The Ganges flows west-east and, along with its tributaries, regularly floods parts of the Bihar plain. The main northern tributaries are the Gandak and Koshi, which originate in the Nepalese Himalayas, and the Bagmati, which originates in the Kathmandu Valley. Other tributaries are the Son, Budhi Gandak, Chandan, Orhani and Phalgu.