The existence of enormous supplies of waste in the valley of a river profoundly influences its action. The energy of a stream is limited, and its excess is chiefly spent on transportation and corrasion. It will therefore be evident that terrace-forming must be connected in some way with the load a stream carries. If the load is excessive, there will be no energy left for lowering its bed, and hence for forming permanent terraces. Many of the laws governing streams may be studied by examining the miniature fans and deltas formed at the roadside or in other places after heavy rain. The following order of events is apparently true for a minature fan as for our large rivers:—
During flood-time the stream is fully loaded with waste from the surrounding country, but drops it on the gently sloping ground, thus raising its bed. Terraces are absent. When the height of the flood is past, the supply of waste falls off—only smaller particles are carried; and there is an excess of energy left over for corrasion, and the fan is terraced, on a small scale it is true, but the processes and the sequence of events are just the same as on a large scale. If this is so, the degradation of its bed by a river which is fully loaded in flood-time will occur principally as the flood is falling, and will continue till the river is running clear again and carrying no sediment. I have re-peatedly noticed this order of events on shingle fans, and I have received confirmation of these facts from engineers whose business it is to supervise the fords across the streams on the Christchurch-Hokitika Road. It must be remembered that our rivers when in flood are undoubtedly highly charged with waste, and therefore differ greatly from the condition of ordinary streams when in flood. These may be discoloured by fine particles, and may even move stones along; but the supply of waste on the Canterbury mountains is exceptional in amount, therefore our rivers in flood-time are comparable to the excessively charged streams of a small fan, and the sequence of events is apparently the same, although the conditions are somewhat different.
I think it can be proved that when the volume of a stream diminishes, the transporting power falls off in a slightly greater ratio than the energy. The result of this will be that, when a stream is fully loaded, on a diminution in volume there will be an excess of energy left over for corrasion, and the stream will therefore channel its bed. The explanation of this phenomenon may be due to the fact that with a falling volume the larger
particles are dropped first, and if there is not an approximately equal quantity of smaller material for the river to move in place of the material dropped there will be an excess of energy left over for corrasion. Under ordinary circumstances there is an insuffcient supply, and so the river-channel is lowered.
The supply of waste has such an important bearing on the corrasive power of a river that a consideration of the circumstances which control the supply in the Canterbury mountains will be relevant here. One of their most striking features is the vast supply of débris supplied by their slopes exposed to frost erosion. This effect is so marked that whole mountainsides are covered with angular débris, which is continually moving downwards, but especially so in the case of shingle-slips. These are often from 2,000ft. to 3,000ft. in height, and may be as much as a mile wide. The reasons for this excessive supply of waste are as follows: (1.) The jointed character of the rocks in the drainage-basins of the rivers. (2.) Owing to intense folding of the rocks, they frequently dip at very steep angles, and therefore the weakest beds are exposed to the atmosphere without being protected by more resistent beds. (3.) The age of the folding dates back to Mesozoic times, and therefore weathering agents have been able to exert their influence to a marked extent. (4.) The range, both annual and diurnal, of the temperature is very great. (5.) The absence of close plant-covering over large areas. All these causes promote extensive disintegration, and any explanation of the life history of our rivers must take them into account.
One of the principal factors determining the production of waste is the extent of mountain-slope not protected by a close covering of vegetation. the area of most vigorous denudation is between the snow-line and the upper limit of this covering. The snow protects the underlying rocks to a certain extent; but, nevertheless, even here the denudation is rapid, but especially on those steep faces where snow cannot lie. When the snow is turned to ice the effect is somewhat similar. Erosion will not proceed as rapidly under the ice as on the slopes at a higher and lower level free from ice, but exposed to the action of frost. The effect of elevation of the land will be to make the area above the snow-line greater and to expose a much greater area to the influence of frost. The part affected in the Southern Alps is principally that between the 3,500 ft. and the 7,000ft. contour lines. If the land were raised, the country affected would be approximately that between the same levels, but the area included would be very much greater; although this would be diminished by the accumulation of ice in hollows where it could not melt. Large areas below the snow-line would be
covered with glaciers; but, in spite of this, the area exposed to frost action would be more extensive, and therefore the supply of waste would be in excess. A very large amount of erosion due to glaciers, as estimated by the proportion of sediment in the rivers flowing from their terminal faces, is due primarily to the action of frost on the hillsides above the glaciers.
The supply of waste in this case would be increased during elevation, owing to the previous loosening action of the plants on the rocks rendering them subject to other weathering agencies; again, if this were also attended with a general desiccation of the climate on the mountains fronting the east, the supply of waste would be further increased owing to the disappearance of the protective plant-covering.
From a general survey of the country in the upper basins of our rivers I am of opinion that the period of maximum weathering has passed. The old and mature shingle-slips are far larger than those now existing. Vegetation in many cases has got the better of the moving shingle, and in some cases the old fans are completely covered with forest. Our shingle-slips at the present time are diminishing in extent, and they will continue to do so unless the plant-covering is destroyed either by nature herself or by man.
The excess of waste during a period of elevation accounts for the present form of the Canterbury Plains. They have been formed by the overlapping fans of great glacier streams, as can be conclusively proved by carefully contouring their surface. The contour lines show them to have been formed in exactly the same way as an ordinary shingle fan, except that their grade is more gentle. They were built up to their present height when the rivers were overloaded with sediment, during a time of high land, severe glaciation, and acute frost action. On the land being depressed, the supply of waste would fall off, and the rivers would begin to terrace their old deposits in a manner analogous to that in which a stream terraces its fan during a falling flood. This action was certain to occur unless the volume of the river fell off in a relatively greater proportion. I believe that such would not occur in Canterbury, owing to the excessive amount of waste which would be poured into the rivers falling off in a greater ratio than the decrease in snow or rain.
It will be noted in every case that the grade of the rivers is less than that of the plains; the rivers are therefore able to do their work on a gentler slope than formerly. This can only be due to—(1.) Elevation of the interior of the country since the plains were formed. (2.) Rivers having a greater volume, and therefore power to move their load on a gentler grade: this is extremely unlikely. (3.) A diminution in the supply of waste:
this last appears to me the most satisfactory explanation. No doubt the erosion of its bed which the river is enabled to perform owing to the diminution of the supply of waste would tend to be neutralised by the depression of the land proved on page 32. If the land had been low, and the former supply of waste comparatively small, this depression would have been sufficient to produce aggradation instead of corrasion. But the land is still high, the rivers are still powerful torrents, and the supply of waste fast diminishing. These factors are sufficiently great to nullify the effect of depression in the higher portion of the river course; but the rivers have now reached such a stage in their development that in their lower course aggrading is now going on: hence depression has made its influence apparent. This is what might reasonably have been expected; and, if depression continues, this effect will become more and more marked, so that the terraces will tend to disappear. However, should the slight elevation which has taken place recently continue, aggrading in the lower portion of the river-course will cease and terracing will be resumed.
I have been confirmed in my conclusion that the supply of waste is a controlling factor in the terrace-development of our rivers by observation of the history of shingle fans. In their youthful stage they are built up by an aggrading stream; in their vigorous middle period they are partly channelling their fans and partly building them up on their outskirts; when they reach their mature stage they become channelled and terraced by the stream that runs through them. This terracing closel [ unclear: ] resembles that on the plain course of our rivers. It is more marked near the apex of the fan, and falls off towards the fringe. This may be due to the fact that the river is more confined near the apex of the fan, and therefore more capable of vertical corrasion. But it is also due to the fact that in former times of excessive supply of waste that waste was chiefly deposited just below the gorge. It may perhaps be due to increase in volume of the river as it enlarges its drainage-area. Howeover, increase in volume will not explain the fact that after every freshet a stream apparently terraces its fan on a diminishing volume.
In his accounts of the formation of the Canterbury Plains, Captain Hutton maintained that they had been levelled by the sea and subsequently raised, so that the rivers were able to terrace them. If this were the case, terracing should progress up-stream, should show a maximum development near the sea, and not, as in this case, near the gorges. If, however, the loess is not marine but of æolian origin, as seems very probable, and since it is incapable of resisting marine erosion, there can-
not have been any recent elevation of more than a few feet. The general recent direction of land movement has been down-ward, and this is indicated also by the aggradation going on in the Lower Waimakariri and Rakaia.
The evidence afforded by Otago, where river-terracing is also shown on a gigantic scale, points distinctly to a sinking land. Unless there has been at the same time an increase in the rainfall—and as long as conditions have been the same over the Tasman Sea there seems to be no reason why this should have increased on the mountains—we are at once driven to consider the supply of waste to be a predominating factor in terrace-formation in the valleys of the Canterbury rivers. If we consider those parts of the world where terraces are greatly developed—e.g., British Columbia, the Rocky Mountains region, the Himalayas, and Patagonia—we must be struck by the fact that they have all passed through a severe glaciation, when waste filled the valleys, and now terracing is actively going on. Elevation of the land has had an important effect in some cases, but not in all. It seems that too little consideration has been given to the control exerted by excessive waste-supply.