Mangde Chhu or Tongsa river flows in central Bhutan traversing roughly north-south. The river rises in Wangdue Phodrang district (or dzongkhag in Dzongkha), near Gangkhar Puensum, Bhutan's highest peak at 7,546 metres (24,757 ft). Bhutan's main east-west highway crosses the Mangde Chhu about 8 kilometres (5 mi) west of Trongsa. South of Trongsa, the river drains the eastern slopes of the Black Mountains (Bhutan), and forms the eastern boundaries of Black Mountain National Park and Royal Manas National Park. Another highway follows the river south from Trongsa to Shemgang. The highway leaves the river at Tingtinbi, and eventually reaches the Indian border at Gelegphug. South of Tingtinbi, the Mangde is joined by the Bumthang River from the east, and is known as the Tongasa.