Ground water now stands low. The run-off washes directly to the
streams, with the least delay and loss by evaporation in ponds and
marches; the discharge of the river is therefore at its height.
The entire region is dissected by stream ways. The area of valley
slopes is now largest and sheds to the streams a heavier load of
waste than ever before. At maturity the river system is doing its
greatest amount of work both in erosion and in the carriage of
water and of waste to the sea.

LATERAL EROSION. On reaching grade a river ceases to scour its
bed, and it does not again begin to do so until some change in
load or volume enables it to find grade at a lower level. On the
other hand, a stream erodes its banks at all stages in its
history, and with graded rivers this process, called lateral
erosion, or PLANATION, is specially important. The current of a
stream follows the outer side of all curves or bends in the
channel, and on this side it excavates its bed the deepest and
continually wears and saps its banks. On the inner side deposition
takes place in the more shallow and slower-moving water. The inner
bank of bends is thus built out while the outer bank is worn away.
By swinging its curves against the valley sides a graded river
continually cuts a wider and wider floor. The V-valley of youth is
thus changed by planation to a flat-floored valley with flaring
sides which gradually become subdued by the weather to gentle
slopes. While widening their valleys streams maintain a constant
width of channel, so that a wide-floored valley does not signify
that it ever was occupied by a river of equal width.

THE GRADIENT. The gradients of graded rivers differ widely. A
large river with a light load reaches grade on a faint slope,