One way would be to install a large culvert for the water to flow through. Have a thick steel cap ready made so when construction is done just place the cap over the inlet end.
Could do the same thing with a pipe and valve, but large valves could be cost prohibitive and overkill for what you need to do.
You could even have a welder weld a nipple in place and a smaller valve so if you want to drain the pond some day. But valves tend to quit working after a few years, especially if not used. One reason I personally do not like valves in the bottom of a dam unless a person has a specific use for them and uses them somewhat regularly. Otherwise they are likely not going to work when you need them anyway.
