Moreover, a bird's-eye view or a map of a region shows the
significant fact that the valleys of a system unite with one
another in a branch work, as twigs meet their stems and the
branches of a tree its trunk. Each valley, from that of the
smallest rivulet to that of the master stream, is proportionate to
the size of the stream which occupies it. With a few explainable
exceptions the valleys of tributaries join that of the trunk
stream at a level; there is no sudden descent or break in the bed
at the point of juncture. These are the natural consequences which
must follow if the land has long been worked upon by streams, and
no other process has ever been suggested which is competent to
produce them. We must conclude that valley systems have been
formed by the river systems which drain them, aided by the work of
the weather; they are not gaping fissures in the earth's crust, as
early observers imagined, but are the furrows which running water
has drawn upon the land.

As valleys are made by the slow wear of streams and the action of
the weather, they pass in their development through successive
stages, each of which has its own characteristic features. We may
therefore classify rivers and valleys according to the stage which
they have reached in their life history from infancy to old age.

YOUNG RIVER VALLEYS

INFANCY. The Red River of the North. A region in northwestern
Minnesota and the adjacent portions of North Dakota and Manitoba
was so recently covered by the waters of an extinct lake, known as
Lake Agassiz, that the surface remains much as it was left when