River delta

“ This process has been going on for ages, and as the delta has enlarged eastwards, the river has always kept a channel for itself behind. Wherever we see an island all sand, or with only one layer of mud in it, we know it is one of recent formation, and that it may be swept away at any time by a flood; while those islands which are all of mud are the more ancient, having in fact existed ever since the time when the ebbing and flowing tides originally formed them as parts of the delta. ”

“ If we were to plait branches with small limbs and strengthen such mats with heavy posts against the shores of the Delta to keep the river from spreading, then by its own force would it dig a deeper channel for itself as it goes to the sea just as this stream is doing. ”

“ We saw the delta gradually sink beneath the sea, and corals and stone-lilies cluster thick over its submerged area, to form the limestones of the Roslyn group. Again, the mud-bars of the river crept out to sea, and tangled forests waved green as of old, washed by the sea or inundated by the river. How this last period came to a close, we shall probably never know, and have no possible means of conjecturing. We pass at one step from the ancient era of the coal to the comparatively modern one of the drift—from a verdant palæozoic land, to an icy post-tertiary sea. ”

“ This triangular tract is formed from the fine silt brought down in suspension by a muddy river and deposited when the river reaches the sea. When tidal currents are feeble, the delta frequently advances some distance seawards, forming a local prolongation of the coast. ”

“ Where a river enters a large body of water its current is checked. Some of its sediment is consequently dropped, and by slow accumulation forms a delta (q.v.) . On land, every lake in mountain districts furnishes instances of this kind of alluvium. But the most important deltas are those formed in the sea at the mouths of the larger rivers of the globe. Off many coast-lines the detritus washed from the land gathers into bars, which enclose long strips of water more or less completely separated from the sea outside and known as lagoons. ”

“ Maritime and terrestrial advantages are concentrated where a river joins the sea; especially characteristic of such districts are deltas, at an early date rendered more efficient for purposes of commerce through canals and dredging. The fertility of the alluvial soil, the lack of forest occasioned by frequent floods, and the protection afforded by the islands of the delta, may have had not a little influence on the choice of such regions as settlements for man. At all events, estuaries and deltas, both small and great, were in the earliest times centres of civilisation. ”

“ But these formations, as well as the Port Hudson Clay, have nothing to do with the present problems of the delta, beyond serving as the floor on which it is built forward. The depth of the sands and silts of the true delta is practically from thirty to forty feet, and rarely reaches above sixty feet. That so great a river should show so small a depth of alluvium, when compared with such rivers as the Nile, Ganges, Hoangho and others, at first appears incredible ”

“ When I first arrived at Pará, I was struck with the fact that the Amazons, the largest river in the world, has no delta. All the other rivers which we call great, though some of them are insignificant as compared with the Amazons,—the Mississippi, the Nile, the Ganges, and the Danube,—deposit extensive deltas, and the smaller rivers also, with few exceptions, are constantly building up the land at their mouths by the materials they bring along with them. Even the little river Kander, emptying into the lake of Thun, is not without its delta. ”

“ The minor channels, blocked with driftwood which formed natural rafts; the sand-bars and mud-banks; the great curves of the river, the "bayous" [114] and isolated pools; the natural banks built up by the sediment arrested at flood-time by the herbage near the river brink; the floating timber and the "snags"—all provided valuable illustrations of the physical [Pg 163] features of a great river delta, and supplied him with material which afterwards was worked up into newer editions of the "Principles" and the "Elements." ”

“ It is only in bays like that of Menzaleh, that any great amount of new land has been gained, the general advance of the delta being checked by a strong current of the Mediterranean, which, running from the west, sweeps eastward the sediment brought down by the great river, and prevents the land from encroaching farther on the sea. The slow subsidence also of the land is another cause which checks the advance of the delta, and the raising and desiccation of the inland country. ”

“ Wherever there has been a markedly rapid sinking upon a delta region, and depressions are common in delta territory, no doubt as a result of the loading down of the crust, the river may present the paradoxical condition of flowing at a higher level than the surrounding country. ”

“ Almost all great river mouths are true lowlands—the Egyptian delta, the delta of the Ganges and the Indus, for instance, (the two latter being separated by the very moderate plateau (100 feet) between Delhi and Mooltan;) to these we may add the delta of the Euphrates, the east shore of China, between the Blue and the Yellow Rivers, and Senegambia, between the Senegal and the Gambia. And in America, the same thing occurs in the Mississippi, Orinoco, Amazon, and La Plata, where the immense mass of water which they send to the sea passes through lowlands of very great extent. ”

“ When the West river emerges from the high lands, with its burden of silt, to join its waters with those of the North and East rivers, it has entered a vast delta plain some eighty miles from east to west and nearly as many from north to south, and this has been canalized, diked, drained and converted into the most productive of fields, bearing three or more crops each year. As we passed westward through this delta region the broad flat fields, surrounded by dikes to protect them against high water, were being plowed and fitted for the coming crop of rice. ”

“ Not all the rich sediment the Nile brings down is left on the level flood plain along its course. A vast quantity is dumped at the river's mouth, where the tides of the Mediterranean check the river's current. Thus the great delta is formed. The broad river splits into many mouths that spread out like a fan and build higher and broader each year the mud-banks between the streams. Upper Egypt consists of river swamps. Lower Egypt, from Cairo to the sea, is the delta built by the river itself on sea bottom. ”

“ On the seaward face of its delta are the open roadsteads of Negapatam and French Karikal. The only navigation on any portion of its course is carried on in boats of basket-work. It is in the delta that the real value of the river for irrigation becomes conspicuous. This is the largest delta system, and the most profitable of all the works in India. ”

“ At the north, the Mackenzie enters the nearly tideless Arctic Ocean, where floating ice almost completely counteracts the tendency of the wind to produce currents, and a great delta is being extended seaward. The river divides on its delta into many distributaries and enters the sea by several mouths. The sea near the mouths of the river is reported to be shallow, and obstructed by many sand-banks and islands. ”

“ It often happens that a delta is prevented from extending further seawards owing to the prevalence of some marine current that comes sweeping along the coast-line and cuts away the accumulations thrown down by the river. The sediment thus removed is often carried to great distances, and eventually settles down as a fine mud along the floor of the sea, entombing any fucoids, infusoria, shells, corals, fish-bones, or other relics that may lie at the bottom. ”

“ The line where a land-born river meets the sea tends to become a sandbar or a delta, created by the river-borne silt and the wash of the waves, a form intermediate between land and sea, bearing the stamp of each, fluid in its outlines, ever growing by the persistent accumulation of mud, though ever subject to inundation and destruction by the waters which made it. The alluvial coastal hems that edge all shallow seas are such border zones, reflecting in their flat, low surfaces the dead level of the ocean, in their composition the solid substance of the land ”

“ Delta is the name for a tract of land at the mouth of a river shaped like the Greek letter Δ, which land owes its existence to a gradual deposit by the river of sand, stones, and [Pg 301] earth on one particular place at its mouth. As the Deltas are continually increasing, the accession of land they produce may be very considerable, and such accession is, according to the Law of Nations, considered an accretion to the land of the State to whose territory the mouth of the respective river belongs, although the Delta may be formed outside the territorial maritime belt. ”

“ 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. ”

“ The Delta of the Indus is free from sand-banks; from Tatta to Hydrabad, they occur every where, and, as the sides of the river are here more frequently shelving than steep, it is difficult to discover the deep channel, which [256] perplexes the navigator. Many of these sand-banks are but knee deep in the water, and are constantly shifting their position; the current being less rapid than near the sea, they are not easily swept away. ”

“ In fact, rivers, like mountain torrents, often run for a long distance on the summit of a ridge built up by their own deposits. The delta of the Mississippi is a regular cone, or rather mountain, of dejection, extending far out into the Gulf of Mexico, along the crest of which the river flows, sending off here and there, as it approaches the sea, a system of lateral streams resembling the fan-shaped discharge of a torrent. ”

“ His investigations have completely exploded the false notions so long entertained regarding the delta and its inhabitants. The land is not all an impassable swamp. Much of it is so elevated above [81] the river that it is never reached by the high floods of the Orinoco.
Its soil is even more fertile than that of the Nile valley and produces every tropical fruit and tree in the greatest profusion. Game is abundant, and the Indians have extensive rancherias—plantations—which supply them grain, fruits and vegetables of all kinds. ”

“ The number of people in the delta does not exceed 30,000; and the parts away from the river, both to the east and west, are thinly peopled, for pastoral countries are not populous. The villages within reach of the inundation are, however, large and numerous; and, including the whole face of the country, there cannot be less than a million of human beings. ”

“ Not many up-river boats would answer a hail from Delta, but she will, for she'll see they're from this boat and that it's your wish. There she comes round the bend now. Yes, Delta's a lot safer for 'em than Helena with its wharf-boat and daylight crowd and those three red-hots going ashore with 'em. On the Westwood they can put up with any yarn that'll carry 'em through. They're actors and used to that sort o' thing. ”

“ In some such method as this was once formed 182a delta, now constituting the Wealden formation in our own country. What is now the south-east of England, including the Weald clay, the Hastings sand, and the Purbeck beds of limestone and marls, in short, the greater part of the counties already named, with part of Dorsetshire, was once the delta of a mighty river, that so long and so uninterruptedly flowed through this then uninhabited part of the planet, that its accumulated deposits average 1,000 feet in thickness. ”

“ Sometimes the river carries this dirt clear out into the sea, and sometimes it piles it up at its mouth so that a delta is formed."
"Oh, yes," said Donald, "we studied about that in geography when we had school, but I didn't know a delta was made that way."
"Are there any deltas in this part of the river?" asked Susie.
"There may be," replied Uncle Robert, "wherever one stream flows into another."
[Illustration: The mill and dam.]
"Is there one at the mouth of our creek?" asked Frank.
"We will look when we go back," replied Uncle Robert. "Shall we take a walk now?" ”

“ For ages the Wealden river flowed, and over its vast delta laid down its depth of river mud. The land was gradually sinking; for continually strata of river mud were laid down over the same area, all shallow-water strata, yet counting hundreds of feet in thickness in all. At last a change came. The land sank more rapidly, and in over the delta the sea water flowed. The sign of coming change is seen in the limestone band made up of small oysters near the top of the Wealden strata. Marine life was beginning to appear. ”

“ This delta is formed by deposits at the point of discharge of the river St. Clair, into Lake St. Clair, and occurs at the estimated distance of about thirty- [Pg 323] six miles above the city. The flats are fan-shaped, and spread, I am inclined to think, upward of fifteen miles, on the line of their greatest expansion.
There are three principal channels, besides sub-channels, which carry a depth of from four to six fathoms to the very point of their exit into the lake, where there is a bar in each. ”