Tidal river

“ To secure a good and fairly uniform depth on a tidal river, it is essential that the flood and ebb tides should follow the same course, in order to combine their scouring efficiency, and form a single, continuous deep channel. ”

“ When the tide is in, the breakers dash against these rocks even in calm weather with a force that sends the spray very many feet high, and any one then attempting to get round the point does so at the imminent peril of his life; the river also becomes swollen by the tidal inflow, and although a ferryman lives on the opposite bank, he wisely forbears from venturing to cross in his canoe until the tide has well run out again. ”

“ The causeway which might therefore be necessary to approach the stream from the south in all states of the tide need only have been such a hardening of the track over the mud as is necessary between the high- and low-water mark of any tidal river where a ferry is to be established, and we must believe that the river at high water washed the gravel spit. ”

“ Stand on the bank of an estuary or river up and down which a great tidal current ebbs and flows; you [42] will see the water copiously charged with sediment which the tide is bearing along. Engineers are well aware of the potency of the tide as a vehicle for transporting stupendous quantities of sand or mud. A sand-bank impedes the navigation of a river ”

“ The formation and keeping open of large estuaries are due to the combined influence of tidal currents and rivers; for when the tide rises, a large body of water suddenly enters the mouth of the river, where, becoming confined within narrower bounds, while its momentum is not destroyed, it is urged on, and, having to pass through a contracted 338 channel, rises and runs with increased velocity, just as a stream when it reaches the arch of a bridge scarcely large enough to give passage to its waters, rushes with a steep fall through the arch. ”

“ John pours out its great flood into the harbor. An immense body of water passes through this channel, representing the natural river drainage of over 25,000 square miles of country. It is not a fall as ordinarily understood, nor a sheer drop from high to low level. The outlet, though wide, is not sufficiently large to admit the inflow of the tidal water. The tide packs up, therefore, and falls into the river. In the same way the tide recedes in the harbor much faster than it can escape from the river channel, and at such time the fall is outward. ”

“ A tidal wave that propagates up a relatively shallow and sloping estuary or river in a solitary wave. The leading edge presents an abrupt rise in level, frequently with continuous breaking and often immediately followed by several large undulations. An uncommon phenomenon, the tidal bore is usually associated with very large ranges in tide as well as wedge-shaped and rapidly shoaling entrances. ”

“ It is never high water at all points simultaneously. To show the actual state of affairs at various stages of the tide, series of lines must be drawn as in fig. 67, where the firm lines show the flood, and the dotted lines the ebb tide.
The flow in the tidal reach of a river is the same as if the water was alternately headed up by a movable weir and then allowed to flow freely and be drawn down. If the water carries silt, the tendency for deposit to occur is (Chap. V., Art. 2) no greater than if there was no heading up or drawing down. ”

“ Whereas the size of tideless rivers depends wholly on their fresh-water discharge, the condition of tidal rivers is due to the configuration of their outlet, the rise of tide at their mouth, the distance the tide can penetrate inland, and the space available for its reception. ”

“ It is true that, in this oceanic river system, the tidal action has an annual, instead of a daily, ebb and flow; that its rise and fall obey a larger light, and are regulated by the sun, and not the moon ”

“ Tidal basins, as they are termed, are generally interposed in the docks of London between the entrance locks and the docks, with the object of facilitating the passage of vessels out of and into the docks before and after high water, by lowering the water in the basin as soon as the tide has risen sufficiently, Tidal and half-tide basins.and opening the lock gates directly a level has been formed with the tide in the river. Then the vessels which have collected in the basin, when level with the dock, are readily passed successively into the river. ”

“ It is well known that the tides on our coasts sometimes rise to an unusual height. Those who dwell on low ground adjoining tidal rivers are painfully aware of this fact by the floods which are often produced. Such occurrences generally take place at the time of new moon or of full moon. At first quarter or last quarter the tides are even below the usual height. A fisherman who has to regulate his movements by the tides will know full well that at certain times the tides rise higher and fall lower than at other times. ”

“ Extensive areas of marsh or meadow, protected by a high shingle-beach or chain of sand-dunes, are not uncommon, especially along our eastern coast. These marshes may be quite fresh, and even have trees growing on them, below the level of high tide, [Pg 5] as long as the barrier remains unbroken. The reason of this is obvious. The rise and fall of the tide allows sea water to percolate landward and the fresh water to percolate seaward; but the friction is so great as to obliterate most of the tidal wave. ”

“ The very bottom of the deep bays near shore—dark and eternally silent, prisoned under the restless waste of waters—is thickly carpeted with strange and many-coloured forms of animal and vegetable life. But the beaches and tide-pools over which the moon-urged 130 tides hold sway in their ceaseless rise and fall, teem with marvels of Nature’s handiwork, and every day are restocked and replanted with new living objects, both arctic and tropical offerings of each heaving tidal pulse. ”

“ These tremendous tides cause peculiar phenomena; they make the rivers seem to actually run up-hill at times, while the tidal "bore" or wall of water, which is the advance of the flood, moves up the streams and across the extensive mudflats with the speed of a railway train, often catching the unsuspecting who may be wandering over them. The elaborate wharves made for boat-landings are 278 built up like three-story houses, with different floor-levels, so as to enable the vessels to get alongside at all stages of the tide. ”

“ It is high-water in the stream, on the days of the new and full moon, between two and three o'clock; and the tide rises, upon a perpendicular, between three and four fathoms. The reason of the tide's being greater here than at other parts of this coast, is easily accounted for. The mouth of the river being situated in a corner of the coast, the flood that comes from the ocean is forced into it by both shores, and by that means swells the tide to a great height. ”

“ To see it, the nearest way led through a lane past the blacksmith's. A small tidal stream, curving between two broad, low hills, it ran deep with the ebb and bent round a crag into the hidden sea; with the flood, it crept up shallow, pale, spreading inland among green-wooded points and fields whitened by daisies. ”

“ In the Ouse the tide rises and falls, so it is necessary to stop the mouths of the great drains going into it, which is done by a structure of timber or brick-work like the gates of a dock, which close at high tides to keep out the sea-water, and open at low tides to let out the land water. Either from insufficient workmanship or bad locality, or unusual pressure, or from burrowing animals, the land water, or sea water, or both, made a flank movement, and burst up the whole sluice, so that the tide, instead of being confined to the river, obtained free way up the drain also. ”

“ John is still further harassed by prodigious tides that rise here to a height of twenty-five feet. An engineer who in 1761 witnessed the phenomenon of a river turned back upon its course twice in every day, reported: "The current runs down till half-flood, and up till half-ebb. The falls are smooth every half tide for fifteen to twenty minutes. The greatest rise at the rapids is equal to half the rise of the tide." At low tide, the piled-up waters of the river are higher than the sea. At full tide the incoming flood of Fundy is higher than the river. ”

“ You must understand, then, that all tide takes its rise in the free and open ocean under the action of the moon. No ordinary-sized sea like the North Sea, or even the Mediterranean, is big enough for more than a just appreciable tide to be generated in it. The Pacific, the Atlantic, and the Southern Oceans are the great tidal reservoirs, and in them the tides of the earth are generated as low flat humps of gigantic area, though only a few feet high, oscillating up and down in the period of approximately twelve hours. ”

“ At low tide there is very little river to be seen—it is reduced to a tiny stream that seems to trickle with difficulty through vast stretches of mud. But when the tide does come up it alters the whole appearance, and the place seems to come to life again as the strong current pushes its way up—running far up the little streams, and beneath the houses, which are built out over the river bed, at the bridge, on high wooden gates—giving a wonderfully picturesque effect, and reflecting all shades of color. ”

“ The tidal movement of the ocean, deep-sea currents, and the agitation of inland waters by the wind, lift up the sands strewn over the bottom by diluvial streams or sent down by mountain torrents, and throw them up on dry land, or deposit them in sheltered bays and nooks of the coast—for the flowing is stronger than the ebbing tide, the affluent than the refluent wave. ”

“ At the upper end of the Bristol Channel the range of the tide is double the range in the sea outside the channel. The Bay of Fundy is another place where a similar phenomenon occurs. When a river or estuary is shallow and the range of the tide is great, so that its rise is rapid, the flood tide in some cases advances in the form of a wave or “bore,” causing a sudden rise in the water-level and a sudden reversal of the flow of the stream. A bore is most pronounced at spring tides. ”

“ In Hindûstan, and some parts of Africa, rivers exist, which, though they flow night and day, are, from the accession of snow-water, the greatest by day. Those rivers also which fall into the sea have their level daily varied by the tidal wave for some distance from their mouths, the extent through which the influence of the tides is felt being modified by the breadth and shape of their channels and the force of their current. The wider and more direct the bed of a stream communicating with the ocean, and the slower its motion the farther the tide will penetrate ”

“ The momentum of the tide running up the centre of the river is in excess of that along the banks, so that the current changes near the shore before it does in the middle, and, as the sea water is of greater specific gravity than the fresh, weighing 64 lb per cubic foot against 62-1/2 lb, it flows up the bed of the river at the commencement of the tide, while the fresh water on the surface is running in the opposite direction. ”

“ At such times he would wonder how the tide stood in the estuary at home. Was the flood swirling up from the Channel, bringing with its clear eddies the smell of the ocean as it hurried in and out among the piles of the old pier? Or was it the time of the ebb when stretches of damp sand come out at the foot of cliffs and when ridges of rock, dripping with cool weed, emerge once more into the sun? What a moment for a swim! ”

“ This is Tide Water Day and men of the river, like Vikings of old, are gathering on the coast to pay tribute to tide water counties, and as it was in the wilderness period, so it is now, every path of the river leads to the sea.
The tide water men from each shore town. ”

“ Tidal water is, indeed, unlimited, in volume; but, unlike the drainage waters which must be discharged into the sea, it only flows up rivers where there is a channel and space available for its reception. Consequently, it is possible to exclude the tide by injudicious works, such as the sluices which were erected long ago across the fen rivers to secure the low-lying lands from the inroads of the sea; the tidal influx is also liable to be reduced by accretion in an estuary resulting from training works. ”

“ A sailing vessel would have difficulty in getting in without a leading wind from the southward, but a light breeze would probably be sufficient; although there is a heavy body of water in the river, it does not flow with such impetuosity into the sea as to make much debatable ground between it and the tide. The natural effect of the existence of the Lake must be to reserve the water for a time on the occasions of storms, and to feed the river more gradually than if it flowed directly from among the hills. ”

“ The tidal information becomes important and must be considered in navigation or in anchoring in waters where the available depth at low water approximates the draft of the vessel. In the general use of coast charts it is also important to observe the effect of the stage of tide on the appearance of many features. Rocks rising some feet above low water may be entirely submerged at high water. In some areas the aspect may be radically changed between high and low water by the baring of extensive shoals or reefs. ”

“ The result will be that the particles will travel a little farther out from the shore each time it is swung to and fro in the alternating movement of the tide.
The effect of tidal movement in nurturing marine life is very great. It aids the animals fixed on the bottoms of the deep seas to obtain their provision of food and their share of oxygen by drawing the water by their bodies. All regions which are visited by strong tides commonly have in the shallows near the shores a thick growth of seaweed which furnishes an ample provision of food for the fishes and other forms of animal life. ”