. If it were found to be true, it
could be advantageously taken into consideration in connection with
measures for the prevention of flood damages. As the Pompton had no such
effect upon the flood flow at Dundee dam in two consecutive historic
floods, the writer is inclined to believe that the idea is entirely
erroneous.

[Illustration: FIG. 1.--Comparative flood run-off at Dundee dam, March,
1902, and October, 1903.]

Since the flow curves in fig. 1 were drawn it has been found by careful
observation that the depressions which occur in the rise of every flood
over Dundee dam are probably due to the carrying away of the flashboards
which are placed upon the dam crest in times of low water. A review of
the gage heights recorded by floods for several years past shows that
the break occurs when the height of water over the dam crest reaches
from 40 to 60 inches. The flashboards used upon this dam are usually 18
inches wide, and as they are supported by iron rods, which are of
approximately the same strength and are placed upon the dam by one crew
of workmen, it may be safely assumed that they are of approximately
equal stability and might be expected to fail almost simultaneously
along the length of the dam crest. So sudden a decrease in the effectual
height of the dam must lower the water on the dam crest markedly, and as
every other probable cause has been eliminated in the case of the recent
flood, the explanation of the check in the progress of floods over this
dam may be safety accepted as due to carrying away of flashboards. This
effect should be apparent in the gage-height records only.

In the flow diagrams (figs. 1 and 2) the effect would not be the same,
but the curve would rise more sharply. Similarly, the measurements at
the beginning are not correct, as they are calculated according to gage
heights measured from the stone crest of the dam. Therefore, a true
flood curve at this point would be much flatter at the beginning and
rise sharply at a period coincident with the carrying away of the
flashboards.

An important difference between the two floods is that the earlier
continued longer, but the later one was much higher. The flood of 1902
was caused by the turning of an equivalent of approximately 6 inches of
precipitation into the main channel during a period of six days. In the
deluge of 1903 there fell 11.74 inches of rain, the greater part of
which was precipitated in 36 hours. Thus it is seen that the