I'm not sure where it is, but I think it is part of the dam/spillway. Churn may not be the best word for it. It really an airation system. The in the inlet and outlet ports may be well below the surface. My knowledge of this is really just a regurgitation of what a coworker explained to me as the reasons why some water supplies allow human contact and some don't. He's an avid sierra club member, and seemed to know what he was talking about, so I took what he was saying as truth. He said Lopez has a pump that airates and margarita doesn't. The airation may very well promote algae growth, hence the stain in Lopez and lack of stain in Margarita. This makes sense, because the purpose of the airation is to promote groth of things that will consume human waste, bacteria, etc. It seems like plants/algae would do that nicely.