I notice several of them clearly have water vegetation the way a pond or stream that's safe to swim in has. I'm guessing they act as natural filters the way they do in the wild. Plus, only those of us in Northern climes would, I guess, match the European ponds and streams, but .... you couldn't tell this winter. There is a house near where I grew up that had a natural swimming pool...a stream ran into a pond and from the pond into the pool. But by the early 1960's the pool had been long ago closed and was full of the usual detritus--branches, tree limbs, leaves. I guess it was from early in the 20th century. Satellite photos show that it was filled in, but I haven't been by there in a couple of years...and haven't looked at the property since the early 70's. I have no idea if it actually worked as a pool when it was new.