Danube
The Danube flows through many cities, including four national capitals (shown below in bold), more than any other river in the world. Ordered from the source to the mouth they are:
The Danube is navigable by ocean ships from the Black Sea to Brăila in Romania and by river ships to Kelheim, Bavaria, Germany; smaller craft can navigate further upstream to Ulm, Württemberg, Germany. About 60 of its tributaries are also navigable.
Internationalization of the Danube River
In 1616 an Austro-Turkish treaty was signed in Belgrade wherein the Austrians were granted the right to navigate the middle and lower Danube, at that time under the control of the Ottoman Empire. Under the 1774 Treaty of Küçük Kaynarca, ending a war between Russia and Turkey, Russia was allowed to use the lower Danube.