Young Adult book: rain forest girl leaves her tribe and finds they are all living in a huge dome

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, but no one could identify it: Rain forest girl leaves her tribe and learns they are living in a huge dome (August 2012).

Thank you in advance!