You can see that there was a sufficient flow of water in the stream from which this sketch of a section of a beaver dam was taken; otherwise the dam would have been plastered with mud to conserve the supply. The longest slope, of course, was up-stream—a fundamental principle in beaver bridge engineering.
In building his dam the beaver uses, for the most part, slender green poles trimmed and cut in lengths; but mixed with these are small untrimmed trees which he places with the butt end up-stream, and propped with mud and sticks so that the up end will be a foot or so higher than the down end. In this way, you see, the branches are made to resist the push of the waters against the butt end; while, if they were placed the other way, the current would have a pulling purchase on the butt end. The raising of the ends also lessens the pushing force of the water as it doesn't strike the butt of the tree "full on," as it would otherwise do. And the branches not only help to hold the trees in place, but, together, form a kind of foundation on which to pile and intermix the trimmed poles.
The timbers, being cut green, become water-soaked. This makes them heavier and so causes them to sink and helps to hold them in place; while the branches and twigs of the untrimmed trees form a kind of basketwork that catches the sediment and drift of the stream, and so the dam lets less and less water through. The upside stream is plastered by the beavers with mud in cases where the flow of water in the stream is meagre. Otherwise it is left unplastered. You see Mr. Beaver's idea is not to make the dam absolutely water-tight, for then it would be running over all the time and so be worn away. What he wants is a dam that will let the water through slowly and at the same time keep a proper level.
BEAVER HOME WITHOUT TIME LOCK
Here is a beaver home as it looks before the time lock is put on in the Fall.
Mr. Beaver's chief purpose in building these dams seems to be to keep his front-door yard full of water. This may look like a funny idea at first, but in this, as in other things, Mr. Beaver shows he has a very wise head on his shoulders; for one peculiarity of his life is that he is obliged to come and go through the cellar door. As he doesn't want any of his enemies—the wolf, the coyote, and all that class of people—to use this door, he keeps it under water. And in winter-time, when he goes out to the wood-pile to get something to eat, the water must be deep enough so that the pond doesn't freeze solid to the bottom.
