The main stream of the Mekong River is 4,908 kilometers long and is the most important transnational river system in Asia. It is the ninth longest river in the world and the seventh longest river in Asia. The main source is Zaqu, which originates in Zaduo County, Yushu Tibetan Autonomous Prefecture, Qinghai Province, China. It flows through Yunnan Province, Laos, Myanmar, Thailand, Cambodia and Vietnam, and flows into the South China Sea in the province south of Ho Chi Minh City, Vietnam. The Amazon River flows through Peru, Ecuador, Colombia, Venezuela, Guyana, Suriname, Bolivia and Brazil, and finally flows into the Atlantic Ocean in Brazil. The Ural River is the boundary river between Asia and Europe. It originates from the southern mountains of the Ural Mountains in Russia. The upper source flows south along the eastern foot of the South Ural Mountains, passing through heavy industrial cities such as Magnitogorsk along the way, and turns west to Orsk. Below Orenburg, after the largest tributary of Na, the Sakmara River, it continues to flow west into Kazakhstan. It turns south again at Uralsk and flows into the Caspian Sea at the river mouth of Guryev. The Danube is the second longest river in Europe after the Volga River. It originates in southern Germany and flows from west to east, passing through Austria, Slovakia, Hungary, Croatia, Serbia, Bulgaria, Romania, Moldova, Ukraine, and finally flows into the Black Sea.