Anything that size in Texas during the CW would have been on the coast, probably at the mouth of a river to take out the ships offshore unloading troops to the smaller boats, probably row boats, to come up the river.
There were small paddle wheelers that traveled the Trinity, but they would be sitting ducks no matter what was used. Only the smallest of boats could turn around. Take one out and the rest would be dead in the water because they probably couldn't get around it.
Once in the river the small boats could be taken out with small artillery pieces which are quicker to load.
The more I've looked into it the more I believe it's not from the CW but was Mexican. Santa Anna had the army to move and man a piece this size.
It probably wasn't as heavy as the CW pieces since it wouldn't need to be used at long range, Texans didn't have but a few cannon to fire back with and they were small.
Wow. That would still be a big gun to drag all over the place. I do recall that a rather large Mexican cannon was used or was set to be used at the Alamo ... but that may be Disney flight of fancy.
Please post a picture of your find, with something to help with the scale. You may have a historically significant hunk of iron on your hands. How would one go about tracing the manufacture of such a projectile?