Unless there was some way to make the water fall a ways, I don't know if they could keep the trash fish out of the BOW. I haven't seen evidence of carp or Gizzards jumping up a waterfall like Salmon.
I have. At a nearby lake the city built a series of wetlands to help clean the water before it entered the lake. The wetlands were supposed to be fish free, so they put a 2' drop from the wetlands to the lake. They aren't fish free. There are so many carp in them vegetation can't grow.
We seined one of the wetlands for a class I was in. One year after the wetlands were built the wetland closest to the lake contained common carp, bluegill, green sunfish, emerald shiner, largemouth bass, yellow perch, and yellow bullhead (listed in order from most abundant to least abundant). People may have moved some, but that is a lot of species to move. All of the fish seined were under 8" long, with most under 4", which were probably born in the wetland. A few adult carp were also observed, but not captured in the bag seine.
In the two wetlands further back from the lake (the three wetlands were in series with waterfalls between 1 and 2 feet in between) we found mostly carp with very few bluegills and GSF.
As part of the lab report I found a journal article on a study of green sunfish trying to move up a culvert. A grid of squares with bags to catch the sunfish was placed around a culvert that the GSF were trying to jump into. The culvert was something like 1.5' above the pool/stream the GSF were in. Many fish were caught in the bags nearest to the culvert. One GSF ended up in one of the bags about 3' above the water. Amazing.
The point is a waterfall would need to be taller than most people would think to prevent fish migration. If it were my pond, 5' would be where I would start.
