Popular Science Monthly/Volume 38/December 1890/The Pororoca or Bore of the Amazon
|THE "PORORÓCA," OR BORE, OF THE AMAZON.|
STATE GEOLOGIST OF ARKANSAS.
I ONCE had an opportunity, while traveling upon the Amazon, to observe some of the effects of a remarkable phenomenon which occurs at the northern mouth of that river in connection with the spring tides. It is known to the Indians and Brazilians as the pororóca, and is, I believe, generally supposed to be caused in the same manner as the "bore" of the Hoogly branch of the Ganges, of the Brahmapootra, and of the Indus, I regret very much that, like Condamine, who passed through this region in 1740, I could not observe this phenomenon in actual operation; but the gentleman whose guest I was at the time, and upon whose boat I was a passenger, was fairly horrified at my suggesting such a thing, while his boatmen united in a fervent "God forbid that we should ever see the pororóca!" and ever afterward doubted my sanity. I give some of the results of my observations, however, as collateral evidence, and in order that those who in the future visit this particular part of the Amazon Valley, concerning which so little is known, may be able to see and establish as far as possible the rate of destruction and building up here being carried on.
I was upon a trip from Macapá—a small town on the northern bank of the Amazon, and about one hundred miles from its mouth—down the river to the ocean, and thence up the Rio Araguarý as far as the last might be navigable. The one inhabited place on the Araguarý is a very small military colony, called the Colonia Militar Pedro Segundo. At Macapá I became acquainted with the then director of this colony, Lieutenant Pedro Alexandrino Tavares, and was invited by him to visit the Araguarý.
The trip from Macapá was by a small sail-boat down the Amazon to the ocean, and thence up the Araguarý. Our departure was so timed that we should reach that part of the region disturbed by the pororóca exactly at the time of the month when there would be the least probability of its being met with—that is, at the time of the neap tides. The voyage down the river was in the face of the wind, and it was only five days after leaving Macapá that we put into a channel on the island of Porquinhos to wait for the turning of the tide. I had already seen islands said to have been half washed away, and others built up, by the pororóca; and I had seen upon the shores the evidences of its destructive power in carrying away forests and cutting away banks; but it was on this island that I was first able to see some of its effects near at hand and at my leisure. After having seen so much, I was only the more anxious to see the pororóca itself; but my suggestions in regard to it were answered by an ominous silence on the part of the director, and my requests by additional expressions of horror.
As I shortly afterward met and conversed with a man who had seen the pororóca, I can not do better than give his description of it. This man was a soldier in the Brazilian army, and, on the occasion referred to, was going with a few other soldiers from the colony to Macapá in a small open boat. Arriving at the mouth of the Araguarý, they went down with the tide, and anchored just inside the bar which crosses the mouth of this stream, to await the turning of the tide, which would enable them to pass the shallows, and then carry them up the Amazon. Shortly after the tide had stopped running out, they saw something coming toward them from the ocean in a long white line, which grew bigger and whiter as it approached. Then there was a sound like the rumbling of distant thunder, which grew louder and louder as the white line came nearer, until it seemed as if the whole ocean had risen up and was coming, charging and thundering down upon them, boiling over the edge of this pile of water like an endless cataract, from four to seven metres high, that spread out across the whole eastern horizon. This was the pororóca! When they saw it coming, the crew became utterly demoralized, and fell to weeping and praying in the bottom of the boat, expecting that it would certainly be dashed to pieces, and they themselves drowned. The pilot, however, had the presence of mind to heave anchor before the wall of waters struck them; and, when it did strike, they were first pitched violently forward, and then lifted, and left rolling and tossing like a cork on the foaming sea it left behind, the boat nearly filled with water. But their trouble was not yet ended; for, before they had emptied the boat, two other such seas came down on them at short intervals, tossing them in the same manner, and finally leaving them within a stone's-throw of the river-bank, where another such wave would have dashed them upon the shore. They had been anchored, before the waves struck them, near the middle of the stream, which at this place is several miles wide.
But no description of this disturbance of the water can impress one so vividly as the signs of devastation seen upon the land. The silent story of the uprooted trees that lie matted and tangled and twisted together upon the shore, sometimes half buried in the sand, as if they were nothing more than so many strings or bits of paper, is deeply impressive. Forests so dense that I do not know how to convey an adequate idea of their density and gloom, are uprooted, torn, and swept away like chaff; and, after the full force of the waves is broken, they sweep on inland, leaving the débris with which they were loaded heaped and strewn through the forests, or lodged in the very tree-tops. The most powerful roots of the largest trees can not withstand the pororóca, for the ground itself is torn up to great depths in many places, and carried away by the flood to make bars, add to old islands, or build up new ones. Before seeing these evidences of its devastation, I had heard what I considered very extravagant stories of the destructive power of the pororóca; but, after seeing them, doubt was no longer possible. The lower or northern ends of the islands of Bailique and Porquinhos seemed to feel the force of the waves at the time of my visit more than any of the other islands on the southeast side of the river, while on the northern side the forest was wrecked and the banks washed out far above Ilha Nova.
The explanation of this phenomenon, as given by Condamine, appears to be the correct one—that is, that it is due to the incoming tide meeting resistance in the form of immense sand-bars in some places and narrow channels in others. So long as the tide advances through a deep ocean, it moves freely and swiftly; but when it passes suddenly from the deep waters of the open ocean to the near-shore shallows, it stumbles upon them, as it were, and the waters are heaped up.
Most persons who mention the pororóca say that it breaks as far up the Amazon as Macapá; and, indeed, the people of Macapa themselves often refer to the rapid cutting away of the riverbanks near their city as the work of the pororóca. It is true that these banks are being rapidly cut down; and it is even a common thing to see, in this part of the country, the stilted houses—the floors being nearly two metres from the ground—that were originally built one, two, or three hundred feet from the water, gradually encroached upon until they fall into the stream. A portion of the old fort at Macapá was, at the time of my visit, about to fall, on account of the land upon which it was built being washed away; but all this is the work of a rapid current, for the surf of the pororóca does not reach Macapá. Moreover, there is a marked difference in character between the washing done by the pororóca and that done by the ordinary river or tide current. The latter works from below, and, by undermining and softening the bank, causes what is known through the Amazon Valley as terras cahidas, or fallen banks. The land falls into the stream in sections of various widths, and not infrequently these form temporary terraces miles in length. These terras cahidas are most common and most extensive on the upper Amazon during high water; but they may be seen on a small scale at various places through the valley. From this it is clear that the work of destruction goes on entirely below the surface. With the pororóca, on the contrary, the water is dashed fairly against the banks, the earth is washed away from above as well as from below, and the shore is left clear of loose débris. The depth to which the banks are cut shows that this disturbance is also a profound one; so much so, indeed, that on the northwest side of Porquinhos the deepest place in the channel of the river was, in 1881, close to this island, where the action of the pororóca was most violent.
Throughout this region of the Araguarý the pororóca is largely instrumental in the rapid and marked changes that are constantly going on. The water of the Amazon is notoriously muddy, and, as would naturally be expected, these disturbances in comparatively shallow places make it much more so, and fill it with all the sediment it can possibly carry. Even when I entered the Araguarý, a time when there was the least possible tidal disturbance, the water near the mouth of this stream was so muddy that a thick sediment would settle in the bottom of a vessel of it left standing a single minute; though the water of the Araguarý proper, as far down as the Veados, is of a clear, dark color. But the work of tearing down and that of building up is equally rapid, and the vegetable world takes quick possession of what the sea offers it; and, while some islands are being torn away, others are being built up, old channels being filled, islands joined to the mainland, and promontories built out. To the northwest of Faustinho is an island known as the Ilha Nova (New Island), about ten miles long by about three wide, when I saw it, and which, I was assured by several trustworthy persons, did not exist six years before. In 1881 it was covered by a dense forest. The young plants were sprouting at the water's edge, those behind were a little taller, and so on; so that the vegetation sloped upward and backward to a forest from twenty to thirty metres high in the middle of the island. On the southern side of the month of the Araguarý was a point of land nearly or quite six miles in length, and covered with vegetation, from young shoots to bushes six metres high. I was told that one year before this was nothing more than a sand-bar, without a sign of vegetation on it. The western end of the island of Porquinhos was once known as Ilha Franco; but the channel that separated it from the Porquinhos has been filled up gradually, and the two islands are now one, though the upper end of it is still known as Franco. The point in the mouth of the Araguarý known as the Ilha dos Veados (Deer Island) was, at the time of my visit, fast being joined to the mainland. A couple of years before, boats navigating the Araguarý passed through the channel on the south side of the island. In 1881 it was no longer navigable, and the Veados was rapidly being made part of the right bank of the river.
Owing to this shifting of material the pilots never know where to find the entrance to the Araguarý River. One week the channel may be two fathoms deep on the north side, and the next it may be in the middle; or it may have disappeared altogether, leaving the river-bed perfectly flat, with only one fathom of water across the whole mouth. The bar was in this last-mentioned condition when I passed over it in 1881. At this time another bar extended eastward from the eastern end of Bailique, while a little farther out was another just south of the same line. The shifting nature of the sand-bars about the mouth of the Araguarý renders it unsafe for vessels drawing more than one fathom to enter this river, except at high tides; but, as high tides and the pororóca come at the same time, only light-draught steamers can enter by waiting well outside the bar until the force of the pororóca is spent.
With the few canoes or small sailing vessels that enter this stream (probably less than half a dozen a year) it is the custom to come down past Bailique with the outgoing tide, and to anchor north of the bar that projects from the southern side of the Araguarý, and there to await the turn of the tide to ascend the latter river. Care is always taken to pass this point when the tides are least perceptible.
Although the pororóca breaks as far up the Araguarý as midway between the Veados and the entrance to the Apureminho, its violence seems to be checked by the narrowing of the stream below the Veados, by the turns in the river, and by the vegetation along the banks.
This vegetation is of a kind against which it seems to be least effective—namely, bamboos. They grow next the stream from near the mouth to the foot of the falls above the colony, and for much of the distance form a fringe to the heavy, majestic forest behind them, than which nothing could be more strikingly-beautiful. The clusters next the stream droop over till their graceful plumes touch the surface of the water, and, as the plants grow older, they droop lower, until the stream is filled with a yielding mesh of canes. I measured a number of these bamboos, and the longer ones, taken at random, were from twenty to twenty-five metres in length and from seven to ten centimetres in diameter. A more effectual protection against the pororóca could hardly be devised.
On Bailique and Brigue I found the forests very different from any I had hitherto seen in the tropics. These islands, like all the others in this part of the country, are flooded at high tide during part of the year, and, as a consequence, they are very like great banks of mud covered with the rankest kind of vegetation. This vegetation varies with the locality. All around the borders the island of Brigue is fringed with tall assai palms, bamboos, and various kinds of tall trees, all of which are hung with a dense drapery of sipós (lianes) and vines, which form an almost impenetrable covering. Inside of these are several palms, the most common being the ubussú (Manicaria saccifera). The next in order are the murumurú (Astrocaryum murumuru), urucurý (Attelea excelsa, the nut of which is used for smoking rubber), and ubim (Geonoma). But, unlike most tropical forests, this one has very little or no undergrowth, except upon the borders. Most of the ground was under from one to six inches of water, while the exposed places were covered with fine sediment deposited by the standing muddy waters of the Amazon. I walked several miles through this forest without finding any palms except the ones mentioned. The little ground above water was covered with the tracks of deer, pacas, cutias, and of many kinds of birds, mostly waders; but the death-like stillness was unbroken, save for the little crabs that climbed vacantly about the fallen palm leaves or fished idly in the mud for a living.
This half-land and half-water condition of the country is common not only in the immediate vicinity of the mouth of the river, but through a very large part of the valley of the Amazon, and is one of the most impressive features of this wonderful region. But, instead of adding to what has already been written upon this subject, I will quote a few words from two writers, whose descriptions are entirely trustworthy: "All that we hear or read of the extent of the Amazons and its tributaries fails to give an idea of its immensity as a whole. One must float for months upon its surface in order to understand how fully water has the mastery over land along its borders. Its watery labyrinth is rather a fresh-water ocean, cut up and divided by land, than a network of rivers. Indeed, this whole valley is an aquatic, not a terrestrial basin."
"This belt... can not be called either land or sea, island or archipelago. It is a veritable labyrinth of streams, canals, gulfs, islands, and lakes, combined in such a fashion as to impress one as to the caprice of man rather than as the work of Nature."
This vast expanse of muddy water, bearing out into the ocean immense quantities of sediment; the pororóca, breaking so violently on the shores, and carrying away the coarser material to the open sea, and burying uprooted forests beneath newly formed land; the rank vegetation of islands and varzea rapidly growing and as rapidly decaying in this most humid of climates; the whole country submerged for a considerable part of the year by the floods of the Amazon—impress one with the probability of such phenomena having been in past ages, and still being, geological agents worthy of study and consideration. Across the mouth of the Amazon, a distance of two hundred miles, and for four hundred miles out at sea, and swept northward by ocean-currents, beds of sandstone and shale are being rapidly deposited from material some of which is transported all the way from the Andes, while in many places dense tropical forests are being slowly buried beneath the fine sediment thrown down by the muddy waters of the great river.
So many random and erroneous statements concerning the pororóca have been made by writers upon Brazil that I take this occasion to refer to and correct some of the most glaring of them.
Prof. William H. Edwards, who visited the Amazon region in 1846, has made way with it altogether, and says that "no one knows of such terrible phenomena nowadays," although he "inquired of several persons accustomed to piloting in the main channel, and of others long resident in the city of Pará" But, with the exception of a very few who have business relations in that direction, the people of the city of Pará, as a rule, know as little of the northern mouth of the Amazon as they do of the mouth, of the Nile. And no wonder; for the Araguarý region can not be considered an attractive one in any respect, while the relations of the Paraenses with the ontside world are all through the Pará, River, which is the main channel, and the only one used nowadays by vessels visiting the Amazon, whether stopping at Para or going farther up the valley.
M. A. de Belmar tells how ships coming up the Amazon to Pard, avoid the pororóca. Prof. Orton says it rises suddenly along the whole width of the Amazon; while a writer in the Bulletin de la Société' de Géographie (November, 1871) says it is washing away the shore at the Salinas lighthouse, southeast of the mouth of the Pará River. In reply to all this I have only to repeat that the pororóca proper is confined to the northern mouth of the Amazon, in the vicinity of the Rio Araguarý.
It is well known that the tide is felt as far up the Amazon as Obidos. Mr. Belmar has erroneously attributed this to the pororóca. One authority, in describing this phenomenon, represents the waves as breaking upon the rocks. I can say, from personal observation, that there is not a rock to be seen from a short distance below Macapa to near the colony on Araguarý. I can not speak positively of what may be found in the vicinity of Cape North, but I very much doubt there being many rocks exposed there, if any at all.
All that has been written upon this subject by persons having visited the theatre of its action in Brazil is limited to the notes of Condamine on the great pororóca of the Amazon and Araguary, to those of Bernardino de Souza, and Dr. Alfred R. Wallace on the small one of the Rio Guamá. Dr. Marques also gives something regarding its occurrence on the-Rio Mearim, in the province of Maranhão.
So far as I am able to ascertain, the pororóca itself in its greatest development has never been seen by a white man.
- Pronounced paw-raw-raw'ca. This word, which is of Tupý or native Brazilian origin, is the one invariably used by the Brazilians. Father João Tavares says it is probably a frequentative form derived from the Tupý word opoe, which means "to break with a noise."
- Similar phenomena, though on a much smaller scale, occur on the Garonne in France, on the Wye, Severn, and Trent in England, and on the following streams in Brazil: Rio Guamá, Capim, and Mojú in the province of Pará, in the Rio Purús in the province of the Amazonas, and on the Mearim in the province of Maranhão.
- Prof. Hartt attributes the pororóca a of the Rio Mearim in Maranhão to the form of the channel. It can not be questioned that the form of the channel may modify, and does modify, the force with which the surf strikes the shore; but the single fact of its great violence along the shores between the Araguarý and Cape North, where the whole coast is exposed to the open sea save for the protection offered by shallows, is sufficient to show that form of channel is not its sole cause.
- For a good description of the terras cahidas, see The Naturalist on the Amazon, by Bates, fifth edition, p. 249.
- The plants growing upon this newly formed land are all of one kind. They are called Ciriúba, or Xiriúba, by the inhabitants, and belong to the family Verbenaceæ, genus Avicennia.
- Probably the only steamers that have entered the Araguarý have been Brazilian menof-war of light draught. But in 1881 there was nothing to take a steamer, however small, into this region; for, although the forests below the falls contain an abundance of rubber trees, and although cacao trees form extensive forests, there was at that time next to no population on the stream, while the malaria and the mosquitoes made it almost impossible to live there—indeed, this region is noted for being the most unhealthful on the lower Amazon. Some rubber is gathered above the falls, but it is carried overland from Porto Grande to the Rio Matapý and thence by canoes to Macapá.
- A Journey in Brazil, by Prof. and Mrs. Louis Agassiz, p. 256.
- Major João Martins da Silva Coutinho, in the Bulletin de la Société de Géographie, October, 1867, p. 330.
- Voyage fait dans l'intérieur de l'Amerique, par M. de la Condamine. Paris, 1745, pp. 193-195.
- Lembranças e Curiosidades do Valle do Amazonas, pelo Conego Francisco Bernardino de Souza. Pará, 1873, pp. 126, 127.
- The Amazon and Rio Negro, by Alfred R. Wallace, pp. 114 et seq., where it is spoken of as a "piroróca."
- Diccionario da Provincia do Maranhão, por Cezar Augusto Marques. Maranhão, 1870, pp. 385, 386.