Both, dpending on context.
If the child is in the water, it is in the river
If the child is in a boat, it is on the river.
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Student or Learner
What is correct?
The child is in the river.
The child is on the river.
This is the second thread you have started for the same question.
Please, stick to your thread until your question is answered. If nobody seems to anwer, ask again on the very same thread and do not start a new one.