When filling a salt water pool, the salt was put in before the water. This resulted in a layer of highly dense salt water sitting at the bottom of the pool with less dense water floating on top. The leaves are slightly denser than the fresher water and sink in it, however are not as dense as the salt water and are therefore floating on top of the layer of salt water. As they are not lying on top of a solid surface, they float upright with the denser part of the leaf hanging downwards. I just noticed this as I was refilling my pool and thought it would be interested to share!