Discussion on Mr. Travers's paper “On the Great Flood of February, 1868,” read at the previous meeting.
The President explained, for the information of those who had not been present when the paper was read, that the chief points brought forward by the author were, that the flood referred to was of such magnitude as had probably never occurred previously in the particular locality, and had been so destructive as to alter the surface features of the country, thereby leaving a permanent record of its severity.
Mr. Chapman agreed that the flood in question was exceptional, but there was not sufficient evidence brought forward to prove that such had not previously occurred—indeed, he believed there had been quite as severe floods in the south and west, but not doing such damage, probably because the rivers in those localities were better adapted to earry off such heavy rains. Such a flood as the author described would on the West Coast have been unimportant.
Mr. Maxwell gave instances of exceptional floods, which had been due to the occurrence of great land-slips, and, from his knowledge of the locality and the enormous masses of driftwood reported, he thought that the extraordinary effects produced by this flood had been produced by heavy land-slips, blocking up the rivers temporarily, rather than the exceptional amount of rainfall. In the construction of public works, it would never do to provide against exceptional extremes, but only average extremes; such a rainfall as 13in. in twenty-four hours could only be local, and it would not pay to construct works to provide in all parts of the country against the effects of such rainfall.
Mr. Marchant had surveyed that block of country, and believed that the destruction of the timber had a baneful effect in increasing the rapidity with which the storm water ran off the mountatns. He instanced the case of the Rimutaka and Tararua ranges, and stated that if the clearing of the forests was continued, the result would be the scouring out of all the valuable lands in the Hutt and Wairarapa valleys. Bush reserves were now being made to avert this disastrous result.
Mr. Cox took exception to the geologieal reasoning in the paper, and thought that the proofs of this flood being of an unexampled character were not sufficient. He argued that the formation of secondary cones in lateral streams, on which Mr. Travers chiefly relied for support of his argument, was in reality no proof, as it was only when these lateral streams had cut sufficiently deep through the main terrace to be dammed back by the floods in the main river, that they would have any tendency to cut fresh outlets for themselves, and thus form fresh cones. It thus came to be a question of years, and not thousands of years, since the conditions were favourable for the formation of fresh cones, and in reality, before this was brought about, these lateral creeks might have carried far greater volumes of water in flood-time, and yet left no trace of their having done so.
Mr. Bull was in the Ashley district at the time of the flood, and gave an account of its enormous spread, destroying farms by covering them over many square miles with a thick deposit of shingle. The Maoris had told him that there had been a similar flood about 50 years before, but there was no evidence of the deposit of drift-wood or shingle to anything like the same extent. The storm was preceded by remarkable water-spouts and whirlwinds which he described.
Mr. Travers, junr., mentioned the floods in the Motueka district in 1876, which were caused by land-slips damming the rivers, so that the hil-sides were cleared of timber, and the driftwood thickly covered the sea in Blind Bay.
Mr. McKay pointed out that the evidence derived from the thickness of the silt deposit at the mouth of the Pahau River, instanced by the author, was only proof of the extent to which it had been dammed back by the flood in the Hurunui, and that the flood was not due to landslips amongst the mountains in which the larger rivers take their rise, as streams such as the Pahau, which rise much nearer to the east coast, were relatively as much flooded as the Hurunui and the Waiau-ua. He was of the opinion that the new shingle-fans formed by some of the creeks in the Waiau-ua Gorge were not necessarily evidence that the flood was of a wholly unprecedented character, and endeavoured to show how circumstances irrespective of the mere amount of rainfall might have brought about the results mentioned by Mr. Travers. Viewed as a geological fact, he thought it likely that greater floods had occurred within the recent period, or since the gravel terraces were formed within the Waiau-ua Gorge.
The President, before calling on Mr. Travers to reply, remarked that he thought the paper was one of great importance to those in charge of public works that were proposed, especially in this particular district. He had since last meeting looked up the meteorological records for the period when this flood took place. These early records were not so complete as those now made, but he had obtained sufficient proof of the passage over the middle part of New Zealand of a great atmospheric depression, from the 2nd to 4th February, 1868, and that the centre passed N.E. of that part of the islands which suffered most, which fully accounted for the unusual direction of the wet wind, which on this oceasion came from the eastward. The depression revolved round a low pressure of 28.6, and by means of the accompanying diagram of the isobaric lines he showed reasons for believing
that it reached New Zealand from the tropical parts of the Pacific Ocean lying to the N.E. of New Zealand, instead of having had the more common course from the westward. At the period referred to an extensive anticyclonic area prevailed over Australia, producing a difference of pressure of no less than two inches between there and New Zealand. The cyclonic disturbance that produced the floods was revolving along the eastern edge of this area of high pressure as it did not affect Australia. The translation of a mass of warm and moisture-laden tropical air to higher latitudes, and its impingement on the eastern flanks of the New Zealand mountains, sufficiently accounts for the extraordinary character of this flood.
Mr. Travers, in reply, thought that it was immaterial to attribute the effects he had described to landslips—these were an effect, not a cause. The fact was that the rain was so great that the river channels could not carry the waters. It was during the first few hours that the rivers rose most rapidly. All the driftwood was dead and seemed to have been the accumulation of years. The flood was not by any means local, as the Hutt River also brought down driftwood, so as to cover the whole harbour and discolour the sea outside the Heads for two miles, for several days. Mr. Cox, he thought, misunderstood his argument, as the accumulations of detritus formed during the flood were entirely new at that date, and now form permanent objects in the district. Such a flood could never have occurred before, or similar marks would have been left. He gave further particulars respecting the whirlwinds mentioned by Mr. Bull, and stated that large tracts of forest in the Nelson district had been completely overthrown by them.
“Notes on the Mineralogy of New Zealand,” by S. Herbert Cox, F.C.S., F.G.S., Assistant Geologist. (Transactions, p. 418).
On the Alpine Flora of New Zealand,” by John Buchanan, F.L.S. (Transactions, p. 342).
“On the Occurrence of of the Salmon Trout (Salmo trutta) in Nelson Harbour,” by Dr. Hector. (Transactions, p. 211).
Mr. Travers said he had reason to believe that similar fish occurred in the lower part of the Hutt River.