What River would that be? Is it wide enough to handle small boat traffic? Could be a cannonball from a swivel cannon (maybe older than Civil War). I have one that the barrel would fit that plus have a cannonball the same size as yours.
You're pretty close to 'large canister' in both size and weight… considering the rarity of 32 pounders in the field, I'd bet it's a 24-pounder that slung it.
You've kind of got that backwards: canister is about quarter-sized in diameter and fired from field guns; grape is 1-2 inches and fired from naval guns. No artillery used anything like buckshot (although there were buck and ball loads used in smoothbore muskets).