THE DAM oo 
foot or two long, made to head off a side flow of water, to 
the one mentioned by Mills, on Jefferson River, near 
Three Forks, Montana. This was 2140 feet long, most of 
it old; more than half was under six feet high; two short 
sections were twenty-three feet wide at the base, five on top, 
and fourteen feet high. This is, of course, extremely 
unusual, and unfortunately Mills gives no further details 
which might explain the reason for this extraordinary length. 
Undoubtedly it was the work of many generations of beavers, 
and probably constructed intermittently. 
Shiras gives a picture of a dam forming the western end of 
Echo Lake, which occupies a mile in the center of Grand 
Island, Lake Superior. The legend under the picture says: 
“The bank on the left, covered with second growth trees, 
is an ancient beaver dam 1500 feet long, probably 400 
years old, and forming the lake, originally.” 
I think it doubtful if dams are ever of any great length 
when first constructed. The great majority are under fifty 
feet long at first, though I have seen a dam in Colorado 
which was nearly a hundred feet long as first built, if being 
necessary for it to be of this length in order to cross the 
stream. Another below it was seventy-six feet in a straight 
line from end to end, but curved twice, first down, then 
upstream, starting from the right bank and ending short of 
the left. Below this dam, and setting out diagonally into 
the stream from the left bank, was a short one twenty feet 
long. The water flowed around the end of the long upper 
dam, and thence around the free end of the short lower one, 
thus throwing the main body of water back to the right bank 
where it would naturally have been if undisturbed by any 
obstruction, and where it was much deeper than on the oppo- 
site side, and therefore keeping the water deep below the dam 
as well as above. When I first described this place I made 
the statement that the downward curve of the dam near 
