increase the depth and expansion of the pond, and thus the dam grows
from a small one of a few yards in length to a big one of several
hundred feet--sometimes to even four or five hundred feet in
length--that may bank up the water four or five feet above the stream
just outside the dam, and turn the pond into a great reservoir covering
hundreds of acres of land.

The dam is more often built of branches laid parallel to the current
with their butts pointing up stream, and weighted down with mud and
stones; thus layer after layer is added until the structure rises to
the desired height and strength. Some dams contain hundreds of tons of
material. They are usually built upon a solid bottom, not of
rock--though big, stationary boulders often are included in the
construction for the extra support they furnish. When thus used,
boulders often cause the beavers to divert the line of the dam out of
its usual graceful and scientific curve that well withstands the
pressure from even a large body of water.

The beavers excavate canals--sometimes hundreds of feet in length--to
enable them to reach more easily and float home the wood they have cut
from freshly felled trees lying far beyond the reaches of their pond.
The canals measure from two to three feet in width and a foot to a foot
and a half in depth, and are not only surprisingly clean-cut and
straight but occasionally they are even provided with locks, or rather
little dams, to raise the water from one level to another--generally
about a foot at a time--to offset the disadvantage of the wood lying on
higher and more distant ground than is reached by the waters of the
residential pond. Sometimes their canals are fed by springs, but more
often by the drainage of rainwater. The building of many of their dams
and canals displays remarkable skill and a fine sense of engineering,