Remember the Rainforest 1
The following observations are the ones we determine for how fast the river is and, in fact, always at a short distance from the bank, because in the middle of the course no experience can be obtained.
The speed of the Amazon, on average, would be 0.775 feet per second; but this number is of little value, because most observations refer to the extreme ebb level, and because not all refer to the speed in the middle of the bed, but only along the margins.
In the Pauxis strait, the speed remains almost constant; as we found ourselves: 1.2 feet per second. One should perhaps consider as medium such rapidity along the margins, and the double, therefore 2.4 per second, in the midst of the current. According to La Condamine, the Maranhao,
at the navigable point, carries a canoe 7.5 feet per second, very fast, much higher than that of numerous alpine streams at only 5 feet per second. These La Condamine calculations have given rise to the estimated average Amazon speed at 7 feet per second, although it is undoubtedly too high in the general rule.
When in the month of March, I was sailing upstream in Madeira, I noticed in the first two days of travel the current of its very full waters, 20 to 26 feet per minute, so average speed per second at 0.38 feet. Spix noticed in Rio Branco very weak current; its waters carried the manned canoe with 9 people at an average speed of 0.27 feet per second. The rapidity of the Negro River current depends not only on its flooding, but also on the speed of the Amazon, because when it is flooding, it seems to be very weak because its waters are repelled by those of the more caudal river.
At the most, the speed of Japura, according to my calculation, consisted of 5.6 to 7 feet per second; in the lower half only half that amount. The Amazon River has never had the extraordinary current of the Cassiquiare Canal in which the waters flow 8,111 feet per second.
Among the local conditions, which properly determine water movements, I mention in particular the case of waters that, before the mouth of a channel, find no space in it, and then, retreating in part, form whirlwinds (in tupi, hy-jebyra , that is, "water coming back"). Of this genus are the so-called caldeiroes (in tupi, hy-coarana) of the upper mouth of the Nhanmunda River.
In places, where the waters revolve inwards in large bays (coves, tupi, sabaa), the current is doubled, and even tripled; for near the bank the water flows upstream, and flows all the more downstream in the center of the river (in tupi, parana-pyterpe), and at the extremity of these opposite movements are formed the sand islands around which the river slowly swirls.
La Condamine refers to certain water movements on the surface and bottom, on the coast, upstream, in the middle of the riverbed, downstream, etc., which can only be clarified and presented in their physical characteristics, taking into account the influence from the seas after long-term observations.
How would it be to wish that all of these and other similar grandiose phenomena occurring here would be examined by physicists as soon as possible! Certainly, one could find no better model of hydraulic conditions than in this immense realm of fluid element.
The Obidos Strait has achieved a certain fame because of the fact that the seas have been felt so far inland. The village of Obidos is situated at the northernmost corner of a knee-shaped bend that the river makes from southwest to northeast and thence to southwest again;
It is thus influenced by the height of the water, the ocean, and cannot easily extend further west. Certainly, however, the seas would not be felt until now if the river were not narrowed and thus subjected to the ebb and flow of the waters. Also the change is already insignificant here; It is not realized by the delaying or equalization of the river waves, the hours of the highest tide, and the sudden and faster flow in the ebb, but only by the fact that the regularity of the high and low tide causes the water to rise or fall on the steep coasts. From Obidos downwards, the seas are becoming more pronounced, with their daily, monthly and annual variations; however, you only need to take them into account when traveling in channels. In Almeirim, the high spring tide rises to three feet in height.