76 FORESTS, WOODS, AND TREES 
silted up. At present, on our treeless catchment areas, as 
Mr. Margerison (6) pointed out, “ the heads of great reser- 
voirs are being very seriously and rapidly silted up, and 
some day they will cost an enormous sum for cleaning out. 
Where is the silt to be deposited so that it is not again 
quickly washed back? And what are those corporations to 
do with it who do not own the adjoining freeholds ? Herein 
afforestation will prove beneficial. Plantations will prevent, 
or greatly minimise, silt-laden surface-water rushes by block- 
ing them and by promoting percolation and filtration.” It 
has been objected that wooded catchment areas may pollute 
the water supply on account of the enormous masses of dead 
leaves which decay on the watersheds, or are blown about 
and carried down by streams to rot in the reservoirs. The 
humus in the forest is, however, antagonistic to pathogenic 
bacteria, and the decayed leaves themselves are absolutely 
innocuous. In order to prevent the leaves being blown 
into the reservoir, a band of ground next the water may be 
planted with gorse and other small-leaved shrubs, which 
form traps for the leaves of the trees. A narrow belt of 
spruce trees, which have dense, small, evergreen leaves, is a 
more efficient screen for this purpose. Only a small part of 
the foliage of the spruce is shed annually, namely, the leaves 
on the oldest and innermost parts of the branches. These 
leaves, as they are shed, fall on the ground beneath and 
are not blown to a distance. The falling leaves of oak, 
beech, ash, and other broad-leaved trees are blown to con- 
siderable distances by the wind, and, in the absence of a 
screen of spruce, may constitute somewhat objectionable 
litter in the water of the reservoir. 
To sum up, the effect of trees on the yield and quantity 
of the water supply collected on upland areas cannot be 
otherwise than beneficial In the US. Year-Book of 
Department of Agriculture, 1902, it is laid down: “ A forest 
furnishes the best possible cover for watersheds of storage 
reservoirs. For this reason fully as much as the financial 
one, several water companies are planting extensively in the 
Eastern States, especially in Massachusetts and Connecticut.” 
