t his escape to water would be
cut off by thick ice. So he plans a big claw-proof house with no
entrance save a tunnel in the middle, which leads through the bank to
the bottom of his artificial pond. Once this is frozen over, he cannot
get out till the spring sun sets him free. But he likes a big pond,
that he may exercise a bit under water when he comes down for his
dinner; and a deep pond, that he may feel sure the hardest winter will
never freeze down to his doorway and shut him in. Still more
important, the beaver's food is stored on the bottom; and it would
never do to trust it to shallow water, else some severe winter it
would get frozen into the ice, and the beavers starve in their prison.
Ten to fifteen feet usually satisfies their instinct for safety; but
to get that depth of water, especially on shallow streams, requires a
huge dam and an enormous amount of work, to say nothing of planning.

Beaver dams are solid structures always, built up of logs, brush,
stones, and driftwood, well knit together by alder poles. One summer,
in canoeing a wild, unknown stream, I met fourteen dams within a space
of five miles. Through two of these my Indian and I broke a passage
with our axes; the others were so solid that it was easier to unload
our canoe and make a portage than to break through. Dams are found
close together like that when a beaver colony has occupied a stream
for years unmolested. The food-wood above the first dam being cut off,
they move down stream; for the beaver always cuts on the banks above
his dam, and lets the current work for him in transportation.
Sometimes, when the banks are such that a pond cannot be made, three
or four dams will be built close together, the back-water of one
reaching up to the one above, like a series of locks on a canal. This
is to keep the colony together, and yet give room for play and
storage.

There is the greatest difference of opinion as to the intelligence
displayed by the beavers in choosing a site for their dam, one
observer claiming skill, ingenuity, even reason for the beavers;
another claiming a mere instinctive haphazard piling together of
materials anywhere in the stream. I have seen perhaps a hundred
different dams in the wilderness, nearly all of which were well
placed. Occasionally I have found one that looked like a stupid piece
of work--two or three hundred feet of alder brush and gravel across
the widest part of a stream, when, by building just above or