Stream (hydrology)

“ When there is a gradual rise lasting for a considerable time—and this is most likely to cause a high flood—there is no rise of the flood-level downstream of the embanked reach, except such as is due to the increase in the discharge of the stream consequent on the absorption and evaporation being less than before, owing to the reduced area of flooding in the embanked reach. ”

“ The advancing end of a flood wave may, while the 152 wave is rising and being formed, travel rapidly, but when the wave has been formed it travels at the ordinary rate of flow of the risen stream. The advancing end of a trough may, while it is being formed, travel rapidly, but after formation it travels at the ordinary rate of the fallen stream (Hydraulics, Chap. IX., Arts. 3 and 4) . Thus the rate at which a change in water-level travels down a stream depends at first on the amount of the rise or fall, but afterwards on the water-level of the risen or fallen stream. ”

“ In the dry season the stream runs at the bottom of a narrow and deep groove, whose sides are polished and fluted by the boiling action of the water in flood, like the rims of ancient Eastern wells by the draw-ropes. The breadth of the groove is often not more than from forty to sixty yards, and it has some sharp turnings, double channels, and little cataracts in it. As we steamed up, the masts of the "Ma Robert," though some thirty feet high, did not reach the level of the flood-channel above, and the man in the chains sung out, "No bottom at ten fathoms." ”

“ The available rainfall in a catchment area is the total fall less the quantity which is evaporated or absorbed by vegetation. The evaporation does not chiefly take place directly from the surface. Rain sinks a short distance into the ground, and is subsequently evaporated. The available rainfall does not all flow directly into the streams. Some sinks deep into the ground and forms springs, and these many months later augment the flow of the stream and maintain it in dry seasons. ”

“ They also show that in small streams the proportion of mud in the flood-flows is greater, and the average duration of floods is shorter, than in larger streams. In other words, the differences between flood- and low-water flows are greatest in small streams, and gradually become less as the size of the stream increases.
When a stream is used for water-works purposes in the usual way, a certain quantity of water is taken from the stream each day, which quantity is nearly constant, and is not dependent upon [Pg 126] the condition of the stream, or the volume of its flow. ”

“ In arid regions the ground-water surface lies so low that for the most part stream ways do not intersect it. Streams therefore are not fed by springs, but instead lose volume as their waters soak into the thirsty rocks over which they flow. They contribute to the ground water of the region instead of being increased by it. Being supplied chiefly by the run-off, they wither at times of drought to a mere trickle of water, to a chain of pools, or go wholly dry, while at long intervals rains fill their dusty beds with sudden raging torrents. ”

“ 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. ”

“ The increase in the magnitude of streams, due to violent rains and the melting of the snows, changes the position of the substances that compose their bed, and the more easily suspended materials are often held until the stream actually meets the ocean. In such sudden increases, the streams often overflow their usual banks, and make their deposits laterally, until the constant succession of such deposits raises the adjacent ground high enough to set bounds to the further spreading of the stream. ”

“ 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. ”

“ While the stream is often turbid with surface waste washed into it by rains, the spring remains clear; its water has been filtered during its slow movement through many small underground passages and the pores of rocks. Commonly the spring differs from the stream in that it carries a far larger load of dissolved rock. Chemical analysis proves that streams contain various minerals in solution, but these are usually in quantities so small that they are not perceptible to the taste or feel. ”

“ Sludge consists of a stratum of detached ice crystals, or of snow, or of the smaller fragments of brash-ice, floating on the surface of the sea.
A stream is an oblong collection of drift or bay-ice, the pieces of which are continuous. It is called a sea-stream when it is exposed on one side to the ocean, and affords shelter from the sea to whatever is within it. ”

“ This stream flow consists of the rainfall over the whole watershed less the amount absorbed by the earth or evaporated from the surface, and every such stream is a potential source of power. The possible water-power of a country or district is, therefore, primarily dependent on rainfall, but also, of course, on absorption and surface evaporation. In places where the land is approximately flat, the tendency to concentrate rainfall into streams would be small, as the water would tend to lie rather in swampy low pools, or form innumerable tiny, slowly moving brooks. ”

“ This wave has first to overcome the water flowing down the river, which is acting in opposition to it, and in so doing causes a banking up of the water to such a height that the inclination of the surface is reversed to an extent sufficient to cause a tidal current to run up the river. The momentum acquired by the water passing up-stream carries it to a higher level towards the head of the river than at the mouth, and, similarly, in returning, the water flowing down the river gains sufficient impetus to scoop out the water at the mouth and form a low water below that in the sea adjoining. ”

“ A part of the rain-water is there held, until it returns by evaporation, to the clouds, while a part slowly percolates downward, finding its way into swamps and springy plains, and finally, after days or weeks of wandering, slowly, but surely, finds its outlet in the stream or pond. ”

“ When a silt-bearing stream overflows its banks the depth of water on the flooded bank is probably small and its velocity very low, and a deposit of silt takes place on the bank. When the deposit has reached a certain height it acts like a weir on the water of the next flood, which flows quickly over it and, instead of raising it higher, deposits its silt further away from the stream. In this way a strip of country along the stream gradually becomes raised, the raising being greatest close to the stream. The country slopes downwards in going away from the stream. ”

“ 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. ”

“ But the magnitude of a stream is more especially regulated by the extent of country which forms the declivities of its basin, though there is no invariable proportion here, for a small basin in a humid region will yield a greater quantity of water than one much more considerable in a different situation. High mountains, a humid climate, and a wide superficial drainage, are the three physical circumstances which lead to the accumulation of vast bodies of water, the magnitude of which will be proportionate to the degree in which these causes are in combined operation. ”

“ When the bed of the greater river is also the deepest any flood descending by the other channel when the larger stream is low must flow with greater force into that which is deeper, and in a soft and yielding soil may thus increase the width of its own channel. On the contrary a flood coming down the greater river while the minor channel may happen to be dry must first flow upwards some miles and so fill this channel and, being thus affected both by the rising and subsidence of the greater stream, this process would have had a tendency to deepen and widen the lower part of the Darling. ”

“ The rapidity of the increase is accounted for by the fact that every field it passes whose surface inclines towards it is a watershed from which an unseen but considerable drainage takes place. When no brook passes through the fields the water stands and soaks downwards, or evaporates slowly: directly a ditch is opened it fills, and the effect of a stream is not only to collect water till then unseen, but to preserve it from evaporation or disappearance into the subsoil. ”

“ In districts where there is no settled population, and in any case where the stream is ill-defined, and the flow fitful, the rainfall figures may afford the only, or at least far the best, means of estimating the discharge.
The rainfall to be considered in all these cases is the maximum likely to fall in a short period of time. The catchment areas dealt with are small, say 5 square miles or less. It must be assumed that the fall of rain extends to all parts of the catchment area, and that its duration is sufficient for the water from all parts of it to reach the site of the work. ”

“ The first stream captured in this way must necessarily be the nearest to the large stream. The diversion of the considerable volume of headwaters, H, to the channel of the small subsequent branch, G, causes it to deepen its channel rapidly; the same effect is perceptible in H for a distance above its point of capture and diversion: the increased load of sediment thus given to G will be in great part dropped in a fan-delta where it enters the flat valley of the master stream, A, (fig. 6) . ”

“ When the earth's crust is thrown up in mountain folds, and between them valleys are formed, the level of rivers is sometimes lowered and the rapidity of their flow is checked. A stream which has torn down its walls at a rapid rate becomes a sluggish water-course, its current clogged with sediment, which it has no power to carry farther. When such a river begins to build and obstruct its own waters it bars its progress and may form a lake as the outlet of its tributary streams. Many ancient rivers have been utterly changed and some obliterated by general movements of the earth's crust. ”

“ In many cases the natural fall can be increased by a dam or weir thrown across the stream. The engineer will also examine to what extent the head will vary in different seasons, and whether it is necessary to sacrifice part of the fall and give a steep slope to the tail race to prevent the motor being drowned by backwater in floods. Streams fed from lakes which form natural reservoirs or fed from glaciers are less variable than streams depending directly on rainfall, and are therefore advantageous for water-power purposes. ”

“ Where a stream has all the load of any given degree of fineness that it is capable of carrying, the entire energy of the descending water is consumed by the transportation of the water and its burden, so that none is applied to erosion. If it has an excess of load, its velocity is thereby lessened and its power to transport is diminished ”

“ 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 ”

“ With favorable conditions, as a stream carrying abundance of fine silt, the water moving sluggishly over the inundated surfaces will drop its earthy burden upon them in thin, loose layers, evenly at first, but, as shallows are formed, irregularly modified, or as sudden freshets produce stronger currents, eddies and ripple-bars gather or disturb the sediment. New lands in wet banks and knobs will begin to appear over the highest ridges. ”

“ A river channel, indeed, generally suffices approximately to carry off the average flow of the river, which, whilst comprising considerable fluctuations in volume, furnishes a sufficiently constant erosive action to maintain a fairly regular channel; though rivers having soft beds and carrying down sediment erode their beds during floods and deposit alluvium in dry weather. As the velocity of a stream increases with its fall, the size of a channel conveying a definite average flow varies inversely with the fall, and the depth inversely with the width. ”