The second kind of dam is for swift streams. Stout, ten-foot brush is
the chief material. The brush is floated down to the spot selected;
the tops are weighted down with stones, and the butts left free,
pointing down stream. Such dams must be built out from the sides, of
course. They are generally arched, the convex side being up stream so
as to make a stronger structure. When the arch closes in the middle,
the lower side of the dam is banked heavily with earth and stones.
That is shrewd policy on the beaver's part; for once the arch is
closed by brush, the current can no longer sweep away the earth and
stones used for the embankment.
The third kind is the strongest and easiest to build. It is for places
where big trees lean out over the stream. Three or four beavers gather
about a tree and begin to cut, sitting up on their broad tails. One
stands above them on the bank, apparently directing the work. In a
short time the tree is nearly cut through from the under side. Then
the beaver above begins to cut down carefully. With the first warning
crack he jumps aside, and the tree falls straight across where it is
wanted. All the beavers then disappear and begin cutting the branches
that rest on the bottom. Slowly the tree settles till its trunk is at
the right height to make the top of the dam.
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