A visit to Lake Rotoroa in Nelson Lakes National Park is rewarded with a remarkable sighting.

Sea monsters are real enough; I have even caught one. Years ago a friend and I found a live oarfish (Regalecus glesne) stranded on a reef in Tasman Bay. This is one of the candidates for sightings of the Great Sea Serpent and at over 5m long it was certainly impressive. It was still alive though injured by its struggles on the rocks.

However the oceans are large enough to support populations of giant creatures, and new ones are still being found. The colossal squid is a fairly recent example.

Lake monsters however are supernatural beasts. Enthusiasts seem to imagine they are seeking a single individual that could hide in a large body of water. But animals exist as populations and no lake is large enough to support a population of giant creatures. Lakes are also ephemeral; very few are more than tens of thousands of years old; Lake Baikal is a rare exception to these rules.

However this has not stopped people from claiming sightings of monsters in lakes. They have suggested plesiosaurs (which died out around 65 million years ago) exist in lakes that are less than 20,000 years old. The Loch Ness Monster is perhaps the best known of all these myths. It was a deliberate fabrication or rather a series of fabrications for the simple purpose of making money. There is an extensive literature on the subject.

It is perhaps odd that New Zealand with a large number of lakes has had so few claimed sightings of lake monsters. The Taniwha is part of Maori superstition, but few claim to have seen one, much less describe it.

The overseas literature includes a large number of sightings in lakes which were thought mysterious by the observer. Otters, clumps of vegetation, ducks seen in mist, offer simple explanations for many sightings, but these are rejected by those of mystical leanings. One favourite is an odd number (usually three or five) of dark humps showing above the surface and apparently moving through the water. Anybody used to small boats will recognise this common phenomenon as the intersection of two wakes from craft that may not be visible.

However nearly 40 years ago I did see monsters in Rotoroa in Nelson Lakes National Park. It was a very close encounter yet for some time I could not identify what I was seeing.

We used to have an annual fishing trip to the head of Rotoroa, which is accessible only by boat or a walk of nearly 18km. One year we had a weekend of heavy rain; both the D’Urville and Sabine Rivers were unfishable and pouring muddy water into the lake. I fished the lake edge with some success on the Saturday but the next day, which was calm and cloudy, I decided to experiment. We drifted at the edge of the deep water and jigged. That is, we bounced small heavy lures vertically under the boat and close to the bottom. I had not brought my small portable echo sounder and we were not very successful because the technique works best where there is a sharp change of depth. The sounder is needed to find the right spots.

The water was gin-clear once away from the river mouth and we could see many vertical metres down, though not to the bottom which we discovered was about 60 metres below by measuring the length of line needed to reach it.

I was peering over the stern when a huge pale green-brown object rose out of the depths. It got to within a few metres of the surface then turned down and dived out of sight. I was literally speechless. Then another one rose. This time I looked for a head, fins, anything, but it seemed featureless. Down it went like the first but soon another came up – there was an endless procession of monsters.

I looked up and saw that we had drifted to about 250 metres directly off the mouth of the D’Urville River and the truth dawned. The monsters were waves of muddy water appearing much greener than the brown river when seen through the clear upper water of the lake. It was April (our trips were always at this time of year); the lake surface water was warm, but the river water very cold.

In summer, surface water warms up but unless the lake is shallow, the main body of water stays cold. The warm water floats on the denser cold water and there is often a sharp temperature gradient called the thermocline.

Under the calm conditions, dense, cold, river water was not mixing with the lake surface but flowing in below the warm water. What I was seeing were ‘thermocline waves’, a well-known phenomenon but one which is rarely so dramatically visible. Furthermore, relatively small, fast-moving thermocline waves like these are probably not terribly common. The lake surface was glassy, so I had not been thinking about waves.

This was a remarkably convincing monster display and I expected that eventually somebody else would spot this phenomenon and claim they had seen genuine lake monsters. I would then be able to counter with a rational explanation. However I have never seen such a claim either in NZ or overseas. Mind you I have not looked too hard; ‘Lake Monsters’ in a web search produces over 7.5 million results and it is clearly not worth wading through such a huge amount of nonsense.

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