Isn't it from scene when Littlefoot found Valley alone and run back to the others to Burning Mountains?
The thing is, I don't know. All I wrote above is speculation.
What I say now is pure hypothesis too and only based on the motion picture soundtrack of the movie (so... the one you can purchase, not the one you hear in the movie itself since there are significant differences due to the cut scenes

)
When I listen to the part of the soundtrack that is supposed to be playing when Littlefoot arrives at the Great Valley the
first time, it sounds like that scene ends rather abruptly with Littlefoot's decision to go back to save his friends. There is no music playing that suggests Littlefoot is running through that waterfall on his way to rescue his friends. The soundtrack of James Horner is so excellent because you can tell by the music alone what is happening even in a scene you don't know. A scene of Littlefoot running through that waterfall would surely use the water theme or some more dramatic scene, considering the situation.
What stumbles me though is Littlefoot's expression on that screenshot... he's happy. Only... why? I can't quite come to an unambiguous answer to that. Is he happy because he's just discovered the Great Valley? I'm not sure how Littlefoot would feel in such a situation. I mean, he could also be worried about his friends being alright? I don't know... :unsure:
Another possibility that comes to my mind is, that this scene is actually IN the Great Valley though not very likely in my opinion.
I'll probably have to take a very close look at the waterfall scene again and make a screenshot to have a comparison...