STREAM REGULATION

Evening Post, Volume CIII, Issue 53, 4 March 1922, Page 9

STREAM REGULATION

FOREST CONSERVATION

AN OFT-NEGLECTED ASPECT.

(Contributed.) The passing of a resolution by the Society of Civil Engineers "to support the efforts of the Forest Service in preventing the deforestation of the mountainous slopes at the head-waters of the New Zealand rivers, as an essential measure for stream.regulation," cannot be allowed to pass without some explanation and comment of appreciation.

This aspect of our forest policy is one too often.neglected, and it is hoped that this action on the part of an unbiased body of professional men will succeed in arousing public interest in the matter. ■The relation between forests and .streams involves two main considerations. First, the relation between forests and the regularity of flow of streams, and, second, the relation between forests and the erosion of the soil and the consequent silting up of the rivers. While the usual practice, it is inadvisable to approach.either of these problems from a consideration of the relative merits of two areas, one forested and the other deforested. There are a number of classical foreign investigations of this character which prove undoubtedly the advantage of the forested area over the deforested, but the process of reasoning from particular to general results often leads to erroneous conclusions. In the'absence, therefore, of New Zealand investigations , the explanation given is based on a purely deductive form of reasoning. Now the stream flow of any area is dependent on the amount of rainfall and run-offj and it is therefore desirable to consider these and other factors separately. First, the effect of forests on. rainfall. It has been demonstrated conclusively that the forests cool the air,, and also that a forested area evaporates or transpires- a greater total amount of moisture than the same area without forests. As the coolness and humidity of the atmosphere are the determining conditions for rain it is reasonable to suppose that the effect of the forests is to increase precipitation. RAINFALL AND RUN-OFF. Rain, however, does not fall uniformly throughout the year, and any agency which serves to store water during periods of excessive rainfall to- be used later for Supplementing the supply in periods of deficient precipitation is of the utmost importance. A stream is fed by two,sources: by the water which percolates into the ground and which reaches the stream from springs or from the ground water, and by tne water which flows immediately into the stream from the surface of the ground. In general the greater the amount of water from the former source and the less from the latter, the more regular the flow of the stream. The only demonstration necessary to show how this is effected by the forest is to observe any forested area during a rain-storm, and then to observe the open ground under the same conditions.. There is no doubt that on the steep slopes the forests retard the discharge of surface water, facilitate percolation into the ground, and so contribute to the regulation of streams.

The navigability of streams depends not simply upon the regularity of flow, but also upon the sediment carried, the consequent silting up, and the necessity for maintaining a channel of sufficient depth by means of dredging.' In regard to this which reduces to the erosion of the soil, the effect of the forest is just as clear, if not more clear than ,in the case of regularity of flow. It is only necessary to say that anyone who has ever been in a forest on a steep mountain slope knows from observation that the effect of the forest "is to hinder, and even to .prevent, the erosion of the soil. It is the erosion of the mountain slopes which is the principal cause of the silting up of our rivers, even though the material which immediately forms the river-bars does not come from the mountain slopes, but from the caving banks along the nearer reaches of the stream. A little thought will show this. The stream and rivulets which flow ■ down the steep mountain sides . with great velocity may carry large particles of earth with them, or even gravel or stones, transporting some and rolling others along the bed. When they reach a point In the stream where the slope is not sufficient to give a velosity which would carry these particles, they are deposited in the stream bed, and filHt up. In order to discharge the same volume of water, therefore, the stream has to form for itself a new bed. This is done by cutting into the adjoining banks, and acquiring thereby a load of sediment 'consisting of small particles which it carried down until its velosity becomes so reduced that it can no longer carry even these, which, in turn, being deposited,. obstruct the channel at this lower point, and cause as before an ero- | sion of the banks there. In this way, in the course of many yearsi material is transported gradually down the stream, the particles become worn and rounded, weathered and reduced in size, and, finally, at the mouth of the stream it carries into the ocean a load of fine silt which is deposited as a bar. This is, of course, not a complete discussion of the causes of the caving of river banks. It is simply an indication that this is to some extent due to erosion further up the stream. EXPERIENCES ELSEWHERE. At the tenth Congress of International Navigation, held at. Milan in 1905, the reporting engineers were unanimous on this question. A few remarks of some of the delegates to this conference are of interest. Mr. Ponti, of Italy, reports as follows:— < '

"In Sicily the consequence of cutting away the forests on a vast scale in the province of Messina has been also to raise^ sensibly the bed of the streams, and maiiy 'of these beds are now above the adjoining fields."

Mr.' Riedel, of Vienna, with reference to Germany, states that " the unquestioned circumstance that a large number of rivers now carry down more material than formerly is a consequence of the: extensive denudation and careless clearing of plantations. ,The slopes of the hills lose a large part of their fruitful soil, and in many cases earth slides, and even extensive subsidences 'of whole slopes, take place, while considerable areas of ground in the valleys are smothered up and rendered useless."

M.. La Fosse, the French delegate, says: "If the destruction of forests is to be deplored, it is most of all on the mountain that the cutting away of timber is to be feared. It is not alone the supply of the springs and the discharge of the streams which are in danger, it is the very existence of the rivers themselves. The stream which can be utilised disappears to give place to the devastating torrent." The importance of this question is well illustrated by the comprehensive plans now being adopted in India, Switzerland, Austria-Hungary, Norway, Sweden, Denmark, Rumania, Germany, and Japan for the prot3ct:on of the forests at tho headwaters o? mountain streams. It will be noticed that another aspect of the question has been introduced by the foregoing quotations of the delegates of the Navigation Conference. Many instances are available ;n New Zealand, where fruitful lands of only twenty years ago are now forming the bedr, of our riverc.. A VTT.IL PROBLEM. The problem is, therefore, a vital one to the farming community, and one which should call for the heartiest sup-

port of our Forest Service policy. Let it not be said of us that we have been unable to profit by the experience of others, but have had to laarn by dear experience the same lessons which they have learned, and which we might have learned from them.

Flat lands having good soil are needed for gro\ying crops and other purposes, and will be so needed in great measure as population increases. We should exercise discrimination in the utilisation of our lands, cultivating for growing crops those which ate best suited therefor, and reserving for forests steep slopes and mountain sides, and those areas unsuited for cultivation of crops. We should recognise that our forests are a valuable national asset, and that, independent of their production of timber, they regulate flow and prevent erosion.

Our thanks are due to the. New Zealand Society of Civil Engineers for their action in this matter. The dependence of our prosperity as a nation upon the engineer as a pi'ofessional man should be better realised. His advice should be sought in an increasing degree by the nation, the State,'and by the people. In this' way the benefit of scientific training and discovery will be directed to this problem, and it is not impossible that new methods and discoveries may be brought to light which will reduce our enormous waste, and make the prospects for our descendants more cheerful.