River current

“ When the tide begins to flow up the river there may be a low-level salt water current moving landward and a high-level fresh water current moving seaward, but this is quite a temporary state of affairs. The surface slope is landward, and the water moving seaward is not moving in obedience to the surface slope. ”

“ Where a large river empties into the sea, if the current of the ocean flows parallel to the coast, and strikes the river current at a right angle, all the mud is carried with the ocean current, and is quickly placed on the bottom. By the arming on their lead, navigators may tell, at night or in a fog, on which side of the mouth of the river they are as they near the coast, provided they study the ocean currents. ”

“ Great rivers often change their courses, especially where they flow through a region of made earth, such as is the case with the lower Mississippi River, and in other great rivers of the world. The loose earth is continually shifted by the current, and where the current is not very strong it will often hold the water back to such an extent of accumulated weight that the flood will break over at some weak point on its banks and make a new course for itself. ”

“ As it is the property of water to follow a descent, and the greatest descent that occurs in its way, the course of a river points out generally the direction in which the land declines, and the degree of the declination determines in part the velocity of its current, for the rapidity of the stream is influenced both by its volume of water and the declivity of its channel. ”

“ Hence one river often pours its tide into another without causing any perceptible enlargement of its bed, the additional waters being disposed of by the creation of a more rapid current, for large masses of water travel with a swift and powerful impetus over nearly a level surface, upon which smaller rivers would have only a languid flow. ”

“ To reach the lowest level as soon as possible is the end each river is striving toward. If it could, each river would cut its bed to this depth at the first stage of its course. Its tools are the rocks it carries, great and small. The force that uses these tools [Pg 87] is the power of falling water, represented by the current of the stream. The upper part of a river such as the Missouri or Mississippi engages in a campaign of widening and deepening its channel, and carrying away quantities of sediment. The lower reaches of the stream flow through more level country ”

“ Every stream has its load of sediment, as the silt- and sand-filled reservoirs along the edges of the valley so effectively testify. Each sandy river bed is an aggregate of rolling grains, moving with the current, slow where it is slow and faster where the current is accelerated, but travelling always towards the sea. ”

“ Owing to a flow of upland water down a river the ebb lasts longer than the flood tide by a period, increasing in length as the distance from the mouth of the river increases; and, similarly to the sea, the current may continue to run down a river after the tide has turned and the level of the water is rising. ”

“ The regulation of a river consists in straightening its channel by cutting off bends, securing its banks from erosion by floods, and, where necessary, by constructing embankments to confine the waters and prevent them from overflowing and stagnating upon the low grounds which skirt their current. ”

“ But we may also employ the method of synthesis, and study the manifest process of valley-building which takes place simultaneously. The river is always loading up on one side, and unloading on the other. The deepest part of the river near the high banks, as it sweeps round the great bends, is also the swiftest. The current grows slower and slower in the direction of the opposite shore, and at the same time the water grows more and more shallow, until at last a sand-bar is reached gradually rising out of it. ”

“ An enlargement of the bed of the river, principally by deepening it, in order to increase its discharging capacity sufficiently to prevent inundations, is precluded by the cost, and also, in rivers bringing down sediment, by the large deposit that would take place in the enlarged channel from the reduction in the velocity of the current when the flood begins to subside. ”

“ In these and similar resting-places, our little natural philosophers, instinctively aware that the current of a stream is less below than above, and along the sides than in the centre, remain for several months during spring, and the earlier portion of the summer, till they gain such an increase of size and strength as enables them to spread themselves abroad over other portions of the river, especially those shallow places where the bottom is composed of fine gravel. ”

“ In treating of formations in valleys, we cannot always confine our illustrations to the operations of fresh waters, because continued research appears, in several instances, to show that what appeared at first to be due to lacustrine fluctuation or river currents was really the effect of water-movement in an ancient arm of the sea. ”

“ These are the natural consequences which must follow if the land has long been worked upon by streams, and no other process has ever been suggested which is competent to produce them. We must conclude that valley systems have been formed by the river systems which drain them, aided by the work of the weather; they are not gaping fissures in the earth's crust, as early observers imagined, but are the furrows which running water has drawn upon the land. ”

“ Stately and slow that river moves down to the bay, into which it empties its load. The rain that falls on hundreds of square miles of territory flows into the streams that feed this trunk. The little spring that is the headwater of the system is but one of many pockets in the hillsides that hold the water that soaks into the ground and give it out by slow degrees. Surface water after a rain flows quickly into the streams. It is the springs that hold back their supply and keep the rivers from running dry in hot weather. ”

“ The thirsty soil absorbs, and the unclouded sun of summer evaporates, the moisture in its progress; and this, I take it, is the solution of the apparent paradox, with respect to rivers—that they are sometimes greater at their source than at their mouth. Such is the state of the river in summer; but what must it be in winter, when every valley and ravine pours forth its tributary streams into one common channel, the sole outlet of the accumulated waters of an extensive district? ”

“ If we would discover how a watercourse is formed, from the very first bog-streams up in the mountain, we must follow a multitude of tiny rills, receiving one fresh stream after another from every side, running together into burns, which grow and grow and form little rivers, till we come to the end of the wooded hillside and are suddenly face to face with the great river in the valley below. ”

“ They flow between low, rounded hills, and are joined by quiet streams, that occupy the separating grooves between the hills. This is the oldest type of river. It has done its work. Rainfall and stream-flow have brought the level of the land nearly to the level of the stream. Very little more is left to be ground down and carried away. The landscape is beautiful, but it is no longer picturesque. Wind and water have smoothed away unevennesses. Trees and grass and other vegetation check erosion, and the river has little to do but to carry away the surface water that falls as rain. ”

“ The rain continuing, and the stream rapidly rising, we halted at some farm-houses in the midst of a long rice-field. Fording the river is difficult work; the water rushing down at headlong speed, renders it necessary to exert one’s utmost strength to avoid being carried away: the pole in both hands, placed well to seaward, one foot advanced cautiously before the other, to avoid the slippery rocks and loose stones. I found that this fatigued me more than the walking. The water became much cooler as we approached the mountain, while the land is rapidly increasing in elevation. ”

“ Now, the power of this moving water is a mighty wide-spread agent of change in the physical condition of the globe. For wherever water is in motion over the surface of the land, whether it be a rippling stream, or a mountain torrent, or a majestic river, it is surely wearing away the channel through which it flows, and carrying along in its course particles of clay, or sand, or gravel. ”

“ Taking a few centuries or a few thousand years, no doubt, we can attribute to the action of rivers, whether in ordinary flow or in flood, little power of modifying the region which they drain. But taking that wider survey (in time) of fluviatile work which modern science requires, dealing with this form of aqueous energy as we deal with the earth’s vulcanian energies, we perceive that the effects of river action in the course of long periods of time are not limited to the course which at any given time a river may pursue. ”

“ Torrential streams tend to excavate or to raise their beds according to the inclination, and to the character of the material they transport. No general law on this point can be laid down in relation to the middle and lower courses of rivers. The conditions which determine the question of the depression or elevation of a river-bed are too multifarious, variable, and complex, to be subjected to formulae, and they can scarcely even be enumerated. ”

“ More mobile and more searching than ice or rock rubbish, the trickling drops are guided by the deepest lines of the hillside in their incipient flow, and as these River systems. lines converge, the stream, gaining strength, proceeds in its torrential course to carve its channel deeper and entrench itself in permanent occupation. Thus the stream-bed, from which at first the water might be blown away into a new channel by a gale of wind, ultimately grows to be the strongest line of the landscape. ”

“ Thus, by tracing a river backwards, from its end to its real beginning, we come at length to the sun; for it is the sun that produces aqueous vapour, from which, as we have seen, clouds are formed, and it is from clouds that water falls to the earth to become the sources of rivers. [Pg 268]
There are, however, rivers which have sources somewhat different from those just mentioned. They do not begin by driblets on a hillside, nor can they be traced to a spring. Go, for example, to the mouth of the river Rhone, and trace it backwards. ”

“ Whether a river begins its life on a lake plain, as in the example just cited, or upon a coastal plain lifted from beneath the sea or on a spread of glacial drift left by the retreat of continental ice sheets, such as covers much of Canada and the northeastern parts of the United States, its infantile stage presents the same characteristic features,—a narrow and shallow valley, with undeveloped tributaries and undrained interstream areas. Ground water stands high, and, exuding in the undrained initial depressions, forms marshes and lakes. ”

“ A river flowing to the sea or emptying itself into an inland lake, carries its burden of gravel, sand, and mud, and sooner or later, as the rate of flow slackens, it deposits the materials in the river-bed or on the floor of the sea or lake. ”

“ This deposition of material takes place particularly at the sides of the stream where the velocity is least, and the banks are in consequence raised above the main channel, so that the river becomes lifted bodily upwards in its bed, and flows above the level of the surrounding country. In flood-time the muddy water flows over the river’s banks, where its velocity is at once checked as it flows gently down the outer side, causing more material to be deposited there, and a long alluvial ridge, called a natural levée, to be built up on either side of the stream. ”

“ The physical circumstances of the river easily account for the possession of this attribute. The air above is pure and serene. But little rain falls upon the country through which the greater part of its course is prosecuted, and no snow or hail. Hence there is little drainage into it from the surrounding land, and its waters are kept free from any noxious taint derived from earths and minerals, except from those in its immediate channel. ”

“ More to the purpose of the present moment, the government engineers have expelled the quicksands and confined the waters so that from the vagaries of running waves of sand and shallow films of water, true channels have been forced through, and the old two or three feet of available low-water depth has been increased to eight or ten feet, practically guaranteed up a thousand miles of river. ”

“ By a curious law of compensation, the stream which, at flood, scoops out cavities in its bed, often fills them up again as soon as the diminished velocity of the current allows it to let fall the sand and gravel with which it is charged, so that when the waters return to their usual channel, the bottom shows no sign of having been disturbed. ”

“ BRAIDED CHANNELS. A river actively engaged in aggrading its valley with coarse waste builds a flood plain of comparatively steep gradient and often flows down it in a fairly direct course and through a network of braided channels. From time to time a channel becomes choked with waste, and the water no longer finding room in it breaks out and cuts and builds itself a new way which reunites down valley with the other channels. Thus there becomes established a network of ever-changing channels inclosing low islands of sand and gravel. ”