Some of the rivers don't physically show up until you have enough information to know that there is a site in that area, but they are visible on the map before then.

Also! If you've already visited a moon and it's now 'locked,' the fork will be unavailable to you unless it's part of a path to an 'allowable' destination.

This is deeply frustrating, developers! I can (grudgingly) understand not letting us return to the moons after our allotted # of turns have passed, but not even letting us sail by them is aggravating.

Hear hear, make it a option to visit a place, though it is Useless for the gameplay in the third sail around or so #developers_please_listen :-)

Man, such a good game, but I so desperately want to go back to a certain something after it saves your life, and the fact that I can't is legit so upsetting