Ohinemuri Regional History Journal 25, November 1981
By C. J. Gwilliam
Before embarking on such a subject one must delve into the past and marry the basins of both the Waihou, Piako and Ohinemuri Rivers, for all three had a major part in filling the vast heavily wooded valley that once stretched between the Colville Range and the westward hills. Today we refer to the lower half of this area as the Hauraki Plains, but there are still some who remember it as an almost impossible swamp, mostly at or below water level.
To arrive at its formation we must go back in aeons of time, when the Waikato river flowed more or less gently via Hinuera, Te Poi, and Te Aroha, and out into the Hauraki Gulf.
Then, came the Taupo cataclysm, a massive out-throw of pumice which clogged the path of many streams and changed their courses. The Waikato was one. As the crater (Lake Taupo) filled, a new overflow formed and the new Waikato eased its way around the northern edge of the pumice field and followed a new course to its destination - the sea.
That it followed the outer edge of the pumice deposit became evident at the time of the near disaster at Arapuni when the spillway uncovered the remains of a vast primeval forest. Here was shown the devastative power of water when unrestricted.
A vital question now surfaces here. Supposing another Taupo outburst, or even an earthquake of Napier severity were to occur in the vicinity of the upper or any of the Waikato Hydro Dams, what of the towns and districts below. These are thoughts to be kept well in mind when attempting to restrict any natural flow of water. It is the natural phenomena of water to pursue its way to the lowest point of finality.
Let me now turn to our own area, the basin of the Ohinemuri River - its origins, its travails, its troubles. We must go back once again to bygone ages, long, long before the Maori even began to settle in the far off Pacific Islands. We are back in the age when the Colville and Kaimai Ranges formed a continuous backbone from the Great Barrier to almost the East Cape.
From small beginnings beneath the sea rose the volcanic mass of Mayor Island (Tuhua), with massive earth movements causing the shattering and displacement of the "back-bone". That this so very evident from the broken geological pattern within a three to five mile radius of the Karangahake Gorge.
It is not generally known or realized that at one time, after this land movement, that the Waihi Plains area was also a vast lake into which the young Ohinemuri, Waitawheta, Waiteti [Waitete – E], and countless others poured their sediment, so filling up until some way out to lower levels and the sea be found.
There were three outlets. Which came first?
Out through Waimata to Tauranga Harbour, out and down towards Waihi Beach, or through towards Paeroa? Both the former two are obvious today, but it is not generally realized that the first outlet of Ohinemuri water came northwards, or rather west-wards on the line of the old Rahu road and valley.
This outlet became blocked by another earth-movement when a great mass split from what was originally a high mountain, now more known as Trig Hill, Karangahake. This vast blockage helped to refill the Lake and assist with the deepening of the two Eastern outlets. Gradually the overflow began again with the Ohinemuri and Waitawheta waters forcing a new pathway through the loose rubble and eventually draining the Lake, with the Ohinemuri cutting its own new pathway, the present Gorge.
I spoke of the mountain being split apart. It is a well known fact that the Waihi Goldseekers had an early eye on the White Rocks (Karangahake) in the hope that a major reef would be found in the rail tunnel. However there was no trace, so the railway went through instead of taking the route of the present highway. The only other location of the White Rock (a type of Calcite) is high up on Trig Hill. Portions of the high Dubbo Reef system can be found scattered deep below the original Rahu Gorge, so scattered and broken as to be not worth attempting to recover.
Earlier I mentioned lake sediment. The fact that the whole of the Waihi Plains enclosed between the higher country is practically level bears out the conclusion of having been water-borne. Again, the content and width of the declination towards present river level at Queen's Head Rock points to a rather fast draining of the lake at this vicinity. An examination of the roadside banks particularly at the Waiteti Stream bridge shows a heavy conglomeration of river-worn metal which one would expect to find on a bottom course.
Coming down Snake Hill to the overhead rail bridge there are great protrusions of rock in the grass-land. This is reputed to be of ancient origin, and is possibly the result of solidified sand together with volcanic ash (Mayor Is). Indeed you have here a very similar sandstone to that in the old Waikato basin at and around Hinuera. This hard sandstone stretches right across the valley from the bluff above the rail line at Victoria Rock to the site of Gordon's Lily Gardens.
From here onwards through Waikino the country is of a softer mixture until it reaches the mouth of the present Gorge where again the waters rose and found egress via the Rahu Valley. Now came the mountain collapse, another water build up until a passage was forced through, forming the present river bed.
So though the 1980 flood was the most severe in human memory, this river has consistently flooded from time immemorial, and will continue to do so.
There are only two answers to the solution.
Divert the upper reaches of the river back towards its original flow, via Waihi Beach gorge or Waimata gorge to the sea, OR -
Shorten the river from Paeroa to the Gulf.
The first is obviously the most difficult but the second is feasible provided it is done on a national scale, and this must be at full river depth, not merely as a by-pass to take only flood water.
There are two major bends in the Ohinemuri, Pereniki and Waimarie, which had they been put through at full depth when first mooted, would have given full flood and bank protection and a good workable and economic waterway transport system.
In the past it was the churning propellers of this form of transport which kept the sediment constantly moving seawards.
The economics of our whole country depend upon land being fully productive ALL of the time, not just some of the time.