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, 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 are able to subdue the