THE DAM 43 
good-sized area. In 1923 I found the place in the same con- 
dition as when last visited in 1921. Evidently the beavers 
had decided they did not wish a pond there. 
Beavers make mistakes at times in building dams, some- 
times because the stream is too turbulent for them to make a 
permanent structure, or because there is not material for 
them to work with, or the dam does not make a pond of 
sufficient size or depth. 
Mills mentions a dam which by successive additions came 
to extend three-fourths of the way around the pond. I 
saw a small abandoned pond near Yancey’s, the dam to which 
partially encircled the pond and described an are of 225 
degrees, though far from circular. The pond was situated 
where the North Fork of Elk Creek entered the valley. 
The pond was so filled up with gravel that but little could be 
learned about the history of the dam. 
Dugmore says that alders are most frequently used in 
building dams. I presume that refers to the regions where 
most of his observations were made, Canada and Newfound- 
land. If I were to go according to my own work, I should 
say willow, and as a matter of fact I think the animals use 
the materials which are most available. He states that a 
week of steady work on the part of a family will see a thirty 
or forty foot dam raised two feet or more. 
Ponds are often abandoned because the supply of food. 
trees has become exhausted. After a new crop has grown 
the beavers may return and repair the dam and reestablish 
the colony, but very likely the newcomers may be entirely 
unrelated to the previous residents. 
Dugmore says that spillways are often made on dams to 
take care of the overflow, though I have never seen anything 
of the sort myself. He mentions an instance of a dam under 
construction being threatened by a flood. The beavers 
made a large enough opening in the dam to permit the flood 
