Not sure if this would work, but 1 tile thick water is considered a "stream" and wont spawn anything at all, idk if you can connect streams to ponds or if that turns them to rivers. If it is possible maybe add some flowers right below the waterfall and add stream tiles 1 away from the base of the waterfall. Pretty sure this wouldnt work but hey, worth a shot right?
It has to be small, not connect to another body of water, and have no waterfall. I find starting with a rough shape and then fishing to see what I get helps me determine quickly if I need to adjust the size.