Meeting of the Rivers

25 June 2018:

On our last morning on board the ship we stopped at the meeting of the Rio Negro and the Amazon, just outside of Manaus. Both are very large rivers at this point. The Rio Negro is a much darker color, has high acidity due to tannins in the water, is quite warm, and flows at a slow rate (1 mph). The Rio Amazonas (the local name is the Solemoise) is brown in color, low acidity, is relatively  cooler, and flows at a much faster rate (3 mph). Rio Negro is the older river. Manus is on the Rio Negro. When they meet, the two rivers take many kilometers to merge because of the large difference in density. The boundary between the two rivers is clearly seen in these two pictures.

The local fresh water dolphins have learned a neat trick to gather fish to eat. The fish species in each river are quite specific to each because of the large differences in characteristics. I think especially because of the acidity (Ph of 4.3 vs. 7.2, if I remember). So fish in one river cannot breathe in the other. The dolphins, being air breathing, are not bothered by this difference. The dolphin use the river boundary as a fence, and herd the fish up against the boundary. The fish have two not very nice choices - asphyxiate or be eaten. We did see dolphin in the area, and in many other places at well, but did not see this feeding behavior.

Canon 5D