Comprehensive survey and analysis of the headwaters of the Mekong River by use of GPS, GIS and SRS show that the source of the Mekong River is the Zha'aqu River which starts from the terminal of a small glacier on the Guozongmucha Mountain in Zaduo County, Qinghai Province The altitude of the source is 5244m, lying between 94°41′44″E and 33°42′31″N The Mekong River flows through six Asian countries and ranks as the 12 th largest river in the world From the source to its first convergence of bigger tributaries of Zha'aqu River and Zha'naqu River, Ganasongduo ( 94°36′40″E and 33°12′33″N ), the Zha' aqu River has a length of 101.1km, a drainage area of 2634 0km²and a discharge of 117.4m3 /s which was measured on July 12, 1999 From Zhanahuohuozhudi ( 93°52.926′E and 33°16.534′N ) to Ganasongduo, the Zha'naqu River has a length of 93 0km, a drainange area of 1999 3km²and a discharge of 32.3m³/s which was measured on July 12, 1999 At the joint ( Ganasongduo ), the current direction of the Zha'naqu River is from west to east and the Zha'aqu River is from north to south After jointing, the river is called Zaqu River and It's current direction is nearly from north to south The run of the Zha'aqu River is similar to that of the Zaqu From Ganasongduo to Changdu in Tibet Autonomous Region, the Zaqu River is 518 km in length Down from Changdu to the outlet of the river inside China, the river is called Lancangjiang River, which has a drainage area of 167487km², a length of 2161km and a mean annual discharge of 2180m³/s