Una gran lección pero tengo preguntas . . .

Question

With: "Both rivers, the Amazon and the Orinoco, and their respective basins", why does Amazon get pluralized to "Amazonas"? I've seen that the masculine is "el Amazonas" and the feminine is "la Amazona". Why is that, please? I'm wondering about the use of "el río Amazonas" versus "la selva Amazona" and "la selva Amazonica".

How does "se nos" change the meaning of "Uno de esos ríos que enseguida viene a la mente" -v- "Uno de esos ríos que enseguida se nos viene a la mente".

And the use of "ello" . . . does "ello" not mean "it" or "that"? Is the use of "ello" as "this" merely the uneducated English useage where "this" and "that" and their appropriate relationships to time and place become misused? : "An example of that is Caño Cristales, a natural sanctuary."

And why the use of the future tense "existirán en otros lugares del mundo"?

Accepted Answer

Hola Garry
When we say "el Amazonas" we are referring to the river - it's its name "el río Amazonas" (=el Amazonas).
La selva amazónica, la Amazonia or la Amazonía refers to a big region in South America (tropical jungle) that involves different countries: Brasil, Peru, Colombia, Venezuela, Ecuador, Bolivia, Guyana, Surinam y French Guayana. 
I don't think we call it "la selva amazona", I've never heard it that way; we call it la selva amazónica.
In this sentence using "se nos...": "Uno de esos ríos que enseguida se nos viene a la mente" the "se" is an emphatic pronoun that doesn't change the meaning, it is simply reinforcing the meaning of the verb (venir algo a la mente = venirse algo a la mente) - the use of "nos" is as an indirect object pronoun as in saying "it comes to OUR mind", it comes to us. This can be considered a "dativo de interés"; we tagged this lesson in the exercise. Have a look here to see what sort of effect it's doing in this sentence. 
As for "ello", it is a slightly more formal than "eso/esto"; this is why the translation is using "of that" - we also have a lesson on ello/eso. 
The use of the future in "que no existirán" is a bit tricky to explain. It is really referring to a present, hence the English "that don't exist,  but the use of the future tense in Spanish here is as if we were saying "that you will not find in any other place" - for some reason the future doesn't work in English with the verb to exist but it's OK to use it in Spanish. Of course, we can always use the present too: que no existen en otros lugares del mundo. 
I hope this clarified your doubts. 
Saludos cordiales
Inma