This little stream was right outside the cabin in which my family stayed during our Spring Break vacation to Gatinburg/Smoky Mountains last week.
I've been wanting to try this common slow shutter effect with moving water, but haven't had the opportunity until this trip.
I visited this location just about every night we were there and didn't get these until the last night we were there.
Mainly, it was a matter of getting out at the right time of evening -- when there was enough twilight left in the sky to provide the right amount of light. Not too much when I couldn't drag the shutter enough for the 'slow water' effect; and not too little when I was depending almost entirely on erratic and distant street lamps for light.