contour as well as by its color and thickness.

The Grand Canyon of the Colorado is often and rightly cited as an
example of the stupendous erosion which may be accomplished by a
river. And yet the Colorado is a young stream and its work is no
more than well begun. It has not yet wholly reached grade, and the
great task of the river and its tributaries--the task of leveling
the lofty plateau to a low plain and of transporting it grain by
grain to the sea--still lies almost entirely in the future.

WATERFALLS AND RAPIDS. Before the bed of a stream is reduced to
grade it may be broken by abrupt descents which give rise to
waterfalls and rapids. Such breaks in a river's bed may belong to
the initial surface over which it began its course; still more
commonly are they developed in the rock mass through which it is
cutting its valley. Thus, wherever a stream leaves harder rocks to
flow over softer ones the latter are quickly worn below the level
of the former, and a sharp change in slope, with a waterfall or
rapid, results.

At time of flood young tributaries with steeper courses than that
of the trunk stream may bring down stones and finer waste, which
the gentler current cannot move along, and throw them as a dam
across its way. The rapids thus formed are also ephemeral, for as
the gradient of the tributaries is lowered the main stream becomes
able to handle the smaller and finer load which they discharge.

A rare class of falls is produced where the minor tributaries of a
young river are not able to keep pace with their master stream in
the erosion of their beds because of their smaller volume, and