etain
water after a rainfall. In some drainage areas the ground is more or
less impermeable to water, and in this case the water runs readily off,
causing a sudden rise in the river; and as suddenly it reaches the
low-water mark. In other drainage areas the ground is very permeable to
water, so that the rain penetrates to a greater depth into the earth,
where it is held, and by a slow process drains into the rivers, while
much more of it is carried off by evaporation and into vegetation than
is the case in the drainage district before mentioned.

The courses of rivers are determined by the topography of the country
through which they flow. The sinuous windings, that are found to be a
characteristic of nearly all rivers, are caused by the water, through
the force of gravity, seeking the lowest level, and avoiding
obstructions, which they can flow around more easily than remove.

Great rivers often change their courses, especially where they flow
through a region of made earth, such as is the case with the lower
Mississippi River, and in other great rivers of the world. The loose
earth is continually shifted by the current, and where the current is
not very strong it will often hold the water back to such an extent of
accumulated weight that the flood will break over at some weak point on
its banks and make a new course for itself.

One of the great rivers of China--the Hwangho--often causes dire
destruction to life and property owing to change in its bed from time to
time. It is estimated that between the years of 1851-66 this river
caused the loss of from 30,000,000 to 40,000,000 lives through drowning
and famine by the destruction of crops.

Floods in rivers are occasioned from various causes. Of course the
primary cause is the same in all cases, that is, from precipitation of
moisture in the form of rain or snow. Some rivers are so related to the
area of rainfall and to the permeability of the soil that there is but
little variation in the amount of discharge throughout the year. The
great river of South America, the Amazon, is an instance of a river of
this class. A certain number of the smaller rivers that feed it lie in
the area of rainfall during the whole of the year; for instance, the
streams of the upper Amazon are being fed by rains at one season of the
year, when those feeding the river lower down are at the lowest stage.
When the rainy season prevails in the upper section of the river the dry
season prev