Art. XXXV.—The Northern Wairoa.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 36, 1903, Page 453

Art. XXXV.—The Northern Wairoa. By E. K. Mulgan,M.A. [Read before the Auckland Institute, 21st September, 1903.) The Northern Wairoa is in several respects a wonderful river. Rising in a narrow peninsula, it is wholly confined to a relatively small extent of country. The entire catchment-area, indeed, does not measure more than fifty miles by forty miles, and yet this gathering-ground is able to furnish sufficient water to supply a river which is navigable for the largest ships for upwards of fifty miles, and for smaller craft for nearly double that distance. Can any other river in New Zealand show a similar record? Throughout the lower part of its coarse the fall is so slight that the influence of the tide is felt beyond the “Junction” (i.e., junction of Mangakahia and Wairua Rivers), a distance of more than a hundred miles from the Kaipara Heads. The term “Northern Wairoa” is used in the title of this paper in a somewhat wider sense than that in which it is generally employed, this name properly belonging to that portion of the river between the Kaipara Heads and the “Junction.” The streams forming its head-waters are known as the Mangakahia and Wairua (the latter in. its upper reaches receiving the name of the Waiotu), both of which drain the northern slopes of the basin, the former to the west and the latter to the east. The two streams gradually converge, and finally meet at what is known as the “Junction.” Other tributaries are the Kaihu and Tangowahine, flowing south, and the Mangonui, draining the basin to, the south-east. The stream known in its lower course as the Wairua rises a few miles to the east of Kawakawa, and not far from the upper waters of the Bay of Islands. From here it flows in a

southerly direction, keeping well to the east of the peninsula,and receiving on its way the Whakapara and Mangahahuru Streams, both of which come from the east, and drain the country almost to the seaboard. The Mangakahia rises on the southern slopes of the watershed separating the Hokianga and Kaipara basins, and, gathering supplies from the northern and eastern slopes of Tutamoe, skirts the Tangowahine range, receives further, down the Hikurangi from the north, and finally unites with the Wairua at the Junction. The Kaihu, which drains the basin to the west, is worthy of note in that it receives supplies from streams some of which rise not far from the source of the Mangakahia, and others almost at the coast. The Kaihu itself, moreover, keep parallel to the coast until it reaches the Wairoa at Mangawhare. The Mangonui rises on the western slopes of the hills to the west of Waipu, and, flowing north-west, is joined by another large stream, the Tauraroa. From here its course is first west and then north until the Wairoa is reached. It will be seen from this that the catchment-area practically includes the whole of the land from coast to coast, and stretches from a line joining the mouth of the Hokianga Harbour and the upper waters of the Bay of Islands as far south as the Kaipara Harbour. That is to say, bearing in mind the configuration of the country, the river-basin comprises the maximum area available. These singular conditions have been made possible by the distribution of the high land. It usually happens that the highest land of a country occurs in the centre, and the lower levels are found near the coast. In the case of the Wairoa basin these conditions, although not entirely reversed, yet are considerably modified. The greater part of the land is high; indeed, nearly the whole of the central portion of the basin consists of broken, bush-clad ranges, some of which reach an elevation of from 1,500 ft to 2,000 ft. (Tutamoe, the highest peak north of Auckland, has an elevation of 2,576 ft.). These ranges, whose general trend is from north to south, are cut across in many places by narrow valleys, and flanked on either side by low hills running parallel to the seaboard. Thus, the whole of the rainfall on this portion of the peninsula, instead of being carried to the sea by a number of different streams, is collected to furnish the supply for a single river. The size and configuration of the catchment-area would not in themselves account for the volume of water discharged by the river. A more important factor than either is the rainfall, which in this particular case must reach phenomenal

proportions, seeing that the area over which it is distributed is so small in extent. In point of fact, the rainfall across that portion of the Auckland Peninsula is exceptionally great. The prevailing westerly winds, after their passage over the Tasman Sea, reach the relatively high land heavily laden with moisture, where the inevitable lowering of temperature brings about a constant precipitation of rain. I was unable to obtain reliable statistics touching the rainfall, but several well-known facts pointed to its great abundance. When the surveyors and parties were cutting the track for the road through the Awatuna and Marlborough Settlements they, found the ground very damp and soft, and the tree-trunks covered with moss and, lichen. The ground was frequently buried to a depth of several feet in moss, and was everywhere saturated wish moisture. The men employed on the work suffered much from the wet and damp, some of them being obliged to give up their employment in consequence. Then, again, the settlers experience great difficulty in burning off the bush. “It will not burn,” they say—a statement to which the half-burnt logs and ill-cleared land bear ample witness. One cannot help noticing the readiness with which the scrub (chiefly makomako—Aristotelia racemosa) comes up in the clearings, and the small extent of land which is really freed from bush, as also the extreme moistness of the land itself. It seems unnatural to find soft boggy soil at such a high elevation—to find it, in fact, on hilltops. That there is very considerable rainfall cannot be doubted, and this, added to the size and configuration of the catchment-area, accounts for the dimensions of the river, which at first sight would appear out of all proportion to the conditions, under which it exists. The fall of the river from the Junction to the sea is very slight—only a few feet—for the effects of the tide are felt considerably beyond that point This implies that the velocity of the stream itself is small, hence we would expect to find, as is actually the case, a wide flat-bottomed valley filled with detritus. From the Junction to the sea the Wairoa has lost the power of vertical erosion. The banks of the river, however, are soft, and the load of sand, and mud carried by the water is very great, conditions highly favourable for lateral erosion, hence one is not surprised to find that the river, has cut for, itself a very winding channel. It is well, to remember, in considering the work done by rivers, that running water is only the motive power, and that the instrument which really does the work is the sand and mud carried along by the water. A perfectly clear stream running over a bed of sand and pebbles which do not move is doing no work at all; it is only

when its waters are loaded with sediment that it becomes such a powerful agent of erosion. In the course of the Wairoa from the Junction seawards a number of very interesting loops occur, any one of which would furnish a profitable subject for investigation, and well repay the trouble spent in unravelling its history. Take, for instance, that large loop below the junction of the Mongonui Stream. In this case the river, flowing north-west, was deflected at Te Pango by an elevation which, although slight, was sufficient to give direction to the current. Meantime its waters spread out over the low ground on what is now its left bank, where, the current being checked, deposition took place, gradually bringing the depressed area up to its present level. This area, which must have been a large one, grew towards the north, and in so doing gradually pushed the river-bed in the same direction, until the process was arrested by the low range of hills along the right bank, and the river was confined to its present channel. The net result has been the formation of a large flat nearly circular in shape with a series of low hills running across its narrow end. The power of the stream to erode laterally is well seen in several places, notably at Dargaville. Here the line of greatest velocity has impinged against the right bank, cutting into it for a considerable distance. One is surprised at first that it has not gone further and cut back as far as the high ground behind Mangawhare. But the fact is that the process of erosion here has not been long in operation. It was not until the growth of the opposite bank—to be explained further On—forced the current into its present course that the banks on the Dargaville side began to disappear The presence of the Kaihu Stream no doubt brought about at an earlier time the deposits of the Dargaville and Mangawhare flats. Erosion and deposition are both going on; indeed, in a river of the kind where the fall is not great this is inevitable. At Dargaville, for instance, if the right bank is disappearing, the left bank is gradually being pushed out into the stream. The erosion on the Dargaville side is now arrested to a large extent by heaps of stones placed at intervals along the bank. The effect of this has been to narrow the channel, and hence to increase the velocity and erosive power of the current, with the result that a sandbank of considerable size, nearly opposite Mangawhare, has lately disappeared. The appearance of the banks, too, is very characteristic. A deep channel runs pretty close to the right bank, where, moreover, the ground is hard and presents a vertical face to the water even at high tide. The left bank, on the other hand, consists of a low shelving deposit of mud. One of the first things that strikes an observer is the

extreme turbidity of the water, a turbidity, moreover, from which it is never free. The load of mud carried is very great—indeed, it is difficult to conceive of its being increased to any large extent. The particles of which the mud consists are extremely small, so much so that a good deal of the deposit is carried far out into the Kaipara Harbour before coming to rest on the sea-floor. That an immense quantity of detritus has been carried down by the river is abundantly proved by the deposits of alluvium along its banks, and the source of supply may no doubt be looked for in the higher ground of the river-basin, where the low ranges consist partly of sedimentary and partly of volcanic rock. The outcrops of these are much decomposed and weathered, and the flanks of the hills are steep, conditions favourable for the removal of a disintegrated surface. Much of the country, however, is covered with bush, which serves to bind together the surface-soil and prevent its being washed away. It is not surprising, therefore, that the tributary streams draining the area in question do not show any noticeable turbidity—indeed, they present the appearance of clear running brooks—and yet the waters of the Wairoa are always heavily laden with mud. Clearly, then, the present supply of detritus carried by the river is not directly derived from its tributaries, but is largely the result of lateral erosion. Some of it is carried down and spread out on the harbour-floor, but much of it is washed backwards and forwards by the tide and current before being finally deposited where the banks are growing. So free from suspended impurities are most of the tributary streams that the question arises, Would a supply of material such as now finds its way into the river be sufficient to furnish the deposits of alluvium found in the valley; in other words, have conditions become greatly modified since the deposits were laid down? There do not seem to be any strong reasons for supposing that they have, for the drainage-area is large enough to furnish all necessary supplies if only time were allowed and the river-valley where the deposits occur is relatively small. Still, there may have been a time when deposit, owing to altered conditions, was more rapid than now-when, for instance, a good deal of the country at present bush-clad was comparatively bare, and when in consequence the weathered surface was readily and easily swept away. The country may then have stood at a much higher level, which in itself would account for a more scanty vegetation, and the reduction in height may have been brought about partly by erosive agencies and partly by depression. To reach a period when such conditions prevailed one would have to go back a considerable time, if

we may judge from the growth of some of the larger of the forest-trees, which indicate an age of between two and three thousand years—perhaps more. The left bank of the river near its mouth consists of an immense tract of flat land containing several thousand acres, and known as the Tokatoka Swamp, the whole of which, no doubt, has been carried down and deposited by the river. The swamp is on the north-eastern side of the Kaipara Harbour, and the prevailing winds are south-westerly. The effect of these has been to cause the water, with its load of mud, to spread out towards the opposite shores. In this way the north-eastern side was gradually filled up and converted into land. The process of deposit has by no means stopped. The southern portion of the swamp gradually becomes lower until it ends in a wide mud-flat covered with mangroves, where a constant dental deposit is going on, with the result that the bank is being pushed out further and further into the harbour. Owing to the continual deposit the river-channel is now confined to the shore-line along the western side. The land of which the swamp consists is extremely fertile, and has given magnificent returns to those who have been, fortunate enough to secure it. A good deal of work in draining and surveying has been done by the Government, and already several blocks have been disposed of by ballot. Some difficulty was at first experienced in obtaining water, but this has since been overcome by sinking artesian wells. The result of the borings put down are not without interest as throwing light on the origin of the deposit and the rate at which it was laid down For the following tables giving an account of the borings at Raupo I am indebted to Mr. R. F. Baff, who personally superintended and directed the operations. It may be explained that Raupo is the name given to that portion of the swamp already settled. Details of Borings. No, 1, at Raupo. Ft. 50 ft. soft clay or mud (blue) 50 45ft. soft sands and mud 95 7 ft. hard clay light-grey in colour 102 33ft. sandy clay 135 19 ft. quicksand. (Dribble of water, drop at a time, rises 2 ft. 6 in. above ground: tried pump—no better) 154 22ft. soft sandy mud 176 19ft. grey clay, changing to brown, with woody particles 196 17 ft. sharp sand. (Good flow of water, which rises 12 ft. above surface of ground, and at 3 ft. above ground gives 20 gallons per minute, or 28,800 gallons per twenty-four hours) 212

No. 2, about two miles and a half from Raupo. Ft. 10ft. clay or mud 10 52ft. soft sand and shell 62 58ft. clay or mud 120 12ft. coarse sand. (Water rising near surface) 132 18ft. clay (blue) 150 8ft. sand, same as before 158 18ft. soft sandy papa (blue), varies in thickness 176 8 ft. hard pure papa. (Stopped sinking. Drew pipes back to 120ft. (see * above). Water rises 12 ft. above ground, gives 3,000 gallons per twenty-four hours—clean water free from salt) 184 No. 3, about three miles south, of Raupo. 10ft. soft clay 10 80ft. dark quicksand 90 6ft. hard clay 96 17ft. white sand (sharp) 113 56ft. brown and blue clay 169 41 ft. fine white sand, with flow at 2 ft. above surface—good strong flow, gives 14,400 gallons per twenty-four hours clean and good water 210 At Mangawhare the boring operations, which were carried on with the same object as those at Raupo, and which, moreover, were not successful, in that the supply of water obtained was insufficient, disclosed the following details: Beginning at the surface, the pipes passed through 80ft.soft wet river-mud, 50 ft. fairly hard argillaceous limestone, 80 ft. calcareous marl (papa). There were no traces of marine or fresh-water organisms found in these (Mangawhare) deposits. The papa rock referred to both, here and at Raupo is an indurated calcareous, marl of presumably Cretaceo-tertiary age, which outcrops at various places across this portion of the peninsula. Taking, this as the basement rock, we have in the case of the Tokatoka Swamp a deposit of upwards of 200 ft. of sea-borne and river-borne sediment; indeed, it is highly probable that the depth of this deposit in places is not far short of 300 ft., an immense depth when, it is remembered that it occurs in a relatively small land-locked harbour. The material passed through in the borings, however, is not altogether the product of the river. The eastern limit of the sandstone series forming the land between the river and the coast no doubt extended at one time as far as the flanks of the hills to the east of the Kaipara Harbour. The upper members have been removed by denudation, but the lower beds still underlie the swamp-deposits. An examination of the tables

will show this. Near the surface the borings invariably passed through soft clay or mud, after which the material met with always contained more or less sand. To attribute to the agency of the river the whole of the material passed through in the Tokatoka Swamp borings would be to demand a continuance of conditions under which a gradual subsidence took place with sufficient slowness to enable the river to fill up the depression, This appears to be highly improbable; there is nothing to warrant the supposition, which, moreover, is not required to explain existing conditions. The great depth of river-deposit at Mangawhare (80ft.) is a more difficult matter to account for, and until farther evidence is forthcoming we may attribute it to some local depression. It is difficult to arrive at any accurate knowledge as to the rate at which the deposit was laid down. As affording some guide to this it may be mentioned that two clubs of black-maire (Olea cunninghamii)—the kind Used by the natives in crushing fern-root—were found about 13 ft. from the surface by some men whilst cutting a drain. It is not at all likely that these clubs were buried to that depth by the natives; it is much more probable that they were dropped into the waters of the old swamp or harbour and were gradually covered up. The wood of which they were made would at once sink to the bottom. This is very slender evidence to build on, but if it is to be accepted at all it would go to show that the deposit has been a rapid one, and that the last 13 ft. have been laid down within a period which could not have extended over many generations. The borings at Mangawhare appear to have gone through nothing but river-mud before reaching fairly hard argillaceous limestone, a rock similar to that which outcrops at Arapohue, Okahu, and other places along the eastern side of the river. The age of this limestone is considerably greater than that of the river-deposit, so it may here be taken as the basement rock. Hence at Mangawhare the depth of alluvium is about 80 ft.; and, as no remains of marine organisms are found, we may conclude that the deposit is of fluviatile origin. This no doubt applies to all of the material forming alluvial flats along the river north of Te Kopuru. It is not improbable that the sea once flowed over the land now known as the Tatarariki Flat, and that the high ground from Te Kopuru south at one time formed part of the western shore-line of the harbour. On the eastern bank of the river there occur some very remarkable volcanic outcrops, rising through the sedimentary rocks at short intervals and extending for several miles in a line running approximately north and south. The first of these met with is at Maungaraho, evidently the remains of a huge dyke 720 ft. in height and dipping towards the north-

west. Seen from the south it presents a broad surface like that of a wall viewed ac right angles to its length; from the north-east or south-west it has the appearance of a wedge, broad below and tapering above. About two miles south of Maungaraho is the Tokatoka Peak, a well-preserved volcanic neck rising through the shales to a height of 580 ft. The base is relatively large, but the summit tapers to a point with an area of a few square yards. In the upper portions well-defined columnar structure is seen; but this disappears some little distance from the top. The rock can be traced to the river, where it forms a low cone-shaped boss. Between the Peak and the boss-known as Cemetery Hill—is a well-marked depression. None of the rock here presents the appearance of a lava-flow, but rather that of the remains of the upper portion of a subterranean reservoir. To the east the volcanic rocks pass under the shale and reappear again about a quarter of a mile further on. The rock here is much, weathered, but lying on and showing through the surface are a number of boulders compact and fresh-looking. They consist of a hard close-grained andesite with large well-defined augite crystals. A short distance further on the junction of the shale and volcanic rock can be seen in an old road-cutting. At this point the rock is much decomposed, but the remains of the augite crystals can be plainly seen. About a mile to the south of the Peak the same rocks again outcrop in a low rounded hill, which is evidently a boss. The rocks here cross the river in a south-west direction, reappearing in a well-defined conical hill rising through the river-deposit, known as the “Green Hill,” and consisting of a hard-grained andeaite very similar to the boulders found to the east of the Peak. Much of the rock at this exposure shows but little sign of decomposition, and will in the future furnish valuable material for road-metal. A little more than a mile to the south of this spot another similar outcrop occurs, bare at low tide and a source of much danger to vessels. In the Tokatoka Swamp itself, about a mile and a half back from the landing at Raupo, these rocks again rise to the surface, where quarrying operations are carried on. The rock is broken in a steam-crusher and used for ballasting the railway-line running through the swamp, as well as for spreading on the roads. There is little or no doubt, that these outcrops are connected, and have had their origin in some common reservoir. The land between the Wairoa River and the coast consists for the most part of soft sandstone arranged in horizontal layers, which under the action, of the weather crumble into white sand composed in places almost entirely of silica. At the coast the rocks are very soft, and are readily acted on by the atmospheric agents of erosion. Great quantities of the

looser material are washed down by the rain and spread out in the form of fans along the beach. The harder portions then tumble off and are rapidly broken to pieces and carried away by the action of the waves. The work of erosion here must be rapid, for the prevailing westerly winds at times drive in the waves with great fury. This strip of land, extending from the Kaipara Heads to the Maunganui Bluff, and having a width of about seven miles, appears to have its eastern limit along the river. It is evidently the remains of a much larger and higher area which at one time extended westward, and has gradually been reduced to its present size under the action of the weather and the sea. In none of the exposures were any fossils to be seen, but the general appearance of the rocks, their well-defined stratification, and the presence of a band of compressed vegetation some 6 ft. or 8 ft. in thickness outcropping at intervals for miles along the coast remove all grounds for doubt as to their origin. The whole of this area is an immense gumfield, and yet there are no kauri-trees to be seen. The forests which must have covered it have long since disappeared, and the land is now clothed with a stunted growth consisting chiefly of fern, Leptospermum scoparium, Leucopogon fasciculatum, L. fraseri, Pomaderris elliptica, P. filicifolia, and other plants usually found where these comprise the prevailing vegetation. The kauri seems to thrive best on rugged, broken country where the elevation is considerable. Any pronounced lowering of the general level would bring about the destruction of the trees, and this is apparently what has happened. As the land subsided the forests died out, leaving behind these extensive deposits of gum which of late years have become so valuable. These beds of sandstone no doubt in former times bad their eastern extension along the flanks of the hills running north and south on the left bank of the river, and overlay unconformably portions of the older shales of which most of these hills consist. The upper portion of this part of the series has now been completely removed by the river, which in all probability originally flowed along the junction of the older and younger rocks, and hence carried away first those portions forming the eastern extension of the beds. On the beach at the west coast is found an interesting and valuable burrowing mollusc, Mesodesma ventricosa, known locally as the “toheroa.” In size it varies greatly, but some of the larger forms measure as much as 6 in. by 3 ½ in. Its habitat occurs between high- and low-water mark— at least, that is where it is found. When the tide is in it comes to the surface of the sand, and, thrusting out its siphons, obtains the necessary nourishment from the sea-water. As the waves retreat the siphons are withdrawn, and

the animal again buries itself to a depth of from 12 in. to 15 in. This it accomplishes by thrusting from the ventral surface of the anterior end of the shell a powerful wedge-like foot, which it forces into the sand by means of a peculiar wriggling movement. As soon as the root has penetrated as far as it can reach it is made to contract and, drag the animal after it. It is surprising how few of these contractions are required to bury the animal completely, and with what rapidity it disappears. The settlers and natives consider the toheroa a great delicacy, and, as it is easily obtained, great numbers are, dug up and used as food. At Mangawhare a tinning-depot has been established, and already a considerable output is disposed of. It is said, that the animal is confined to a small portion of the coast reaching from the Murewai Caves to the Maunganui Bluff; but further evidence is necessary before accepting this statement, though, from all accounts, it has not been found in any quantity elsewhere If it be true that its habitat does not extend beyond this limited range, an extremely interesting subject awaits investigation, for there seems to be no barrier to prevent its spreading all along the ocean-beaches of the northern portion of the Island. Its restricted range may be caused by a food-supply being confined to the area over which it is found, or by the sand in which it burrows being in some way specially fitted for its needs. This last conjecture does not appear to be a likely one, for the sand on the west coast is constantly moving. Another peculiarity is that it is found in patches, these often being separated by considerable distances. An explanation of this may be that the animal lives some distance from the shore, and only leaves the deeper water at breeding-time. Living together in great numbers on special feeding-grounds, the animals may migrate at intervals to shallow water, each migration being shared in by a large number of individuals.* Since this was written I have learnt that the toheroa occurs along the coast to the north and south of the Raglan Harbour, and to the north of Porirua Bay; but in none of these places is it found in such abundance as on the ocean-beach opposite Dargaville. The Northern Wairoa has long been the seat of a large and important timber industry, the immense kauri and kahikatea forests with which much of the river-basin was-clothed furnishing an accessible and readily marketable supply. The kauri forests on the hills along the river and in other places easily reached have long since been removed, and a very few years will see the complete disappearance of the kauri as a timber-producing tree in the Northern Wairoa. The rapid removal of the timber may be attributed

partly to its value and partly to the fact that in nearly all cases its conveyance to the mills was a matter of comparative ease. The numerous tributaries of the river, many of them deep, well supplied with a constant flow of water, and reaching far back from the main stream, furnish the best of all waterways for the conveyance of logs. When the natural supply of water is not sufficient to float these they are forced along by driving-dams or hauled out of the bush, either by bullocks or steam-engines. Great ingenuity is brought to bear on the removal of trees from difficult positions—huge logs, weighing many tons, are hauled from the bottoms or sides of steep gullies or lowered from the tops of lofty hills. Timber whose position a, few years ago would have rendered it valueless is now worked with profit; indeed, there are few patches of kauri sufficiently inaccessible to be beyond the reach of the axe and the wire rope. A good deal has been written on the slow growth of the kauri. The following contribution to the literature of this subject by a northern settler may be of interest: “In 1896 a number of trees, measured 7 ft. from the ground, girthed something under 3ft. 4in. Ten of these trees, measured under similar conditions in 1901 (five years afterwards), girthed as follows: 3ft. 5m., 3ft. 6in., 3ft. 7 in., 3ft. 8in., 3ft. 10in., 3ft. 11in., 4ft., 4ft. 1 in., 4 ft. 2 in., 4 ft. 3 in. This would give an average girth of 3 ft. 10 in., and hence an average increase of girth of 6 in. in five years, slightly over 1 in. per annum. These trees were in sheltered positions; in those growing on exposed situations there was no perceptible change.” On the low land bordering the river there is still a great quantity of kahikatea—a valuable timber for some purposes, easily obtained and easily worked. An immense kahikatea forest, estimated to contain between ninety and a hundred million feet of timber, covers the northern and eastern portions of the Tokatoka Swamp. A mill has been erected and is now in full operation at Raupo, where many years of work await the enterprising owners.