When the work of a river is completed, the line which marks the profile of its bed should have a gentle and continuous slope downwards to base level. The existence of irregularities, of breaks in the smoothness of the slope, means that the work of excavation has not proceeded far, that the river is young. But it is not necessary to proceed to the laborious drawing of a profile in order to determine the extent to which the process of excavation has been carried. The existence of rapids, of waterfalls, the alternation of swift and slow-flowing reaches are all proofs that it has not been carried far. In short, if a river is navigable, the navigable reach at least is mature; if it is capable of furnishing power, that region at least is youthful. If, as sometimes happens, the middle course is navigable and slow-flowing, and the lower course broken by rapids and falls, then the probability is that earth movements have occurred, so that the two regions are of different age. This is a condition which occurs relatively often in the case of large rivers.
One other point is worth notice, because it illustrates another way in which the analogy of youth and maturity holds good. The youthful river, with its interrupted slope, its lakes and falls, does not permit the water to flow off with the same regularity as the mature river with its smoothed outlines. The mature river is thus a more perfect instrument of drainage.
It is not necessary for our purpose to consider in detail the characteristic forms of river erosion. It may be sufficient to notice that rapids and waterfalls are due to the varying hardness of the rocks forming the bed of the river, and that the normal course of events is the transition from waterfall to rapid, and from rapid to stream flowing quietly at the bottom of a rocky gorge. Long gorges or canyons tend to occur in regions where river erosion is not greatly assisted by the other eroding agents. As a general rule, as the river cuts its way down, the other agents cut back the walls so exposed, so that a wide valley is formed.
But a river does not only eat out its bed in its valley track. A necessary consequence of this erosion is that it is also able to eat back the slope on which it is rising, as a result of the smoothing out of the curves of its bed, so that its source retreats further and further into the mountain. In regions of abundant rainfall every slope is abundantly supplied with streams, and therefore those streams which cut back their region of origin most rapidly will necessarily encroach upon their neighbours' territory. They therefore tend to tap some of the tributaries of the other streams, a phenomenon which has sometimes considerable human importance, and has been extensively studied of late years under the name of river-capture.