Chapter III: The Work of Running Water (5)

There is another way of looking at the same question. Earthy matter is being continually transferred from land to sea, and chiefly from high land. Rarely does it start from any point distant from the shore and move uninterruptedly to it. It is transported a short distance and lodged, to be again picked up, carried forward another step in its journey, and lodged again. For a very large part of the earth’s surface it would be true to say that its mantle rock is material in transit from higher land to the sea.

=Alluvial plains.=—Most streams, whether heading in mountains or not, have gentler gradients in their lower courses than in their upper, and in spite of increasing volume are usually unable to carry to their debouchures all the material gathered above. The excess of load is dropped chiefly on the flood-plains of the streams and constitutes them _alluvial plains_.

The making of an alluvial plain usually involves both erosion and deposition. When a stream has cut its channel to grade, downward erosion ceases, or more exactly, downward cutting is, on the average, counterbalanced by deposition. So long as a stream is cutting downward rapidly, it carries away whatever débris descends the side slopes. When it approaches grade, the débris which descends the side slopes tends to accumulate at their bases, and the V-shaped cross-section of the valley becomes U-shaped (see Fig. 172). At about the same time the stream begins to meander, for, having lost something of its former velocity, it is more easily turned from side to side. As it begins to meander, it widens the bottom of its valley. This is the initial stage in the development of the valley flat (2 and 3, Fig. 172). In its meandering the stream encroaches on the talus accumulations at the bases of its valley’s slopes. The side-cutting may remove all the loose débris and even undercut the bluff as at _a_, Fig. 173. The stream’s meanders shift their positions from time to time so that the valley flat is successively widened at different points. By lateral planation, therefore, a stream tends to develop a flat as soon as it reaches grade. This is the initial part of erosion in the making of a river flat, but a flat developed by erosion alone is not an alluvial plain.

So soon as the flat developed by a stream exceeds the width of its channel, the water (except in times of flood) does not cover it all at the same time. On any part which it temporarily abandons, some débris (alluvium) is likely to be left. This deposit of alluvium constitutes the valley flat an alluvial plain (Fig. 174). It will be seen that the valley flat is commonly an alluvial plain from the beginning.

Once the valley flat and alluvial plain are begun, their further development is easily followed. The stream in flood overflows the banks of its channel. The velocity of the overflowing water is reduced, and if it has much load a part of it will be dropped and the plain aggraded. Meantime meandering and lateral planation continue. Thus the flood-plain is widened by erosion, and aggraded by alluviation, the two processes going on simultaneously.

Flood-plains, chiefly the result of planation, but partly of aggradation, are a normal feature of river valleys, after a certain stage of development has been reached. This stage is that at which downward erosion becomes slight in comparison with lateral erosion. It follows that an alluvial plain normally begins its development where the valley is first brought to grade, that is, in its lower course. As the development of the valley goes on, the head of the flood-plain advances up-stream, and at the same time its older parts become wider.

=Flood-plains due to alluviation only.=—Exceptionally, an alluvial plain is developed by deposition only. Thus if a stream becomes overloaded while its valley is still narrow, as sometimes happens, deposition follows, and, as aggradation proceeds, the narrow valley acquires a progressively wider bottom (Fig. 175). Wide valley plains are sometimes developed in this way. Flood plains developed wholly by alluviation are sometimes formed under conditions which are independent of the stage of a valley’s development. Thus if a stream suddenly acquires an exceptional supply of detritus in its upper course, the development of an alluvial plain begins immediately below the point of overloading.

The overload might be acquired in various ways. (1) If a stream taps another (piracy) which carries a large quantity of sediment, carrying off both water and sediment to a channel with a lower gradient, deposition may take place where, under the earlier conditions, there was none. (2) Again, when a stream cuts through a barrier near its head waters, its velocity, and, therefore, its eroding power, may be so increased in its upper course that sediment enough is acquired to occasion deposition below, where none took place before. (3) In working back through formations of varying degrees of resistance, a stream’s head may presently reach a formation or a region which yields abundant sediment, even though there was no especial barrier below. (4) If an advancing glacier should reach the head waters of a stream, its discharge to the stream would greatly increase the load of the latter, and, although its volume would be augmented at the same time, deposition might result. As a matter of fact, streams carrying glacial drainage are usually aggrading streams. In general, anything which greatly increases the load of a stream near its head is likely to cause deposition, and so the development of a flood plain, at some point farther down the valley.

Streams which are actively aggrading their valleys are likely to anastomose (Figs. 176, 177). This results from the filling of the channels until they are too small to accommodate all the water. The latter then breaks out of the channel at few or many points. The new channels thus established suffer the same fate.

=Flood-plains due to obstructions.=—Again, any obstacle in a stream’s course is likely to cause deposition above. Thus dams built across rivers entail the deposition of sediment above. Where a stream flows over the outcropping edges of strata of different strength, the more resistant serve, in some sense, as dams. Above them the stream cuts its bed to a low gradient, and, becoming sluggish, drops more or less of the detritus brought down from above. Obstacles of any sort across a stream’s channel, therefore, favor the development of alluvial plains.

=Levees.=—As the stream in flood escapes its channel and overspreads its plain, its immediate banks are the site of active deposition, for it is here that the velocity of the overflowing water is first notably checked. On the banks of the channel, therefore, low alluvial ridges, called _natural levees_, are built up (Fig. 178, and Pl. XV). They may be narrow, or hundreds of feet in width, and are often several feet above the plains behind them, giving the latter a slope away from the channel of the stream. They are sometimes high enough to control the courses of tributary streams, as shown by numerous tributaries to the Mississippi below the Ohio. The Yazoo, for example, flows some 200 miles on the flood-plain of the Mississippi before it joins that river near Vicksburg. The levees even become divides, directing drainage away from the streams they guard (Pl. XV). Streams sometimes build levees faster than their tributaries aggrade their channels. The latter are then ponded, giving rise to lakes. The lakes on the lower courses of the tributaries to the Red River of Louisiana are examples.[70] They are sometimes built up above their natural level and kept in repair by human agency so as to confine the streams in time of flood. This is a source of danger unless they be steadily maintained, for the breaking of such levees often occasions great destruction. A case in point is the breaking of the levees of the Mississippi near New Orleans in 1890. The water broke through the levees at the Nita and Martinez crevasses (Fig. 187) and flowed eastward (from the former) with a current of 15 miles per hour, spreading destruction in its path. The water flowed eastward through Lakes Pontchartrain and Borgne, and entered Mobile Bay with such volume, velocity, and load of mud, as to destroy for a time the oyster and fish industries of that locality.[71]

NEAR HAHNVILLE, LOUISIANA.
U. S. Geol. Surv.]

Fig. 1. MISSOURI.
U. S. Geol. Surv.

Fig. 2. MISSOURI.
U. S. Geol. Surv.

Fig. 3. MISSOURI.
U. S. Geol. Surv.]

=Flood-plain meanders. Cut-and-fill.=—A stream with an alluvial plain is likely to meander widely (Pl. XVI). In general terms this may be said to be the result of low velocity, which allows it to be easily turned aside. Were the course of such a stream made straight, it would soon become crooked again. The manner of change is illustrated by Figs. 180 and 181. If the banks be less resistant at some points than at others, as is always the case, the stream will cut in at those points. If the configuration of the channel is such as to direct a current against a given point, _a_ (Fig. 180), the result is the same, even without inequality of material. Once a curve in the bank is started, it is increased by the current which is directed into it. Furthermore, as the current issues from the curve, it impinges against the opposite bank and develops a curve at that point. The water issuing from this curve develops another, and so on.

Once started, the curves or meanders tend to become more and more pronounced (compare Figs. 180 and 181). In the case represented by Fig. 1, Plate XVI (Missouri River near Brunswick, Mo.) the narrow neck of land between curves is almost cut through. When this is accomplished, the stream will abandon its wide curve. A later stage in the process is shown in Fig. 2, Plate XVI (the Osage River near Schell, Mo.).

The straightening of the channel is often accomplished in another way. Even before the meanders reach the stage represented by Fig. 1, Plate XVI, the position of the channel becomes unstable. In time of flood, the whole flat is covered with flowing water. The greater depth of water in the channel tends to give it a velocity greater than that of the water on the flat outside. But the distance from _a_ to _c via b_ (Fig. 181) is much greater than that in a direct line. It follows that the slope from _a_ to _c_ direct is greater than that by way of _b_. If the current between _a_ and _c_ in time of flood be strong enough to erode, it may deepen its bed, and thereby increase the volume of water following this course. The increased volume gives increased velocity, and the result may be the opening of a channel between _a_ and _c_ direct. The channel may be worn so deep that when the flood subsides, the stream will follow it. So long as the abandoned channel-curve remains unfilled with sediment, it is often called a _cut-off_. If it contains standing water and has the proper form, it is called an _ox-bow lake_ (Fig. 182), or sometimes a _bayou_. The water-filled portions are not always bows (Fig. 183, Osage River, near Butler, Mo.). Cut-offs, with or without standing water, are of common occurrence along most rivers with wide plains. Meandering is not confined to streams which are near sea-level. Even small creeks at high altitudes may meander, if so situated as to have slight velocity. Trout Creek in the Yellowstone Park (Fig. 184) is an example.

There seems to be some relation between the width of the belt within which a stream meanders, and the width of the stream itself. Recently it has been estimated that the ratio between them is 18 : 1.[72]

During the development of the meanders it is to be noted that lateral planation on the one side of a stream is accompanied by deposition on the other. This is _cut-and-fill_. The sediment eroded from the curve which is concave toward the stream is shifted down-stream, while that deposited in the curve which is convex toward the stream is brought down from above. Thus even in the development of meanders, the material which is dislodged is shifted down-stream. Since the current directed against the down-stream side of a growing meander is on the average stronger than that directed against the opposite side, the meander itself has a tendency to migrate down-stream (Fig. 182).

In their evolution, the curves of a stream’s channel often reach and undermine the valley bluff (Pl. VII). Since the meanders are, on the average, shifted down-stream individually, and since meanders are frequently developed in new places, it follows that a meandering stream tends to widen its valley throughout. Widening is also effected in other ways, for a stream with a flood-plain sometimes abandons its channel altogether for miles at a stretch, and the new course chosen may be against one of the bluffs of the valley. Such changes are most likely to take place where deposition along channel and levees has brought the part of the flood-plain (though not necessarily the bottom of the channel) adjacent to the stream above the level of that farther from it (Fig. 178). The change is likely to be effected in time of flood.

Flood-plains often attain great size. That of the Mississippi below the Ohio (Fig. 179) has a width ranging from rather more than 20 miles at Helena (Ark.), to something like 80 miles in the latitude of Greenville (Miss.).[73] Below the Ohio its area is something like 30,000 square miles, and its entire area has been estimated at about 50,000 square miles.[74]

Theoretically, the rotation of the earth should affect the erosion of streams, increasing it on the one bank (the right in the northern hemisphere and the left in the southern) and decreasing it on the other.[75] The streams doubtless accommodate themselves to the rotation of the earth in the original development of their gradation-plains and flood-plains, and the later effects of rotation are usually inconspicuous.

=Scour-and-fill.=—It has already been shown that aggrading streams cut laterally at the same time that they build up their plains. It is now to be added that they periodically deepen their channels to a notable extent, and that the deepening of the channel takes place at the very time when the flood-plain is being aggraded. In other words, the stream in flood aggrades its plain, and degrades its channel. This follows from the fact that the current is sluggish in the former position, where the water is shallow, and rapid in the latter, where it is deep. When the flood subsides, the channel, deepened while the current was torrential, is filled again by the feebler current which follows. This alternate deepening and filling is known as scour-and-fill. It is well illustrated by the Missouri River. At Nebraska City, scour is believed to occasionally reach depths of 70 to 90 feet.[76] At Blair, about 25 miles above Omaha, the same river is believed to cut to bed-rock (about 40 feet below the bottom of the channel in low water) twice a year, that is, during floods.[77] Fig. 185 shows the changes recorded in the channel of the river at this point during the year 1883. It shows that the scour-and-fill during this year amounted to almost 40 feet. All streams similarly situated do a like work. The material thus eroded is shifted down-stream, some of it for short distances only, and some of it to the sea. Even an aggrading stream therefore is not without erosive activity; it is a stream whose fill exceeds its scour, not one which has ceased to erode.

=Materials of the flood-plain.=—As a result of its varying velocities in flood and low water, a stream may deposit coarse material at one time and fine at another. A similar sequence of deposits takes place in the flood-plain of a meandering stream, irrespective of floods. Flood-plain deposits are often therefore very heterogeneous, as shown in Fig. 186, which represents the constitution of the alluvium of the Missouri River at Omaha. The deposits of the streams range from the finest clay, through sand to gravel, and even bowlders. In general they become finer down-stream. In a given plain, they are usually coarser below and finer above.

=Topography of the flood-plain.=—The flood-plain is nearly, but not altogether, flat. It has a gentle slope down-stream, and often for a distance from the sides toward the center (Fig. 174). This latter slope is the result of deposition by waters descending to the plain from the sides. It is destroyed wherever a meandering stream reaches its bluffs. When levees are well developed, there is a slope from them toward the sides of the valley (Fig. 178), but it rarely continues to the limiting bluffs. Since a stream with a well-developed flat frequently shifts its course, old levees and abandoned channels lend variety to the topography of the flood-plain.

=The topographic adjustment of tributaries.=[78]—The meandering and shifting of a main stream affects its tributaries. If a main stream swings against the bluff through which a tributary enters, the latter brings its channel into topographic adjustment by lowering its end to the level of the main. If now the main stream opposite the tributary swings to the other side of its valley, the tributary must make its way across the flat with a very low gradient. Not only this, but the flat of the main valley through which the tributary must flow is likely to be aggraded by the main in time of flood. The result is that the tributary stream becomes an aggrading stream at its debouchure, and topographic adjustment is not established until it has filled up the lower end of its valley to some notable extent. The filling of the lower end of the tributary likewise affects the lower ends of its lower tributaries.

If the main stream again swings over to the point where the tributary issues from its valley, the tributary stream and all its affected tributaries again become eroding streams. Thus scour-and-fill are not confined to the valley of the main stream.

=River-lakes.=—While rivers are in general hostile to lakes, they sometimes give origin to them. Oxbow lakes (Fig. 182 and Pl. XVI), due to the cut-offs of meandering streams, have already been referred to. Lakes formed in the same way have other forms (Pl. XVI and Fig. 183). Rivers also give rise to lakes through the deposits they make. If a main stream obstructs its tributaries by deposition at their debouchures, their lower courses are ponded and converted into lakes. The lakes along the tributaries to the Red River of Louisiana have already been cited as examples. If a tributary brings more load to its main than the latter can carry away, the detritus constitutes a partial dam, ponding the river and causing it to expand into a lake above. Such is the origin of Lake Pepin already referred to. In mountain regions, the alluvial cones of tributary valleys sometimes pond their mains.

Rivers may be dammed in other ways, as by lava flows, by landslides, by glacial drift, etc. In all such cases, lakes may come into existence, but they are not due primarily to the activity of the river itself.

=Deltas.=[79]—Where a stream enters standing water, or a slower stream, a special form of plain, _the delta_, is sometimes built up (Figs. 169, 187, and 188). Deltas and alluvial fans have much in common, and their only notable differences are those imposed by the differences in the conditions of deposition. The current of the stream is checked, but not altogether stopped, at its immediate debouchure. If it carries abundant sediment, much of it will be promptly dropped where the decrease in velocity is first felt. Such flow as there is beyond the debouchure is not confined to a definite channel, and the deposits made are therefore spread more or less on either side of the line which represents the continuation of the stream’s course.

As the depth of the water into which the stream flows increases, the current diminishes. Out to the point where the depth of the standing water is less than the depth of the current, the latter affects the bottom, and the surface of the deposits made slopes gently seaward; but where the depth of the standing water is such that the projected stream current is ineffective at the bottom, all the load rolled along the bottom is dropped, and a depositional slope is established (Fig. 188), its upper edge being below sea-level by an amount corresponding roughly to the depth of the current which brings the detritus. The outer slope is relatively steep and well-defined where the detritus is coarse, and relatively gentle and ill-defined where it is fine. Thus the stream tends to construct a sort of platform in the water just beyond its debouchure. The successive deposits on the outer abrupt slope will dip conformably with its surface (Fig. 189). The finest sediment will be carried beyond the steep slope, and conform to the topography of the bottom beyond (_c_, Fig. 189).

At the beginning, the top of the delta platform is at the level of the bottom of the stream’s channel at the point of debouchure, but it is gradually aggraded as water continues to flow over it. Its landward margin is presently built up to sea-level and then above it, and as the delta grows the delta-land is extended seaward (compare Figs. 188 and 189). At the same time the channel of the stream above the original head of the delta is aggraded, for the current there is checked by the aggradation of the delta. Thus alluvial deposits continuous with the delta are extended landward.

The projection of the direction of the lower end of the stream may be said to be the axis of the extra-debouchure current. From this axis, where the flow is strongest, the movement diverges more or less to right and left. Since the velocity of the diverging water is reduced more rapidly than that of the water which follows the axis of flow, deposition is likely to take place faster on either side of the axis than along the axis itself. The result is that the extra-debouchure current tends to build up levee-like ridges on either side, making a sort of sluice-way for itself. This sluice-way is gradually extended seaward, and at the same time gradually filled. As its capacity is reduced, more and more of the water flows over its sides. Presently the escape of the water over the little side-levees will develop a break at some point, and a line of distinct flow then diverges from the main current. This distributary repeats the history of its main. Thus the processes of levee-building, channel-filling, and levee-breaking follow one another, until some such system of currents as shown in Fig. 190 is developed. The result is that a delta’s growth is not simply in the line of extension of the main stream, but in a more or less semicircular area, the center of the circle being a point slightly below the position of the debouchure of the stream when the delta began. At any stage in its development the margin of the delta is more or less crenate (Fig. 191), or characterized by delta fingers (Fig. 190), the projections corresponding to the positions of the debouchures of the latest streams flowing across it. The extreme ends of the delta lobes (Fig. 190), and of groups of the delta fingers, often have something of the shape of the Greek letter from which the name originated, but the resemblance in form between a well-developed delta and the Greek letter is not striking. Deltas are sometimes built in bays, and in such cases their forms are predetermined on all sides but one. The head of a delta is sometimes arbitrarily located at the point where the first distributaries are given off. Since this point shifts widely with time, the definition can hardly be accepted. On this basis the head of the Mississippi delta is about 200 miles above its lower end. In reality it is much farther north.

The structure of a delta, shown in Fig. 189, shows its history. At any stage in its growth the river discharges its sediment across that part of the platform already built. The sediment rolled at the bottom of the current is dumped on reaching the steep slope, and constitutes the inclined _fore-set_ beds shown in Fig. 189. The material in suspension is carried farther, settles more gradually, and constitutes the _bottom-set_ beds (_c_, Fig. 189). In time the bottom-set beds, originally deposited some distance beyond the debouchure, may come to be overlain by the fore-set beds, deposited at a later time. While the fore-set beds are being deposited on the steep slopes of the delta, and the bottom-set beds beyond, deposition is also taking place on the top of the delta. These _top-set_ beds are laid down in a nearly horizontal position, and their seaward margin is gradually extended. Thus the delta comes to have the threefold structure shown in Fig. 189.

That part of the delta which is above the abrupt slope of its front corresponds in all essentials to an alluvial fan; but the delta as a whole differs from the fan in its abrupt and crenate or digitate margin.

It is to be noted that the delta is not wholly the product of a stream’s activity. The stream supplies the material, but the lake or sea renders at least passive assistance in its disposition. Not all rivers opening into the sea build deltas, and their failure is often the result of waves or shore currents which carry off the river sediment. Deltas are, however, sometimes formed in tidal seas, as at the debouchures of the Yukon; the Mackenzie, where the tidal range is three feet; the Niger, where the range is four feet; the Hoang-Ho, where the range is eight feet; and the Brahmaputra and Ganges, where the range is sixteen feet.[80] Since lakes, bays, gulfs, and inland seas have weaker waves and currents than the open sea, they are more favorable than the latter for the growth of deltas. Hence occur such deltas as those of the Mississippi, the Nile, the Po, and the Danube.

Deltas are likely to be absent, or confined to the heads of bays, on coasts which have recently sunk. Their general absence on the Atlantic coast of the United States is a case in point.

The following figures give some idea of the extent of deltas, and of their importance in land building. The Mississippi delta is advancing into the Gulf at the rate of about 100 yards per year, or a mile in 16 or 17 years. Its length is more than 200 miles, its area more than 12,000 square miles, and its depth at New Orleans has been estimated at 700[81] to 1000[82] feet. This great depth is believed to be the result of subsidence, and so of the superposition of one delta on another.[83] The delta of the Yukon has a sea margin of 70 miles, and extends more than 100 miles inland. The delta of the Rhône has also had a remarkable growth, considering the size and the history of the stream. Arles, near the debouchure of the stream, was 14 to 16 miles inland in the fourth century +B.C.+, and is now 30 miles inland.[84] The Rhône has also built a great delta in Lake Geneva, and its lower delta is built of sediment gathered below the lake. The Po has built a delta 14 miles beyond Adria, the port which gave its name to the Adriatic Sea. The extension of this delta has been at the average rate of about 50 feet per year, but recently, on account of artificial embankments, the rate has been much more rapid.[85] The Ganges and Brahmaputra together have made a delta of great size. Its area is sometimes estimated to be as high as 50,000 or 60,000 square miles, and its head is more than 200 miles from the sea.[86] The head of the Nile delta is 90 miles from the sea, and it has a coastal border of 180 miles. The head of the delta of the Hoang-Ho is about 300 miles from the coast, and its seaward border has a length of about 400 miles, though with some highland interruptions.[87]

After a delta has been built into a lake, the lake may disappear, leaving the delta out of water. Such “fossil” deltas, if so recently exposed that erosion has not destroyed their distinctive features, are readily recognized by their flat tops, their abrupt and lobate fronts, and their characteristic structure. They are often a means of determining the former existence of extinct lakes,[88] or the former higher levels of lakes which still exist.[89] Elevated deltas on seashores show either a rise of the land or a depression of the sea-level.

The material which is carried along the coasts or shores from the mouths of rivers may take on various and peculiar forms, according to the strength, direction, and relations of waves and currents. The consideration of these forms belongs more properly to the work of the sea than to that of rivers, since rivers are not concerned in their construction except in supplying material.

=Delta lakes.=—Delta-building streams sometimes help to form lakes by throwing their deposits around an area which fails to be aggraded to sea-level. Lake Pontchartrain, and other lakes in the delta of the Mississippi are examples (Fig. 187).

STREAM TERRACES.

Stream terraces[90] are bench-like flats or narrow plains along the sides of valleys (Fig. 192). They are usually narrow, but sometimes have great length in the direction of the axis of the valley. They originate in various ways.

=Due to inequalities of hardness.=—Reference has already been made (p. 140) to the effect of hard horizontal layers in the development of terraces and terraciform projections on the sides of valleys (Fig. 120). Such terraces are the result of differential degradation, and the upper surface of the hard layer marks the lower limit of the terrace, which commonly has a distinct slope toward the stream. Except where interrupted by tributary valleys, such terraces are likely to be continuous in a valley so long as the structure remains the same and the stream sustains the same relation to it. Such terraces would first show themselves in the older part of the valley. The effect of inclination of the hard stratum on the development of such terraces will be readily inferred. Terraces and benches of this sort are not equally distinct at all stages of a valley’s history. For great distinctness, the hard layer should have been exposed long enough to allow the general processes of erosion to have effected considerable differential wear, but not long enough to allow the topographic effects of unequal resistance to be obliterated.

=Normal flood-plain terraces.=—It has been seen that deposition in a river valley stands in more or less definite relationship to the stage of its development, and that the deposition which leads to the development of an alluvial plain is likely to take place where the higher gradient of the upper course gives place to the gentler gradient of the lower. It has also been seen that as a stream’s history advances, the stretch where the gradient is high recedes up-stream, and that the point which marks the head of active deposition follows. It follows that a river flat or flood-plain normally begins in the lower part of a valley, and works progressively headward, its upper end following, at some considerable distance, the head of the valley itself.

The commoner river terraces are remnants of former flood-plains, below which the streams which made them have cut their channels. It has already been pointed out (p. 184) that processes of erosion and deposition work together in the development of flood-plains, and that some flood-plains have but little alluvium (Fig. 174), while others owe their origin wholly to stream deposits (Fig. 175). It follows that terraces developed from flood-plains may be of rock, of alluvium, or of rock covered with alluvium.

The amount which a river channel must be deepened in order to change the remnants of its flood-plain to terraces cannot be definitely stated. When a channel is so deep that the remnants of a former flood-plain are no longer flooded, they would be called terraces, especially if a lower flood-plain has been developed. Even though not above the reach of floods, they are often called terraces if they are notably above the channel and separated from it by a lower plain. Thus the flat at _b_, Fig. 193, would be called a terrace, even though covered by water in exceptional floods; but the flat at _c_, but slightly above the channel, would hardly be called a terrace.

The question now arises why a stream, having once developed a flood-plain, should sink its channel to a lower level, leaving parts of the old flood-plain as terraces. This may be brought about by the operation of various causes.

(1) In the first place, the head of the valley-plain where the first notable deposition takes place normally advances up-stream. After the advance has been considerable, the descending stream may, on reaching the head of its valley-plain, lose so much of its load as to be able to sink its channel into the flood-plain farther down the valley (Fig. 194).

(2) Ordinarily a stream does not drop all its load at the head of its plain, but only its excess; but it will always drop coarse sediment to take fine, if fine be available. For a relatively small amount of coarse material dropped, a relatively large amount of fine may be taken up (p. 179). Other things being equal, it follows that when a stream drops coarse material to take fine, its channel is degraded unless there is at the same time a great reduction in the stream’s energy. Such reduction is likely to go with the decreasing declivity down-stream; but this is partly, or sometimes wholly, counterbalanced by the increasing volume of water. By the exchange of load, therefore, a stream may ultimately sink its channel below the flood-plain which the earlier and perhaps smaller stream had developed.

(3) Again, so long as a stream is actively eroding at its head, there is likely to be some aggradation below. At a later stage in the stream’s history, when active erosion at the head has ceased because of the reduction of the surface, less material will be carried from the upper part of the valley, and the stream on the flood-plain below, formerly loaded with material from up the valley, is now free to take up and carry away material temporarily left on the flood-plain. The result is a deepening of the channel.

(4) Any stream which has reached the flood-plain stage is likely to meander. After the flood-plain has become wide, the width of the belt within which the stream meanders is less than the width of its plain. In the Lower Mississippi, for example, the meander belt is often no more than a third to a tenth of the width of the flood-plain. It has already been pointed out that the meanders migrate down the valley. In so doing they depress the meander belt, the tendency being to reduce it to the level of the channel, and, therefore, below the level of the flood-plain. As the meander belt widens, the depression which it develops becomes more and more capacious. Presently it may attain such dimensions as to hold the water of ordinary floods. At this stage, or even before, such parts of the earlier flood-plain as remain, are terraces.

These several tendencies conspire to partially destroy river flood-plains, and to transform such parts as remain into terraces _in the normal course of a river’s history_. They appear first in the lower part of the valley, and migrate headward, following the course of nearly every other phase of activity in a stream’s history. The heads of the terraces follow, at a respectful distance, the head of the flood-plain, just as the head of the flood-plain follows at a distance the head of the valley. The second and subsequent flood-plains and the terraces to which they give origin follow the same course.

Terraces developed by the normal activities of a stream are always low, and it is improbable that they would ordinarily be conspicuous. The vertical distance between the first (highest) and second is greater than that between the second and third. The principles developed on page 65 et seq., in connection with the erosion of the hypothetical island, are applicable here.

=Flood-plain terraces due to other causes.=—Certain other causes, accidental rather than normal to a stream, result in the development of terraces from flood-plains. (1) If there be uplift in a region where the rivers have flats, the streams are rejuvenated, and the remnants of their former flood-plains become terraces. (2) If an alluvial flood-plain has been built as the result of an excessive supply of sediment (p. 186), the exhaustion or withdrawal of the excessive supply would leave the stream again relatively clear, and free to erode where it had been depositing. It would forthwith set to work to carry away the material which it had temporarily unloaded on the plain. The plains built up in many valleys in the northern part of our continent during the glacial period, when the drainage from the ice coursed through them, have subsequently been partially destroyed by erosion, and their remnants have become terraces. A notable reduction in the amount of available sediment, even when the earlier supply was not excessive, produces a similar result. (3) A notable increase in the volume of a stream, without corresponding increase in load, as when one stream captures another, may occasion the development of terraces by allowing the stream to deepen its channel. (4) Above any barrier which dams a stream, a flood-plain is likely to be developed. When the barrier is removed the stream will cut more or less deeply into the plain above, leaving terraces. (5) The recession of a falls through a flood-plain converts such parts of it as remain, into terraces.

In conclusion, it may be stated that many river terraces, mostly very low, are normal features of valley development, coming into existence at definite stages in a valley’s history. They are generally composed, in large part, of river alluvium. Others result from more or less accidental causes, working singly or in conjunction, and to this class belong all of the more conspicuous terraces developed from flood-plains. The structure of a terrace often affords some clue to its origin (Fig. 196).

=Discontinuity of terraces.=—When a stream sinks its channel into its flood-plain, it does not follow that a terrace remains on each side. Where the stream’s deepened channel is in the middle of its flood-plain, there is, temporarily, a terrace on either side; but wherever the deepened channel is at one margin of its flood-plain, a terrace remains on the other side only. Even where continuous at the outset, terraces soon become discontinuous, for all processes of subaërial erosion conspire to destroy them. A stream is likely to meander on its second and later flood-plains, as on its first and highest one. Wherever the meanders on its second flood-plain reach the borders of the first flood-plain, the terrace at that point disappears, and since the meanders are continually migrating, terraces are continually disappearing. The same would be true of the second terrace, if a second were developed. The removal of portions of a terrace by the sweep of meanders is likely to leave the remnants cuspate toward the stream.[91] Again, tributary streams, in bringing their channels into topographic adjustment with their mains, cut through the terraces of the latter. New gullies develop on the faces of the terraces and their heads work back across them, dissecting them still further. At the same time, sheet erosion and other phases of slope wash tend to drive the scarps of the terraces back toward the bluff beyond. By the time a second series of terraces is well developed, no more than meagre remnants of the first may remain.

From the foregoing considerations it is clear that the extent to which river terraces once developed, now remain, is dependent in part on the length of time which has elapsed since the river sank its channel below them. Other things being equal, the greater their age the more meagre their remnants.

Terraces developed from river plains formed chiefly by alluviation stand a better chance of long life than most other alluvial terraces. This is because of the configuration of the original valley, the aggradation of which gave origin to the plain. The principle involved is illustrated by Fig. 197. In developing the second flood-plain the river encounters the rock wall of the valley. This greatly retards lateral erosion, and the terrace above, _defended_[92] by the rock, is likely to be long-lived.

Alluvial terraces, like rock shelves, are popularly thought to mark “old levels of the river.” In one sense this is true, but not in the sense in which the expression is commonly used. Every level, from the crest of the bounding bluffs to the bottom of a valley, is a level at which water ran for a longer or shorter time; but the terrace does not mean that the river was once so much larger than now as to fill the valley from its present channel to the level of the terraces.

=Termini of terraces.=—From the mode of development of terraces it will be seen that, traced up-stream, each terrace should theoretically grade into a flood-plain at its upper end (Fig. 194), and that the upper end of the second (from the top) terrace, where there are two, would not be so far up-stream as the upper end of the first (highest). This is represented diagrammatically in Fig. 198.

The down-stream termini of terraces are rarely distinct. This is partly because the notable meandering of the streams in their lower courses is antagonistic to the preservation of terraces. If all terraces once developed remained, and if delta-building proceeded without interruption from waves, the relations should be somewhat as follows: Traced down-stream, the cliff between the oldest (highest) terrace and the next younger becomes gradually lower until it finally disappears, and the continuation of the two is found in a common plain. The cliff between the second and third terraces should disappear in the same way, and below its disappearance the plain representing their continuation is continuous with that representing the continuation of the first and second. The cliff between the second and third terraces may or may not continue farther down-stream than that between the first and second. The plains below the terraces finally become continuous with the lowest flood-plain and with the delta. These relations can rarely be seen because of the destruction of the older terraces, and because of the erosion by waves along shore.

The topography of terraces is similar to that of flood-plains, except in so far as modified by erosion. While flat in general, the terrace may slope either toward or from the valley bluff, and its surface may be marked by all the minor irregularities which characterize a flood-plain.