Art. XLIII.—Note on the Rate of Erosion of the Hooker and Mueller Glaciers.
[Read before the Otago Institute, 6th August, 1912.]
Much discussion, and even controversy, has taken place amongst geological observers in all countries in regard to the importance of glacial erosion. By some, almost unlimited powers have been ascribed to it; by others, almost all effectiveness has been denied to it. Spirited as the discussions have been, there have yet been but few attempts to estimate the rate at which glacial erosion is acting at the present time. Whereas elaborate measurements have been made in regard to the rate at which rivers and all flowing water lowers the land-surface, glaciers and glacial streams have been to a great extent immune from such inquisitive activity.
In New Zealand in particular no attempts have been made to estimate the amount of sediment contained in the water of rivers that flow from glaciers. It was with the idea of initiating local inquiry that the following records were made. It was originally intended to take samples of the water of the Mueller River where it issued from beneath the ice of the glacier. It was, however, found that the outlet of the river had so changed its position that it was practically impossible to obtain samples of the water. This forced me to take samples from beneath the Hooker Bridge, a quarter of a mile below the Mueller outlet, and two miles below the source of the Hooker River. While this is somewhat unsatisfactory, it is thought that the amount of sediment contained in the water is practically the same as at the point where the rivers acually issue from beneath the ice.
It is, of course, recognized that the amount of sediment suspended in glacial water represents the amount of rock removed by one only of several different activities that are effective in glacial regions. The suspended matter is of an extremely fine and impalpable nature and of a pale-grey colour, and it almost certainly represents the material derived from the filing action of rock-fragments dragged over the rocky bed of the glacier during the slow movement of the ice. The amount and nature of the erosion due to the transport of morainic matter, to the plucking action of the ice, and to solution are not here referred to. The quantities of sediment were as follows:—
|Nov. 17, 1911||1 part in 9,009 of water.|
|" 18, "||"18,220 "|
|" 19, "||"14,188 "|
|" 20, "||"11,815 "|
|" 23, "||" 16,982 "|
|" 25 "||" 14,000 "|
|Feb. 3, 1912||" 15,765 "|
|" 7, "||" 885 "|
|" 17, "||" 9,459 "|
|" 29, "||" 9,700 "|
|Mar. 15, "||" 27,046 "|
|" 25, "||" 39,141 "|
The average of these estimations is 1 part of sediment in 15,523 parts of water.
Records of a similar nature made in other countries are not very numerous. The following are quoted from Hess, “Die Gletscher”: The average for the Rhone Glacier is 1 part in 1,611 of water; the Arve, 1 in 4,166; the Aare, 1 in 7,042; the Muir Glacier, 1 part in 77 of water. From such records the inference has been drawn that the rate of glaciation depends largely upon the degree of glaciation, which is, of course, intense in Alaska, where the Muir Glacier flows.
Further records show that the annual variation is considerable. Thus, in 1890 the Bossons Glacier delivered water that contained 1 part of sediment in 437 parts of water, but in 1891 it contained only 1 part in 2,078 of water; the Tour Glacier in 1890 contained 1 part in 4,115, and in 1891 1 part of sediment in 32,193 parts of water.
It is probably in these records that an explanation is to be found of the low average of sediment in the water of the Hooker River. In November, when most of my samples were taken, the weather was extremely cold, and light snow fell on several occasions at the Hermitage, though the total precipitation was quite small, and the river during the whole of my visit was very low—not higher than winter level, in the opinion of the guides. Chief Guide A. Graham, who kindly took the later samples for me, states, “The river, on the whole, has not varied much during the summer, so the samples that I am sending you, with those taken by yourself, should give a fair average for this summer. With a summer like that of last year I am sure that the sediment would have been in much greater quantity. I took a sample of the water during one small fresh in February.”
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It is, of course, well known that the summer of 1911–12 was the coldest that has been experienced in New Zealand for a considerable time. Calculations based on the amount of sediment stated above show that if the erosion were supposed to be of equal amount over the whole of the névé and glacier area the rock-bed would be undergoing removal at the rate of 1/63 in. annually, or 1 ft. in 756 years. This is an extremely slow rate when compared with the action of the Muir Glacier, which lowers its bed by ¾ in. annually. Despite the small amount of sediment in the water, the high precipitation in the region causes the rate of erosion to be more than twice as rapid, as that of the Aare Glacier, which lowers its bed 1 in. in 170 years, or 1 ft. in about 2,000 years. It is, however, probably true that the effect of erosion is far more pronounced in the bed of the glacier, where the ice is thickest, than elsewhere; and if it is supposed that the action is so distributed that half the total of sediment is derived from 5 square miles of glacier-floor, then that floor would be eroded a depth of 4,000 ft.—the depth of many of our glacial valley—in about 1,000,000 years. The action appears to be somewhat more rapid than stream-action in the same region.
Objection may be offered to these statements on the ground that some of the sediment is carried to the ice by streams that flow down the steep glacial walls in that part of the glacial valley that lies below the ordinary snow-line, and contribute water-derived sediment to the ice. This suggestion is discounted by the very nature of the sediment in rivers that have a glacial origin: it is unoxidized, and so fine that no settlement takes place in twenty-four hours. The hillsides mentioned are covered with snow for many months of the year, and their water is filtered by passing through the lateral moraine before they join the glacier. Even when flooded after heavy summer rain the water of these streams is almost clear of sediment.
It is hoped that, with the assistance of Chief Guide Graham, further records will be made in future years.