Downstream

“ Though history shows us countless instances of upstream expansion, nevertheless owing to the ease of downstream navigation and this increasing historical importance from source to mouth, the direction of a river's flow has often determined the course of commerce and of political expansion. ”

“ Immediately downstream of an abrupt change scour is often severe. An abrupt change is one, whether of sectional area or direction of flow, and whether or not accompanied by a junction or bifurcation, which is so sudden as to cause eddies. ”

“ It is especially so as regards the most important of points, that of the underground connection. Of course, wherever there is running water at hand, a river, or even a tiny stream that never dries, the matter is easy enough, but as in the great majority of buildings to be protected such water does not exist, the solution of the question becomes more difficult, and frequently one of the greatest perplexity. It tends even to be more and more so in consequence of the progress of sanitary arrangements under which towns and villages are ‘drained’ until the soil has been made as dry as a rock. ”

“ If a stream has constant fountains at its head, and numerous tributaries joining it in its course, and flows withal through a country of gradual descent, such a stream will never fail; but if the supplies do not exceed the evaporation and absorption, to which every river is subject, if a river dependant on its head alone, falls rapidly into a level country, without receiving a single addition to its waters to assist the first impulse acquired in their descent, it must necessarily cease to flow at one point or other. ”

“ The mouth of a large river system is the converging point of many lines of inland and maritime navigation. The interests of commerce, especially in its earlier periods of development, demand that the contact here of river and sea be extensive as possible. Nature suggests the way to fulfill this requirement. The sluggish lowland current of a river, on approaching sea level, throws out distributaries that reach the coast at various points and form a network of channels, which can be deepened and rendered permanent by canalization. ”

“ A torrent that issues from its source in the mountains is not the river which reaches the sea. On its long journey from highland to lowland it receives now the milky waters of a glacier-fed stream, now a muddy tributary from agricultural lands, now the clear waters from a limestone plateau, while all the time its racing current bears a burden of soil torn from its own banks. Now it rests in a lake, where it lays down its weight of silt, then goes on, perhaps across an arid stretch where its water is sucked up by the thirsty air or diverted to irrigate fields of grain. ”

“ 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 ”

“ The course of water flowing underground is not strictly analogous to that of rivers on the surface, there being, in the one case, a constant descent from a higher to a lower level from the source of the stream to the sea; whereas, in the other, the water may at one time sink far below the level of the ocean, and afterwards rise again high above it. ”

“ 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. ”

“ When a river is embanked at a given point, and, consequently, the water of its floods, which would otherwise spread over a wide surface, is confined within narrow limits, the velocity of the current and its transporting power are augmented, and its burden of sand and gravel is deposited at some lower point, where the rapidity of its flow is checked by a dam or other artificial obstruction, by a diminution in the inclination of the bed, by a wider channel, or finally by a lacustrine or marine basin which receives its waters. ”

“ 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. ”

“ The stream whose water spreads over its low banks is without depth and current and power. But confine the waters between high, narrow banks, the bed of the stream is deepened, and its mighty current supports animal life and turns the wheels of mill and factory. ”

“ The laws of the distribution and utilization of underground water are as simple as those controlling the surface supply, but the popular fallacies concerning them are appalling. The most prevalent of these is that the waters originate at some remote point from their outlet, and flow in subterranean streams like the "blood in the human body," as a farmer once said, and that these streams must be tapped by the well borer or digger before water can be obtained. ”

“ If it shall be found that nearly all the rainfall should go into the soil and thence into the under-drainage, coming out slowly and steadily by seepage and by springs into the streams, clear and pure, these streams should present nearly ideal conditions for water-food, for water-power and for stream-navigation. An ideal solution of the soil problem may therefore solve the greater part of the whole complex of problems of which navigation is the last term. ”

“ The conservation of water upon the farm remains for our consideration; but, if we may be permitted to draw a conclusion at this stage, it is this:—Whether we deal with head-water reservoirs for the regulation of stream flow, with water-power development and the incident pools, with reclamation or irrigation projects, with the dredging and damming for navigation purposes, or with stream pollution by any means, we find a vital relation to fishery problems and to fish-cultural operations. ”

“ The distance to which a new obstruction to the flow of a river, whether by a dam or by a deposit in its channel, will retard its current, or, in popular phrase, "set back the water," is a problem of more difficult practical solution than almost any other in hydraulics. ”

“ 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. ”

“ The removal of obstructions, whether natural or artificial, from the bed of a river furnishes a simple and efficient means of increasing the discharging capacity of its channel, and, consequently, of lowering the height of floods; for every impediment to the flow, in proportion to its extent, raises the level of the river above it so as to produce the additional artificial fall necessary to convey the flow through the restricted channel, thereby reducing the total available fall. ”

“ A stream may acquire the name of a river, in the channel of which, at some seasons of extreme drought, no water flows. For a great portion of the year parts of a stream may flow in great abundance, in which, during a very dry season, we may find only standing pools. It would be against all usage to say that the general source of the river was at that point in its channel from which the water always flows. ”

“ Low Water Level cannot be absolutely known, because a river reaches its lowest level only at rare intervals, and because alterations in the cultivation of the land, the drainage, the removal of forests, the removal or erection of obstructions in the river bed, &c., gradually alter the conditions of discharge. ”

“ The water of subterranean currents and reservoirs, as well as that of springs and common wells, is doubtless principally furnished by infiltration, and hence its quantity must vary with every change of natural surface which tends to accelerate or to retard the drainage of the surface-soil. ”

“ The natural consequence is a stream of the most broken and impetuous character, a stream whose rushing, roaring, and foaming drowns all sounds contiguous to it; rapid after rapid of seemingly tempest-tossed and crested billows, of whirlpools and eddies, of backwaters and heavings into surface currents, and never a still pool to be found anywhere. ”

“ Along the course of the larger streams huge chambers occur; for the ever-eddying water, bearing sand along in its course, eats out the sides of its channel, or, revolving stones in its bed, carves out the pothole by friction. Or some pendent mass of rock has its support undermined and comes crashing into the streamway, only to be broken up and carried away by the ceaseless energy of the stream, so ever enlarging the chambers upwards towards the light of day. But whilst this action is going on underground, a more potent factor is at work where the subterranean stream first sees the light. ”

“ It is a fixed rule that water, descending in the soil, will find the lowest outlet to which there exists a channel through which it can flow, and that if, after heavy rains, it rise too near the surface of the ground, the proper remedy is to tap it at a lower level, and thus remove the water table to the proper distance from the surface. This subject will be more fully treated in a future chapter, in considering the question of the depth, and the intervals, at which drains should be placed. ”

“ Closure of a Flowing Stream.—The closure of a flowing stream by means of a dam is usually attended with some difficulty and sometimes with enormous difficulty. There may be little trouble in running out dams from both banks for a certain distance, but as soon 76 as the gap between the dams becomes much less than the original width of the stream, the water on the upstream side is headed up and there is a rush of water through the gap, which tends to deeply scour the bed and to undermine the dams. The smaller the gap becomes the greater is the rush and scour. ”

“ As the streams that flow on the surface of the earth are constantly altering their courses, thus also the subterranean waters are ever active in excavating new channels in the bosom of the rock. Finding at length new outlets on a lower level, they abandon their ancient beds, and the explorer now wanders dry-footed where once a foaming river gushed along. ”

“ In all geological times, as at the present, the work of streams has been to destroy the lands, and in so doing to furnish to the ocean the materials from which the lands of future ages were to be made. Before we consider how the waste of the land brought in by streams is rebuilt upon the ocean floor, we must proceed to study the work of two agents, glacier ice and the wind, which cooperate with rivers in the denudation of the land. ”

“ The work of streams is of three kinds,—transportation, erosion, and deposition. Streams transport the waste of the land; they wear, or erode, their channels both on bed and banks; and they deposit portions of their load from time to time along their courses, finally laying it down in the sea. Most of the work of streams is done at times of flood. ”

“ Any newly formed land surface, like a recently drained lake bed or part of the marginal sea bottom which has been raised into land, has a drainage system developed upon it. In the early or youthful stage of such a new land area lying well above sea level, under ordinary climatic conditions a few streams only form and these tend to follow the natural or initial slope of the land. ”

“ The greater also would be the probability of the stream, on the cessation of the flood, not returning to its original channel, which might have become obliterated or filled up, but of its flowing along some new course, it may be miles away from its former channel. Even when not flooded so as to overflow their banks, rivers along which a larger body of water flowed than there does at present, would, so long as they were not confined within deep valleys, have a tendency to wander over a much wider tract of country than that now occupied by their valleys. ”

“ But, frequently, a very large marsh is traversed by a tortuous stream which occupies a large area, and which, although the tidal water which it contains gives it the appearance of a river, is only the outlet of an insignificant stream, which might be carried along the edge of the upland in an ordinary mill-race. ”

“ So long as the river is confined between narrow banks, the high rate of the current is sufficient to force forward the thickened and heavy fluid; but the instant that the banks are over-topped and the river expands over an increased area, the rapidity is reduced, and the water, no longer able to contain the earth in solution, deposits alluvium, and produces a delta, which must necessarily increase upon every future inundation. ”