454 Life-histories of Northern Animals 



The process of building, as nearly as I can tell from having 

 seen many dams in all stages of growth, is as follows : 



The Beaver and his wife first decide on the stream they 

 propose to make into a pond — and it is always a small one, 

 sometimes a mere spring. Morgan says^ there is no instance 

 known of a dam made across a stream having a greater depth 

 than two feet at the site of the structure when the water is at 

 its lowest level. 



Next they select a place where the bed is hard clay or 

 gravel, neither rock nor bog being desirable, and then begin 



Fig. 128 — Section of dam showing mud face up stream. 



the dam by cutting and laying quantities of brushwood length- 

 wise in the deepest part of the stream bed, butts against the 

 current. Each stick as it is laid is partly covered near the 

 thick end with mud, stones, or clay to hold it down and the 

 process carried on until the wall is raised, and would in 

 sections be somewhat as shown in Fig. 128. But very rarely is 

 a log used, and never a stake. By this time the original bed of 

 the stream is blocked and the water flooding the shore calls 

 for a still wider dam. At both wings it is spread, and here we 

 see why the curvature against the stream is usually given. 



The force of the current, especially at high water, is such 

 that a new formation not up stream is surely swept away, thus 

 the only correct plan is forced on the Beaver. Exceptions 

 occur, but there is generally an obvious explanation. Some 

 log, root, stump, or hillock was there to suggest a different line 

 and offer assistance in making it safe. 



Night by night the Beaver family works on the dam, piling 

 up sticks and burying them in mud that is full of fibrous roots, 

 or anchoring them down with stones of one to six pounds 



^ Ibtd., p. 105. 



