The third kind is the strongest and easiest to build.  It is for places where big trees lean out over the stream.  Three or four beavers gather about a tree and begin to cut, sitting up on their broad tails.  One stands above them on the bank, apparently directing the work.  In a short time the tree is nearly cut through from the under side.  Then the beaver above begins to cut down carefully.  With the first warning crack he jumps aside, and the tree falls straight across where it is wanted.  All the beavers then disappear and begin cutting the branches that rest on the bottom.  Slowly the tree settles till its trunk is at the right height to make the top of the dam.  The upper branches are then trimmed close to the trunk, and are woven with alders among the long stubs sticking down from the trunk into the river bed.  Stones, mud, and brush are used liberally to fill the chinks, and in a remarkably short time the dam is complete.

When you meet such a dam on the stream you are canoeing don’t attempt to break through.  You will find it shorter by several hours to unload and make a carry.

All the beaver’s cutting is done by chisel-edged front teeth.  There are two of these in each jaw, extending a good inch and a half outside the gums, and meeting at a sharp bevel.  The inner sides of the teeth are softer and wear away faster than the outer, so that the bevel remains the same; and the action of the upper and lower teeth over each other keeps them always sharp.  They grow so rapidly that a beaver must be constantly wood cutting to keep them worn down to comfortable size.

Often on wild streams you find a stick floating down to meet you showing a fresh cut.  You grab it, of course, and say:  “Somebody is camped above here.  That stick has just been cut with a sharp knife.”  But look closer; see that faint ridge the whole length of the cut, as if the knife had a tiny gap in its edge.  That is where the beaver’s two upper teeth meet, and the edge is not quite perfect.  He cut that stick, thicker than a man’s thumb, at a single bite.  To cut an alder having the diameter of a teacup is the work of a minute for the same tools; and a towering birch tree falls in a remarkably short time when attacked by three or four beavers.  Around the stump of such a tree you find a pile of two-inch chips, thick, white, clean cut, and arched to the curve of the beaver’s teeth.  Judge the workman by his chips, and this is a good workman.

When the dam is built the beaver cuts his winter food-wood.  A colony of the creatures will often fell a whole grove of young birch or poplar on the bank above the dam.  The branches with the best bark are then cut into short lengths, which are rolled down the bank and floated to the pool at the dam.