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dam is short and comparatively low. It is enlarged 

 as conditions may require. As the trees in the 

 edge of the pond are harvested, the dam is built 

 higher and longer, so as to flood a larger area ; 

 or as sediment fills the pond, the dam is from 

 time to time raised and lengthened in order to 

 maintain the desired depth of water. Thus it may 

 grow through the years until the possibilities of 

 the locality are exhausted. The dam may then be 

 abandoned. It may be used for a few years or it 

 may be used for a century. A gigantic beaver 

 dam may thus represent the work of several gen- 

 erations of beaver. It often occurs that one or 

 more generations may use a dam and yearly add 

 something to its size. By and by these beaver 

 may die or emigrate. The old dam remains, fall- 

 ing to ruin in places. Years go by and other 

 beaver come upon the scene. The old dam is 

 then used for the foundation for a new one. The 

 appearance of some old dams indicates that they 

 have been repeatedly used and abandoned. 



New dams, being made largely of coarse mate- 

 rials, appear very unlike old ones. Decay, settling, 

 repairs, and other changes come rapidly. The 

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