Fig. 11.—Oxbows and cut-off. Showing the changes in the course of a river in its alluvial plain.Fig. 11.—Oxbows and cut-off. Showing the changes in the course of a river in its alluvial plain.
When the curves of a river have been developed to a certain point (see Fig. 11), when they have attained whatis called the "oxbow" form, it often happens that the stream breaks through the isthmus which connects one of the peninsulas with the mainland. Where, as is not infrequently the case, the bend has a length of ten miles or more, the water just above and below the new-made opening is apt to differ in height by some feet. Plunging down the declivity, the stream, flowing with great velocity, soon enlarges the channel so that its whole tide may take the easier way. When this result is accomplished, the old curve is deserted, sand bars are formed across their mouths, which may gradually grow to broad alluvial plains, so that the long-surviving, crescent-shaped lake, the remnant of the river bed, may be seen far from the present course of the ever-changing stream. Gradually the accumulations of vegetable matter and the silt brought in by floods efface this moat or oxbow cut-off, as it is so commonly termed.
As soon as the river breaks through the neck of a peninsula in the manner above described, the current of the stream becomes much swifter for many miles below and above the opening. Slowly, however, the slopes are rearranged throughout its whole course, yet for a time the stream near the seat of the change becomes straighter than before, and this for the reason that its swifter current is better able to dispose of thedébriswhich is supplied to it. The effect of a change in the current produced by such new channels as we have described as forming across the isthmuses of bends is to perturb the course of the stream in all its subsequent downward length. Thus an oxbow cut-off formed near the junction of the Ohio and Mississippi may tend more or less to alter the swings of the Mississippi all the way to the Gulf of Mexico.
Although the swayings of the streams to and fro in their alluvial plains will give the reader some idea as to the struggle which the greater rivers have with thedébriswhich is committed to them, the full measure of the work and its consequences can only be appreciated by those who have studied the phenomena on the ground. A riversuch as the Mississippi is endlessly endeavouring to bear its burden to the sea. If its slope were a uniform inclined plane, the task might readily be accomplished; but in this, as in almost all other large water ways, the slope of the bed is ever diminishing with its onward course. The same water which in the mountain torrent of the Appalachians or Cordilleras rolled along stones several feet in diameter down slopes of a hundred feet or more to the mile can in the lower reaches of the stream move no pebbles which are more than one fourth of an inch in diameter over slopes which descend on the average about half a foot in a mile. Thus at every stage from the torrent to the sea the detritus has from time to time to rest within the alluvial banks, there awaiting the decay which slowly comes, and which may bring it to the state where it may be dissolved in the water, or divided into fragments so small that the stream may bear them on. A computation which the present writer has made shows that, on the average, it requires about forty thousand years for a particle of stone to make its way down the Mississippi to the sea after it has been detached from its original bed. Of course, some bits may make the journey straightforwardly; others may require a far greater time to accomplish the course which the water itself makes at most in a few weeks. This long delay in the journey of the detritus—a delay caused by its frequent rests in the alluvial plain—brings about important consequences which we will now consider.
As an alluvial plain is constructed, we generally find at the base pebbly material which fell to the bottom in the current of the main stream as the shores grew outward. Above this level we find the deposits laid down by the flood waters containing no pebbles, and this for the reason that those weightier bits remained in the stream bed when the tide flowed over the plain. As the alluvial deposit is laid down, a good deal of vegetable matter was built into it. Generally this has decayed and disappeared. On the surface of the plain there has always been growing abundant vegetation, the remains of which decayed on the surface in the manner which we may observe at the present day. This decomposing vegetable matter within and upon the porous alluvial material produces large quantities of carbonic acid, a gas which readily enters the rain water, and gives it a peculiar power of breaking up rock matter. Acting on thedébris, this gas-charged water rapidly brings about a decay of the fragments. Much of the material passes at once into solution in this water, and drains away through the multitudinous springs which border the river. As this matter is completely dissolved, as is sugar in water, it goes straight away to the sea without ever again entering the alluvium. In many, if not most, cases this dissolving work which is going on in alluvial terraces is sufficient to render a large part of the materials which they contain into the state where it disappears in an unseen manner; thus while the annual floods are constantly laying down accumulations on the surface of these plains, the springs are bearing it away from below.
In this way, through the decomposition which takes place in them, all those river terraces where much vegetable matter is mingled with the mineral substances, become laboratories in which substances are brought into solution and committed to the seas. We find in the water of the ocean a great array of dissolved mineral substances; it, indeed, seems probable that the sea water contains some share, though usually small, of all the materials which rivers encounter in their journey over and under the lands. As the waters of the sea obtain but little of this dissolved matter along the coast, it seems likely that the greater share of it is brought into the state of solution in the natural laboratories of the alluvial plains.
Here and there along the sides of the valleys in which the rivers flow we commonly find the remains of ancient plains lying at more or less considerable heights above the level of the streams. Generally these deposits, which from their form are called terraces, represent the stages ofdown-wearing by which the stream has carved out its way through the rocks. The greater part of these ancient alluvial plains has been removed through the ceaseless swinging of the stream to and fro in the valley which it has excavated.
In all the states of alluvial plains, whether they be the fertile deposits near the level of the streams which built them, or the poorer and ruder surfaced higher terraces, they have a great value to mankind. Men early learned that these lands were of singularly uniform goodness for agricultural use. They are so light that they were easily delved with the ancient pointed sticks or stone hoes, or turned by the olden, wooden plough. They not only give a rich return when first subjugated, but, owing to the depth of the soil and the frequency with which they are visited by fertilizing inundations, they yield rich harvests without fertilizing for thousands of years. It is therefore not surprising that we find the peoples who depended upon tillage for subsistence first developed on the great river plains. There, indeed, were laid the foundations of our higher civilization; there alone could the state which demands of its citizens fixed abodes and continuous labour take rise. In the conditions which these fields of abundance afforded, dense populations were possible, and all the arts which lead toward culture were greatly favoured. Thus it is that the civilization of China, India, Persia, and Egypt, the beginnings of man's higher development, began near the mouths of the great river valleys. These fields were, moreover, most favourably placed for the institution of commerce, in that the arts of navigation, originating in the sheltered reaches of the streams, readily found its way through the estuaries to the open sea.
Passing down the reaches of a great river as it approaches the sea, we find that the alluvial plains usually widen and become lower. At length we attain a point where the flood waters cover the surface for so large a part of the year that the ground is swampy and untillable unlessit is artificially and at great expense of labour won to agriculture in the manner in which this task has been effected in the lower portion of the Rhine Valley. Still farther toward the sea, the plain gradually dips downward until it passes below the level of the waters. Through this mud-flat section the stream continues to cut channels, but with the ever-progressive slowing of its motion the burden of fine mud which it carries drops to the bottom, and constantly closes the paths through which the water escapes. Every few years they tend to break a new way on one side or the other of their former path. Some of the greatest engineering work done in modern times has been accomplished by the engineers engaged in controlling the exits of large rivers to the sea. The outbreak of the Yellow River in 1887, in which the stream, hindered by its own accumulations, forced a new path across its alluvial plains, destroyed a vast deal of life and property, and made the new exit seventy miles from the path which it abandoned.
Below the surface of the open water the alluvial deposits spread out into a broad fan, which slopes gradually to a point where, in the manner of the continental shelf, the bottom descends steeply into deep water.
It is the custom of naturalists to divide the lower section of river deposits—that part of the accumulation which is near the sea—from the other alluvial plains, terming the lower portion the delta. The word originally came into use to describe that part of the alluvium accumulated by the Nile near its mouth, which forms a fertile territory shaped somewhat like the fourth letter of the Greek alphabet. Although the definition is good in the Egyptian instance, and has a certain use elsewhere, we best regard all the detritus in a river valley which is in the state of repose along the stream to its utmost branches as forming one great whole. It is, indeed, one of the most united of the large features which the earth exhibits. The student should consider it as a continuous inclined plane of diminishing slope, extending from the base of the torrents tothe sea, and of course ramifying into the several branches of the river system. He should further bear in mind the fact that it is a vast laboratory where rock material is brought into the soluble state for delivery to the seas.
The diversity in the form of river valleys is exceedingly great. Almost all the variety of the landscape is due to this impress of water action which has operated on the surface in past ages. When first elevated above the sea, the surface of the land is but little varied; at this stage in the development the rivers have but shallow valleys, which generally cut rather straight away over the plain toward the sea. It is when the surface has been uplifted to a considerable height, and especially when, as is usually the case, this uplifting action has been associated with mountain-building, that valleys take on their accented and picturesque form. The reason for this is easily perceived: it lies in the fact that the rocks over which the stream flows are guided in the cutting which they effect by the diversities of hardness in the strata that they encounter. The work which it does is performed by the hard substances that are impelled by the current, principally by the sand and pebbles. These materials, driven along by the stream, become eroding tools of very considerable energy. As will be seen when we shortly come to describe waterfalls, the potholes formed at those points afford excellent evidence as to the capacity of stream-impelled bits of stone to cut away the firmest bed rocks. Naturally the ease with which this carving work is done is proportionate to the energy of the currents, and also to the relative hardness of the moving bits and the rocks over which they are driven.
So long as the rocks lie horizontally in their natural construction attitude the course of the stream is not much influenced by the variations in hardness which the bed exhibits. Where the strata are very firm there is likely to be a narrow gorge, the steeps of which rise on either side with but slight alluvial plains; where the beds are soft the valley widens, perhaps again to contract where in thecourse of its descent it encounters another hard layer. Where, however, the beds have been subjected to mountain-building, and have been thrown into very varied attitudes by folding and faulting, the stream now here and now there encounters beds which either restrain its flow or give it freedom. The stream is then forced to cut its way according to the positions of the various underlying strata. This effect upon its course is not only due to the peculiarities of uplifted rocks, but to manifold accidents of other nature: veins and dikes, which often interlace the beds with harder or softer partitions than the country rock; local hardenings in the materials, due to crystallization and other chemical processes, often create indescribable variations which are more or less completely expressed in the path of the stream.
When a land has been newly elevated above the sea there is often—we may say, indeed, generally—a very great difference between the height of its head waters and the ocean level. In this condition of a country the rivers have what we may call a new aspect; their valleys are commonly narrow and rather steep, waterfalls are apt to abound, and the alluvial terraces are relatively small in extent. Stage by stage the torrents cut deeper; the waste which they make embarrasses the course of the lower waters, where no great amount of down-cutting is possible for the reason that the bed of the stream is near sea level. At the same time the alluvial materials, building out to sea, thus diminish the slope of the stream. In the extreme old age of the river system the mountains are eaten down so that the torrent section disappears, and the stream becomes of something like a uniform slope; the higher alluvial plains gradually waste away, until in the end the valley has no salient features. At this stage in the process, or even before it is attained, the valley is likely to be submerged beneath the sea, where it is buried beneath the deposits formed on the floor; or a further uplift of the land may occur with the result that the stream is rejuvenated; or once more endowed with the power to create torrents, build alluvial plains, and do the other interesting work of a normal river.
It rarely, if ever, happens that a river valley attains old age before it has sunk beneath the sea or been refreshed by further upliftings. In the unstable conditions of the continents, one or the other of these processes, sometimes in different places both together, is apt to be going on. Thus if we take the case of the Mississippi and its principal tributaries, the Ohio and Missouri, we find that for many geological ages the mountains about their sources have frequently, if not constantly, grown upward, so that their torrent sections, though they have worn down tens of thousands of feet, are still high above the sea level, perhaps on the average as high as they have ever been. At the same time the slight up-and-down swayings of the shore lands, amounting in general to less than five hundred feet, have greatly affected the channels of the main river and its tributaries in their lower parts. Not long ago the Mississippi between Cairo and the Gulf flowed in a rather steep-sided valley probably some hundreds of feet in depth, which had a width of many miles. Then at the close of the last Glacial period the region sank down so that the sea flooded the valley to a point above the present junction of the Ohio River with the main stream. Since then alluvial plains have filled this estuary to even beyond the original mouth. In many other of our Southern rivers, as along the shore from the Mississippi to the Hudson, the streams have not brought in enough detritus to fill their drowned valleys, which have now the name of bays, of which the Delaware and Chesapeake on the Atlantic coast, and Mobile Bay on the Gulf of Mexico, are good examples. The failure of Chesapeake and Delaware Bays to fill withdébrisin the measure exhibited by the more southern valleys is due to the fact that the streams which flow into them to a great extent drain from a region thickly covered with glacial waste, a mass which holds the flood waters, yielding thesupply but slowly to the torrents, which there have but a slight cutting power.
In our sketch of river valleys no attention has been given to the phenomena of waterfalls, those accidents of the flow which, as we have noted, are particularly apt to characterize rivers which have not yet cut down to near the sea level. Where the normal uniform descent which is characteristic of a river's bed is interrupted by a sudden steep, the fact always indicates the occurrence of one of a number of geological actions. The commonest cause of waterfalls is due to a sudden change in the character of horizontal or at least nearly level beds over which the stream may flow. Where after coursing for a distance over a hard layer the stream comes to its edge and drops on a soft or easily eroded stratum, it will cut this latter bed away, and create a more or less characteristic waterfall. Tumbling down the face of the hard layer, the stream acquires velocity; thedébriswhich it conveys is hurled against the bottom, and therefore cuts powerfully, while before, being only rubbed over the stone as it moved along, it cut but slightly. Masses of ice have the same effect as stones. Bits dropping from the ledge are often swept round and round by the eddies, so that they excavate an opening which prevents their chance escape. In these confined spaces they work like augers, boring a deep, well-like cavity. As the bits of stone wear out they are replaced by others, which fall in from above. Working in this way, the fragments often develop regular well-like depressions, the cavities of which work back under the cliffs, and by the undermining process deprive the face of the wall of its support, so that it tumbles in ruin to the base, there to supply more material for the potholing action.
Waterfalls of the type above described are by far the commonest of those which occur out of the torrent districts of a great river system. That of Niagara is an excellent specimen of the type, which, though rarely manifested in anything like the dignity of the great fall, isplentifully shown throughout the Mississippi Valley and the basin of the Great Lakes. Within a hundred miles of Niagara there are at least a hundred small waterfalls of the same type. Probably three quarters of all the larger accidents of this nature are due to the conditions of a hard bed overlying softer strata.
Falls are also produced in very many instances by dikes which cross the stream. So, too, though rarely, only one striking instance being known, an ancient coral reef which has become buried in strata may afford rock of such hardness that when the river comes to cross it it forms a cascade, as at the Falls of the Ohio, at Louisville, Ky. It is a characteristic of all other falls, except those first mentioned, that they rarely plunge with a clean downward leap over the face of a precipice which recedes at its base, but move downward over an irregular sloping surface.
In the torrent district of rivers waterfalls are commonly very numerous, and are generally due to the varying hardness in the rocks which the streams encounter. Here, where the cutting action is going on with great rapidity, slight differences in the resistance which the rocks make to the work will lead to great variations in the form of the bed over which they flow, while on the more gently sloping bottoms of the rivers, where thedébrismoves slowly, such variations would be unimportant in their effect. When the torrents escape into the main river valleys, in regions where the great streams have cut deep gorges, they often descend from a great vertical height, forming wonderful waterfalls, such as those which occur in the famous Lauterbrunnen Valley of Switzerland or in that of the Yosemite in California. This group of cascades is peculiar in that the steep of the fall is made not by the stream itself, but by the action of a greater river or of a glacier which may have some time taken its place.
Waterfalls have an economic as well as a picturesque interest in that they afford sources of power which may be a very great advantage to manufacturers. Thus alongthe Atlantic coast the streams which come from the Appalachian highlands, and which have hardly escaped from their torrent section before they attain the sea, afford numerous cataracts which have been developed so that they afford a vast amount of power. Between the James on the south and the Ste. Croix on the north more than a hundred of these Appalachian rivers have been turned to economic use. The industrial arts of this part of the country depend much upon them for the power which drives their machinery. The whole of the United States, because of the considerable size of its rivers and their relatively rapid fall, is richly endowed with this source of energy, which, originating in the sun's heat and conveyed through the rain, may be made to serve the needs of man. In view of the fact that recent inventions have made it possible to convert this energy of falling water into the form of electricity, which may be conveyed to great distances, it seems likely that our rivers will in the future be a great source of national wealth.
We must turn again to river valleys, there to trace certain actions less evident than those already noted, but of great importance in determining these features of the land. First, we have to note that in the valley or region drained by a river there is another degrading or down-wearing action than that which is accomplished by the direct work of the visible stream. All over such a valley the underground waters, soaking through the soil and penetrating through the underlying rock, are constantly removing a portion of the mineral matter which they take into solution and bear away to the sea. In this way, deprived of a part of their substance, the rocks are continually settling down by underwear throughout the whole basin, while they are locally being cut down by the action of the stream. Hence in part it comes about that in a river basin we find two contrasted features—the general and often slight slope of a country toward the main stream and its greater tributaries, and the sharp indentation of the gorgein which the streams flow, these latter caused by the immediate and recent action of the streams.
If now the reader will conceive himself standing at any point in a river basin, preferably beyond the realms of the torrents, he may with the guidance of the facts previously noted, with a little use of the imagination, behold the vast perceptive which the history of the river valley may unfold to him. He stands on the surface of the soil, thatdébrisof the rocks which is just entering on its way to the ocean. In the same region ten thousand years ago he would have stood upon a surface from one to ten feet higher than the present soil covering. A million years ago his station would have been perhaps five hundred feet higher than the surface. Ten million years in the past, a period less than the lifetime of certain rivers, such as the French Broad River in North Carolina, the soil was probably five thousand feet or more above its present plane. There are, indeed, cases where river valleys appear to have worked down without interruption from the subsidence of the land beneath the sea to the depth of at least two miles. Looking upward through the space which the rocks once occupied, we can conceive the action of the forces in their harmonious co-operation which have brought the surface slowly downward. We can imagine the ceaseless corrosion due to the ground water, bringing about a constant though slow descent of the whole surface. Again and again the streams, swinging to and fro under the guidance of the underlying rock, or from the obstacles which thedébristhey carried imposed upon them, have crossed the surface. Now and then perhaps the wearing was intensified by glacial action, for an ice sheet often cuts with a speed many times as great as that which fluid water can accomplish. On the whole, this exercise of the constructive imagination in conceiving the history of a river valley is one of the most enlarging tasks which the geologist can undertake.
Where in a river valley there are many lateral streams, and especially where the process of solution carried on bythe underground waters is most effective, as compared with erosive work done in the bed of the main river, we commonly find the valley sloping gently toward its centre, the rivers having but slight steeps near their banks. On the other hand, where, as occasionally happens, a considerable stream fed by the rain and snow fall in its torrent section courses for a great distance over high, arid plains, on which the ground water and the tributaries do but little work, the basin may slope with very slight declivity to the river margins, and there descend to great depths, forming very deep gorges, of which the Colorado Cañon is the most perfect type. As instances of these contrasted conditions, we may take, on the one hand, the upper Mississippi, where the grades toward the main stream are gentle and the valley gorge but slightly exhibited; on the other, the above-mentioned Colorado, which bears a great tide of waters drawn from the high and relatively rainy region of the Rocky Mountains across the vast plateau lying in an almost rainless country. In this section nearly all the down-wearing has been brought about in the direct path of the stream, which has worn the elevated plain into a deep gorge during the slow uprising of the table-land to its present height. In this way a defile nearly a mile in depth has been created in a prevailingly rather flat country. This gorge has embranchments where the few great tributaries have done like work, but, on the whole, this river flows in an almost unbroken channel, the excavation of which has been due to its swift, pebble-bearing waters.
The tendency of a newly formed river is to cut a more or less distinct cañon. As the basin becomes ancient, this element of the gorge tends to disappear, the reason for this being that, while the river bed is high above the sea, the current is swift and the down-cutting rapid, while the slow subsidence of the country on either side—a process which goes on at a uniform rate—causes the surface of that region to be left behind in the race for the sea level. As the stream bed comes nearer the sea level its rate of descentis diminished, and so the outlying country gradually overtakes it.
In regions where the winters are very cold the effect of ice on the development of the stream beds both in the torrent and river sections of the valley is important. This work is accomplished in several diverse ways. In the first place, where the stream is clear and the current does not flow too swiftly, the stones on the bottom radiate their heat through the water, and thus form ice on their surfaces, which may attain considerable thickness. As ice is considerably lighter than water, the effect is often to lift up the stones of the bed if they be not too large; when thus detached from the bottom, they are easily floated down stream until the ice melts away. The ice which forms on the surface of the water likewise imprisons the pebbles along the banks, and during the subsequent thaw may carry them hundreds of miles toward the sea. It seems likely, from certain observations made by the writer, that considerable stones may thus be carried from the Alleghany River to the main Mississippi.
Perhaps the most important effect of ice on river channels is accomplished when in a time of flood the ice field which covered the stream, perhaps to the depth of some feet, is broken up into vast floes, which drift downward with the current. When, as on the Ohio, these fields sometimes have the area of several hundred acres, they often collide with the shores, especially where the stream makes a sharp bend. Urged by their momentum, these ice floes pack into the semblance of a dam, which may have a thickness of twenty, thirty, or even fifty feet. Beginning on the shore, where the collision takes place, the dam may swiftly develop clear across the stream, so that in a few minutes the way of the waters is completely blocked. The on-coming ice shoots up upon the accumulation, increases its height, and extends it up stream, so that in an hour the mass completely bars the current. The waters then heap up until they break their way over the obstacle, washing its top away, until the whole is light enough to be forced down the stream, where, by the friction it encounters on the bottom and sides of the channel, it is broken to pieces. It is easy to see that such moving dams of ice may sweep the bed of a river as with a great broom.
Sometimes where the gorges do not form a stationary dam large cakes of ice become turned on edge and pack together so that they roll down the stream like great wheels, grinding the bed rock as they go.
In high northern countries, as in Siberia, the rivers, even the deepest, often become so far frozen that their channels are entirely obstructed. Where, as in the case of these Siberian rivers, the flow is from south to north, it often happens that the spring thaw sets in before the more northern beds of the main stream are released from their bondage of frost. In this case the inundations have to find new paths on either side of the obstructed way. The result is a type of valleys characterized by very irregular and changeable stream beds, the rivers having no chance to organize themselves into the shapely curves which they ordinarily follow.
The supply which finds its way to a river is composed, as has been already incidentally noted, in part of the water which courses underground for a greater or less distance before it emerges to the surface, and in part of that which moves directly over the ground. These two shares of water have somewhat different histories. On the share of these two depends the stability of the flow. Where, as in New England and other glaciated countries, the surface of the earth is covered with a thick layer of sand and gravel, which, except when frozen, readily admits the water; the rainfall is to a very great extent absorbed by the earth, and only yielded slowly to the streams. In these cases floods are rare and of no great destructive power. Again, where also the river basin is covered by a dense mantle of forests, the ground beneath which is coated, as is the case in primeval woods, with a layer of decomposing vegetation a foot ormore in depth, this spongy mass retains the water even more effectively than the open-textured glacial deposits above referred to. When the woods, however, are removed from such an area, the rain may descend to the streams almost as speedily as it finds its way to the gutters from the house roofs. It thus comes about that all regions, when reduced to tillage, and where the rainfall is enough to maintain a good agriculture, are, except when they have a coating of glacial waste, exceedingly liable to destructive inundations.
Unhappily, the risk of river floods is peculiarly great in all the regions of the United States lying much to the east of the Rocky Mountains, except in the basin of the Great Lakes and in the district of New England, where the prevalence of glacial sands and gravels affords the protection which we have noted. Throughout this region the rainfall is heavy, and the larger part of it is apt to come after the ground has become deeply snow-covered. The result is a succession of devastating floods which already are very damaging to the works of man, and promise to become more destructive as time goes on. More than in any other country, we need the protection which forests can give us against these disastrous outgoings of our streams.
In considering the journey of water from the hilltops to the sea, we should take some account of those pauses which it makes on its way when for a time it falls into the basin of a lake. These arrests in the downward motion of water, which we term lakes, are exceedingly numerous; their proper discussion would, indeed, require a considerable volume. We shall here note only the more important of their features, those which are of interest to the general student.
The first and most noteworthy difference in lakes is that which separates the group of dead seas from the livingbasins of fresh water. When a stream attains a place where its waters have to expand into the lakelike form, the current moves in a slow manner, and the broad surface exposed to the air permits a large amount of evaporation. If the basin be large in proportion to the amount of the incurrent water, this evaporation may exceed the supply, and produce a sea with no outlet, such as we find in the Dead Sea of Judea, in that at Salt Lake, Utah, and in a host of other less important basins. If the rate of evaporation be yet greater in proportion to the flow, the lake may altogether dry away, and the river be evaporated before it attains the basin where it might accumulate. In that case the river is said to sink, but, in place of sinking into the earth, its waters really rise into the air. Many such sinks occur in the central portion of the Rocky Mountain district. It is important to note that the process of evaporation we are describing takes place in the case of all lakes, though only here and there is the air so dry that the evaporation prevents the basin from overflowing at the lowest point on its rim, forming a river which goes thence to the sea. Even in the case of the Great Lakes of North America a considerable part of the water which flows into them does not go to the St. Lawrence and thence to the sea. As long as the lake finds an outlet to the sea its waters contain but little more dissolved mineral matter than that we find in the rivers. But because all water which has been in contact with the earth has some dissolved mineral substances, while that which goes away by evaporation is pure water, a lake without an outlet gradually becomes so charged with these materials that it can hold no more in solution, but proceeds to lay them down in deposits of that compound substance which from its principal ingredient we name salt. The water of dead seas, because of the additional weight of the substances which it holds, is extraordinarily buoyant. The swimmer notes a difference in this regard in the waters of rivers and fresh-water lakes and those of the sea, due to this same cause. But in thoseof dead seas, saturated with saline materials, the human body can not sink as it does in the ordinary conditions of immersion. It is easy to understand how the salt deposits which are mined in many parts of the world have generally, if not in all cases, been formed in such dead seas.[5]
It is an interesting fact that almost all the known dead seas have in recent geological times been living lakes—that is, they poured over their brims. In the Cordilleras from the line between Canada and the United States to central Mexico there are several of these basins. All of those which have been studied show by their old shore lines that they were once brimful, and have only shrunk away in modern times. These conditions point to the conclusion that the rainfall in different regions varies greatly in the course of the geologic ages. Further confirmation of this is found in the fact that very great salt deposits exist on the coast of Louisiana and in northern Europe—regions in which the rainfall is now so great in proportion to the evaporation that dead seas are impossible.
Turning now to the question of how lake basins are formed, we note a great variety in the conditions which may bring about their construction. The greatest agent, or at least that which operates in the construction of the largest basins, are the irregular movements of the earth, due to the mountain-building forces. Where this work goes on on a large scale, basin-shaped depressions are inevitably formed. If all those which have existed remained, the large part of the lands would be covered by them. In most cases, however, the cutting action of the streams has been sufficient to bring the drainage channels down to the bottom of the trough, while the influx of sediments has served to further the work by filling up the cavities. Thusat the close of the Cretaceous period there was a chain of lakes extending along the eastern base of the Rocky Mountains, constituting fresh-water seas probably as large as the so-called Great Lakes of North America. But the rivers, by cutting down and tilling up, have long since obliterated these water areas. In other cases the tiltings of the continent, which sometimes oppose the flow of the streams, may for a time convert the upper part of a river basin which originally sloped gently toward the sea into a cavity. Several cases of this description occurred in New England in the closing stages of the Glacial period, when the ground rose up to the northward.
We have already noted the fact that the basin of a dead sea becomes in course of time the seat of extensive salt deposits. These may, indeed, attain a thickness of many hundred feet. If now in the later history of the country the tract of land with the salt beneath it were traversed by a stream, its underground waters may dissolve out the salt and in a way restore the basin to its original unfilled condition, though in the second state that of a living lake. It seems very probable that a portion at least of the areas of Lakes Ontario, Erie, and Huron may be due to this removal of ancient salt deposits, remains of which lie buried in the earth in the region bordering these basins.
By far the commonest cause of lake basins is found in the irregularities of the surface which are produced by the occupation of the country by glaciers. When these great sheets of ice lie over a land, they are in motion down the slopes on which they rest; they wear the bed rocks in a vigorous manner, cutting them down in proportion to their hardness. As these rocks generally vary in the resistance which they oppose to the ice, the result is that when the glacier passes away the surface no longer exhibits the continued down slope which the rivers develop, but is warped in a very complicated way. These depressions afford natural basins in which lakes gather; they may vary in extent from a few square feet to many square miles. When a glacier occupies a country, the melting ice deposits on the surface of the earth a vast quantity of rockydébris, which was contained in its mass. This detritus is irregularly accumulated; in part it is disposed in the form of moraines or rude mounds made at the margin of the glacier, in part as an irregular sheet, now thick, now thin, which covers the whole of the field over which the ice lay. The result of this action is the formation of innumerable pools, which continue to exist until the streams have cut channels through which their waters may drain away, or the basins have become filled with detritus imported from the surrounding country or by peat accumulations which the plants form in such places.
Doubtless more than nine tenths of all the lake basins, especially those of small size, which exist in the world are due to irregularities of the land surface which are brought about by glacial action. Although the greater part of these small basins have been obliterated since the ice left this country, the number still remaining of sufficient size to be marked on a good map is inconceivably great. In North America alone there are probably over a hundred and fifty thousand of these glacial lakes, although by far the greater part of those which existed when the glacial sheet disappeared have been obliterated.
Yet another interesting group of fresh-water lakes, or rather we should call them lakelets from their small size, owes its origin to the curious underground excavations or caverns which are formed in limestone countries. The water enters these caverns through what are termed "sink holes"—basins in the surface which slope gently toward a central opening through which the water flows into the depths below. The cups of the sink holes rarely exceed half a mile in diameter, and are usually much smaller. Their basins have been excavated by the solvent and cutting actions of the rain water which gathers in them to be discharged into the cavern below. It often happens that after a sink hole is formed some slight accident closes thedownward-leading shaft, so that the basin holds water; thus in parts of the United States there are thousands of these nearly circular pools, which in certain districts, as in southern Kentucky, serve to vary the landscape in much the same manner as the glacial lakes of more northern countries.
Some of the most beautiful lakes in the world, though none more than a few miles in diameter, occupy the craters of extinct volcanoes. When for a time, or permanently, a volcano ceases to do its appointed work of pouring forth steam and molten rock from the depths of the earth, the pit in the centre of the cone gathers the rain water, forming a deep circular lake, which is walled round by the precipitous faces of the crater. If the volcano reawakens, the water which blocks its passage may be blown out in a moment, the discharge spreading in some cases to a great distance from the cone, to be accumulated again when the vent ceases to be open. The most beautiful of these volcanic lakes are to be found in the region to the north and south of Rome. The original seat of the Latin state was on the shores of one of these crater pools, south of the Eternal City. Lago Bolsena, which lies to the northward, and is one of the largest known basins of this nature, having a diameter of about eight miles, is a crater lake. The volcanic cone to which it belongs, though low, is of great size, showing that in its time of activity, which did not endure very long, this crater was the seat of mighty ejections. The noblest specimen of this group of basins is found in Crater Lake, Oregon, now contained in one of the national parks of the United States.
Inclosed bodies of water are formed in other ways than those described; the list above given includes all the important classes of action which produce these interesting features. We should now note the fact that, unlike the seas, the lakes are to be regarded as temporary features in the physiography of the land. One and all, they endure for but brief geologic time, for the reason that thestreams work to destroy them by filling them with sediment and by carving out channels through which their waters drain away. The nature of this action can well be conceived by considering what will take place in the course of time in the Great Lakes of North America. As Niagara Falls cut back at the average rate of several feet a year, it will be but a brief geologic period before they begin to lower the waters of Lake Erie. It is very probable, indeed, that in twenty thousand years the waters of that basin will be to a great extent drained away. When this occurs, another fall or rapid will be produced in the channel which leads from Lake Huron to Lake Erie. This in turn will go through its process of retreat until the former expanse of waters disappears. The action will then be continued at the outlets of Lakes Michigan and Superior, and in time, but for the interposition of some actions which recreate these basins, their floors will be converted into dry land.
It is interesting to note that lakes owe in a manner the preservation of their basins to an action which they bring about on the waters that flow into them. These rivers or torrents commonly convey great quantities of sediment, which serve to rasp their beds and thus to lower their channels. In all but the smaller lakelets these turbid waters lay down all their sediment before they attain the outlet of the basin. Thus they flow away over the rim rock in a perfectly pure state—a state in which, as we have noted before, water has no capacity for abrading firm rock. Thus where the Niagara River passes from Lake Erie its clean water hardly affects the stone over which it flows. It only begins to do cutting work where it plunges down the precipice of the Falls and sets in motion the fragments which are constantly falling from that rocky face. These Falls could not have begun as they did on the margin of Lake Ontario except for the fact that when the Niagara River began to flow, as in relatively modern times, it found an old precipice on the margin of Lake Ontario, formed by the waves of the lake, down which the waters fell, andwhere they obtained cutting tools with which to undermine the steep which forms the Falls.
Many great lakes, particularly those which we have just been considering, have repeatedly changed their outlets, according as the surface of the land on which they lie has swayed up and down in various directions, or as glacial sheets have barred or unbarred the original outlets of the basins. Thus in the Laurentian Lakes above Ontario the geologist finds evidence that the drainage lines have again and again been changed. For a time during the Glacial period, when Lake Ontario and the valley of the St. Lawrence was possessed by the ice, the discharge was southward into the upper Mississippi or the Ohio. At a later stage channels were formed leading from Georgian Bay to the eastern part of Ontario. Yet later, when the last-named lake was bared, an ice dam appears to have remained in the St. Lawrence, which held back the waters to such a height that they discharged through the valley of the Mohawk into the Hudson. Furthermore, at some time before the Glacial period, we do not know just when, there appears to have been an old Niagara River, now filled with drift, which ran from Lake Erie to Ontario, a different channel from that occupied by the present stream.
The effects of lakes on the river systems with which they are connected is in many ways most important. Where they are of considerable extent, or where even small they are very numerous, they serve to retain the flood waters, delivering them slowly to the excurrent streams. In rising one foot a lake may store away more water than the river by its consequent rise at the point of outflow will carry away in many months, and this for the simple reason that the lake may be many hundred or even thousand times as wide as the stream. Moreover, as before noted, the sediment gathered by the stream above the level of the lake is deposited in its basin, and does not affect the lower reaches of the river. The result is that great rivers, such as drain from the Laurentian Lakes, flow clear water, are exemptfrom floods, are essentially without alluvial plains or terraces, and form no delta deposits. In all these features the St. Lawrence River affords a wonderful contrast to the Mississippi. Moreover, owing to the clear waters, though it has flowed for a long time, it has never been able to cut away the slight obstructions which form its rapids, barriers which probably would have been removed if its waters had been charged with sediment.