That would not be good where the weather freezes. I do have plastic gate valve in a pit for my first pond to keep water in the pipe from freezing, but have never used it and bet it would break if I did. Having the valve on the pond side also prevents any possible leaks in the drain pipe from mattering. I would just cap the pond side of the drain plug, but I want to be able to drain it without diving if the bottom turns out not to be sealed. There was a very thin sand lens between the red and grey clay layers that I hope I sealed up.
