WAIKATO RIVER

Waikato Independent, Volume XL, Issue 3677, 7 March 1940, Page 5

WAIKATO RIVER

ABNORMAL CHANGES IN COURSE ITS GEOLOGICAL HISTORY A paper on the Waikato River basin, written by L. Cussen,; and read before the Auckland Institute on 17th December, 1888, contains interesting reference to that part of the river which concerns Cambridge and the Central Waikato district. ' After remarking that the river seems to have been subject to apparently abnormal changes in its course from an early period in its history, and giving considerable detail regarding changes further up stream, the writer goes on: The next remarkable change in the course of the Waikato, and that which was attended by the most serious results in the great middle basin, is that which took place at Pairere, about 14 miles above''Cambridge. Anyone travelling the road from Cambridge to Oxford (Tirau) could scarcely fail to remark the well-defined broad valley, bordered by steep cliffs, which runs in a north-east direction by Hinuera towards Matamata. There is little doubt that the Waikato River once flowed down this naturally sloping valley, and. thence to the sea at the Hauraki Gulf, But from some cause it again left its old bed, and timing to the westward, passed through the gorge, between Maungatautari and .Hinuera ranges for six miles, and debauched into the great middle basin at Cambridge.

Lake Miles Long

The river formed a sinuous lake in the valleys above, extending backwards for a distance of eight miles, and covering the Waipa plains, which were evidently the bed of a lake. We find the remains of a deep alluvial deposit, chiefly of light pumice sands, which filled the valleys running in between the spurs in level plains. ... It runs into valleys between the ridges, varying in depth from 120 ft. downwards. ...

The river in working its channel deeper through the barriers on the Maungatautari Gorge, gradually drained off the waters of the lake, leaving behind, in the valley above eight rows of terraces, which fringe the river on either side, indicating each different stage in the lowering of the bed.

Land Level Higher

The height of the land through the gorge which the river now traversed was certainly above 200 ft. above the present (1888) river-bed, and through this to the bedrock the river has eroded its channel. . .

The broad plain in central-Wai-kato known as the "Waikato middle basin/' has an area of five hundred square mile's. We find an alluvial deposit all over the lower areas of this valley. In places it is 150 feet in depth. The deposit ... is made up mainly of rounded particles of pumice, inter-stratified with layers of clay and . . . sarid; and enclosing trunks of trees placed horizontally. That these deposits were brought down by the Waikato River seems unquestionable. ' The surface height of the land at Cambridge is 220 ft above the sea, at Hamilton it is 120 feet . . . and at Taupiri only 39 feet. Now we find the alluvial deposits have been carried by the Waikato waters to the Roto-o-rangi swamps eight miles almost in a south-east direction from Cambridge, whilst the natural fall of the country is in the direction of Taupiri and one 7ft to the mile. In Dr Hochstetter's opinion geological evidence points to the fact that the whole middle Waikato basin was but recently a shallow bay of the sea. Growing Trees Buried

If this be so, as the land rose and the sea receded a channel or channels would be left in the estuary and through those the rivers and streams would naturally continue to flow into the Haiiraki Gulf. Trunks of trees standing as they grew and some horizontally and other evidence tends to show that the stratified land could only have been laid down by the action of very slow-moving water in a lake or sea.

The most interesting example of this character, because of the most recent occurrence, is that shown in Mr E. B. Walker's property at Monavale, four miles south-west from Cambridge. A drain was cut about a mile in length through a neck of dry land, to drain the Mona Vale swamp into a dry gully, which led to the Waikato River. During a heavy flood some years ago a scour was started in ' this

drain, which soon formed a gully from 60ft to 70ft in depth and in some places several chains across, at the bottom of this gully the ancient land surface was exposed to view. Old Land Surface It consists of stiff brown, marly looking soil, apparently of excellent quality. The trunks of many trees are lying on the old land surface, and some were found standing, with their roots penetrating the old soil, as they grew. The present land surface is perfectly level, whilst the ancient surface is found at various depths from 30t't to 00ft, showing the old contour. of the land. . . .

Now it seems very evident that these deposits in the middle Waikato basin could never have been laid on the bed of a lake nor by the waters of the Waikato at all, with the levels of the land as we now find them. The lake would have four outlets—one at Morrinsville, one at Hapukohe, one at Matahura, and one at Taupiri, all 100 ft below the level of the bed of the lake in the centre; therefore the water could not be impounded to place the deposits. Neither is the action of the sea admissible. ... To local movement or oscillation in the level of the land within the basin, it would seem we must attribute the phenomena. The Waikato, on debauching through the Maungatautari Gorge, would probably occupy about 350 square miles of the low areas in the middle basin, in the form of a broad shallow lake dotted with numerous islands, which are now the clay hills and ridges of the valley. At that time the land to the east, north and west sides of the hasin stood higher than it now does —sufficiently so to enclose the waters of the shallow lake, and then were the alluvial deposits of the valley laid down, and subsequently a tilt in the surface level of the valley took place, emptying the lake.

The almost direct course of the Waikato River from Cambridge to Ngaruawahia and the absence of a wide river valley, may be taken as indicating the rapid formation of the river bed. Among the proofs of subsidence quoted include the similar forest in the lower Waikato 40 or 50 miles from the sea; also in building the railway bridge at Ngaruawahia and Hamilton (where at Ngaruawahia) cylinders are sunk several feet below low water mark in the Auckland Harbour, and at this depth river pebbles and shingles were found, including an ancient river bed which must have been higher than it is now to flow to the sea. (The same evidence wal seen at Hamilton). Remnants of the- old lake still remain in the J akelets, lagoons and lake-like swamps which occupy the depressed areas in the valley and many of them are fast drying up.. Facts as quoted prove first a depression and subsequent elevation of the land.

The Waikato River may have occupied the bed it now does, before the submergence. . . In the subsequent elevation the river might have first found its way to the sea on the east coast through the Waiotapu valley. As (he land gradually and uniformly rose the river would erode its bed deeper in the loose materials. The Waikato would for a time flow through the Hinuera valley but subsequently would resume its former course into the middle basin.