Learn English – Using “had just arrived”

You can use "forgot" or "have forgotten", but not normally "had forgot" or "forgotten" by itself.

Forgot is usually used to indicate an instance of forgetfulness, for example:

I forgot to do my homework last night.

Joe forgot to set his alarm clock on Tuesday.

On the other hand, forgotten refers to being in a state of forgetfulness (particularly an ongoing one) - and hence always goes with an auxiliary verb; normally have:

I have forgotten how to speak French.

We have forgotten the ways of our ancestors.

In your specific instance, you probably want to use the word forgotten, because you want to say that the not only did we forget, but that the forgetfulness is ongoing. Consequently:

Humans have forgotten the rivers

As commented by Catija, the first three sentences are perfectly OK and as to the last one, the simple present tense is actually the correct usage.

According to grammar, when you are talking about the future, you should use the present simple in the time (when) clause, not a future form. For example, I'll come when I finish my work. So the correct sentence is:

I want to spend a year travelling when I finish university.

(Reference: The Free Dictionary - the use of "when").