A narrative of travels on the Amazon and Rio Negro,

by Alfred Russel Wallace

The Iça and Japurá have waters very similar in colour to the Amazon. The Rio Branco, a branch of the Rio Negro from the north, is remarkable for its peculiar colour: till I saw it, I had not believed it so well deserved its name. The Indians and traders had always told me that it was really white, much more so than the Amazon; and on descending the Rio Negro in 1852, I passed its mouth, and found that its waters were of a milky colour mixed with olive. It seemed as if it had a quantity of chalk in solution, and I have little doubt of there being on its banks considerable beds of the pure white clay which occurs in many parts of the Amazon, and which helps to give the waters their peculiar whiteness. The Madeira and Purús have also white waters in the wet season, when their powerful currents bring down the alluvial soil from their banks; but in the dry season they are a dark transparent brownish-olive.

All the rivers that rise in the mountains of Brazil have blue or clear water. The Tocantíns, the Xingú, and the Tapajóz, are the chief of this class. The Tocantíns runs over volcanic and crystalline rocks in the lower parts of its course, and its waters are beautifully transparent; the tide however enters for some miles, and renders it turbid, as also the Xingú. The Tapajóz, which enters the Amazon about five hundred miles above Pará, is clear to its mouth, and forms a striking contrast to the yellow flood of that river.

It is above the Madeira that we first meet with the curious phenomenon of great rivers of black water. The Rio Negro is the largest and most celebrated of these. It rises in about 2° 30´ N. lat., and its waters are there much blacker than in the lower part of its course. All its upper tributaries, the smaller ones especially, are very dark, and, where they run over white sand, give it the appearance of gold, from the rich colour of the water, which, when deep, appears inky black. The small streams which rise in the same district, and flow into the Orinooko, are of the same dark colour. The Cassiquiare first pours in some white or olive-coloured water. Lower down, the Cababurís, Maravihá, and some smaller white-water streams help to dilute it, and then the Rio Branco adds its flood of milky water. Notwithstanding all this, the Rio Negro at its mouth still appears as black as ink; only in shallow water it is seen to be paler than it is up above, and the sands are not dyed of that pure golden tint so remarkable there.

On the south of the Amazon there are also some black-water streams—the Coary, the Teffe, the Juruá, and some others. The inhabitants have taken advantage of these, to escape from the plague of the mosquitoes, and the towns of Coary and Ega are places of refuge for the traveller on the Upper Amazon, those annoying insects being scarcely ever found on the black waters. The causes of the peculiar colour of these rivers are not, I think, very obscure; it appears to me to be produced by the solution of decaying leaves, roots, and other vegetable matter. In the virgin forests, in which most of these streams have their source, the little brooks and rivulets are half choked up with dead leaves and rotten branches, giving various brown tinges to the water. When these rivulets meet together and accumulate into a river, they of course have a deep brown hue, very similar to that of our bog or peat water, if there are no other circumstances to modify it. But if the streams flow through a district of soft alluvial clay, the colour will of course be modified, and the brown completely overpowered; and I think this will account for the anomalies observed, of streams in the same districts being of different colours. Those whose sources are pretty well known are seen to agree with this view. The Rio Negro, the Atabapo, the Isanna, and several other smaller rivers, have their sources and their whole course in the deep forest; they flow generally over clean granite rocks and beds of sand, and their streams are gentle, so as not to wear away the soft parts of their banks.

The Iça, Japurá, and Upper Amazon, on the contrary, flow through a long extent of alluvial country, and, having their sources on the slopes of the Andes, are much more liable to sudden floods, and by their greater velocity bring down a quantity of sediment. In fact it seems clear, that a thorough knowledge of the course of each river would enable us to trace the colour of its waters to the various peculiarities of the country through which it flows.

With the exception of the streams rising in the Andes, the boundaries of the Amazon basin, or the most distant sources of its tributaries on the north and south, are comparatively little elevated above the level of the sea. The whole basin, with the exception of a very small portion, is one great plain of the most perfect and regular character.

The true altitude of the source in the Lake Lauricocha has not been ascertained. At Tomependa Humboldt states it to be 1320 feet above the sea: this is as near as possible 2000 miles in a straight line from the mouth; so that the average rise is only eight inches in a mile. But if we take the height at Tabatinga, on the boundary of Brazil, which, according to Spix and Martius is 670 feet, we shall find, the distance being about 1400 miles, that the rise is only five and a half inches per mile. If we had the height of Barra do Rio Negro accurately, we should no doubt find the rise to that point not more than two or three inches in a mile. The distance is, in a straight line, about seven hundred miles, and we may therefore probably estimate the height at less than 200, and perhaps not more than 150 feet.

This height I am inclined to believe quite great enough, from some observations I made with an accurate thermometer, reading to tenths of a degree, on the temperature of boiling water. This instrument I received from England, after leaving Pará. The mean of five observations at Barra, some with river- and some with rain-water, gave 212·5° as the temperature of boiling water; a remarkable result, showing that the barometer must stand there at more than thirty inches, and that unless it is, in the months of May and August, considerably more than that at the sea-level, Barra can be but very little elevated above the sea.

For the height of the country about the sources of the Rio Negro, Humboldt is our only authority. He gives 812 feet as the height of São Carlos; he however states that the determination is uncertain, owing to an accident happening to the barometer; I may therefore, though with great diffidence, venture to doubt the result. The distance, in a straight line, from the mouth of the Rio Negro to São Carlos, is rather less than from the same point to Tabatinga, whose height is 670 feet. The current however from Tabatinga is much more rapid than down the Rio Negro, the lower part of which has so little fall, that in the month of January, when the Amazon begins to rise, the water enters the mouth of the Rio Negro, and renders that river stagnant for several hundred miles up. The falls of the Rio Negro I cannot consider to add more than fifty feet to the elevation, as above and below them the river is not very rapid. Thus, from this circumstance alone, we should be disposed to place São Carlos at a rather less elevation than Tabatinga, or at about 600 feet. My observations up the Rio Negro gave consistent results. At Castanheiro, about five hundred miles up, the temperature of boiling water was 212·4°, at the mouth of the Uaupés 212·2°, and at a point just below São Carlos, 212·0°. This would not give more than 250 feet for the height of São Carlos above Barra; and, as we have estimated this at 200 feet above the sea, the height of São Carlos will become 450 feet, which I think will not be found far from the truth.

The velocity of the current varies with the width of the stream and the time of the year; we have little accurate information on this subject. In a Brazilian work on the Province of Pará, the Madeira is stated to flow 2970 braças, or about three and a half miles, an hour in the wet season. At Obidos I made an observation in the month of November, when the Amazon is at the lowest level, and found it four miles an hour; but this by no means represents the current in the rainy season. On descending to Pará, in the month of June, 1852, I found that we often floated down about five miles an hour, and as the wind was strong directly up the river, it probably retarded us, rather than helped us on, our vessel not being rigged in the best manner.