Fig. 50. Drainage Maps
A, an area in its infancy, Buena Vista County, Iowa;
B, an area in its maturity, Ringgold County, Iowa
Fig. 51. Successive Longitudinal Profiles of a Stream
am, initial profile, with waterfall at w, and basins at l and l´, which at first are occupied by lakes and later are filled or drained; b, c, d, and e, profiles established in succession as the stream advances from infancy toward old age. Note that these profiles are concave toward the sky. This is the erosion curve. What contrasting form has the weather weather curve ([p. 34])?
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.
Fig. 52. A V-Valley,—the Canyon of the Yellowstone
Note the steep sides. What processes are at work upon them? How wide is the valley at the base compared with the width of the stream? Do you see any river deposits along the banks? Is the stream flowing swiftly over a rock bed, or quietly over a bed which it has built up? Is it graded or ungraded? Note that the canyon walls project in interlocking spurs