As ganga was flowing from heaven to earth and by the path shown by bageeradh. It happened it flowed through muni named jahnav (or some thing that syllabelled the same way.) In order to reduce its speed and to protect his ashram from the tremendous flow. The rishi swallowed the entire gangajaal. After pleading from bageeradh he released ganga through his ear. Since after that it flowed from that muni ganga acquired the name jahnavi. As far as I remember. B'coz they say the same story for river godavari in Andhra Pradesh also.