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.