Fluss vs. Strom
In German, both Fluss and Strom can be translated as river, but they are not interchangeable. The choice between them depends mainly on the size, scale, and significance of the body of water. The distinction is similar to the difference between river and major river or mighty river in English.
Fluss
A2Fluss is the general and most common German word for a river. It is the default term for any medium-to-large-sized river and is used when the massive scale is not the primary point of emphasis.
Die Isar ist ein Fluss, der durch München fließt.
(The Isar is a river that flows through Munich.)
Wir machten eine Kanutour auf einem kleinen Fluss in Brandenburg.
(We went on a canoe trip on a small river in Brandenburg.)
Viele Städte in Deutschland liegen an einem Fluss.
(Many cities in Germany are located on a river.)
Der Lachs schwimmt den Fluss hinauf, um zu laichen.
(The salmon swims up the river to spawn.)
Eine Brücke führt über den Fluss in die Altstadt.
(A bridge leads over the river into the old town.)
Strom
B1Strom designates a very large, major river, typically one that flows into a sea or an ocean. It emphasizes the river's immense size, power, and economic or geographical importance. This word also means electricity or current, but the context makes the intended meaning clear.
Die Donau ist der zweitlängste Strom in Europa.
(The Danube is the second-longest river in Europe.)
Der Rhein ist ein industriell stark genutzter Strom.
(The Rhine is a heavily used river for industrial purposes.)
Der Amazonas ist der wasserreichste Strom der Erde.
(The Amazon is the most voluminous river on Earth.)
Große Containerschiffe navigieren den mächtigen Strom.
(Large container ships navigate the mighty river.)
Der Nil ist ein legendärer Strom in Afrika.
(The Nile is a legendary river in Africa.)
Summary
In essence, the difference is one of scale and significance. Use Fluss as the general-purpose word for most rivers. Reserve Strom for the world's great and mighty rivers, like the Danube, Rhine, Nile, or Amazon, to emphasize their vast size and importance. If you are ever in doubt, Fluss is generally the safer and more common choice.