Hey Fishfirst, nice to see someone from Wisconsin!
I'm from Milwaukee myself.
Well, the filter in question has a continuous suction rate (not sure how many gallons per hour) that is pretty good. It's a relatively new Whisper 20, and it seems like it's going to be pretty efficient over the years. The filter just picks up the water with the intake tube, then the water gets filtered through a spongy filter cartridge, then drops back into the tank. There will be a stream (constructed from pond liner, just to keep drainage into the gravel at a minimum) underneath the overflow, and I'll make sure it catches all the water. Hopefully, that will simulate a current down the stream (and eventually over the edge into the waterfall), but even if there isn't a current the water will naturally go over the edge (since there's nothing holding it back). It's just a question of whether it will look like a good waterfall due to the strength of the current from the filter - it may just dribble down the edge. If it does, it's not a big deal, it just won't look as good. I've taken the liberty to draw another badly done diagram, this time focusing on the stream & waterfall: