side.

Usually a river first attains grade near its mouth, and here first
sinks its bed to near baselevel. Extending its graded course
upstream by cutting away barrier after barrier, it comes to have a
widened and mature valley over its lower course, while its young
headwaters are still busily eroding their beds. Its ungraded
branches may thus bring down to its lower course more waste than
it is competent to carry on to the sea, and here it aggrades its
bed and builds a flood plain in order to gain a steeper gradient
and velocity enough to transport its load.

As maturity is past and the relief of the land is lessened, a
smaller and smaller load of waste is delivered to the river. It
now has energy to spare and again degrades its valley, excavating
its former flood plains and leaving them in terraces on either
side, and at last in its old age sweeping them away.

ALLUVIAL CONES AND FANS. In hilly and mountainous countries one
often sees on a valley side a conical or fan-shaped deposit of
waste at the mouth of a lateral stream. The cause is obvious: the
young branch has not been able as yet to wear its bed to accordant
level with the already deepened valley of the master stream. It
therefore builds its bed to grade at the point of juncture by
depositing here its load of waste,--a load too heavy to be carried
along the more gentle profile of the trunk valley.

Where rivers descend from a mountainous region upon the plain they
may build alluvial fans of exceedingly gentle slope. Thus the
rivers of the western side of the Sierra Nevada Mountains have