River channel

“ The stream may also have to be trained, by means of guide banks or spurs, so as to remain in one channel and flow past the canal head and not form shoals against it. Where the river is unstable, it may shift its course so as to strike the weir obliquely and this may cause excessive heading up at one side of the weir. ”

“ The stream is a unit; most streams have a single channel as the last goal of their descent. Others may have double channels, which contend with each other for the superiority. If they are double only for a part of the whole length of the river, and in the upper or middle course flow together, and form one main channel, they can be called twin head-streams. We have an example in the Danube and Inn, which are equally long and equally large. Other instances are the Rhone and the Saone, the Volga and Kama, the Missouri and Mississippi, the Blue and White Nile, the Ganga and Jumna. ”

“ This mole stretched its long strong low back to a rock a good way out, breaking the force of the waves, and rendering the channel of a small river, that here flowed into the sea across the sands from the mouth of the canal, a refuge from the Atlantic. But it was a roadway often hard to reach. In fair weather even, the wind falling as the vessel rounded the point of the breakwater into the calm of the projecting headlands, the under-current would sometimes dash her helpless on the rocks. ”

“ 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 [95] 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.
Fig. 73. Terraced Valley of River in Central Asia ”

“ For about two hundred miles from its mouth, it has many long and narrow lakes, of about the width of the river itself; and probably they were formerly its channel. These lakes generally have an outlet into the river, [Pg 87] and these so much resemble it, that a person not well acquainted with the stream, would be puzzled to know what channel to take. The river occupies too much ground for its quantity of water, and for about half of the year, it is a difficult stream to navigate. ”

“ The deviations which rivers are constantly making in their course are, it appears, determined by fixed laws, which engineers should be competent to find out and regard. To understand them more fully, we may imagine our river straight, evenly broad and deep, with no marked channel, and without tributaries. The formation of a channel begins as soon as a bit of the earthy constituents contained in the water is deposited on the ground under it. ”

“ A strong and winding channel, made it a labor of active watchfulness for the canoe-men, to keep our frail vessels from being dashed against boulders, or torn in pieces by fallen timber or overhanging trees. Chopping with the axe, was frequently necessary to clear the passage, and no small labor was imposed by getting through the drift wood, piled up at almost every sudden bend. We were almost imperceptibly drawn into a series of rapids and petty falls where the stream was more compressed, and the water deepened ”

“ While the width and depth of the stream will be fixed so as to be suitable for the navigation, the ratio of depth to velocity should be so arranged, if this is possible, as to minimise trouble connected with silting or scour (Chap. IV., Art. 6) . A remodelled channel is, in short, designed in exactly the same way as a new channel. The depth of water exercises the greatest effect on the discharge, and the gradient the least. The weak point in a scheme which includes weirs is the difficulty of dealing with floods. ”

“ A dead river is a channel formerly occupied by a running stream, but now filled up with earthy or rocky matter, and is not to be confounded with a channel that is open and remains dry during the greater part of the year because of a lack of water, or that has been abandoned by the stream for a deeper channel elsewhere. ”

“ They made meandering channels across the flat land and gradually entrenched themselves into tortuous canyons. As the land continued to rise, the streams cut ever deeper canyons.
The main purpose, or the driving force, of a stream is to make the shortest distance between two points, that is, a straight line. Every creek and river attempts to make a straight channel with an even grade from its source to its mouth. Hills, ridges, blocks of rock, or any other obstacles which a stream must bypass are gradually worn away as the stream makes a channel more to its choice. ”

“ Most river navigations beyond the reach of tides, are impeded by shallows in summer, by floods in winter, by artificial rights of mills, drainages, &c., all of which are avoided by the Grand Junction Canal, which, availing itself of cross valleys, and perforating narrow ridges, has opened an inland navigation from the metropolis 11 to the British Channel, to the Irish Sea, and to the German Ocean. ”

“ This volume depends on the size of the stream and on its velocity. In navigation canals in England the bank on the side opposite the towing-path is usually 4 to 6 feet wide and 11⁄2 feet above the water. In irrigation canals in India the bank of a very large canal is 2 feet above the water and 20 feet wide, while that of a small canal with 6 feet of water is 8 or 10 feet wide and 11⁄2 feet above the water, and that of a small distributary channel with 3 feet of water is 4 feet wide and 1 foot above the water. The soil is often poor. ”

“ They are several hundred in number, and are separated by as many shallow channels, some of which are as wide as the main channel, and into which persons who were not acquainted with the River have frequently run, and have been lost among the shallows for many hours. ”

“ A succession of gentle bends, of any considerable angle, cannot of course occur within a moderate length of channel.
When there is head to spare there is clearly no objection to bends, except that the bank may need protection. At a place where the bank has in any case to be protected, e.g. at a weir, there is no objection to an elbow.
9. General Tendencies of Streams.—Since the velocity is greater as the area of the cross-section is less, a stream always tends to scour where narrow or shallow, and to silt where wide or deep. The cross-section thus tends to become uniform in size. ”

“ The banks of the stream are sandstone and sand, and the channel scarcely three yards wide, with a strip of grassy thicket twenty yards in width along the stream, which is the only feed near the river, as the plain through which it runs produces nothing but scrub and banksia with a few grass-trees. ”

“ The new channel is 933 feet wide, 8·3 to 11·4 feet in depth, and has a mean slope of 1 in 2272, the speed of the current varying according to the state of the river. The side slope has an inclination of 2 to 1, and is riveted throughout in stone 93⁄4 inch thick, with a banquette on the top 39 inches wide. The ground on the right bank has been raised so as to reach the same height as the dyke on the left, thus protecting the country round about from inundation. ”

“ We have an unknown distance yet to run, an unknown river to explore. What falls there are, we know not; what rocks beset the channel, we know not; what walls rise over the river, we know not. Ah, well! we may conjecture many things. ”

“ The broadest parts of its channel lie in the central portions of its valley. Its depth is great in all its lower parts, and increases as it flows on to the Gulf, and its general descent and velocity are such as to appear very striking characteristics. [7] Noble views arrest the eye of the observer, in every part of its diversified course. Originating in a heavy and extensive bed of diluvial soil, superimposed upon primitive strata, it soon wears its channel down to the latter, and after running over them for several hundred miles, plunges at length, at the Falls of St. ”

“ In rivers which always have a fair fresh-water discharge, or a small fresh-water flow combined with a tidal flow and ebb, the channel sometimes has its direct outlet closed, and is deflected parallel to the shore till it reaches a weak place in the line of beach, through which a new outlet is formed; or, where the current is strong enough to keep the outlet open, a bar is formed across the entrance by the littoral drift, reducing the navigable depth. ”

“ 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. ”

“ The tide flows in and the rivers flow out, making an inextricable confusion of channels, creeks, canals, waterways, of every shape, size, and direction. The water seems to flow every way at once. The traveller is perhaps being rowed up a channel, and his men are straining at the oars against a strong current. ”

“ The upstream face of a rapid is vertical, or has a steep slope.
Fig. 40.
5. Weirs with Sluices.—The long weirs built across Indian rivers below the heads of irrigation canals generally extend across the greater part of the river bed. In the remaining part—generally the part nearest the canal head—there is, instead of the weir, a set of openings or “under-sluices” (fig. 40) with piers having iron grooves in which gates can slide vertically. The piers may be twenty feet apart and five feet thick. The gates are worked by one or more “travellers,” which run on rails on the arched roadway. ”

“ Canalization of Rivers.
Rivers whose discharge is liable to become quite small at their low stage, or which have a somewhat large fall, as is usual in the upper part of rivers, cannot be given an adequate depth for navigation by regulation works alone; and their ordinary summer level has to be raised by impounding the flow with weirs at intervals across the channel (see Weir) , while a lock (see Canal and Dock) has to be provided alongside the weir, or in a side channel, to provide for the passage of vessels (fig. 8) . ”

“ As the dividing channels of the lower course point to the feasibility of amplifying the connection with the ocean highway, so the spreading branches of a river's source, which approach other head waters on a low divide, suggest the extension of inland navigation by the union of two such drainage systems through canals. Where the rivers of a country radiate from a relatively low central watershed, as from the Central Plateau of France and the Valdai Hills of Russia, nature offers conditions for extensive linking of inland waterways. ”

“ Whenever, as a man told me who was standing near, it is necessary to lay the bed of the canal dry, in the immediate neighbourhood for the purpose of making repairs, the floodgate to the canal is closed, and the one to the lower part of the weir is opened, and then the water from the pond flows into the Dee, whilst a sluice, near the first lock, lets out the water of the canal into the river. The head of the canal is situated in a very beautiful spot. To the left or south is a lofty hill covered with wood. ”

“ Its survey crosses no rivers; but were a canal to do so, it would, if on the same level, let the river run into the canal and act as a water-feeder. If not on a level, either the river or the canal would be crossed by an aqueduct. Mountains are tunnelled, or the route laid out around them. ”

“ The weir, you will observe, is provided with sluices, by which the accumulated silt in front of the heads of the canals may be scoured through the body of the work, these sluice-heads for the supply of the canals on both banks of the river being built at the two ends of the weir. This system has generally been applied to the deltas of the Madras rivers, and the more gentle slope of the country has enabled these canals to be largely used for navigation as well, to the great advantage of the traffic; the fall of the bed is often only six inches per mile, or even less. ”

“ At the same time it increases the actual supply. Rain, or the signs of rain, in any part of a canal system ought always to be reported to the other parts. Owing to changes in the channels, to fluctuation in the water level of the river, especially during the night, to rain or to changes in the temperature and moisture of the air and to lack of continuous attention on the part of the gauge reader, particularly at night, there is a constant, though perhaps small, fluctuation in the water level in all parts of a canal. ”

“ A narrow channel formed of stone, and full to overflowing, guides the course of the stream, which escapes30 from the heart of the mountain at the point where it hems in the gayest and the greenest valley that ever fairy revelled in by moonlight. The channel skirts this valley, until it again passes beneath the living rock, and pours itself into the reservoirs of Broussa—but it is less of the mountain stream, or of the fine old Roman remains, that I desire to speak, than of the lovely glen to which I have just alluded. ”