Travels on the Amazon

by Alfred Russel Wallace

The main stream of the Amazon itself is a white-water river, this name being applied to those waters which are of a pale yellowish olive-colour. This colour does not seem to depend entirely on free earthy matter, but rather on some colouring material held in solution; for in lakes and inlets, where the waters are undisturbed and can deposit all their sediment, they still retain the colour.

The waters of the Amazon continue of the same colour up to the mouth of the Uaycali, when they become blue or transparent, and the white waters are extended up that branch.

This has been taken as an evidence of the Uaycáli being the main stream of the Amazon; but I cannot consider that it has anything to do with the question. It is evident that if equal quantities of clear and muddy water are mixed together, the result will differ very little from the latter in colour, and if the clear water is considerably more in quantity the resulting mixture will still be muddy. But the difference of colour between the white-and blue-water rivers, is evidently owing to the nature of the country they flow through: a rocky and sandy district will always have clear-water rivers; an alluvial or clayey one, will have yellow or olive-coloured streams. A river may therefore rise in a rocky district, and after some time flow through an alluvial basin, where the water will of course change its colour, quite independently of any tributaries which may enter it near the junctions of the two formations.

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 Tocantins, the Xingú, and the Tapajóz, are the chief of this class. The Tocantins 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.