This question's answer largely depends on how you want to convey the river.

Words I might use if..
it's flowing beautifully like a dance: sarhhtha
it's flowing rapidly: hsshran
it's slow and calm: em
it's flowing quickly such that there is danger: isthu'ratha'a

What do you call the river itself?
Is it a large river? - shh'oi heke
Is it small? - pi'heke
Is it a long river? - neh'heke

Each of these would change the translation because of how S'Kra is set up. My best suggestion here would be to capture what you want to convey most about the river and then make your words.

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NaOH+HI

You do enjoy making this difficult. :p

The best I could come up on my own is close to:

pi'neh mhhg'heke hhshranzh

Broken down, you're looking at:
Pi being little, neh being long. This gives the river a long but small size.
Heke being the water, modified by mhhg to give it the winding of a river (like a serpent.)
Hhshran gives it swiftness and the zh negates it. It's not perfect, but it can imply a moderateness since rivers don't stand still.

This is a very detailed way of saying it, but it covers what you're asking for.

Hope this helps out, but in the future, you might want to check out elanthipedia. All of the translations I've done so far have come directly from it. :)

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NaOH+HI