Failure of the Yan Yean Reservoir. 119 
ning, is gradually augmented by innumerable rills and streams 
issuing from the moist rotten soil and from the fissures and 
crevices of the rocks. Both branches, at the base of the 
mountain are discharged into large swamps; and it is not 
until they issue again from these that they unite to form the 
Plenty River, about four miles from Yan Yean. The area of 
the western swamp, according to the estimates of your Com- 
mittee, is 787,000 square yards, that of the eastern, 3,808,000 
square yards. 
These streams have been measured at different times and 
under different circumstances, with the following results. 
The late Mr. Blackburn, in his first report on the water 
supply of Melbourne, dated 9th January, 1851, states that 
he had measured two of the branches of the Plenty, above 
the marshes, and found them to discharge respectively 2,000 
and 1,700 gallons per minute, and he estimated that the whole 
discharge of the tributaries amounted to 5,000 gallons per 
minute, which is equal to 6 feet 7 inches in the reservoir, 
while the united stream below the marshes scarcely gave 2,700 
gallons per minute, or 3 feet 7 inches. There was thusa loss 
by evaporation of 2,300 gallons per minute, which would give 
3 feet in the reservoir, in twelve months. 
There being an unusual drought in the summer of 1851, 
Mr. Blackburn, on two separate occasions, revised his former 
measurements ; on the latter occasion, the 14th February, he 
found the discharge of all streams above the swamps amount 
to 4,040 gallons per minute, which equals 5 feet 4 inches in 
the reservoir, and in the river below the swamps he found 
only 865 gallons per minute, or at the rate of 1 foot 1 inch 
in the reservoir, showing a loss by evaporation of 3,175 gallons 
per minute, which would give 4 feet 14 inch in the reservoir. 
Mr. Hodgkinson, on the 9th December, 1852, after fifteen 
hours’ rain, measured the western arm, where it issues from 
the granite rock, and estimated the discharge at 1,180 gallons 
per minute, or 1 foot 7 inches in the reservoir; he also 
measured the same stream, where it enters the swamps, and 
found it to give 1,700 gallons per minute, or 2 feet 3 inches. 
On the following day he measured the eastern arm, below the 
first waterfall, and found it to discharge 1,980 gallons per 
minute, or at the rate of 2 feet 74 inches in the reservoir. 
The first of Mr. Hodgkinson’s measurements is the one of 
most value in this inquiry, as we may presume that the stream, 
where it issued from the rocks, was little affected by the 
previous rain. The second measurement was taken at a 
