I read this young-adult book in the early 90s, but it was probably published in the 70s or 80s. All I remember is this: the book starts with this girl who lives in a tribe in the rain forest, then something happens and she leaves (possibly with other kids from the tribe). She takes a boat really far down the river and she discovers that her tribe, and the forest they live in, is actually a kind of park inside this gigantic dome in a very futuristic city.
I asked about this book on reddit two years ago here http://www.reddit.com/r/tipofmytongue/comments/10z8e5/tomtbook_rain_forest_girl_leaves_her_tribe_and/ but no one could identify it.
Thank you in advance!