THE BEAVERS OF NORTH AMERICA 48 
August. Then they see what is the minimum 
supply of water, and the dam may be built with the 
least possible trouble. When they finally settle 
on the exact site, the first sticks are laid, and these 
in most cases are freshly cut and not peeled. 
Alders are perhaps the most frequently used, as 
they are found growing along the waterways and are 
therefore easily obtained, while their weight allows 
of their being sunk without much difficulty, and 
their irregular form of growth helps them to hold 
on the bottom against the current of water. How 
quickly a dam is built depends entirely on the 
urgency of the situation. If it is late in the year, 
the building proceeds with incredible speed, little 
more than a week of steady work on the part of a 
family will see a thirty or forty-foot dam raised to a 
height of two feet or more. As soon as it backs 
up the water, the house is commenced and we find 
that all the different tasks are being attended to 
simultaneously—dams, both main and subsidiary, 
lodges, tree-cutting, and storing of winter food. 
Within a few weeks the colony becomes established 
and the face of the land is changed by the work of 
these small animals. Larger and larger the pond 
grows, as the dam is extended, while the banks 
are covered with fresh, white stumps of fallen 
trees, as though a gang of luinber-men were at 
work. Each night sees marked changes resulting 
from the untiring work of these builders and tree- 
cutters. If the colony is successful and the site 
