"Water is running in the stream."
Translation:Vesi juoksee purossa.
Hmm, I think you'd need a bit more context to justify the use of partitive, but you can certainly say "vettä juoksee purossa". Maybe you could continue the phrase with e.g. "tavallista vähemmän" (less/a smaller amount than usual). The use of partitive, to me, indicates that the amount of water in the stream is somehow significant (unusual). Usually subjects are in the nominative.