126 Failure of the Yan Yean Reservoir. 
Thomson’s estimate cannot apply to the Plenty Ranges, but 
their geological formation, and the tropical vegetation with 
which they are covered, are singularly adapted to absorb and 
retain a large proportion of the rain that would otherwise 
flow direct into the watercourses; and it is to this beneficent 
provision of nature in this dry climate that all the rivers that 
take their rise in the primary and granitic formations owe 
their permanency, and not to springs of the ordinary kind, 
that are met with in the secondary and tertiary formations, 
which are almost entirely absent; and were it not for this 
provision the river Plenty, and other similar streams, would 
cease to flow altogether in the summer months. The winter 
rain which is now stored up in the spongy soil, and in the 
caverns and fissures of the rock, maintains a more or less 
constant stream during the whole summer, and it is in this 
manner that we explain the otherwise singular fact, that the 
river Plenty is so little increased in size, during the winter 
months, in ordinary seasons. In my estimate of the discharge 
of the the river I have aliowed an increase of two-thirds. 
But it is not to be supposed that the water thus stored is 
altogether removed from the influence of evaporation. On 
the contrary, from its universal tendency to find a lower level 
it is constantly oozing out over the whole surface of the 
ranges, and leading gullies, which is thus always in a wet 
condition, and always evaporating, and the surface is so wet 
even near the summit, that we found abundance of small 
leeches several hundred yards from the stream. 
The watershed of the Plenty ranges, therefore, differs 
essentially from the watershed of ordinary mountainous 
country, and thus the evaporation from the ranges is, proba- 
bly, fully equal to that from the plains, because while eva- 
poration from the level country is one-third more rapid than 
from the ranges, it ceases nearly altogether for three 
months in the former, while in the latter it is constant 
throughout the year. 
The drainage area of the river above the aqueduct, ac- 
cording to the Survey Maps, may be computed at about 
sixty square miles. The ratio of this surface to the surface 
of the reservoir is as 26 to 1, therefore the whole rainfall, 
including four inches of dew, would give a depth of eighty- 
eight feet in the reservoir, and one-ninth part of this, or nine 
feet nine inches, would give the watershed. 
It may be interesting here to contrast the whole discharge 
of the Plenty, as I have already estimated it, with the 
