THE ADVANCE TO MATURITY. The infantile stage is brief. As a river
advances toward maturity the initial depressions, the lake basins
of its area, are gradually effaced. By the furrowing action of the
rain wash and the head ward lengthening, of tributaries a
branchwork of drainage channels grows until it covers the entire
area, and not an acre is left on which the fallen raindrop does
not find already cut for it an uninterrupted downward path which
leads it on by way of gully, brook, and river to the sea. The
initial surface of the land, by whatever agency it was modeled, is
now wholly destroyed; the region is all reduced to valley slopes.

THE LONGITUDINAL PROFILE OF A STREAM. This at first corresponds
with the initial surface of the region on which the stream begins
to flow, although its way may lead through basins and down steep
descents. The successive profiles to which it reduces its bed are
illustrated in Figure 51. As the gradient, or rate of descent of
its bed, is lowered, the velocity of the river is decreased until
its lessening energy is wholly consumed in carrying its load and
it can no longer erode its bed. The river is now AT GRADE, and its
capacity is just equal to its load. If now its load is increased
the stream deposits, and thus builds up, or AGGRADES, its bed. On
the other hand, if its load is diminished it has energy to spare,
and resuming its work of erosion, DEGRADES its bed. In either case
the stream continues aggrading or degrading until a new gradient
is found where the velocity is just sufficient to move the load,
and here again it reaches grade.

V-VALLEYS. Vigorous rivers well armed with waste make short work
of cutting their beds to grade, and thus erode narrow, steep-sided