Art. XLVII.—On the Physical Geography of the Lake Districts of Otago.
[Read before the Otago Institute, July 19, 1870.]
The Lakes of Otago belong principally to the two great river systems of the Clutha and Waiau. Lakes Hawea, Wanaka, and Wakatipu, belonging to the Clutha River, and Lakes N. and S. Mavora, Te Anau, Manipori, and Monowai, to the Waiau River.
These lakes are of great extent relative to the size of the country. Taking their dimensions from the reonnoissance surveys, we have for the Clutha River system:—
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|Length in miles.||General breadth in miles.||Area in sq. miles.||Alt. above sea level in feet.|
|Lake Wanaka||29||1 to 3||75||974|
|Lake Wakatipu||50||1 to 3½||114||1069|
For the Waiau River system:—
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|Length in miles||General breadth in miles.||Area in sq. miles.||Alt. above sea level in feet.|
|Lakes N. and S. Mavora||9||½ to 1||5||2073|
|Lake Te Anau||38||1 to 6|
|The three western arms or fiords of Te Anau, each||10 to 18||1 to 3||132||694|
|Lake Manipori||18||½ to 6||50||597|
|Lake Monowai||14||to||1||11 not over 500|
It may here be noted that the Te Anau is the largest lake of the Middle Island.
The lakes are all known to be many hundreds of feet deep, but no great attention has been given to the sounding of any of them, excepting Lake Wakatipu. Soundings of this lake have been taken by several persons independently. The greatest depth given is 1400 feet, about the middle of the lake off Collins Bay, and sixteen miles from the south end of the lake.
Having stated the principal survey data of the lakes, we may now refer to the map for their relative positions. It will appear that they all lie along the eastern side of the great western ranges, or Southern Alps; as also Lakes Ohau, Pukaki, and Tekapau, of the Waitaki River system.
It may also be observed that the lakes of Otago and Canterbury, taken as a whole, lie on a line which is roughly parallel to the axis of the Southern Alps, and to the west coast. The length of the lakes greatly exceeds their breadth, and they all lie lengthwise in their valleys, and occupy the full width of the valley, the mountains rising generally from their shore line. Their surfaces do not differ greatly in altitude above sea level, and what difference there is
seems to have a relation to the height of the mountains [the lakes of greatest elevation being in the valleys running down from the highest mountains.] The lakes terminate just where their valleys begin to widen out into plains. Along their sides, and at their southern ends, there are invariably vast collections of shingle and large blocks of rock.
From these similarities it is evident that some great natural cause or law has had a uniform action over this 300 miles of country in producing the lakes.
An observer might have these facts at his command, and yet be sorely puzzled to account for the origin of the lakes, if he had no access to the records of Arctic or Antarctic discovery, or to the Alpine researches of such philosophers as Agassiz and Forbes. But with these as guides, it is plain that the glaciers which now lie far up the valleys and ravines of the mountains are, comparatively speaking, the puny descendants of glaciers that formerly filled valley and lake beds with their vast dimensions, and in their slow but irresistible march, carried forward the spoil of the mountains, and deposited it as lateral and terminal moraines.
Accepting this explanation of the glaciers at one time filling the present lake basins, the question arises: Did the glaciers excavate the basins, or did they simply occupy them for a time, as the lakes now do?
In how far a glacier could excavate a valley out of rock, is necessarily a question very much of speculation. It will be of interest, however, to endeavour a general demonstration of the effect. In a paper read before this Society by Mr. Beal,* attention was directed to the wearing power of ice in motion, and to the rounded outline of the hills so operated on. As an illustration of this action, Peninsula Hill, near Queenstown, was mentioned. This hill is 1700 feet above the level of the lake surface, or about 3000 feet above its bottom. If, then, we suppose that the glacier did not scoop out the lake bed, but simply smoothed its surfaces, it follows that there would be a glacier of from 3000 to 4000 feet in action. Now, can it be conceived that this vast mass slid over the bed of the present lake for a geological era, without working its bed deeper and deeper?
We find, on examining the beds of such rivers as the Kawarau and Shotover, that running water, probably never even in the highest floods more than forty feet deep, can cut or wear channels through hard schist rock, of 200 and 300 feet deep. If running water, and the sand and gravel which it carries along, can produce such effect, it seems easy to arrive at the inference that a glacier, say 1000 feet thick, would, with chips of rock adhering to its under surface, plane down its bed to a depth only limited by the duration of the process.
[Footnote] * Sea Art. XLIX.
The turbid water issuing from glaciers is an evidence of their degrading power. The waters of the Dart and Matukituki—both glacial-fed rivers—are of a milky colour. Dr. Hochstetter, in his New Zealand, states that the Tekapo Lake is always turbid, from the fact of its supply coming from one of the great glaciers of Mount Cook. Forbes explains that this turbid appearance, constantly the same from age to age, is due to the impalpably fine flour of rocks ground in the ponderous mill betwixt rock and ice.
It may elucidate the subject if we suppose the formation of a lake basin about to begin. Let a glacier descend a mountain slope to a valley, then it must either penetrate through the floor of the valley on the line of the mountain slope, or, in its endeavour to do so, be either bent or broken to the slope of the valley. But this effort of the glacier to continue in its initial slope, must necessarily cause great friction, and where there is friction, there must also be degradation of surface. The friction due to a change from a greater to a less slope is in excess of what may be termed the sliding friction, arising simply from the motion of the glacier over a uniform slope. Thus, at the points of greatest friction, there will be a scooping-out process at work. Nor is this an intermittent operation, but will continue as long as the glacier exists. As the process proceeds, the part of the valley first operated on will have been scooped out, and the valley slope assimilated in part to the mountain slope. The condition of excessive friction due to difference of slope will then apply to a more advanced part of the valley, and so on, till temperature arrests the process by melting the ice.
According to this view of the subject, we ought, keeping all modifying causes out of sight, to find that the lake bottoms are a succession of slopes, steepest next the glacier mountains, and gradually less and less as we proceed from them. The bottom of the Wakatipu Lake complies with these conditions. Taking the soundings from the map in the Otago Museum,* and interpolating the soundings taken off Collins Bay, there will result for the first two miles from the head of the lake a fall of 180 feet per mile, for the next four miles a fall of 70 feet per mile, for the next five miles a fall of 50 feet per mile, for the next seven miles a fall of 40 feet per mile, for the next six miles a fall of 6 feet per mile, for the next six miles a fall of 14 feet per mile, for the next three miles a fall of 30 feet per mile, for the next seven miles a rise of 16 feet per mile, for the next nine miles a rise of 96 feet per mile.
It must be mentioned that the slope of 180 feet per mile for the first two miles may be considered greater than the true glacier or lake bottom, on account of the river deposits of the Dart and Rees.
In regard to the ascending slope of the lake bottom for the last sixteen
[Footnote] * Map showing Surface Features of N. W. District of Otago, by Dr. Hector, 1864. Partly reduced, and accompanied by a Section to elucidate this subject, in Plate 19., Trans. N. Z. Inst., Vol. ii.
miles, of which seven miles are at 16 feet per mile, and the last nine miles at 96 feet per mile, it accords with the gradual recession of the glacier, and the deposit of moraines in the same order. It must also be noted that as the glacier retired, more and more of the lake bottom from the south end would be released from the grinding action, while the operation would still be in force in the other parts of the lake.
As already mentioned, there are no soundings of the other lakes available in this investigation. But the very complete Admiralty survey of the West Coast affords a comparison with each of the sounds there. In all of them there is this peculiarity—that, while they are from 700 feet to upwards of 1200 feet deep, in no case is their entrance to the ocean deeper than 350 feet —so that if the West Coast were raised 300 or 350 feet, the sounds would become lakes varying in depth from 400 feet to 900 feet.
If glacial action is sufficient to have excavated the sounds on the West Coast, it may be urged in objection—Why should the glaciers scoop out deeper basins in the hard rock of the West Coast, than in the comparatively less hard rock of the Wakatipu Basin?
This objection might be disposed of by assuming that the West Coast has been gradually sinking. But adhering to the mechanical principle, the objection may be met by considering that the slopes on the western side of the mountains are more steep than on the eastern side, and therefore the glaciers would act more efficiently, having a greater vertical longitudinal pressure, and by the same difference a greater friction. Then in regard to the harder rock. This objection would be of great moment if the ice were the rubbing power; but it is only the frame or machine in which the chips of rock are set to do the grooving. In hard rock these chips will be part of the rock itself, so that the difference as to the nature of the rock is not so great an objection as at first appears.
The difference in the depth of the sounds seems to have a relation to the heights of the surrounding mountains, as though the excavating process was most active where the greatest snow field existed to feed the glacier, and the greatest pressure to urge it forward. The lake at Martin's Bay [Lake McKerrow] is the only lake of any note on the west side. It may be cited as an illustration of the excavation or basin bearing a relation to the efficiency of the glacial action. The Hollyford Valley, in which this glacier would lie, is the largest valley on the west side of Otago. It is surrounded by very high mountains, which would create and maintain a large glacier. Under these conditions, not only has a basin been excavated, but sufficient moraine matter has been carried forward to dam back the sea, and so a lake has been formed, and not a sound.
In a paper by Dr. Hector,* in Trans. N. Z. Institute, Vol. ii., pp. 373–4,
[Footnote] * The author appears to hold the view, now generally abandoned, that the motion of the lower and horizontal part of a glacier is due to vis a tergo. In other respects he adopts the arguments and conclusions of the paper cited, but without fully appreciating their bearing on the question.—Ed.
reference is made to the excavating power of glaciers; but, while acknowledging their efficiency in this respect, the lake basins and sounds of the West Coast are referred to subsidence of the earth's crust. He states:—“This subsidence has been most rapid in the central and western part of the range, so that in the case of a long valley, like that occupied by the Wakatipu Lake, the slope bacame gradually reversed, and what was at first the higher part of a glacier- excavated valley, has become a depression without an outlet.”
The meaning of this seems to be that the depression was subsequent to the glacial excavation of the valley. To realize the condition of the valley at the close of the glacier action, it will be necessary to conceive the bottom of the lake basin lifted up, so that the valley would throughout its entire length slope in the one direction. Applying this to the Wanaka Lake, the bottom at the south end would come near the present level of the lake surface, 974 feef. The height of the saddle at the head of the Makarora Valley is given by Dr. Haast at 1716 feet, and the distance between the two points is about fifty miles. The fall is therefore 742 feet, or say 800 feet, or a mean slope of 16 feet per mile. But the slope would not be uniform, and so be less over a part of the valley. Forbes states that the slope of the glaciers in the Alps is seldom or never under 3 degrees, or 276 feet per mile. He says, however, that the glaciers which transported the blocks of granite from Mont Blanc to the slopes of Jura would not have had a greater mean slope than 1.8 degrees, or 104 feet per mile, and the slope of a great part of its course must have been much less. Reasoning from the semi-fluid nature of a glacier, he does not pretend to fix the limit of mean slope, and he says it might even be as little as 15 min., or 23 feet per mile. From this it will appear that we cannot assume that a glacier would not move with a mean slope so gentle as even 16 feet per mile. But if we are to accept the subsidence theory, it will be legitimate to apply it in such a manner as will best dispose of the objections that may be urged against it. If we, therefore, conceive the subsidence to have taken place before, and not after, the glacial period, the mean slope of the Wanaka glacier would be greatly increased, and so be brought nearer the ascertained facts of glacier motion.
When the great glaciers occupied the valleys and lake beds, the climate must have been much colder than it is at present. The causes of change may have been various. When we contemplate the vast accumulations of denuded material which is now in the valleys, it is evident that the mountains must have been more massive and probably higher before the denudation began. There would therefore be more surface above the snow line, and by the same proportion, greater snow fields and greater cold. But this is inadequate to the
effect, unless we conceive the present mountain system to have been a great table-land. We are precluded from this supposition, however, by the fact that the high angle at which the rocks lie, determines that from the first upheaval of the mountains there must have been elevation and depression, ridge and valley, as at present. And the denuded material already mentioned has resulted from the widening and deepening of the valleys by glacier and river actions. If we look to the disparity of climate between Labrador and North- western Europe, on the same lines of latitude, and resulting from oceanic currents, there can, by analogy, be no difficulty in supposing such a distribution of land and water in these latitudes as would make a south polar current sweep along the shores of this island, and chill its temperature to the required degree. The equatorial rains, which now nourish the great forests of the western and lake districts, would then descend in snow, and the glaciers would reach their furthest limit. It is remarkable how near our latitude, even at the present time, approaches in locality to the glacial period. At Dunedin we are only about 1600 miles from the ice precipices of the Antarctic continent, and the glaciers are said to descend nearly to the sea in the Straits of Magellan.
The glaciers of Otago have not yet been sufficiently explored to ascertain the lowest level of their terminal faces; but it may be stated that some of the secondary glaciers on Mount Aspiring are from 4000 feet to 5000 feet above sea level.* In sheltered situations, some of them will, no doubt, be lower than this. Dr. Haast gives the terminal face of the Tasman Glacier (Mount Cook) at 2774 feet. Forbes gives the terminal face of the Mer de Glace (Mont Blanc) at 3667 feet above sea level.
The snow line in Otago is from 7000 feet to 8000 feet above sea level. There is a considerable extent of country above this line, the principal part lying between Earnslaw, 9165 feet, and Mount Aspiring, 9949 feet. The intervening ranges of the Forbes and Humboldt Mountains are covered in all their high parts and ravines with snow fields and glaciers. In a bright sun- shine they are so dazzling that it is only with an effort that the eye can rest on them. From the melting of these great snow and ice fields, the Dart, Rees, and Matukituki have their waters. The Makarora and Hunter similarly flow from the glaciers in Canterbury. As the supply of these rivers depends almost entirely on the melting of snow and ice, their volume is regulated by temperature. This gives rise to a set of conditions very much the reverse of what obtains with rain-fed rivers. In winter the glacial-fed rivers are very low, especially during frost, while in summer they are high. On a warm summer day, the difference of their volume between morning and evening is very apparent.
[Footnote] * The largest glacier from the west side of Mount Aspiring descends to 1400 feet above the sea, and the Francis Joseph glacier, from Mount Cook, to within 700 feet.—Ed.
The north-west wind, especially if accompanied with a warm drizzling rain, has a very marked effect on this class of rivers. At such times, the Dart, Matukituki, and such like, become rivers of half a mile wide, and of course pour into the lakes vast quantities of water. The valleys on the west side of Te Anau and Manipori Lakes being generally very narrow, the vertical rise and fall of their rivers is quite astonishing. In many of them, unmistakable flood-marks show that this is not less than 20 feet. The actual observation of these facts impresses the mind very forcibly with the value of the lakes as great regulating reservoirs for the Clutha and Waiau Rivers. The Clutha has 237 square miles of lake to regulate its flow, and the Waiau 198 square miles, or, altogether, an area of 435 square miles for the two rivers. This surface, like a great compensating balance, is ever in a state of oscillation up and down between the inflow and outflow of the rivers. But it attains its maximum level for the year in January or February, and its minimum in July or August. The vertical range of lake surface varies in different years, and the lakes have also differences depending on their area and supply. Generally, the range may be stated at from four to nine feet.
In the season of 1865–66, the Wakatipu Lake had a range of no less than ten feet. On the 14th January it attained its highest level, the water standing two feet deep in Rees-street, Queenstown. The lake had probably not been at so high a level for a very long time before, for trees of two and three feet in diameter were sapped by the rising waters, and laid prostrate on the margin of the lake. A cold spring preceded this great flood, and the lake was then nine inches lower than it has ever been known to be before or since. Consequent on this coldness, more snow than usual was reserved for the melting of the summer sun. The lake had reached a high level, when three days of warm wind from the north and north-west, accompanied with rain, raised its surface to the highest observed level.
The fact of the lowest and highest levels occurring in the spring and summer of the same season, and so intimately connected with change of temperature, is an evidence of the value of our equable insular climate in the river system of the country. For with the same temperature, but colder in winter and warmer in summer, the glacial-fed rivers would become still more fluctuating; and such rivers as the Mataura and Taieri, that have their principal sources in high snow-clad mountains, and have no lakes worth mention to regulate their flow, would necessarily become much more irregular in volume than they are under present conditions.
The ordinary condition of a glacial river, such as the Dart, is a rapid stream of three or four chains wide, and from three to four feet deep, with a number of smaller branches running out and into each other as they continue their course along the channel, which is a wide waste of shingle and quicksand, at places a mile broad. This matter is carried forward to the lakes, and by
the delta so formed the channels of the rivers are all advancing into the lakes. This progress is necessarily very slow, from the great depth which has to be filled up. But the amount of matter so deposited is conclusive evidence of the many ages during which the present geographical order of river and lake must have existed. This silting action has gone on principally at the northern ends of the lakes, where invariably a large river enters. The lakes have each in this way been filled up for several miles. The smaller rivers which enter at the sides of the lakes are also encroaching. Thus the Dingle has formed its delta more than half way across the original breadth of Hawea Lake. The continuation of the process, together with the deposits of the Hunter River, will, in time, reclaim several miles of the lake.
The lessening of the lake areas is also promoted by the eroding action of the rivers issuing from them. The high terraces surrounding them show that their surfaces must have been considerably higher at one time. A minor triangulation has been extended over the middle and upper part of Lake Wakatipu. It determines that the terraces at Greenstone, White Point, and Frankton, are each 100 feet above the present level of the lake. There are other terraces, the heights of which have not yet been ascertained. In the accurate determination of the heights of terraces above the present lake level, there is, apart from finding the ancient lake levels, the means of detecting any secular variation of level that may have taken place in the island since the lake system began. If, for instance, the west side of the island is sinking and the east side rising, the old contour marks or terraces of the lake will not be parallel to its present surface. The detection of this difference, supposing the variation to have taken place, would no doubt be a delicate operation if the oscillation has been insignificant. If, on the other hand, the oscillation has been considerable, it could not fail of detection.* There are several other questions concerning Lake Wakatipu which the extension of the trigono- metrical survey will throw light upon; there is the abandoned river bed at Kingston, and there is the supposed leakage of part of the lake waters through the Kingston Moraine to the Mataura River.
From the twofold influence of silting up and erosion, it is plain that the tendency is to transform each lake into a valley, with a river running through it. This process has been already completed on a small scale in some of the higher valleys. A moraine has, in the first place, dammed across the valley, and then the lakes so formed have been silted up, and are now a succession of flats, with a river running through them, and rushing over the moraines as a rapid. In the higher valleys there are also, in some places, masses of rock lying confusedly across the valley, that at first sight appear to be moraines, but
[Footnote] * The terraces are due to alterations in the level of the outfall of the lake, and could not be affected by such oscillation.—Ed.
in reality are the fragments of great rock slips that have been hurled, by the action of frost, from the adjacent mountain steep.
It is difficult to realise, without actual observation, the activity of the disintegrating forces, and the vast amount of matter that the mountains are denuded of even in the course of one year. The phrase “everlasting hills” is really a misnomer, for the forces now unceasingly at work will level the highest mountains and fill up the deepest lakes. Frost is the great disin- tegrator. The melting of the snows, and the moisture from the clouds, penetrate the rocks during the day; the freezing at night splits them, and the result is long streams of angular fragments from the peaks, and talus heaps around the bases of the precipices. In the higher snow fields, the avalanche, with the noise of thunder, hurls its snow and ice masses into the valley below —there to be kneaded into a glacier, or to rush on as a torrent or succession of cascades. The traveller, in making his first acquaintance with such mountains as the Southern Alps, is apt to be more or less bewildered and appalled with the din and potency of forces at work, and with the vast dimensions of the surrounding scene.
The climate of the Lake District, as indeed the whole of the island, is determined by the Southern Alps. They lie directly athwart the track of the equatorial winds, and their cool tops condense the vapours with which these winds are so highly charged, and hence the almost tropical rains of the West Coast. These high mountains so effectively drain the winds, that there is comparatively little left for the interior of the country, and but for the secondary ranges, such as the Dunstan and Hawkdun Mountains, conserving what does fall in the form of snow, the interior plains and valleys, not on the banks of lake-fed rivers, would for a portion of each year be waterless.
Although the Otago portion of the Southern Alps is from 6000 to 10,000 feet high, yet there are numerous saddles in them much lower, from which the valleys run to the West Coast on the one side, and to the lakes on the other. The valleys on the lakes’ sides act as funnels, down which the winds blow and discharge their moisture. The effect of the discharge is seen in the forests which are invariably found in these valleys. In several cases, where the saddle of the dividing range does not exceed 3500 feet above sea level, the forest is continuous from the west to the east side of the mountains. Thus, beginning at Martin's Bay and following up the Hollyford Valley to the Eglinton Pass, thence to the Te Anau Lake, and then down the Waiau Valley to the ocean, there is a continuous belt of forest, 160 miles in length, and, with its ramifications, covering upwards of a thousand square miles. It is worthy of remark, that the forests of Otago are all to be found within the districts enjoying a moist climate. Thus, on the west side of the province there is the country between the West Coast and the lakes; on the east side there is the margin of 30 or 35 miles along the coast over which the south-westers usually
extend. The highest ascertained limit above sea level of the forest is 3500 feet
The comparatively dry interior districts, extending principally between the Clutha and Waitaki Rivers, have no forests. There have been, however, forests in the interior districts at one time, for the charred trunks of trees are still found on the Rock and Pillar Range, and on the Waitaki side of the Kurow Mountains. In the Lake Districts, similar evidence of a greater extension of forest is also found. The apparent cause of the annihilation has been fire, but it is quite probable that natural causes may also have operated.
If we examine any of the forests on the East Coast in their natural state, and before a litter of rejected timber and branches has accumulated in them, it seems difficult to imagine that a fire could make its way through them. But the forests in the Lake Districts, and generally in high altitudes, are free from the tangled undergrowth of the East Coast forests. In place of it the soil has a covering of foggage and moss, often a foot deep. In a dry season this readily ignites, and as it smoulders rathers than burns, the work of destruction is very sure over the surface the fire extends. In this way a portion of the forest in the Te Anau District was destroyed some years ago by a grass fire kindling the foggage.
The Maoris frequently traversed the forests of the Lake Districts in their hunting excursions, and no doubt their fires would cause the destruction of parts of the forest from time to time.
Speaking of the Lake Districts in a general manner, it may be observed that, considering the extent of agricultural, pastoral, and forest land that abounds in them, their mineral products, their delightful climate, and extent of inland navigation, they have within their own borders all the main elements that render communities prosperous and flourishing.