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.

[Illustration: A BEAVER-DAM IN WINTER]

Among the agencies which best perform this service of keeping the
streams ever-flowing, are the forests and the works of the beaver.
Rainfall accumulates in the brooks. The brooks conduct the water to
the rivers. If across a river there be a beaver-dam, the pond formed
by it will be a reservoir which will catch and retain some of the
water coming into it during rainy days, and will thus delay the
passage of all water which flows through it. Beaver-reservoirs are
leaky ones, and if they are stored full during rainy days, the
leaking helps to maintain the stream-flow in dry weather. A beaver-dam
thus tends to distribute to the streams below it a moderate quantity
of water each day. In other words, it spreads out or distributes the
water of the few rainy days through all the days of the year. A river
which flows steadily throughout the year is of inestimable value to
mankind. If floods sweep a river, they do damage. If low water comes,
the wheels of steamers and of manufactories cease to move, and damage
or death may result. In maintaining a medium between the extremes of
high and low water, the beaver's work is of profound importance. In
helping beneficially to control a river, the beaver would render
enormous service if allowed to construct his works at its source.
During times of heavy rainfall, the water-flow carries with it,
especially in u