44 THE BEAVER 
waters to escape, and after the storm had passed and the 
water subsided they closed the gap and completed the dam. 
Morgan distinguished two types of dams, the “‘stick dam”’ 
and the “‘solid bank dam;’’ the former of interlaced stick and 
pole work on the lower face, with mud and earth on the water 
or upper face; the latter usually found lower down on streams 
and containing much more earth and mud, probably much 
older structures than the first. Personally I have never 
been able to make any such distinctions. He thought 
curvature of dams was purely accidental. 
He estimated the amount of material in Grass Lake dam, 
which was 260 feet 10 inches long, greatest height 6 feet 
2 inches, to be upwards of 7,000 cubic feet. 
Morgan knew of no instance in the area studied by him 
of a dam being constructed across a stream having a greater 
depth than two feet at the lowest water level. 
Practically all dam building is done in the fall, in prepara- 
tion for winter. 
Beavers will build dams wherever a supply of water can 
be had to make a pond, and where a supply of food is at hand. 
They will build them across the outlets of natural ponds in 
order to raise the water level. The greater number are built 
on running streams. The previously mentioned dam at 
Crescent Hill was seemingly built to hold the water from 
springs in the gulch above, which are now covered by the 
pond, for there was no visible water supply, though in the 
spring melting snow must supply considerable water for a 
short time. On Tower Creek, also in Yellowstone Park, an 
extensive series of ponds utilized the water from a large 
spring on the creek bottom some distance away from the 
stream, and at the foot of the mountainside which rises 
above the creek valley. There were other series of ponds 
on the same stream which obtained their water supply in 
the same manner from other springs. The stream itself 

