Wow, and thanks everybody! This was an interesting round!

I can now admit that my picture was more or less the result of being in the wrong place at the wrong time with the wrong equipment ;-)

What I had wanted to do: Some "standard" long-exposure waterfalls.

What actually happened: The stream was completely flat, there weren't any waterfalls, and it was around noon with nary a cloud in sight, so my 6-stop ND wasn't really up to the task (can't remember whether I stuck the CPL on, too).

However at f/22 I did manage to get 1 second without over-exposing, the shot was obtained after pointing the lens straight down. The light trails are simply all the reflections of the sun on the surface of the stream, 100% natural!

I was quite surprised myself when I saw the result...