40 THE BEAVER 
short dam across the stream which forms a pond. The water 
rises in this, and presently begins to flow around one or both 
ends of the dam. The beavers extend the dam to cut off the 
escaping water, and these extensions point up or downstream 
according to the lay of the land, thus producing an up or 
downstream curve in the dam, which is thus purely an 
jf 

2 4g ge 32 ge fe iGO Faar: 
Fia. 16. Map or Ponps on THE Nort Fork or ELK CREEK, YELLOWSTONE 
Park, SHOWING THE CHANGES WHICH WERE MADE IN Two YEARS 
accident and not intentional. The varying curves in such 
a long dam as the one I have just mentioned are undoubtedly 
caused by variations in the surface of the ground across 
which the dam was made, the builders carrying the dam 
along from point to point as necessary to stop the overflow. 
Something over a hundred yards upstream from the above 
described dam is one which in 1921 was 115 feet long. In 

