I strolled along the Danube, the longest river in the European Union (2850 km) today. Well, not all of those kilometres ... It was very hot, the heat softened by a light breeze. Pleasant.
The river originates in Germany's Black Forest, passes through ten countries (Germany, Austria, Slovakia, Hungary, Croatia, Serbia, Romania, Bulgaria, Moldova and Ukraine).
The Danube basin was the site of some of the earliest human cultures such as the Linear Pottery Culture (5500–4500 BC). The river was part of the Roman empire's Limes Germanicus. The Romans often used the river Danube as a border for their empire.
Here's more on the river.
My favorite Danube books are Claudio Magris'" Danube - A sentimental journey from the source to the Black Sea, a fascinating mixture of history, literature, philosophy and travelogue, and Inge Morath's photo book Danube.
Everybody knows the Waltz The Blue Danube by Johann Strauss II. The Danube is seldom blue, more often greyish or even brownish, by the way.
Here's an interpretation by Faryl Smith, called "The River of Light".
The German poet Friedrich Hoelderlin called the Danube "a refreshing, melodious river, sometimes foaming with high spirits, at other times dreaming serenely.”