Try with an env_steam.
Use a large number for start size and end size but keep them about the same.
Use a good roll speed like 30.
Make sure the flag "Emissive" is checked in the flags tab so that the color you choose in the steam properties will be used.
The length of the jet (lifetime) should be very long so that the large water particles stay alive long enough and look like they kinda rotate in place and keep all the "speed" and "spread" settings to a low number so that your particles don't fly too far from the location they spawn.
You might need to use more than one env_steam, but give them all the same name so that your trigger/logic_auto only needs to use 1 output to start them all.
Finally, make sure you turn it ON using a trigger or a logic_auto.
Try changing the properties until you got something that fits the size of the waterfall and the effect you want.
MQ