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.
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