the ë (inconsistency)

There's an old man called the Mississippi, that's the old man that I'd like to be! What does he care if the world's got troubles? What does he care if the land ain't free? Old man river, that old man river, he must know something but don't say nothing. He just keeps rolling, he keeps on rolling along. He don't plant taters, he don't plant cotton, and them that plants 'em is soon forgotten. But old man river, he just keeps rolling along.