g is true of all rivers. They flow, gradually or suddenly, from
higher to lower levels. To reach the lowest level as soon as possible is
the end each river is striving toward. If it could, each river would cut
its bed to this depth at the first stage of its course. Its tools are
the rocks it carries, great and small. The force that uses these tools
is the power of falling water, represented by the current of the stream.
The upper part of a river such as the Missouri or Mississippi engages in
a campaign of widening and deepening its channel, and carrying away
quantities of sediment. The lower reaches of the stream flow through
more level country; the current is checked, and a vast burden of
sediment is laid down. Instead of tearing away its banks and bottom, the
river fills up gradually with mud. The current meanders between banks of
sediment over a bottom which becomes shallower year by year. The Rocky
Mountains are being carried to the Gulf of Mexico. The commerce of the
river is impeded by mountain debris deposited as mud-banks along the
river's lower course.

Many rivers are quiet and commonplace throughout their length. They flow
between low, rounded hills, and are joined by quiet streams, that occupy
the separating grooves between the hills. This is the oldest type of
river. It has done its work. Rainfall and stream-flow have brought the
level of the land nearly to the level of the stream. Very little more is
left to be ground down and carried away. The landscape is beautiful, but
it is no longer picturesque. Wind and water have smoothed away
unevennesses. Trees and grass and other vegetation check erosion, and
the river has little to do but to carry away the surface water that
falls as rain.

But suppose our river, flowing gently between its grassy banks, should
feel some mighty power lifting it up, with all its neighbour hills and
valleys, to form a wrinkle in the still unstable crust of the earth.
Away off at the river's mouth the level may not have changed, or that
region may have been depressed instead of elevated by the shrinking
process. Suppose the great upheaval has not severed the upper from the
lower courses of the stream. With tremendous force and speed, the
current flows from the higher levels to the lower. The river in the
highlands strikes hard to reach the level of its mouth. It grinds with
all its might, and all its rocky tools, upon its bed. All the mud is
scoured out, and then the underlying roc