Danube

Growing up on the second longest river in Europe is not a big deal, until you realize that this river was the long-standing frontier of the Roman Empire against the barbaric northern tribes. My hometown Ingolstadt, where I was born and raised is ~30km west of the northern most point of Regensburg, before it meanders southeast into the black sea. It connects 10 European countries: Germany, Ukraine, Hungary, Romania, Austria, Serbia, Bulgaria, Slovakia and Croatia. No wonder, it is 2850km long and – as mentioned above – the second longest river in Europe after the Volga in Russia. The biggest cities along the river are

Vienna (1.9 million inhabitants)
Budapest (1.7m)
Belgrade (1.1m)
Bratislava (475k)
Novi Sad (277k)
Galati (249k)
Linz (204k)
Braila (180k)
Regensburg (152k)
Ruse (145k)
Ingolstadt (136k)
Ulm (126k)

The Danube originates in Germany and is heavily used for transportation of goods, mainly from Regensburg to the Black Sea. It is a vital source of water for more than 20 million people and has been the route for emperors and kings for centuries. River cruises are a newer addition to the river and gaining in popularity because so many different cultures and traditions live along the way.