THE LIFE HISTORIES OF RIVERS

The intricate pattern of river etchings.—The attack of the weather upon the solid lithosphere destroys the integrity of its surface layer, and through reducing it to rock dÉbris makes it the natural prey of any agent competent to carry it along the surface. We have seen how, for short distances, gravity unaided may pile up the dÉbris in accumulations of talus, and how, when assisted by thaw water which has soaked into the material, it may accomplish a slow migration by a peculiar type of soil flow. Yet far more potent transporting agencies are at work, and of these the one of first importance is running water. Only in the hearts of great deserts or in the equally remote white deserts of the polar regions is the sound of its murmurings never heard. Every other part of the earth’s surface has at some time its running water coursing in valleys which it has itself etched into the surface. It is this etching out of the continents in an intricate pattern of anastomosing valleys which constitutes the chief difference between the land surface and the relatively even floor of the oceans.

The motive power of rivers.—Every river is born in throes of Mother Earth by which the land is uplifted and left at a higher level than it was before. It is the difference of elevation thus brought about between separated portions of the land areas that makes it possible for the water which falls upon the higher portions to descend by gravity to the lower. This natural “head” due to differences of elevation is the motive power of the local streams, and for each increase in elevation there is an immediate response in renewed vigor of the streams. The elevated area off which the rivers flow is here termed an upland.

The velocity of a stream will be dependent not only upon the difference in altitude between its source and its mouth, but upon the distance which separates them, since this will determine the grade. The level of the mouth being the lowest which the stream can reach is termed the base level, and the current is fixed by the slope or declivity. The capacity to lift and transport rock dÉbris is augmented at a quite surprising rate with every increase in current velocity, the law being that the weight of the heaviest transportable fragment varies with the sixth power of the velocity of the current. Thus if one stream flows twice as rapidly as another, it can transport fragments which are sixty-four times as heavy.

Old land and new land.—The uplifts of the continents may proceed without changes in the position of the shore lines, in which case areas, already carved by streams but no longer actively modified by them, are worked upon by tools freshly sharpened and driven by greater power. The land thus subjected to active stream cutting is described as old land, and has already had engraved upon it the characteristic pattern of river etchings, albeit the design has been in part effaced.

If, upon the other hand, the shore line migrates seaward with the uplift, a portion of the relatively even sea floor, or new land, is elevated and laid under the action of the running water. As we are to see, stream cutting is to some extent modified when a river pattern is inherited from the uplift. The uplift, whether of old land only or of both old land and new land, marks the starting point of a new river history, usually described as an erosion cycle.

The earlier aspects of rivers.—Though geologists have sometimes regarded the uplift of the continents as a sort of upwarping in a continuous curved surface, the discussions of river histories and the pictorial illustrations of them have alike clearly assumed that the uplift has been essentially in blocks and that the elevated area meets the lower lying country or the sea in a more or less definite escarpment. The first rivers to develop after the uplift may be described as gullies shaped by the sudden downrush of storm waters and spaced more or less regularly along the margin of the escarpment (Fig. 165). These gullies are relatively short, straight, and steep; they have precipitous walls and few, if any, tributaries.

With time the gully heads advance into the upland as they take on tributaries; and so at length they in part invest it and dissect it into numerous irregularly bounded and flat-topped tables which are separated by caÑons (Fig. 166). At the same time the grade of the channel is becoming flatter, and its precipitous walls are being replaced by curving slopes, as will be more fully described in the sequel. It is because of this progressive reduction of grades with increasing age that the early stages of a river’s life are much the most turbulent of its history. The water then rushes down the steep grades in rapids, and is often at times opened out in some basin to form a lake where differences of uplift have been characteristic of neighboring sections. For several reasons such basins in the course of a stream are relatively short lived (Chapter XXX), and they disappear with the earlier stages of the river history.

The meshes of the river network.—From the continued throwing out of new tributaries by the streams, the meshes in the river network draw more closely together as the stages of its history advance. The closeness of texture which is at last developed upon the upland is in part determined by the quantity of rainfall, so that in New Jersey with heavy annual precipitation the meshes in the network are much smaller than they are, for example, upon the semiarid or arid plains of the western United States. Its design will, however, in either case more or less clearly express the plan of rock architecture which is hidden beneath the surface (Chapter XVII).

The upper and lower reaches of a river contrasted.—From the fact that the river progressively invades new portions of the upland and lays the acquired sections under more and more thorough investment, it has near its headwaters for a long time a frontier district which may be regarded as youthful even though the sections near its mouth have reached a somewhat advanced stage. The newly acquired sections of river valley may thus possess the steep grade and precipitous walls which are characteristic of early gullies and caÑons and are in contrast with the more rounded and flat-bottomed sections below. Lateral streams, from the fact that they are newer than the main or trunk stream to which they are tributary, likewise descend upon somewhat steeper grades (Fig. 167).

The balance between degradation and aggradation.—We have seen that the power to transport rock fragments is augmented at a most surprising rate with every increase in the current velocity. While the lighter particles of rock may be carried as high up as the surface of the water, the heavier ones are moved forward upon the bottom with a combined rolling and hopping motion aided by local eddies. Those particles which come in contact with the bottom or sides of the channel abrade its surface so as ever to deepen and widen the valley. This cutting accomplished by partially suspended dÉbris in rapidly moving currents of water is known as corrasion and the stream is said to be incising its valley.

As the current is checked upon the lower and flatter grades, some of its load of sediment, and especially the coarser portion, will be deposited and so partially fill in the channel. A nice balance is thus established between degradation and the contrasted process known as aggradation. The older the river valley the flatter become the grades at any section of its course, and thus the point which separates the lower zone of aggradation from the upper one of degradation moves steadily upstream with the lapse of time.

The accordance of tributary valleys.—It is a consequence of the great sensitiveness of stream corrasion to current velocity that no side stream may enter the trunk valley at a level above that of the main stream—the tributary streams enter the trunk stream accordantly. Each has carved its own valley, and any abrupt increase in gradient of the side streams near where they enter the main stream would have increased the local corrasion at an accelerated rate and so have cut down the channel to the level of the trunk stream.

The grading of the flood plain.—All rivers are subject to seasonal variations in the volume of their waters. Where there are wet and dry seasons these differences are greatest, and for a large part of the year the valleys in such regions may be empty of water, and are in fact often utilized for thoroughfares. In the temperate climates of middle latitudes rivers are generally flooded in the spring when the winter snows are melted, though they may dwindle to comparatively small streams during the late summer. In the upper reaches of the river the current velocities are such that the usual river channel may carry all the water of flood time; but lower down and in the zone of aggradation, where the current has been checked, the level of the water rises in flood above the banks of its usual channel and spreads over the surrounding lowlands. As a deposit of sediment is spread upon the surface, the succession of the annual deposits from this source raises the general level as a broad floor described as the flood plain of the river.

The cycles of stream meanders.—The annual flooding with water and simultaneous deposition of silt is not, however, the only grading process which is in operation upon the flood plain. It is characteristic of swift currents that their course is maintained in relatively straight lines because of the inertia of the rapidly moving water. In proportion as their currents become sluggish, rivers are turned aside by the smallest of obstructions; and once diverted from their straight course, a law of nature becomes operative which increases the curvature of the stream at an accelerated rate up to a critical point, when by a change, sudden and catastrophic, a new and direct course is taken, to be in its turn carried through a similar cycle of changes. This so-called meandering of a stream is accompanied by a transfer of sediment from one bend or meander of the river to those below and from one bank to the other. Inasmuch as the later meanders cross the earlier ones and in time occupy all portions of the plain to the same average extent, a process of rough grading is accomplished to which the annual overflow deposit is supplementary.

The course of the current in consecutive meanders and the cross sections of the channel which result directly from the meandering process will be made clear from examination of Fig. 168. So soon as diverted from its direct course, the current, by its inertia of motion, is thrown against the outer or convex side so as to scour or corrade that bank. Upon the concave or inner side of the curve there is in consequence an area of slack water, and here the silt scoured from higher meanders is deposited. The scouring of the current upon the outer bank and the filling upon the inner thus gives to the cross section of the stream a generally unsymmetrical character (Fig. 168 ab). Between meanders near the point of inflection of the curve, and there only, the current is centered in the middle of the channel and the cross section is symmetrical (Fig. 168 cd).

The scour upon the convex side of a meander causes the river to swing ever farther in that direction, and through invasion of the silted flood plain to migrate across it. Trees which lie in its path are undermined and fall outward in the stream with tops directed with the current (Fig. 169). Whenever the flood plain is forested, the fallen trees may be so numerous as to lie in ranks along the shore, and at the time of the next flood they are carried downstream to jam in narrow places along the channel and give the erroneous impression that the flood has itself uprooted a section of forest (see p. 418).

The cut-off of the meander.—As the meander swings toward its extreme position it becomes more and more closely looped. Adjacent loops thus approach nearer and nearer to each other, but in the successive positions a nearly stationary point is established near where the river makes its sharpest turn (Fig. 170, G, and Fig. 454, p. 417). At length the neck of land which separates meanders is so narrow that in the next freshet a temporary jamming of logs within the channel may direct the waters across the neck, and once started in the new direction a channel is scoured out in the soft silt. Thus by a breaking through of the bank of the stream, a so-called “crevasse”, the river suddenly straightens its course, though up to this time it has steadily become more and more sharply serpentine. After the cut-off has occurred, the old channel may for a time continue to be used by the stream in common with the new one, but the advantage in velocity of current being with the cut-off, the old channel contains slacker water and so begins to fill with silt both at the beginning and the end of the loop. Eventually closed up at both ends, this loop or “ox-bow” is entirely separated from the new channel, and once abandoned of the stream is transformed into an ox-bow lake (Fig. 171 and p. 415).

Meander scars.—Swinging as it occasionally does in its meanderings quite across the flood plain and against the bank of the earlier degrading river in this section, the meander at times scours the high bank which bounds the flood plain, and undermining it in the same manner, it excavates a recess of amphitheatral form which is known as a meander scar (Fig. 172). At length the entire bank is scarred in this manner so as to present to the stream a series of concave scallops separated by sharp intermediate salients of cuspate form.

River terraces.—Whenever the river’s history is interrupted by a small uplift, or the base level is for any reason lowered, the stream at once begins to sink its channel into the flood plain. Once more flowing upon a low grade, it again meanders, and so produces new walls at a lower level, but formed, like the first, of intersecting meander scars. Thus there is produced a new flood plain with cliff and terrace above, which is known as a river terrace. A succession of uplifts or of depressions of the base level yields terraces in series, as they appear schematically represented in Fig. 172. Such terraces are to be found well developed upon most of our larger rivers to the northward of the Ohio and Missouri. The highest terrace is obviously the remnant of the earliest flood plain, as the lowest represents the latest.

The delta of the river.—As it approaches its mouth the river moves more and more sluggishly over the flat grades, and swings in broader meanders as it flows. Yet it still carries a quantity of silt which is only laid down after its current has been stopped on meeting the body of standing water into which it discharges. If this be the ocean, the salinity of the sea water greatly aids in a quick precipitation of the finest material. This clarifying effect upon the water of the dissolved salt may be strikingly illustrated by taking two similar jars, the one filled with fresh and the other with salt water, and stirring the same quantity of fine clay into each. The clay in the salt water is deposited and the water cleared long before the murkiness of the other has disappeared.

By the laying down of the residue of its burden of sediment where it meets the sea, the river builds up vast plains of silt and clay which are known as deltas and which often form large local extensions of the continents into the sea. Whereas in its upper reaches the river with its tributary streams appears in the plan like a tree and its branches, in the delta region the stream, by dividing into diverging channels called distributaries (Fig. 458, p. 420), completes the resemblance to the tree by adding the roots. From the divergence of the distributaries upon the delta plain the Greek capital letter ? is suggested and has supplied the name for these deposits. Of great fertility, the delta plains of rivers have become the densely populated regions of the globe, among which it is necessary to mention only the delta of the Nile in Egypt, those of the Ganges and Brahmaputra in India, and those of the Hoang and Yangtse rivers in China.

The levee.—When the snows thaw upon the mountains at the headwaters of large rivers, freshets result and the delta regions are flooded. At such times heavily charged with sediment, a thin deposit of fertile soil is left upon the surface of the delta plain, and in Egypt particularly this is depended upon for the annual enrichment of the cultivated fields. Though at this time the waters spread broadly over the plain, the current still continues to flow largely within the normal channel, so that the slack water upon either side becomes the locus for the main deposit of the sediment. There is thus built up on either side of the channel a ridge of silt which is known as a levee, and this bank is steadily increased in height from year to year (Fig. 452).

To prevent the danger of floods upon the inhabited plains, artificial levees are usually raised upon the natural ones, and in a country like Holland, such levees (dikes) involve a large expenditure of money and no small degree of engineering skill and experience to construct. So important to the life of the nation is the proper management of its dikes, that in the past history of China each weak administration has been marked by the development of graft in this important department and by floods which have destroyed the lives of hundreds of thousands of people.

Wherever there has been a markedly rapid sinking upon a delta region, and depressions are common in delta territory, no doubt as a result of the loading down of the crust, the river may present the paradoxical condition of flowing at a higher level than the surrounding country. Between the levees of neighboring distributaries there are peculiar saucer-shaped depressions of the country which easily become filled with water. At the extremity of the delta the levee may be the only land which shows above the ocean surface, and so present the peculiar “bird-foot” outline which is characteristic of the extremity of the Mississippi delta, though other processes than the mere sinking of the deposits may contribute to this result (Fig. 173).

The sections of delta deposits.—If now we leave the plan of the delta to consider the section of its deposits, we find them so characteristic as to be easily recognized. Considered broadly, the delta advances seaward after the manner of a railroad embankment which is being carried across a lake. Though the greater portion of the deposit is unloaded upon a steep slope at the front, a smaller amount of material is dropped along the way, and a layer of extremely fine material settles in advance as the water clears of its finely suspended particles (Fig. 174). Simultaneous deposits within a delta thus comprise a nearly horizontal layer of coarser materials, the so-called top-set bed; the bulk of the deposit in a forward sloping layer, the so-called fore-set bed; and a thin film of clay which is extended far in advance, the bottom-set bed (Fig. 174, 2). If at any point a vertical section is made through the deposits, beds deposited in different periods are encountered; the oldest at the bottom in a horizontal position, the next younger above them and with forward dip, and the youngest and coarsest upon the top in nearly horizontal position (Fig. 174, 3).

It has been estimated that the surface of the United States is now being pared down by erosion at the average rate of an inch in 760 years. The derived material is being deposited in the flood plain and delta regions of its principal rivers. Some 513 million tons of suspended matter is in the United States carried to tidewater each year, and about half as much more goes out to sea as dissolved matter. If this material were removed from the Panama Canal cutting, an 85-foot sea-level canal would be excavated in about 73 days. The Mississippi River alone carries annually to the sea 340 million tons of suspended matter, or two thirds of the entire amount removed from the area of the United States as a whole. It is thus little wonder that great deltas have extended their boundaries so rapidly and that the crust is so generally sinking beneath the load.