JamesW, The Hotpool we are refering to gets sand and mud in it when the stream get real high. It had a layer of sand in it to the level I was suggesting by natural causes. Various people cleaning with suction hoses caused the sand level in the pool to go down since the last significant flooding. The sand that could be put at the bottom of the pool to raise the level of its bottom would be sand from the stream bed, the same sand. This would not affect the flow of the hotpool, just raise its bottom maybe a foot or so. As you can see here all the regulars agree that clorine should not be used and as you say, it would be against the law. The USFS has said that we can maintain the pools that exist now, as they are. So if a little sand goes out from cleaning hoses, and a little got put back in, I seriously doubt that they would consider that a problem.