BEAVER 279 



cases, however, there exists a channel, with an opening about as wide as the hand, 

 connecting the burrow with the shore, which is either intentionally made by the 

 beaver, or else accidentally caused by the breaking-down of the grass-covering. 

 When this opening becomes too large, the beaver tries to close it in ; sticks and 

 twigs being heaped up on the top of the treacherous spot, which in a few weeks 

 rise to a height of from 6 to 10 feet. If this artificial wood-pile is made solid 

 by reeds and mud in the autumn, it looks exactly like an American beaver-lodge. 

 Besides these, the beaver sometimes raises temporary wooden structures, with an 

 outlet to the land, and of the shape of a large dog-kennel. When driven by a 

 flood from its usual habitation, the beaver tries to swim to a more elevated place, 

 and from there starts on its excursions for food; if the landing-place does not 

 afford sufficient security, sticks and dry twigs are heaped up into a lodge as a 

 shelter until the original haunt is accessible. 



The safety of the beaver depends in many places on the entrance to the 

 passage being deep enough under water to remain clear of ice, and on the water 

 around being sufficiently deep and open to afford a refuge in case of need. For this 

 reason beavers throw up dams on rivers in which the water-level changes, so as to 

 keep the water at a certain height or form a larger surface. These dams extend 

 from one bank to the other, and are generally begun in the middle where the 

 current is strongest, and where the beaver finds some support, such as a rock or 

 stump from which it can build towards both banks. Many dams curve towards 

 the stream, as the beaver has to let the materials drift downwards from the fixed 

 point before the dam can be closed. This curve is often slight, and sometimes the 

 dams are straight, while occasionally they curve outwards from the stream. They 

 are flat-topped when curving outwards, and steep when curving upwards, and 

 may be from 10 to 500 feet long or even more, and from 6 to 13 feet high. 

 At the base they are from 16 to 20 feet in width, and on the top from 20 

 to 40 inches. Since it is necessary that the dam should be firm and allow 

 the water to pass freely over, and also that it should remain safe, regularly 

 worked outlet-holes are made along the upper edge, except when logs are used 

 and there is enough space between them for the water to trickle through. The 

 largest dams are constructed by the organised co-operation of many beavers, but, 

 as is evident from observations in America, they are built by one family, or perhaps 

 by several families of the same age which have been obliged to migrate together. 

 Later on they are probably repaired and completed by other beavers, so that they 

 form gigantic structures, which, in consequence of the ponding-back of the water, 

 give rise to lakes and swamps or peat-mosses. These mosses often completely 

 cover the dams, and are so laro-e that to some dams an age of more than a thousand 

 years is assigned. Wherever beavers are still in undisturbed possession of the 

 country, forest-streams often overflow the valleys for a great distance, thereby 

 causing the trees which are under water to die or fall, and thus forming ponds and 

 lakes, with lilies and other water-plants. In the nearly dry bed of the river below 

 the dam, traversed only by narrow channels, land-plants begin to grow, and soon 

 form a green carpet ; the water from the swamps flows to these places, and when, 

 after long years, the floods of spring force the water into its old course, they sweep 

 away the dam, but do not remove every trace of the beavers' work, and on the 



