![Thumbnail: [Volume 43, 1910 page 442]](/journals/rsnz/images/rsnz_43/thumbnails/rsnz_43_00_0471_0442_ac_02.gif)
(2.) Erosion and Silting.
Erosion and silting generally go together, and either one or the other happens according to the velocity of the current in a river-bed. When the course of a river is steep, and the soil is of a soft or friable nature, the water in the proportion of its volume and velocity scoops out the bottom, and the excavation works back until it reaches a substance of sufficient hardness to be resistant, when a waterfall or permanent rapid is formed. But as the inclination of the bed becomes less the flow of the stream is retarded, and the substance that has been brought down by the current tends to settle in the bottom. In flood-time, however, large masses of stone are swept down, and by grinding against the rocks in the sides and bottom, as well as by mutual attrition, they are rounded into pebbles, becoming smaller and smaller as they travel along, until they wear down into gravel, and eventually into fine sand, which is carried in ripples along the bottom. Meanwhile all soft rock, clays, and earthy matter are quickly resolved into mud. When the bed approximates so nearly to a level that the rate of the current is less than 6 ft. per second on the bottom, then the river is no longer able to shift the solid material, and only the impalpable particles of mud, which may be almost said to be held in solution, are carried along.
This is the process known as “silting,” and it is easy to see that the quality of the silting must entirely depend on the character of the riverbed and of the nature of the material brought down.
When the bed is short and steep and the incline is continued to the coast, the bulk of the silt is carried down to the sea, and no harm is done unless the mouth of the river be situated in a harbour, when, of course, trouble may arise from the shallowing of the water.
It is when a flooded river traverses an alluvial plain that the silting does most damage. The débris brought down by the head-waters must find a lodgment somewhere, and, as the current loses its velocity on reaching the level country, it is no longer able to bear its burden along. The silt therefore lodges on the bottom, and the bed gradually rises until the water is forced over the banks. Then the water breaks away and cuts a new channel for itself, which in time fills up, and the same thing happens over again.
Numberless instances of this process are found in many parts of both the North and the South Islands. Wherever, as in Hawke's Bay, Canterbury, &c., the alluvial plains are backed by a mountainous country the surface is often torn away, the land is scored in every direction, and the fertile soil covered with a deposit of stones, gravel, and slime. A notable instance occurred during the great Napier flood of 1893, already mentioned, when the River Ngaruroro left its bed, and; joining with the Tutaekuri, cut its way through the road and railway to the sea.
A foolish tradition has prompted local governing bodies and private owners in many places to plant the river-banks with willow-trees, with
![Thumbnail: [Volume 43, 1910 page 443]](/journals/rsnz/images/rsnz_43/thumbnails/rsnz_43_00_0472_0443_ac_02.gif)
the view of protecting them against the scour of the current But the scheme generally defeats its own object, and is often the cause of much mischief, which, moreover, is not always confined to the locality in which the planting takes place. Sooner or later, especially if the river is a rapid one, and runs through alluvial country, the trees are undermined and swept down until they are caught by some obstruction or are stranded in some shallow place. Here they intercept the silt and the floating débris that comes down with every flood, and an island or dam is formed, which drives the current into the banks, or even compels it to seek a new channel. In the Lower Waikato the obstruction of the willow islands has caused the bed of the river to silt up to such an extent that in many places the level land on the banks is flooded every winter, and the Township of Mercer is frequently under water after a few days' heavy rain.
It may be asked whether the damage done by the willow-trees has anything to do with the subject of this paper, which professes to deal with the effect of the disappearance of the forest. The answer is that if the forest had not been removed the damage done by the willows would be comparatively trifling—if, indeed, it would have been considered necessary to plant them. But, as I have endeavoured to show, it is the removal of the forest that is directly responsible for the growing violence of the floods, and therefore for the increasing amount of silt and floating detritus, which the willows intercept.
There is another aspect of the silting question that must not be overlooked—viz., the formation of river-bars and the silting-up of harbours. All the mineral débris, stones, gravel, and mud that are carried down by a stream are immediately deposited on the bottom as soon as the current ceases to act, which it does on reaching the sea, and here it forms a bank or shoal, which is augmented by the sand or other material which the sea washes on to it. If the river falls into landlocked water the finer particles held in suspension are carried out into the stream, and drift up and down with the tide until they are precipitated wherever there is least current. In many river-mouths, estuaries, and harbours the effect of the wholesale forest-clearing is already being severely felt. Of late years, unless where temporarily scoured out by a fresh, there is less water on many of the bars, while in some of the shallower harbours—e.g., those of the Thames, Coremandel, &c.—the wharves have had to be lengthened and the buoys on the shoals moved further out. One of the most fertile sources of harbour-silt, and one not generally taken notice of, is the fine dust that is formed by the action of the sun on the bare hills, and washed by the rain into the creeks. This is, doubtless, one of the principal factors of the extensive mud-flats so frequently found in landlocked waters.