hes saying the port nearest pool lets water flow both ways. Hes prob right hes the poolguy after all LOL. Just by pics it looks like water level in pool is lower than the drain on sunshelf so Im thinking its downhill but poolguy is prob right. Id still have a return in there other than the waterfall if it were me the original 4"depth doesnt make sense to me overall, but it would be great for little ones id think