REBUILDING A BEAVER COLONY 129 
boulder too large to be removed. At a dis- 
tance of eighty-one feet from the lower end 
the canal-builders came in contact with granite 
rock and brought the canal to a stop by en- 
larging the upper end into a basin about ten 
feet across. 
The entire length of this canal was through 
the sediment of a former beaver pond. After 
making a pond beavers must occasionally raise 
the height of the dam to deepen the water, 
and also dredge the mud from the bottom. But 
despite both dredging and dam raising, the 
pond sooner or later fills with sediment and 
has to be abandoned. In due time it is over- 
grown with grass or a forest. 
Food shortage—complete exhaustion of the 
aspen growth—had compelled the abandon- 
ment of the Meadow Colony after it had been 
a beaver settlement for a great many genera- 
tions. Two large ponds, a dozen smaller ones, 
and three houses were left to their fate. Most 
of the smaller ponds were completely lost, being 
overgrown with willows. Two of the houses 
had crumbled and were now low wild flower 
beds. 
Since abandonment a number of aspen groves 
had grown, and although these were some dis- 
tance from the stream, they could be reached 
and would furnish necessary food supply. 
