CHAPTER IV 
THe Dam 
Unless the animals happen to be living in natural ponds or 
lakes, the dam is almost sure to be the first structure made 
by the beaver. It is necessary to their existence Inmany 
cases, and absolutely essential to their welfare. Therefore 
in the founding of a new colony dam-building is the first step, 
and once built, the dam requires constant attention and 
repair to keep it in condition. 
The method of construction appears to be always practi- 
cally the same at the beginning. Branches of willows, 
alders, or whatever brush is most available, and this is usually 
one or both of the two just named, are cut and placed on the 
bottom with the butt ends upstream, and often forced into 
the bottom. Mud and gravel, and frequently stones, are 
put on these branches, this material being obtained from the 
bottom just above the dam. Other layers of brush are 
placed upon the first, each in turn weighted down with mud, 
until the dam reaches the desired height. At first the water 
leaks through the loosely constructed affair, but the stream 
brings down sediment which helps to make the dam water- 
tight, more mud is placed on the upper side and on the crest, 
and soon the dam is holding water, and the pond has filled. 
A drained beaver pond usually, if not always, has a ditch 
or trench immediately above the dam from which the earth 
used in the latter was taken. 
Such, in a few words, is the construction of the usual dam. 
But a dam is never finished as long as the pond is occupied, 
for it not only requires continual attention to keep it in 
ordinary repair, but the pond tends to fill up with silt brought 
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