In Beaver World by Enos A. Mills

To "work like a beaver" is an almost universal expression for energetic and intelligent persistence, but who realizes the magnitude of the beaver's works? What he has accomplished is not only monumental but useful to man. He was the original Conservationist. An interesting and valuable book could be written concerning the earth as influenced and benefited by the labors of the beaver. The beaver is intimately associated with the natural resources, soil, and water. His work is not yet done, and along the sources of innumerable streams he will ever be needed to save soil, to regulate stream-flow, and to provide pools for the fish.

The beaver's conservation work is accomplished by means of the dams he constructs across streams of flowing water and the ponds that are thus formed. These dams and ponds render a number of services: first, they save soil; second, they check erosion; third, they reduce flood damage; fourth, they store water and help to sustain stream-flow; fifth, they provide water-holes for fish; and sixth, they are helpful in maintaining deep waterways by reducing the extremes of both high and low water, and also by reducing the quantity of sediment carried down into river-channels.