The majority of beaver dams are as crooked 

 as a river's course. Now and then one is straight. 

 A few are built from shore to a boulder, from the 

 boulder to a willow-clump, and finally, perhaps, 

 from willow-clump to some outstretching penin- 

 sula on the further shore. It is not uncommon 

 for a short dam to be built and afterwards length- 

 ened with additions on each end which may 

 curve either down or up stream. Sometimes a 

 dam is built outward from opposite shores simul- 

 taneously by separate but cooperating crews of 

 beaver. In swift water these ends are forced down- 

 stream in building, so that when they are finally 

 joined midstream the dam curves noticeably down- 

 stream. 



On one occasion I watched beaver commence 

 and complete a dam in moderately swift water 

 that when finished bowed strongly upstream. 

 This, however, was not the intention of the build- 

 ers. The material for this dam consisted of wil- 

 low and alder poles that were cut some distance 

 upstream. These were floated down as used. 

 This dam was begun against a huge boulder near 

 midstream, and built outward simultaneously to- 



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