There is a nice story behind that. I heard that Lord Vishnu had three wives i.e, Laxmi (The goddess of wealth), Saraswati(The goddess of knowledge) and Ganga. But because of arguments and disagreements among all three wives, it was very difficult to live with all three of them. So Vishnu Bhagwan sent Saraswati to Brahma and Ganga to Shiva.