A river has its stages of development, youth, maturity, and old age, just as has a land surface, each stage displaying its characteristic marks. When an entirely new land surface is upheaved from the sea, it has no rivers, and its drainage must consist merely of the surface rain wash following the initial slopes of the new land. No instance of any considerable area of newly uplifted land has ever been observed, but the sequence of events may be readily inferred from known facts. Since the slopes cannot be absolutely plane nor the material entirely homogeneous, there must be slight depressions along which the rain water will gather into rills, and these will wear out little trenches. The more favourably situated trenches will receive more water and be more rapidly deepened and enlarged into ravines. In this early stage of drainage development there will be many ravines, more or less parallel, which are dry except after rains. Those ravines which are most rapidly deepened will be cut down to the level of the ground water and will there be fed by springs and become permanent streams when a level is reached below which the ground water does not sink in the driest seasons. If the new land is not simply sloping, but folded, valleys for drainage are afforded by the synclines.

The principal valleys are thus longitudinal, the main streams flowing in the synclinal troughs and passing from one syncline to another at the points where the anticlines are lowest, owing to the descending pitch of the folds. Such a drainage system is exemplified in the Jura Mountains of Switzerland. Thus, in a newly upheaved or newly folded land the streams are determined entirely by the slopes of the new surface and are called consequent streams. In its earliest stages a river can drain its territory or basin in only imperfect fashion, and whatever depressions exist in the surface of the new land are filled up with water and form lakes. Tributaries are much fewer than in later stages of development; the divides between the tributaries are obscurely marked, and in plains these divides are broad areas, not lines. The Red River of the North is an example of a stream in a very youthful stage, which flows across the level floor of an abandoned lake. In this plain the divides between the streams are so wide and flat that water gathers on them after heavy rains, having no reason to flow in one direction rather than another.

In northern Minnesota is the watershed or divide between the Mississippi, St. Lawrence, and Hudson's Bay drainage systems, which is hardly visible, the sluggish streams wandering over an almost flat surface, which has countless marshes and lakes.

Two very young grulches, Colorado. (U. S. G. S).

Fig. 247. - Two very young grulches, Colorado. (U. S. G. S).

As the river system becomes somewhat older, the stream channels are deepened, the larger ones being cut down to base-level, and if the region be one of considerable elevation, deep gorges and canons are excavated. If the streams flow across strata of different hardness, waterfalls result where a hard ridge crosses them, but in the main stream these cascades and rapids are ephemeral and soon removed by the stream's wearing down the obstacle. On the head-waters of streams, however, waterfalls may persist for a long period. The river valleys are widened out by atmospheric denudation, and channels are formed on their sloping sides, which gradually grow into side valleys. The lakes are for the most part drained or silted up, only the more important and deeper ones remaining, while the system of tributary streams and rills is greatly expanded. A mature river system is characterized by the complete development of its tributaries and drainage, so that every part of its basin is reached by the ramifying channels, and rivers of the same grade tend to be separated by nearly equal interspaces.

The waterfalls have disappeared, except near the stream-heads, and the stream-channels have sought out and utilized every weakness in the strata, adjusting themselves to the structure of the rocks and the alternations of hard and soft beds.

Valley floors are broadened and deposition begins upon them, and the streams, reaching a condition of equilibrium between erosion and deposition, are said to be graded. In graded streams the slope attained varies greatly; a small stream or one loaded with sediment requires a steeper slope than a large one, or one carrying but a small load. Thus, the lower Mississippi and its tributaries are graded, but while the great river flows in a valley with hardly any slope, the valleys of the smaller streams are still quite steep. In the process of development the stream gradients are continually readjusted, with the general result of diminishing the slope. When the stream has reached base-level and no longer erodes vertically, save in seasons of flood, it continues to cut laterally and to receive and transport the material washed into it by rains, and thus the divides are worn away.

The complete network of streams has enlarged the valley surfaces, which increases the rate of destruction and brings to the river a greater load of sediment to carry. In maturity the river receives its maximum load, sometimes so great that the lower reaches of the main stream are unable to transport it all, and spread the excess out over the flood-plain. The channel of an overloaded stream may be so raised and banked in by its own deposits that some of the tributaries are deflected and made to run for some distance parallel to the main stream, perhaps even reaching the sea independently. An example of this is the Loup Fork of the Platte in Nebraska. "The Platte flows there upon a ridge of its own creation. The Loup comes down into its valley and flows parallel with it for many miles." (Gannett).

The final stages of river development are reached when the base-level is attained, and the drainage basin reduced to a peneplain by the combined action of the streams and weathering. The flood-plain deposits may now be partially or completely removed, for the main trunk no longer receives an excessive load, and hence it is able to carry away some of that sediment which it had previously deposited. With its drainage basin smoothed down into a peneplain, the river's work is done; it has reached old age.

The course of river evolution above described is the ideal cycle of development, which, however, may be and generally is interrupted by diastrophic movements. An elevation of the region may simply rejuvenate the streams and start them afresh upon a career of wearing down the land. But if accompanied by extensive warping or folding of the rocks, the drainage system of the entire region may be revolutionized. A depression of the region will have the contrary effect, checking or stopping the work in which the streams were engaged, drowning their lower reaches and converting them into estuaries. A lowered land surface has less material to lose before it is reduced to base-level, but the work of denudation is accomplished more slowly.

When it was first suggested that rivers had cut their own valleys and had not merely taken possession of ready-made trenches, it was objected that such an explanation required many streams to begin their course by flowing uphill. It is very common to find a stream flowing across a region, cutting its way through ridge after ridge, instead of following the easy path of the longitudinal valleys. This is just what the principal streams of the northern Appalachians, such as the Delaware, the Susquehanna and the Potomac, have done, and at first sight, their course is very difficult to explain. Without going very far back into the history of these mountains, we may simply state that the ridges through which the rivers named have cut are the remnants of a reelevated and dissected peneplain, across which the streams flowed to the sea, cutting transverse valleys that were rapidly deepened into gorges. On the soft strata longitudinal valleys were opened out which, however, were formed after the transverse streams and could not be deepened faster than they, because the main stream flowing in each transverse valley gave a temporary base-level for the tributaries flowing in the longitudinal valleys.

The hard beds were sawed through by the descending streams, but elsewheie these beds stood up as ridges, and thus the ridges are also younger than the streams. The mystery disappears at once, if we simply remember that the transverse streams began their flow upon a sloping plain above which the present ridges did not project.