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 

 Avhlch 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 httle increased in size, during the winter 

 months, in ordinary seasons. In my estimate of the discharge 

 of the the river I have allowed 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 w^e 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 mountamous 

 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- 

 cordino' 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, 

 includino- 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 



