"Dlaczego ta rzeka jest tak czysta?"

Translation: Why is this river so clean?

[SurrealSoul]

...is what corporations say before polluting rivers.

[wiktorka234]

Bo ktoś wyczyścił

[Walkinthedog]

I tried stream, a very commonly used word here for a small river. Is there a polish word for it that's different.

[Jellei]

Yes, "strumień".

[JanKLinde]

In German, a "Strom" is a very large river :D