Delta

“ 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. ”

“ A delta was thus formed, habitable at last for man. It was by nature a wide morass, in which oozy islands and savage forests were interspersed among lagoons and shallows; a district lying partly below the level of the ocean at its higher tides, subject to constant overflow from the rivers, and to frequent and terrible inundations by the sea. ”

“ After the delta, the deluge, as a storm broke over us, giving the invaders of the hills their first, but not their last, nature bath. After the delta and the deluge, the initial bit of stiff up-grade climbing, of nearly a thousand feet, tested strength and breath. ”

“ For centuries, ever since the melting of the great glaciers which once descended the Cascade Range and crept down the sound, the river has been building this delta. It grew rapidly, for immense accumulations of gravels and clays were left by the retreating glaciers. The delta has already spread westward into the sound, until it has enveloped some of the smaller islands. The forests growing upon these islands, which rise from the surface of the delta plain, are in picturesque contrast to the fields dotted with stacks of grain. ”

“ 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. ”

“ They continued to mount, for Potter wanted to spread before her the great reaches of the delta—a world of close-growing wild rice and reeds, a universe of wild birds, myriad tiny islets, with here and there a strip of land high enough above the water to supply a foothold for wind-bent, scraggling trees. Here and there wound a maze of channels, some navigable by small boats, and to the northward another gleaming river, the North Channel, up which the fleets of the Lakes had been compelled to pass before the construction of the ship-canal. ”

“ 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. ”

“ 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. ”

“ Hence, all attempts to arrive at any definite estimate of the number of years since the alluvial plain of the delta began to be formed, are frustrated. But, the more one thinks of the matter, the more does the impression of the antiquity of the plain grow; and I, for my part, have no doubt that the extreme term imagined by Herodotus for the filling up of the Arabian Gulf—twenty thousand years—is very much below the time required for the formation of the delta. ”

“ If a series of earthquakes accompany the growth of a delta, and change the levels of the land from time to time, as in the region where the Indus now enters the sea, the phenomena will depart still more widely from the ordinary type. If, after a protracted period of rest, a delta sinks down, pebbles may be borne along in shallow water near the foot of the boundary hills, so as to form conglomerates overlying the fine mud previously thrown into deeper water in the same area. ”

“ 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. ”

“ After one or two narrow escapes between the rapidly running floes, we reached the delta of Black Cape River, a few miles beyond our former place. But when the tide turned we were obliged to hurry back about a quarter of a mile to the shelter of a grounded berg.
When the hawsers were made fast I went ashore and up to the delta to look at the ice beyond. Not a crack or hole was visible to the north, and the path by which we had retreated to our present position was now a sea of solid ice. Should we ever be able to make the few remaining miles? ”

“ In the south lies the Delta of the Irrawaddy and the wide-stretching deltaic districts of Hanthawaddy, Insein, Pegu and Tharrawaddy. This vast expanse of country is a great alluvial plain. The true Delta includes all the districts of the Irrawaddy division, Bassein, Pyapôn, Ma-u-bin, Myaungmya, Henzada, a flat country intersected by countless rivers and creeks. [4] On the sea board and along the banks of streams and creeks are masses of mangrove and dani palm forest, tidal and swamp vegetation, elephant grass and savannah. ”

“ A flat plane near sea level in the Mississippi [41] delta is where the river has swung right and left against valley walls, over its own flood plain. A flat plain, secured by ice sheets or planed off by encroaching wave action as land sinks is mechanically probable. ”

“ The region occupied by the delta, he adds, was "evidently, at one time, a gulf of the sea. It resembles, to compare small things with great, the parts about Ilium and Teuthrania, Ephesus, and the plain of the Mæander. In all these regions the land has been formed by rivers, whereof the greatest is not comparable in size with any one of the arms of the Nile." ”

“ 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?" ”

“ I am in a very complicated network of creeks, swamps, dense forest, and overflowed lands, forming a delta, which in the work I published in 1861 I named the Delta of the Ogobai. For several days I have been here in a canoe exploring the country by water. What a lonely place! We have not seen a single village, we have met not a single human being; it is a complete desolation, and on the day in question it seemed more desolate than usual. The creek we had got into was narrow, and on both sides there was an interminable forest of palms, that kind which yields bitter nuts to eat ”

“ In some cases, the irregularities are not temporary.364 Thus deltas (p. 198) , though not wholly the work of sea- (or lake-) water, often constitute irregularities of a more or less permanent nature. This is the case where they project beyond the general trend of the coast-line. Where, on the other hand, they are built at the heads of bays, they tend to simplify the coast-line by obliterating the indentation. The delta at the head of the Gulf of California is an example. So too is the delta of the Mississippi, the real head of which is far above the present debouchure of the stream. ”

“ In this manner the stream moves from one place to another, successively building the little scallops and continually carving new channels for itself. Fig. 3 is a photograph of such a delta, some three feet across, taken after the water had been drained away, and reveals its form in a characteristic manner. As we watch its growth, it will become evident that only the coarsest material transported by the stream goes to make up the delta, and that the clay and finest sand are deposited farther awy, where the water is more quiet, or else pass out in the stream draining the pool. ”

“ 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. ”

“ 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. ”

“ Egypt remains in the same position that Nature originally allotted to her; the life-giving stream that flows through a thousand miles of burning sands suddenly rises in July, and floods the Delta which it has formed by a deposit, during perhaps hundreds of thousands of inundations; and it wastes a superabundance of fertilizing mud in the waters of the Mediterranean. As Nature has thus formed, and is still forming a delta, why should not Science create a delta, with the powerful means at our disposal? ”

“ 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. ”

“ 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 ”

“ 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. ”

“ The successive deposits on the outer abrupt slope will dip conformably with its surface (Fig. 189) . The finest sediment will be carried beyond the steep slope, and conform to the topography of the bottom beyond (c, Fig. 189) .
At the beginning, the top of the delta platform is at the level of the bottom of the stream’s channel at the point of debouchure, but it is gradually aggraded as water continues to flow over it. Its landward margin is presently built up to sea-level and then above it, and as the delta grows the delta-land is extended seaward (compare Figs. 188 and 189) . ”

“ It is by gradual depression that delta deposits attain enormous thickness, and, being lowered beneath the level of the sea, are safely preserved from erosion until a movement of the earth’s crust in the opposite direction lifts them to form part of the land. ”

“ The delta region is swampy, and forms, for a distance of from 40 to 70 m. inland, a network of interlacing creeks and broad sluggish channels fringed with monotonous mangrove forests. The main rivers are navigable for ocean-going steamers for a distance of from 15 to 40 m. from their mouths. Beyond the delta firm ground takes the place of mud and the mangroves disappear. The land rises gradually at first, becoming, however, in many districts very hilly, and is covered with dense forests. The Niger at its confluence with the Benue is not more than 250 ft. ”

“ This modern delta is supposed by M. Morlot to have required 10,000 years for its accumulation. At the height of 150 feet above the lake, following up the course of the same torrent, we come to a more ancient delta, about ten times as large, which is therefore supposed to be the monument of about ten times as many centuries, or 100,000 years, all referable to the fourth period mentioned in the preceding page, or that which followed the last retreat of the great glaciers. ”

“ The Delta coast-line, composed of sandhills and, occasionally, limestone rocks, is low, with cape-like projections at the Nile mouths formed by the river silt. Two bays are thus formed, the western being the famous Bay of Aboukir. It is bounded W. by a point near the ancient Canopic mouth, eastward by the Rosetta mouth. Beyond the Delta eastward the coast is again barren and without harbours. It rises gradually southward, merging into the plateau of the Sinai peninsula. The Red Sea coast is everywhere mountainous. ”

“ 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. ”