How long is the Blue Danube? How many countries are there?

Danube is a famous international river, and it is the largest river that flows through the country in the world. It originated in the eastern foot of Heilin Mountain, 679 meters above sea level in southwest Germany. It flows through Austria, Czech Republic, Slovakia, Hungary, Croatia, the former Yugoslavia, Bulgaria, Romania and Ukraine from west to east, and then flows into the Black Sea. With a total length of 2,860 kilometers, the Danube is the second longest river in Europe. The Danube meanders like a blue ribbon across Europe. There are more than 300 tributaries along the Danube River, with a drainage area of 810.7 million square kilometers, which is larger than the Yellow River in China. The Danube has an average annual flow of 6,430 cubic meters per second, and the amount of water entering the sea is 203 cubic kilometers. From the source to Vienna is the upper reaches of the Danube, where the Danube receives several tributaries of the melting snow in the Alps, and the water volume is greatly increased. Here, the scenery is beautiful and idyllic. From Vienna to Tiemen is the middle reaches of the river, where the river valley is generally wide, the riverbed slope is not big, the river channel is curved and there are many tributaries. In the Carpathian Mountains east of Belgrade, a canyon with a total length of1.30km has been formed, including Kasang Canyon and iron gate, which is rich in hydraulic resources. From below the iron gate to the estuary is the lower reaches of the river. The lower reaches of the river are wide and the water flow is stable, reaching 15 to 28 kilometers when approaching the estuary, and the shipping industry is very developed. The Danube Delta is rich in reeds (reeds are good raw materials for paper making). There are also a lot of birds and strange floating islands here, so scientists call it "the largest geological and biological laboratory in Europe". There are many beautiful cities on both sides of the Danube. They are like bright pearls embedded in this blue ribbon.