THE DAM 39 
To return to the dimensions of dams. I measured the 
dam which formed Beaver Lake in Yellowstone Park, and 
found it to be 1054 feet long, this without following accu- 
rately all the bends of the dam, which was quite crooked. 
The longest dam still used by beavers which I have per- 
sonally measured was one on the North Fork of Elk Creek, 
Yellowstone Park. This was three-hundred and fifty feet 

Fig. 15. Map SHowine Two Dams witH Many ANGLES IN TuEM, Nortu 
Fork or ELK CrEEK 
from end to end, and very crooked. In spite of its length 
it made only a narrow pond, one hundred feet wide, because 
of the topography (Fig. 14), the ground being comparatively 
level in the direction of the dam but rising faster up stream, 
most of the water entering and leaving towards one side of the 
pond. This conformation of the surface doubtless explains the 
irregularities of this and similar dams. The beginning is a 
