In Ramayanam Lord SriRama asks about river ganga, Siddhaashram,about tathaka rakshas, about sage Vishwamitra etc. as written in BalaKanda, Lord SriRama is an incarnation of Lord MahaVishnu itself so why the god asks when he is the creator?
It is true that Lord Sri Rama is an incarnation of Lord Vishnu but he was born as an ideal human. You have seen in various places of Ramayana that he have never broken any rules of human beings. He was denoted as "Marayada Purushotam". That's why he acted as he did not know anything in his childhood.