The elevation of
the river at the gap is 168 feet. The slopes in the basin being
gentle up to an elevation of 220 feet and abrupt beyond it, it will
be advisable to take this as the minimum or low-water level of our
reservoir. It is generally estimated that 25 per cent of the volume
of the mean annual rain on a given catchment is sufficient
reservoir capacity to fully utilize the flood flows. We have long
series of observations of rainfall at three points, which may be
taken to fairly represent the Passaic catchment. At Newark the mean
annual rainfall is 46.2 inches, at Paterson, 50 inches, and at Lake
Hopatcong, 42. The last being on the Highlands, like most of our
watersheds, is perhaps the safest to use. Now, 25 per cent of 42.5
inches, 10.62 inches, which, on 420 square miles, give a volume of
10,362,000,000 cubic feet, the necessary capacity of reservoir.
By raising our reservoir to 240 feet when full we secure a capacity
of 10,493,000,000 cubic feet, or ample to utilize the heaviest
floods of the watershed.
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