Make a control sector and give one of the linedefs, a tag and the effect '121'. Make sure the celling and floor heights are how you want them. Then give the sector of your choice, the same tag. Hope this helps!
Are you just using one tag? If so, that may be your problem.
I suggest giving Linedef Type 120 and 121 a shot. There are also some example wads and setup tutorials over on the SRB2 Wiki too.
If you want, I can make it for you, I guess. Just PM me the map and I'll add the waterfall.
This is, uh, really not a point where anyone should be relying on someone else to do it for them. This is basics of the basics stuff here.If you want, I can make it for you, I guess. Just PM me the map and I'll add the waterfall.