THE DAM 41 
1923 it was 250 feet, more than double its former length, 
and the pond was proportionately increased in area, and con- 
tained a lodge. This increase in size was probably made by 
beavers which moved up from the large pond below in order 
to be nearer the aspens, and also possibly because they were 
crowded out of the other place (Fig. 16). 
The height of beaver dams is of course variable. I think 
a large majority are under five feet high, measuring per- 
pendicularly from the crest to the stream bottom below. 
Johnson gives the heights of two dams in the Adirondacks 
as 8 feet 8 inches and 11 feet 1 inch, respectively. These 
are really very much out of the common, if not extraordi- 
nary. One sometimes sees dams but two or three feet high 
which back the water up for considerable distances. This 
depends, naturally, upon the fall of the stream; the greater 
the fall, the higher the dam must be to make a pond of any 
size and depth. Sometimes comparatively short dams will 
make large ponds. One dam in the Yellowstone, 103 feet 
long, was placed at the lower end of a wide, shallow valley, 
below which the gulch fell off rather abruptly, forming a 
pond 350 feet long by 225 feet in its greatest width. At 
Crescent Hill the largest pond, 340 by 800 feet, was formed 
by adam 165 feet long. Of course in both cases the locations 
of the dams were exceptionally favorable for the formation of 
such large ponds. Frequently it is the other way about, long 
dams having narrow ponds above them, sometimes not more 
than a half or a third of the length of the dam in width. 
In a stream with a gentle fall a short dam between the 
banks may make a pond several hundred feet long. I have 
seen a dam sixty feet long back the water up 475 feet, and 
another thirty-five feet long make a pond 350 feet in length, 
and a third dam fifty feet long made a 375 foot pond, all on 
the same stream. At Little Trappers Lake I found a dam 
three feet long which backed water up for 75 feet. Johnson 
