It only took two casts before the Shaver, silhouetted against the silvery surface, slid under. I struck into the heaviest trout of the day. The mighty rainbow leapt clear of the water like a migrating salmon, then crashed back down again, sped upstream through the boulders, cartwheeled through the air...and the line went slack.

So exhilarating was the display that instead of cursing the loss, I found myself laughing out loud. What a fish, and good luck to it! I began winding in to check if the flies were still there, when a big trout flashed straight past my feet...but was that a Shaving Brush trailing a couple of feet behind? It took a moment to register, the rainbow was still on my line! Stripping like mad, I soon had contact, and the battle was resumed. The trout tore upstream again, then U-turned down, leaping clear of the water so many times I lost count. Eventually, after stumbling back over the rocks I had taken such care to walk up earlier, I managed to beach the rainbow about 200 metres downstream. It weighed 6 pounds and turned out to be my best fish for the trip.

Feeling in equal parts exhausted and elated, I eventually staggered back up to the point where I had first hooked the 6 pounder, and resumed searching. A part of me (possibly the legs and feet part) actually hoped the flies would be left alone, at least for the rest of the pool! But it wasn’t to be. On the very next cast, the Shaver had drifted just a metre when a dark snout appeared and confidently clipped it off the surface.

This trout wasn’t quite as big as the previous one, but it still pulled mightily, threatening to once again send me plunging from rock to rock downriver. Just when I was contemplating whether I should do this, or stand my ground and hope, I saw two eels. Each was close to 2 metres long, and as thick as my calf. Apparently alerted to the 4 pounder on the end of my line, they cruised menacingly about a rod length out into the river. Sure enough, as I finally brought the rainbow close enough to land, the monstrous eels raced towards it. I literally started kicking them away, but they barely seemed to notice, although I swear one of them gave me a look that said ‘I wouldn’t do that again if I were you’. Somehow I managed to tail the trout before the eels could, and lifting it while still lashing out with my feet, I ran a few metres downstream. I scanned the clear water for a moment before releasing the fish, and was gratified to observe that the eels appeared to have gone. I only hoped they weren’t clever enough to be hiding beneath the boulders, ready to engulf the poor rainbow as it swam off.

By now the sun had long disappeared from the valley, offering only the faintest pink tinge to the icefields high above. More than half of the bountiful edge still lay before me, but I hooked the Shaving Brush onto the stripping guide, wound in the slack line, and began striding through the crisp autumn evening towards the distant car.