i8o Roosevelt Wild Life Bulletin 



Uses and Construction of Beaver Darns, The dam provides the 

 animals with a sufficient depth of water to insure safety from their 

 enemies and to provide transportation facilities as well as safe 

 storage places for their winter food supply. Since the entrances 

 to the lodges must be rendered secure it is important to maintain 

 a sufficiently high water level to cover them. The water being too 

 deep to freeze to the bottom in winter, the beaver is enabled to move 

 about under the ice and secure his food with the utmost safety. 



The materials used in building dams are usually green boughs, 

 dry - sticks, poles, roots, mud and sand, and occasionally stones 

 (figure 32) are added. When logs are found as part of a beaver 

 dam they have either drifted down or were there before the dam 

 was started, the dam probably being built against them in some 

 cases. The lower side of the dam usually contains exposed sticks 

 and boughs which are arranged generally parallel with the flow, and 

 the upper side is covered with mud, muck and entangling rootlets. 

 Old dams become more or less grass-grown. The length of the dam 

 is very variable and is governed largely by the character of the 

 banks and the duration of the colony in the locality. In some parts 

 of the country old dams have been found measuring thousands of 

 feet in length, the work of many generations of beavers. In the 

 Adirondacks the longest dam which I encountered was about 375 

 feet in length. 



Beaver dams require constant attention by the animals. Water 

 is constantly trickling through or over them, and when neglected 

 they soon disintegrate. 



The height of dams varies within much narrower limits than the 

 length but is governed largely by the same factors. The two highest 

 dams that I saw in the Adirondacks measured 8 feet, 8 inches 

 and 11 feet, 1 inch, respectively, from the bottom of the creek 

 at the main channel to the crest of the dam (figures 33, 34). While 

 the dams are usually substantial and tenacious affairs, easily sup- 

 porting the weight of a man and even of larger animals, they at 

 times have their weak spots and give way before the volume of 

 water above them. The literature on the beaver contains statements 

 to the effect that they sometimes make openings in their dams in 

 times of heavy rains or floods in order to relieve the pressure on the 

 dams. Personally I have never seen any clear evidence of this. 



Dams may be straight, curved upstream or downstream, zig-zag 

 or any other form that the situation and circumstances, accident or 



