67. Streams. Streams usually carry mud and sand along with them;
this is particularly well seen after a storm when rivers and brooks
are muddy. The puddles which collect at the foot of a hill after a
storm are muddy because of the particles of soil gathered by the water
as it runs down the hill. The particles are not dissolved in the
water, but are held there in suspension, as we call it technically.
The river made muddy after a storm by suspended particles usually
becomes clear and transparent after it has traveled onward for miles,
because, as it travels, the particles drop to the bottom and are
deposited there. Hence, materials suspended in the water are borne
along and deposited at various places (Fig. 34). The amount of
deposition by large rivers is so great that in some places channels
fill up and must be dredged annually, and vessels are sometimes caught
in the deposit and have to be towed away.

Running water in the form of streams and rivers, by carrying sand
particles, stones, and rocks from high slopes and depositing them at
lower levels, wears away land at one place and builds it up at
another, and never ceases in its work of changing the nature of the
earth's surface (Fig. 35).

[Illustration: FIG. 34.--Deposit left by running water.]

[Illustration: FIG. 35.--Water by its action constantly changes the
character of the land.]

68. Relation of Water to Human Life. Water is one of the most
essential of food materials, and whether we drink much or little
water, we nevertheless get a great deal of it. The larger part of many
of our foods is composed of water; more than half of the weight of the
meat we eat is made up of water; and vegetables are often more than
nine tenths water. (See Laboratory Manual.) Asparagus and tomatoes
have over 90 per cent. of water, and most fruits are more than three
fourths water; even bread, which contains as little water as any of
our common foods, is about one third water (Fig. 36).

[Illustration: FIG. 36.--Diagram of the composition of a loaf of bread
and of a potato: 1. ash; 2, food; 3, water.]

Without water, solid food material, although present in the body,
would not be in a condition suitable for bodily use. An abundant
supply of water enables the food to be dissolved or suspended in it,
and in solution the food material is easily distributed to all parts
of the body.

Further, water assists in the removal of the daily bodily wastes, and
thus rids the system of