I don't remember seeing a particle object already running when I loaded a scene in C2.
If I was to make a waterfall that was already turned on, I would have done this.
1. Create a new scene with a waterfall and run it.
2. Video capture the waterfall and remove the background, if needed.
3. Export the image as PNG and bring it into C2 as animation.
Then I would either..
4. Arrange the scene so that the waterfall's animation will run for a few seconds before particles take over.
5. Use that PNG animation without a particle object in game.
Another method I could think of is this
1. Create a cut scene in a layer above the waterfall let the particle waterfall run in the background first.
2. Remove the cut scene after a few seconds.