Chris,
I'm not sure of the details of your pond, but here is what is common around here: The main spill tube with the anti-seep color exisits alone. Another, smaller diameter pipe goes through the dam with the valve on the back side. The stand pipe on the pond side exists to keep from sucking mud off the bottom when the valve is open, but the standpipe is usually not removable.
I see you're point with the possibilty of the valve being underwater in the future. I would encourage you to not make the stand pipe removable. You could potentially hurt yourself. Plus...what happens if you can't get it put back together just right? You could lose all of your water. You aren't going to want to be in the water trying to fit the pipe back together. You could get your hand or foot sucked off! Maybe the solution is to place the valve on the backside with a long handle (screw-type) that won't be under water in the event that you build another pond behind it. That way you could still operate the valve no matter the situation.
If the pond is fairly small, you could eliminate the withdrawal pipe altogther and just siphon the pond when needed. Start with a t-connection on top of the dam angling slighty to the back of the dam, with one part of the T facing up, one part facing to the back of the dam, and the last part facing toward the pond. Get a length of pipe, connect to the T and run it to the back of the dam and cap the end. Get another length of pipe, connect it to the T and run it into the pond. Now go back to the top of the dam and fill the top of the T with water until the back section of the pipe is completely full. Cap the top of the T and then pull the cap off the pipe you just filled with water. Note: the backside pipe needs to be longer (assuming you're using the same diameter of pipe) than the pond-side pipe or you won't create the siphon. Flexible, currogated pipe will work in small pond situations. The connections won't be perfectly water tight but that's where duck-tape comes in:) Smiley Face Notice: I have to use 90's technology ":)" because I'm too stupid to figure out how to use the smiley faces that everyone else here uses
If the pond is really tiny, you could just rent a trash pump.
