109 
Take the case of the Hawkesbury lliver, which comes to my mind as I write. 
Everyone knows that in the " early days " this river was navigable as far as Windsor 
for fairly large craft, but the navigation of the river to any but the smallest boats 
has long been impossible. Why ? Simply because the river has silted-up. What 
has caused the silting ? In my view, the indiscriminate cutting down of the forests 
in the watershed of the Hawkesbury, and the cultivation of the land too close to the 
banks of the Hawkesbury and its tributary streams. The quantity of silt that comes 
down the river, even by a " fresh," is surprising. Much of this is deposited in the 
bed of the river, and does not pass out to sea. The same remarks apply to the 
Hunter and other rivers. I am fully aware that much of this cutting down of 
forests and cultivation are quite unavoidable; but I also assert, without fear of 
effective contradiction, that much of this cutting down of trees and clearing away 
of vegetation, and also this greedy cultivation of land towards the margin of streams 
and in the line of vvashaways, is the result of cupidity and ignorance, and must 
result in national impoverishment as certainly as the night follows the day. 
In 1905 a meeting of the International Navigation Congress was held at 
Milan, Italy. 
One of the questions taken into consideration was " the influence which the destruction of forests 
and desiccation of marshes has upon the regime and discharge of rivers," and seven papers bearing upon 
the subject were read and discussed. Of these, three were from Austria, and the others from Germany, 
France, and Russia. The problem as to the effect of forests on the water supply of rivers and of climate is 
of great social importance, on account of the agricultural and commercial interests which are so closely 
connected with the use of timber, and with the utilisation of running water. It is allowed by all the 
authors of these papers that, due to the improvident way in which the forests have been dealt with, there 
has been marked change in the water supply of the neighbouring rivers ; that where forests have been 
cut down brooks have disappeared, and many small rivers that at one time were useful as sources of power 
are so no longer for want of water ; that in the larger rivers torrents have become more impetuous and 
flooding more frequent, while, on the other hand, navigation suffers at times for want of water. 
The greatest harm has been done in the mountain districts, where the steep slopes allow the rain- 
water to run off too rapidly, carrying away the surface soil and transporting pebbles and boulders into the 
rivers, causing shoals, and thus decreasing their capacity to discharge the flood-water. The extent to 
which forests, both on the Continent and in America, are being cut down and destroyed, and large areas 
of land, which at one time were covered with primseval forest, have become barren waste by fire or the 
lumberman's axe, without any attempt at reafforestation, was one of the subjects dealt with in the presi- 
dential address of Mr. J. C. Hawkshaw, at the Institute of Civil Engineers, in 1902 
The question for consideration at the Congress was whether the wholesale destruction of forest land 
for cultivation or for timber supply is having any material effect on the rainfall and consequent water 
supply ; aad the effect of forest destruction on the rivers of the country from which the trees are removed 
was also considered. The physical conditions of forest land are that, owing to the shelter from sun and 
wind, the atmosphere is generally colder and damper than in the open country, and evaporation conse- 
quently less. It is calculated that a hectare of forest land (2i acres) gives off every day 37 cubic metres 
of oxygen and 37 metres of carbonic acid, leading to a great expenditure of heat ; and that from every 
hectare of forest land sufficient heat is abstracted to melt 316 cubic metres of ice. Ligneous plants also 
withdraw from the ground and discharge as vapour more than 40,000 gallons of water per hectare per day, 
which causes a sensible reduction of temperature. When clouds pass over a forest they encounter a cool, 
damp atmosphere, the point of saturation comes closer, and rain is caused. This condition of forest land 
has been remarked on by aeronauts, who find that a balloon is invariably affected, and drops when passing 
over forests 
On the other hand, it has been contended by some of those who have made a study of sylviculture 
that forests do not increase the quantity of water flowing to the springs and rivers, but reduce it. The 
numerous striking facts quoted do not bear out this contention, which is mainly based on the fact that the 
