Along the margin of a shallow lake mosses, water lilies, grasses, and other water-loving plants grow luxuriantly.  As their decaying remains accumulate on the bottom, the ring of marsh broadens inwards, the lake narrows gradually to a small pond set in the midst of a wide bog, and finally disappears.  All stages in this process of extinction may be seen among the countless lakelets which occupy sags in the recent sheets of glacial drift in the northern states; and more numerous than the lakes which still remain are those already thus filled with carbonaceous matter derived from the carbon dioxide of the atmosphere.  Such fossil lakes are marked by swamps or level meadows underlain with muck.

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 gorges only wide enough at the base to accommodate the stream.  The steepness of the valley slopes depends on the relative rates at which the bed is cut down by the stream and the sides are worn back by the weather.  In resistant rock a swift, well-laden stream may saw out a gorge whose sides are nearly or even quite vertical, but as a rule young valleys whose streams have not yet reached grade are V-shaped; their sides flare at the top because here the rocks have longest been opened up to the action of the weather.  Some of the deepest canyons may be found where a rising land mass, either mountain range or plateau, has long maintained by its continued uplift the rivers of the region above grade.