138 
MAMMALS 
2. They cut up the trees into lengths which they can 
handle in the water (Figure 129). 
3. If the trees are large and there is no water near, they 
dig canals leading back to the fallen tree. 
4. They build dams that are eight or ten feet high and 
twenty or thirty feet wide. These dams frequently raise 
the level of lakes so that the shore line is changed (Figure 128). 
Figure 130. — A Beaver House. 
What is it used for ? 
5. If the stream is large and the pressure on their dam 
becomes too great, they build a secondary dam below the 
first one, which provides slack water and thus protects the 
main dam. 
6. Their houses are built of mud and sticks on the margin 
of the ponds after the dam is finished. They live in these 
houses and enter them from below the surface of the water. 
The purpose of the dam is to provide a pond with deep water 
for their protection and use (Figure 128). 
