while a smaller stream heavily burdened with waste requires a
steep slope to give it velocity sufficient to move the load.

The Platte, a graded river of Nebraska with its headwaters in the
Rocky Mountains, is enfeebled by the semi-arid climate of the
Great Plains and surcharged with the waste brought down both by
its branches in the mountains and by those whose tracks lie over
the soft rocks of the plains. It is compelled to maintain a
gradient of eight feet to the mile in western Nebraska. The Ohio
reaches grade with a slope of less than four inches to the mile
from Cincinnati to its mouth, and the powerful Mississippi washes
along its load with a fall of but three inches per mile from Cairo
to the Gulf.

Other things being equal, which of graded streams will have the
steeper gradient, a trunk stream or its tributaries? a stream
supplied with gravel or one with silt?

Other factors remaining the same, what changes would occur if the
Platte should increase in volume? What changes would occur if the
load should be increased in amount or in coarseness?

THE OLD AGE OF RIVERS. As rivers pass their prime, as denudation
lowers the relief of the region, less waste and finer is washed
over the gentler slopes of the lowering hills. With smaller loads
to carry, the rivers now deepen their valleys and find grade with
fainter declivities nearer the level of the sea. This limit of the
level of the sea beneath which they cannot erode is known as
baselevel. [Footnote: The term "baselevel" is also used to
designate the close approximation to sea level to which streams