Art. XLVIII. —On the River Systems of the south portion of the Province of Wellington.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 2, 1869, Page 198

Art. XLVIII. —On the River Systems of the south portion of the Province of Wellington. By J. T. Stewart (With Map.) [Read before the Wellington Philosophical Society, August 14, 1869.] The accompanying map is of a part of the Province of Wellington, and shows the main rivers, with some of the principal lines of watershed, and the ranges of hills. This paper is explanatory of it, and refers particularly to some of the river systems. The portion shown on the map is from the area drained by the Manawatu on the north, and extending southward to the extremity of the province. Of the other portion of the province lying to the north of this, I have not sufficient data to represent its rivers on the map, and consequently do not attempt to describe them; although they present many important and interesting features, coming as they do, at least two of them, the Whanganui and the Wangæhu, from the centre of the island, near Tongariro and Ruapehu. The approximate areas of the portion of the island represented on the map may be stated thus: Area of part of the province shown, 5100 square miles, or 3,264,000 acres, —nearly half the area of the province. Area of part of Hawke's Bay province included, as drained by the Manawatu, 320 square miles, or 204,800 acres. Thus making the whole area treated of, 5420 square miles, or 3,468,800 acres, or nearly three and a half millions of acres. Subdividing this we have: Square Miles. Acres. In the Manawatu drainage area. 1830 1,171,200 In the Ruamahanga, or Wairarapa drainage area. 1300 832,000 On the West Coast, from the Manawatu to Paikakariki, drained by the Horo-whenua, the Ohau, the Waikawa, the Otaki, the Waikanæ, and numerous small streams running directly into the sea. 550 352,000 Between Paikakariki and Palliser Bay, including Porirua, and Wellington, by the Hutt, the Wainuiomata, the Orongorongo, and many smaller streams. 500 320,000 By rivers on the East Coast, from Palliser Bay to the Whareama. 570 364,800 By the River Whareama and its tribu-taries. 250 160,000 By rivers on the East Coast, from the Whareama to the boundary of the Province, at Waimata, by the Waka-taki, the Mataikuna, the Aohanga, the Akitio, and numerous small streams running to the sea directly. 420 268,800 Totals. 5420 3,468,800 Of this total area, about 2500 square miles, or 1,600,000 acres are covered with bush or forest, so that little apprehension need be felt at the prospect of

Shewring the River Systems of the Southern Parts of the Province of Wellington N.Z.

drought being induced in most of our main rivers by the destruction of the bush. There is more liability to such a contingency occurring in the smaller streams, and perhaps in the East Coast rivers, and in some of the Wairarapa rivers, where the country is more scantily furnished with forest. I recollect in the dry summer of 1863–4, observing both the Whareama and the Taueru rivers to have nearly ceased running, consisting of a chain of pools connected by a very small run of water between them. Also the Aohanga river, at a place well inland where it falls perpendicularly over a ledge of overhanging rock for a height of about sixty feet, seemed at that time a mere thread of water, which the gusts of wind at times dissipated into spray before it reached the river bed below. On such rivers the preservation of the bush about their upper courses, and on their feeders, becomes an object of importance. It will thus be seen, from the table of areas, that the Manawatu and the Ruamahanga are the most extensive and important river systems in the part of the province under consideration, yet the areas drained by them differ much in character, and the rainfall over them is affected by different meteorological influences. The Ruamahanga, or Wairarapa area, has much more open country in it than the other, and its supply is derived from the rain falling to the eastward, only, of the main dividing range of the Tararua. It gets most of its water directly from the eastern side of this range, by the head of the Ruamahanga itself, by the Waipoua, the Waingawa, the Waiohine, and the Tauherenikau, which latter falls into the lake. It also gets the drainage from the eastern side of the Rimutaka range, by many streams chiefly discharging into the lake. By the Tauheru and its tributaries it drains a large extent of elevated hilly land, more or less open, lying to the N. E. of the Wairarapa valley. By the Huangarua, the Dry river, the Rahohuru, the Turanganui, and many small streams, it drains the more open country lying on the west side of the watershed between the lower part of the Wairarapa valley and the East Coast. The melting of the snow in summer affects it by the rivers running from the Tararua mountains, and this probably to a greater extent than occurs in the Manawatu area. One noticeable feature in the Ruamahanga is, that it discharges itself, in the first place, into the Wairarapa lake, and flows out of it again not far from where it enters, with the addition of the waters collected in the lake by streams falling into it directly. The river, after a course of a few miles, flows into the lower or smaller lake, which is divided from Palliser Bay by a narrow belt of beach, through which the river flows into the sea by a channel which sometimes is closed entirely by the action of the heavy surf in Palliser Bay, and then the water being dammed back fills the lakes, and floods a large area of low marshy land about their margins, until the accumulated water again forces a passage into the sea, when the lakes subside and relieve the adjoining low levels of the surplus water. The nature of the passage into the sea of this river has withheld from the Wairarapa the advantages of a navigable river, notwithstanding the large area drained, and the numerous and large tributaries of the Ruamahanga. The state of this area has been much modified by its long occupation by European settlers; and the substitution of grasses for the growth of bush, fern, and scrub, to a large extent, must affect the rapidity with which the rainfall finds its way to the streams and rivers. The area drained by the Manawatu system of rivers, on the other hand, is still nearly in a state of nature, except what change the native occupants

have affected, which would not seem to be much, in regard to any effect produced on the subject now under consideration. This river and its tributaries present several interesting features. The main river itself penetrates, by a narrow rocky gorge of picturesque scenery, the main dividing range of this part of the island, and separates it into the Ruahine and the Tararua mountains. In this gorge there occur several reaches of still, deep water, and as the view is shut in at both ends by the winding course, the traveller seems to float in his canoe in a rock-bound mountain lake, with grey lichened cliffs, overhung with ferns and shrubs, and steep wooded slopes, rising above them. These quiet reaches are separated by dangerous rapids, full of boulders and rocks. Both above and below the gorge the country is lower, and the character of the river is a rapid course over wide shingle beds, and this makes the change into the gorge more striking. This river takes the rainfall of both sides of the southern end of the Ruahine range; for twenty-five miles on the east side, by the portion of the Manawatu proper, which runs in the Province of Hawke's Bay; and for thirty miles on the west side by the River Puhangina, which has a course almost parallel with the range, and joins the Manawatu only some one and a half miles to the west side of the gorge; and also by the sources of the Oroua to the north of the head of the Puhangina. The Manwatu also takes the rainfall of both sides of the northern end of the Tararua range; for thirty miles on the east side by the Mongahao river, which runs almost parallel to the range, and joins the Manawatu only a mile or two to the east of the gorge, and also by streams falling into the Forty-mile bush rivers from the hills south of the head of the Mongahao; and for twenty miles of the west side of the range, by the Tokomaru river, and the Kahuterawa, and other large streams falling into the Manawatu on its southern bank. It also gets the drainage of the table-land of the Forty-mile bush, by the Makakahi, Mangatainoko, and other streams falling into the Teraumea, —which joins the Manawatu to the east of the gorge; —and by the Teraumea river, which rises on the east side of the Puketoi range, it gets the rain falling on both sides of the southern end of the Puketoi range; and by the Waitawhiti, the Ihuraua, and other streams it drains a part of the high lands adjoining the heads of the Whareama and the Taueru rivers, which both flow to the eastward part of the province. By the numerous rivers and streams flowing into the Oroua from both sides, into the Puhangina from the west, and into the north side of the Manawatu itself to the west of the gorge, the rainfall over an extensive flat and table country between the Ruahine range and the sea coast also finds its way to the sea by the Manawatu. Drawing its supply from such an extensive area, exposed to so much variety of climatic influences, it would seem that we need not expect all its tributaries to be flooded at one time; as the north-west rains will affect the Oroua, Puhangina, and streams to the west of the dividing range, while the south-east rains will flood the rivers on the eastern side. The south-east or south-west rains, however, produce the heaviest floods, as the rain-drift flies along the line of the main range, and supplies both slopes at once, as well as probably falling more copiously on the area to the east of the range, and on the southern end of the Puketoi mountains, while the north-west rains striking more transversely to the line of the main range, probably fall more heavily on the western slopes than on the eastern. The northern end of the Tararua, falling in height as it approaches the gorge, does not contribute much water from summer melting of snow, but

some supply of this nature is probably derived from the Ruahine at the sources of the Oroua river. The whole area drained by the Manawatu being 1,171,200 acres, we find the very large proportion of over 1,000,000 acres to be bush-covered, also there is much flat country, so floods neither rise nor run off so quickly as in an open country. The dense vegetation of the bush retains a large quantity of the rainfall, and the ranges themselves are chiefly bush, and not very precipitous in general character. For instance, on the Tirohanga hill-track, from the Manawatu to the Forty-mile bush, passing over the Tararua range, after attaining on elevation of about 1200 feet, we find nearly three miles flat before the ascent to the summit is made; several streams flow through this flat, and the ground has a thick, spongy stratum on the surface of roots, moss, and soil. Similar comparatively level tracts, no doubt, exist at many places on the hills at considerable elevations; and thus the water falling on them by no means necessarily finds its way rapidly to the lower levels and the main river bed. From these causes more water must be taken away by absorption and evaporation, than at first might be supposed. One feature in the course of the Manawatu, as of other similar rivers, is the numerous old water-courses abandoned by the river, and now forming semi-circular shaped lagoons of uniform width in the flat bush country. These are found at intervals in a belt of half a mile to a mile and a half in width, on both sides of the river. They have formed old river beds, cut through at the neck by the current, and the ends silted up by the deposits brought down in floods. This process still goes on, general extensive bends having been cut off within my own knowledge, as at Raukawa, and near the mouth of the Tokomaru. A kind of balance is thus probably kept up between the speed and wearing power of the current, and the nature of the soil acted on by it, so that the total length of the river course along its numerous windings, maintains a mean from time to time; the formation of a long bend by the stream eating into the banks at one place, being counterbalanced by the cutting through the neck of a peninsula at another. Some of these lagoons are over a mile long, and form fine sheets of water. They are mostly filled in heavy freshets, by the water backing up the stream flowing from their lower ends, and they, together with a large extent of low land subject to floods, for some miles above the junction of the Oroua, act as storing reservoirs for some of the surplus waters, as also do two large open swampy tracts whose surface is about the level of high floods, —one on the south side, called Makurerua, of some 15,000 acres, and the other lower down on the north side, called Ohotuiti, of some 7000 acres, and both with many shallow lagoons in their area. These are of rich soil, and when drained, of which they are capable of being, will form important flax-growing and meadow lands. The large extent of sand and gravel deposits also, no doubt, absorbs and discharges gradually a large part of the rainfall, and of the waters brought down by river floods. Differing from the Ruamahanga, the Manawatu is navigable for many miles from its entrance, to vessels of six or eight feet draft of water, which the bar at the mouth allows to enter, and the flood tide, when there is no fresh in the river, gives an upward current for fifteen or sixteen miles from the mouth. The course of the Oroua gives a good section of the land lying to the west of the Ruahine range. For ten or fifteen miles of its lower course, it divides the open sandy country of the coast from the alluvial bush land, and here its

current is not so rapid, and its channel is narrow, muddy, and canal-like. Above this it becomes wider and more rapid, with shingle beds, and the banks show gravel deposits, which increase in height as it is ascended, and in the upper course cliffs of 100 feet to 200 feet high, washed by the river, show horizontal well-marked layers of sand, gravel, and clay, with marine shells. These beds preserve their horizontal position until the spurs of the range are approached, when they show a decided dip to the west, in parts. About seventy to eighty miles, by the winding course, from its junction with the Manawatu, the Oroua cuts through a spur by a gorge faced by cliffs of rock, about 150 feet high, and nearly perpendicular, and close to here the first hard rock was observed, coming up the course. The channel here is full of boulders and rocks, and the run is rapid. Looking through this gorge, however, in the direction of the range, the same horizontal strata of sand, gravel, and clay, are again seen in the high river bank; and it is probable the river extends a long way further into the main range, as its volume here seems quite as great as in its lower course, showing, at least, that its main supply comes from the hills surrounding its source in the Ruahine. I have not attempted to estimate the quantity of water discharged by these rivers, although an approximation might be made from the estimated rainfall over the areas drained, and allowing for absorption and ovaporation; yet in the absence of observations on the actual volume of the rivers, at different periods of level of water, such an estimate would not be satisfactory, and there are not yet any observations of the local rainfall on these ranges and extensive table lands. The following notes on the route from the Manawatu river to Masterton, through the forty-mile bush, from a journey undertaken by the author in 1868, indicate the nature of this very important track, which might be opened up with a small outlay. November 17th, 1868—Left Foxton and rode some twenty miles to Kai-ranga, on the Manawatu river. Left horses here and crossed river to south bank; walked four miles over a gradually rising country, and camped at foot of first rise of main range, on the Kahuterawa stream: this is a considerable height above the sea, probably 100 to 200 feet. 18th—Commenced ascent of Tararua range: top of first rise at Tiro-hanga is about 1200 feet above where we left this morning; goes on level for some way, then a rise of 200 feet more at six and three-quarter miles from the Manawatu, again a rise of 490 feet to summit at Tipakirikiri, which is thus 1800 to 1900 feet above foot of range, at camp this morning. Fine view from here over Tongariro, Manawatu, and Rangitikei country, Ruapehu and some of the Forty-mile bush. Descended to foot of range, 1440 feet. Thence to Mongahao river, descending 280 feet further, or in all 1700 feet. from summit to Forty-mile bush country. From the Manawatu by this track to the Mongahao river is twelve miles, passable for horses; cut by Mr. Carkeek, Assistant Surveyor, in 1868. The track stops at Mongahao. Diverged down river a little, and took track to Tutækara clearing and native pa—about four miles. 19th—Followed on the old native track (from Ahuriri to Wairarapa) about four miles to Te Hawero clearing—level country. The track from Manawatu might join here, and there is an old track from here to Alfredtown. Four and a half miles further crossed Mangatainoko river: country level for some distance. At, say, eight and a half miles from Te Hawero track rises on a ridge, about 550 feet, and then falls with a good descent 220 feet. Then across a table-land which I estimate some 1100 feet above the sea. Camped about fourteen and a half miles from Te Hawero.

“20th—A mile on crossed Kahepurapura, and also another strong stream, passing over good slopes, rising and falling, say, 100 feet: reached Makakahi river at 8 p.m., nineteen miles from Te Hawero. This is the boundary of Crown land. Two miles on crossed Mangahinau stream, and followed its course for some way. One and a half miles further crossed Mangahuarere stream. At 11.20 a.m. crossed last stream on this side of watershed, say, twenty-four miles from Te Hawero. The bush along this track is very open and free from supple-jacks and scrub: a good horse road very easily made. The track now ascends the water-shed range—rises some 500 feet to a flat top, ascent pretty good. Flat at top, say one and a half miles, and then a descent of 700 or 800 feet to crossing of Ruamahanga river, at the head of Opaki plain, some fourteen or fifteen miles from Masterton. This descent is steep, but by exploring we found a leading spur from the flat top, going about one and a half miles lower down the Ruamahanga, which gives a good descent: we marked and partially cleared this. Crossed the Ruamahanga at 1. 30 p.m., and walked over the Opaki plain to Masterton, which we reached about 7. 30 p.m.”