Looks like a hand-operated valve, sluice, or something like that. Probably currently open, given the amount of the central shaft sticking out. Probably hasn't been closed in a long time, given the rust on the central shaft's threads. Probably something moderately big or heavy-ish, given the length of the operating arm.
All of the above is just a bunch of guesswork. Some context might help, or some additional angles/views of it.