A narrative of travels on the Amazon and Rio Negro,

by Alfred Russel Wallace

Martius calculates that 500,000 cubic feet of water per second pass Obidos. This agrees pretty well with my own calculations of the quantity in the dry season; when the river is full, it is probably much greater. If we suppose, on a moderate calculation, that seventy-two inches, or six feet, of rain fall annually over the whole Amazon valley, it will amount to 1,500,000 cubic feet per second, the whole of which must either evaporate, or flow out of the mouth of the Amazon; so that if we increase the amount given by Martius by one-half, to take in the lower part of the Amazon and to allow for the whole year, we shall have the evaporation as one-half of the rain falling annually.

It is a fact which has been frequently stated, and which seems fully established, that the Amazon carries its fresh waters out into the ocean, which it discolours for a distance of a hundred and fifty miles from its mouth. It is also generally stated that the tide flows up the river as far as Obidos, five hundred miles from the mouth. These two statements appear irreconcilable, for it is not easy to understand how the tides can flow up to such a great distance, and yet no salt water enter the river. But the fact appears to be, that the tide never does flow up the river at all. The water of the Amazon rises, but during the flood as well as the ebb the current is running rapidly down. This takes place even at the very mouth of the river, for at the island of Mexiana, exposed to the open sea, the water is always quite fresh, and is used for drinking all the year round. But as salt water is heavier than fresh, it might flow up at the bottom, while the river continued to pour down above it; though it is difficult to conceive how this could take place to any extent without some salt water appearing at the margins.

The rising of the water so far up the river can easily be explained, and goes to prove also that the slope of the river up to where the tide has any influence cannot be great; for as the waters of the ocean rose, the river would of course be banked up, the velocity of its current still forcing its waters onward; but it is not easy to see how the stream could be thus elevated to a higher level than the waters of the ocean which caused the rise, and we should therefore suppose that at Obidos, where the tidal rise ceases to be felt, the river is just higher than the surface of the ocean at the highest spring-tides.

A somewhat similar phenomenon is seen at the mouth of the Tapajóz. Here, at the end of the dry season, there is but a small body of water, and the current is very sluggish. The Amazon however rises considerably with the tides, and its waters then become higher than those of the Tapajóz, and they therefore enter into that river and force it back; we then see the Amazon flowing rapidly down, at the same time that the Tapajóz is flowing up.

It seems to be still a disputed question among geographers, whether the Pará river is or is not a branch of the Amazon. From my own observation, I am decidedly of opinion that it is not: it appears to me to be merely the outlet of the Tocantíns and of numerous other small streams. The canal or channel of Tagipurú, which connects it with the Amazon, and by which all the trade between Pará and the interior is carried on, is one of a complete network of channels, along which the tide ebbs and flows, so as in a great measure to disguise the true direction and velocity of its current. It seems probable that not a drop of Amazon water finds its way by this channel into the Pará river, and I ground my opinion upon the following facts.

It is well known, that in a tidal river the ebb-tide will continue longer than the flood, because the stream of the river requires to be overcome, and thus delays the commencement of the flood, while it facilitates that of the ebb. This is very remarkable in all the smaller rivers about Pará. Taking this as our guide, we shall be able to ascertain which way the current in the Tagipurú sets, independently of the tide.

On my journey from Pará to the Amazon, our canoe could only proceed with the tide, having to wait moored to the bank while it was against us, so that we were of course anxious to find the time of our tedious stoppages diminished. Up to a certain point, we always had to wait more time than we were moving, showing that the current set against us and towards Pará; but after passing that point, where there was a bend, and several streams met, we had but a short time to wait, and a long ebb in our favour, showing that the current was with us or towards the Amazon, whereas it would evidently have been different had there been any permanent current flowing from the Amazon through the Tagipurú towards Pará.

I therefore look upon the Tagipurú as a channel formed by the small streams between the Tocantíns and Xingú, meeting together about Melgáço, and flowing through a low swampy country in two directions, towards the Amazon, and towards the Pará river.

At high tides the water becomes brackish, even up to the city of Pará, and a few miles down is quite salt. The tide flows very rapidly past Pará, up all the adjacent streams, and as far as the middle of the Tagipurú channel; another proof that a very small portion, if any, of the Amazon water is there to oppose it.

The curious phenomenon of the bore, or “piroróco,” in the rivers Guamá and Mojú, I have described and endeavoured to explain in my Journal, and need not now repeat the account of it. (See [page 131].)