indicate that they have examined the entire brook and then selected the best place.
As soon as the beaver's brush dam is completed, it begins to accumulate trash and mud. In a little while, usually, it is covered with a mass of soil, shrubs of willow begin to grow upon it, and after a few years it is a strong, earthy, willow-covered dam. The dams vary in length from a few feet to several hundred feet. I measured one on the South Platte River that was eleven hundred feet long.
The influence of a beaver-dam is astounding. As soon as completed, it becomes a highway for the folk of the wild. It is used day and night. Mice and porcupines, bears and rabbits, lions and wolves, make a bridge of it. From it, in the evening, the graceful deer cast their reflections in the quiet pond. Over it dash pursuer and pursued; and on it take place battles and courtships. It is often torn by hoof and claw of animals locked in death-struggles, and often, very often, it is stained with blood. Many a drama, picturesque, fierce, and wild, is staged upon a beaver-dam.
An interesting and valuable book could be written
concerning the earth as modified and benefited by beaver action, and I have long thought that the beaver deserved at least a chapter in Marsh's masterly book, "The Earth as modified by Human Action." To "work like a beaver" is an almost universal expression for energetic persistence, but who realizes that the beaver has accomplished anything? Almost unread of and unknown are his monumental works.
The instant a beaver-dam is completed, it has a decided influence on the flow of the water, and especially on the quantity of sediment which the passing water carries. The sediment, instead of going down to fill the channel below, or to clog the river's mouth, fill the harbor, and do damage a thousand miles away, is accumulated in the pond behind the dam, and a level deposit is formed over the entire area of the lake. By and by this deposit is so great that the lake is filled with sediment, but before this happens, both lake and dam check and delay so much flood-water that floods are diminished in volume, and the water thus delayed is in part added to the flow of the streams
at the time of low water, the result being a more even stream-flow at all times.
The regulation of stream-flow is important. There are only a few rainy days each year, and all the water that flows down the rivers falls on these few rainy days. The instant the water reaches the earth, it is hurried away toward the sea, and unless some agency delays the run-off, the rivers would naturally contain water only on the rainy days and a little while after. The fact that some rivers contain water at all times is but evidence that something has held in check a portion of the water which fell during these rainy days.
