Tropic, I've read and reread this post a few times and still am not sure what you are asking. By it's nature, a waterfall is above the pool (source) water, thereby syphoning is an impossibility. I also don't see how this relates to winterizing.
There is a fair chance that I've missed a fine point in what you posted, but I can't seem to make sense of your question .
Welcome to the forum and please let me/ us know a little more about what you're asking. Heaven only knows how many times I've posted what I believed to be a clear post only to reread it a day or so later and realised that I didn't express what I was trying to clearly