THE STRUGGLE FOR EXISTENCE AMONG VALLEYS AND STREAMS.
It is not to be inferred that every gully becomes a valley, nor that every small valley becomes a large one. Among valleys, as among living things, there is a struggle for existence, and fitness determines growth and survival. At an early stage of its erosion history the number of small valleys in a given area is often great, while at a later stage the number is less and the size of the survivors greater.
PLATE IX.
U. S. Geol. Surv.
Scale, 1+ mile per inch.
NIAGARA FALLS.
PLATE X.
U. S. Geol. Surv.
Scale, 2+ mile per inch.
Fig. 1. YELLOWSTONE PARK.
U. S. Geol. Surv.
Scale, 2+ mile per inch.
Fig. 2. ARIZONA.
Fig. 85.—Diagram illustrating the absorption of one gully by another by lateral erosion. The successive lines represent successive cross-sections.
One phase of the struggle for existence is often well illustrated on a freshly exposed slope of clay. The number of miniature gullies which develop on such a slope, even in a single shower, may be very large ([Fig. 84]); but the history of many of them is ephemeral. If two adjacent ones are of unequal depth the widening of the deeper narrows and finally eliminates the divide between them, and the two become one ([Fig. 85]).
Another phase of the struggle for existence is shown in other situations. Examination of a good map of the north shore of Lake Superior or the west shore of Lake Michigan shows a large number of small streams and gullies ([Fig. 1, Pl. IV]). The valleys are short and narrow, and between and beyond them are considerable areas untouched by erosion. The drainage near the lake is therefore young, and each of the small valleys is growing. This condition of things is perhaps typical of that which has been, is, or will be along the average coast at a certain stage in its erosion history. No equal stretch of coast-line where erosion is far advanced can boast of a number of large rivers comparable to that of the many small ones along the coasts mentioned. It therefore seems evident that of these many small streams a few only will attain considerable size.
Some of the methods by which the growth of the many is arrested are easily understood. Some of the young valleys on a given coast will work their heads back into the land faster than others because of inequalities of slope and material. This will be true of the tributaries no less than of their mains. If valleys develop in ways other than by head erosion (see [p. 73]) the chances are also against their equality of growth. If two streams, such as a and c, [Fig. 86], develop faster than the intermediate stream b, it is clear that their tributaries may work back into the territory which at the outset drained into b, so as to cut off the supply of water from the latter stream (compare a′b′c′, [Fig. 87]). As a result, the growth of b will be checked, and ultimately stopped. Similarly other valleys, such as f, will get the better of their neighbors, and many of the competitors, as b, d, e, and g will soon drop out of the race. Between the stronger streams competition still goes on. If a′ and f′ develop faster than c′ its prospective drainage territory will be preëmpted by its rivals (compare Figs. [87] and [88]). Thus as the result of the unequal rate at which valleys are lengthened, the larger number of those which come into existence are arrested in their development. As a result of growth in the manner indicated, the basins of even the large streams remain narrow at their lower ends while they expand above. This is the usual form of a drainage basin the development of which has been normal.
Fig. 86–88.—Diagrams to illustrate successive stages in the struggle for existence and dominion among streams.
Did valleys grow in length only, competition would not destroy the small ones; it would simply limit them. But valleys widen as well as lengthen, and by widening, adjacent valleys may eliminate the divide between them and become one. The elimination of the intervening ridge may be by lateral planation ([p. 82]), or, if the valleys be of unequal depth, by slope wash (see [Fig. 85]). By these and other processes many young valleys are dwarfed, and many others are destroyed.
Piracy.—Streams do not always hold the courses which they establish for themselves at the outset. If the valley occupied by the stream a, [Fig. 89], is deepened more rapidly than the valley occupied by b, a tributary from the former, c, may work back across the inter-stream area to e and steal the head waters of that stream ([Fig. 90]). The tributary which does the stealing is known as a pirate. Stream f ([Fig. 90]) is said to be beheaded, and its upper portion, de, diverted. The beheaded stream is diminished in volume; or if its total supply of water came in above the point of tapping it would disappear altogether.
Fig. 89 and 90.—Diagrams to illustrate piracy.
The process may not end even here. If after the diversion of de the point in the channel to the left is lowered faster than the channel of the beheaded stream f, the divide between dg and the head of f ([Fig. 90]) will be shifted down the valley of the latter, as shown in [Fig. 91]. The shifting will go on until the divide reaches a position of stability, that is, until erosion on its opposite sides is equal.
Fig. 91.—Diagram showing the shifting of a divide after piracy.
The foregoing case may be called foreign piracy because the valleys of different systems are concerned. Domestic piracy may also take place, as illustrated in the accompanying diagrams (Figs. [92] and [93]). Here a tributary to a crooked river may develop, working back until it taps the main at a higher point, thus straightening the course of the stream. The change takes place only when the highest point in the tributary valley is brought below the surface of the water in the main stream at the point where the tapping takes place. This would be likely to occur only after the main stream had attained a low gradient, for so long as it is deepening its channel notably, the small amount of water flowing through the tributary valley would not be likely to bring it down to the level of the main. In any case the flow of water from the main stream through the new valley would be likely to be started during flood, and at such time the erosion in the new channel would be great. The complete and final diversion of the stream through the new channel might be a slow process.
Fig. 92 and 93.—Domestic piracy. The tributary, a of Fig. 92, develops headward until it taps the main stream at b, giving the result shown in Fig. 93.
Piracy may occur where the material in which the valleys are cut is homogeneous; but, as will be seen later, heterogeneity of material, by determining unequal rates of erosion, stimulates the piratical proclivities of streams.
An actual case of piracy is shown on [Plate XI]. North and South Lakes formerly drained westward to the Schoharie Creek, the present head of which is in the extreme northwest corner of the map. The head of Kaaterskill Creek, which had a much higher gradient, worked back and captured the head of the westward-flowing stream, diverting the drainage from North and South Lakes to itself. Schoharie Creek was thus beheaded.
Plaatekill Creek, near the south limit of the map, appears to have beheaded the creek flowing west and northwest, similarly diverting its head waters. The Dells, Wis., quadrangle (U. S. Geol. Surv.) affords an illustration of domestic piracy.