Immediately after the Sap River joins the Mekong by Phnom Penh, the Bassac River branches off the right (west) bank. The Bassac River is the first and main distributary of the Mekong. This is the beginning of the Mekong Delta. The two rivers, the Bassac to the west and the Mekong to the east, enter Vietnam shortly after this. In Vietnam, the Bassac is called the Hậu River (Sông Hậu or Hậu Giang); the main, eastern, branch of the Mekong is called the Tiền River or Tiền Giang. In Vietnam, distributaries of the eastern (main, Mekong) branch include the Mỹ Tho River, the Ba Lai River, the Hàm Luông River, and the Cổ Chiên River.