Dude, it's a waterfall because according to Wikipedia, a waterfall is a type of gravity bong and since it involves water, therefore it is a waterfall. Maybe it's a regional thing on who calls what. So we're both right in this case as it is both. Most importantly, it works. Here is an excerpt from www.concept420.com which also proves my point. This is exactly what I made.

How to make a Waterfall Bong

This is essentially a variant on the gravity. You take a bottle (I use a
2-litre) and drill a small hole (about 3/8") in the bottom, at the lowest
point. Cover this hole with your finger and fill the bottle up. Then, attach a filled bowl to the top (I use the same Coke-bottle lid as the gravity) and light the dope. Uncover the hole out the bottom, and as the water drains out, the smoke will be drawn in. Keep the bowl lit and let the water drain out, and by the time you're done you have a 2-litre bottle full of concentrated smoke. Just suck from the top and uncover the hole at the bottom to hoot. This is also an extremely efficient design, as very little smoke can escape.