Art. XVI.—On the Destruction of Land by Shingle-bearing Rivers, and Suggestions for Protection and Prevention.
[Read before the Nelson Association for the Promotion of Science and Industry, 6th December, 1871.]
The great loss of land and damage occasioned to property yearly in New Zealand, is a matter which is rapidly assuming a more and more serious aspect; and although at present, as a general rule, the greater portion of the land destroyed is of no great value, nevertheless in many places on the banks of the smaller rivers considerable quantities of valuable land have been lost, in some instances entailing great expense and heavy loss to the owners and occupiers. For instances we need not go far in the Nelson province. Both the Motueka and Waimea rivers have widened their beds considerably; the former was, twenty years ago, only a few chains, but is now in many places nearly half a mile wide, and is rapidly filling up its bed with shingle brought down by the floods, and cutting new channels in the alluvial flats adjoining. Between the lowest ferry and the upper part of Pangatotara, several hundreds of acres have been destroyed during the last few years, and the river is now rapidly destroying some of the best land in Riwaka. The Waimea, although of considerably less volume than the Motueka, is continually encroaching upon the adjoining alluvial lands, every flood doing more or less damage. This has been particularly the case above Appleby, and again near Wakefield and Fox-hill. To take larger examples, we have the Wairau, which has always been a source of great anxiety, danger, and loss, to the inhabitants of the plains near Blenheim. The Waimakariri, in Canterbury, has encroached enormously on the alluvial land lying to the northward, and caused a great amount of damage to the farmers by the loss of crops, homesteads, and land. In fact, all the shingle-bearing rivers of the country are continually altering their beds and destroying adjoining land.
In order to correctly appreciate the causes which make this the rule, it will be necessary to consider the general characteristics of shingle-bearing rivers and the country which they drain. The greater number of the rivers in the Middle Island of New Zealand are, more correctly speaking, mountain torrents, which rise in lofty mountains and run but a short distance before reaching the sea; they are subject to very high and sudden floods, which occur in the spring and early summer, from the melting of the snow by the warm rains. The greatest floods occur generally in the streams which are fed by the glaciers in the Southern Alps, the warm northerly rains which fall on the glaciers and exten-
sive snow-fields occasioning enormous floods. From the rapid fall in the river beds, many of them having a greater inclination than thirty-three feet to a mile in their upper parts, vast quantities of drift shingle and silt are brought from the higher levels and deposited in the lower, where the velocity of the stream is diminished by the lessening inclination of the bed; the fine gravels and sand are borne onward to the sea, and form the bars and shoals which exist at the mouths of all our rivers.
The larger gravels are thrown down as the velocity of the stream diminishes, and rapidly fill up the lower portions of the river bed until it is raised above the level of the adjoining land, when the stream, during some flood, overflows its banks, inundating all the low ground adjoining, and makes a new channel in the lower ground, which in its turn will be rapidly filled up, and the stream overflows again into the lowest ground in the neighbourhood. To this action is due the formation of most of the alluvial flats bordering the lower portions of the river courses. This subject has been very carefully investigated in connection with the formation of the Canterbury plains, by Dr. Haast and Mr. Doyne, C.E., whose reports and maps are most instructive. The process of successive elevations of the river bed is much more rapid in an open country such as Canterbury, where there is nothing to prevent the destruction of the river banks, than it is in a wooded country, such as the Nelson province, but the process is exactly the same in both cases. In a forest-covered country, such as the greater portion of the Nelson province is, the elevation of the river beds is necessarily a slow process. The forest clothing the mountain sides checks the sudden rush of water down hill during rain, besides preventing it from cutting water-channels in the surface, thus preventing a supply of detritus to the river in the first instance; and also the banks of all streams and rivers in a state of nature, before disturbance by the hand of man are, thickly covered with scrub and ferns, which, hanging down into the water, constitute a most effective protection against the destructive action of the rivers. The natural vegetation which covers the surface of a wooded country may be truly said to form the best protection to its surface, and the difference in the manner in which the water of precipitation drains off forest and open land is very striking, and well worthy of attention. When heavy rain falls on forest land, before it begins to flow on the surface it has to saturate all the humid and decaying vegetable matter which lies at the foot of the trees; the surface of the ground is also so covered with a network of roots that the water can only form a series of pools, which overflow from one to another as the rain continues, and a large body of water is thus retained upon the ground, which drains off slowly through the moss and roots. The small water-courses also get filled with trees, masses of twigs and moss, which materially assist in checking the velocity of the streams, and prevent abrasion of the surface—but
the case is very different in open country, where there is nothing but grass to check the flow of water on the surface. After saturating the soil the water rushes without hindrance into the nearest hollow, and, rapidly accumulating volume and velocity, soon forms a dangerous and foaming torrent, which, cutting into the surface of the ground, carries down large quantities of gravel and detritus into the nearest river.
Whenever a river, for the greater part of its course, runs through a wooded country, the changes are effected so slowly in the river bed that the vegetation has ample time to take possession of any ground the river may abandon and convert it into forest land; the scrub and undergrowth also retain the silt borne amongst them by the floods, so that the banks and lowlands are raised and fertilised by every inundation. All this, however, is totally changed as soon as settlement commences and man begins to clear the timber and cultivate the soil. The timber is frequently taken from the river banks first, from the facility of transport by water—cattle feed on and destroy the scrub which clothes the banks, which, denuded of the natural covering, become an easy prey to the action of floods—every ditch that is dug increases the rapidity with which the rainfall is carried into the river, and the floods necessarily rise higher than before from having to carry off a greater body of water in the same time; as the clearing is carried further inland and the hill sides are bared, the water, during rains, can collect rapidly in all the small gullies, which will be converted into foaming torrents, and, no longer prevented by roots and moss from abrading the surface, they carve deep gullies in the mountain sides, bearing down enormous quantities of broken stone and gravel into the main stream below, which, in its turn, will carry on the detritus as far as the strength of the current permits, and then throw it down to fill up the river bed, and add to the destruction already in progress by the cutting away of the river banks. The enormous devastation occasioned by the indiscriminate destruction of forests in the Old World is so clearly shown by Mr. G. P. Marsh, in “Man and Nature,” that I must refer to his work, as it gives a far better idea of the evils to be apprehended from the destruction of forests than any description of the devastations at present in progress in this province.
His descriptions show that most disastrous results may be expected from the felling of timber in the valleys and on the mountain sides in this province, unless steps are taken to prevent the evils thus occasioned by renewed planting, and the conservation of the forests in the upper course of the streams.
The town of Nelson, standing as it does on the banks of a mountain torrent, is particularly liable to damage from inundations. Floods which have already occurred show the amount of damage even a small stream can do in a few hours, when flooded and heavily charged with detritus. Owing to the inaccessibility of the valleys of the Maitai and the Brook, but little clearing
has been done, but, as the means of communication are improved, there is no doubt that a great deal more timber will be obtained from these districts; the extension of the dray road up the Maitai will enable fire-wood cutters to work where they have been unable to do so hitherto, and, if the road was but a good one, there is no doubt that a great deal of wood would be brought in from there. The destruction of the forest in the basin of the Maitai I conceive to be prejudicial to the safety of the town in the highest degree, so much so, that I feel no hesitation in stating it as my opinion that before two-thirds of the water-shed of the Maitai are bared of timber, the destructiveness of the floods will have so increased that all the lower parts of the town will be converted into shingle bed. As the upper parts of the rivers in the province run for the most part through wooded country, composed to a great extent of drift shingle, very great destruction may hereafter be confidently expected if steps are not taken, on some general scheme, to preserve the woods which clothe the mountain drainage basins, and to protect the river banks from damage.
For the protection of the river banks, I would suggest the planting of willows, in great quantities, all along the banks and on the shelving gravel beds. Cattle should be kept from destroying the trees, which should be planted on every available part of the river bed; all low flats should also be planted with useful trees, and every little streamlet and water-course that carries shingle should be well planted. The great object to be attained is to prevent shingle from travelling in the first instance as much as possible, and this can be achieved to a great extent in all open ground by planting along the water-courses. An excellent example of the efficacy of this system can be observed at Stoke, near Nelson, on the property of Mr. Marsden, where a dangerous and troublesome stream has been carefully and judiciously planted in this manner, with willows in the bed and European trees on the bank, and thus changed from a destructive torrent into a pleasant brook, which greatly adds to the beauty of the grounds. In the large streams, where the banks are perpendicular and are at present being undermined, planting could not be executed without other measures were taken in connection with it. The banks would require to be well sloped, or, if the land was sufficiently valuable, it might be worth while to undertake the erection of engineering works to divert the current from the bank until the planting could be properly effected. But it must be borne in mind that so long as the higher parts of the rivers are neglected, whatever may be done on the lower levels will be of very little use, for, if the action of the river is to raise its bed, any protective works that may have been erected on the banks will require to be raised as the bed rises, thus entailing a constant outlay. The streams should be encouraged to meander at first as much as possible over the existing shingle beds, for, by encouraging the length, the fall and velocity are naturally diminished.
If the banks and gravel beds of the Waimea, for instance, were well planted with willows, and the neighbouring low grounds planted with rows of trees and shrubs at right angles to the flow of the water, but little damage could be done by floods, and the trees on the banks would materially assist the deposit of silt during floods, thus raising and fertilising the low ground. In the open country, where timber is very scarce, such as in Canterbury and Otago, the planting could be made to serve a double purpose, for, if properly managed, twenty years after planting a great deal of wood might be cut without in the least endangering the efficacy of the trees as a protection from floods. There are many difficulties in the way of preserving the timber and instituting a general system of planting such as I have suggested; there would also be a difficulty about keeping cattle from destroying the trees,* but these matters are questions which could be readily settled by the inhabitants of the districts likely to be damaged, as soon as the magnitude of the evils, which are certain to follow the clearing of the mountain sides and destruction of river banks, was clearly appreciated. No one who travels much in the Middle Island of New Zealand can fail to be struck by the amount of ground occupied by the river beds, nor fail to observe the rapid increase in size of most of the streams on the banks of which clearing is going on, and it is with the view of directing attention to this important subject that I have ventured to write this paper.
It may be urged that but little loss has been suffered yet, and that it will be time enough to go into the question when it assumes a more serious aspect; but, in answer to this, it must be remembered that preventive measures can hardly be taken too soon; and further, when the destruction has once commenced on a large scale, nothing but time and a very great expenditure can possibly remedy the evil.
[Footnote] * The Acacia dealbata is recommended by Mr. Ludlam, of Wellington, as a good tree for such purposes, and not so liable to be destroyed by cattle as the willow.