Yes, it's an Alligator Snapping Turtle. Your multiple photos make it much easier to be sure. The three distinct keels on the back and, in the last photo, the noticeably pointed snout are good field I.D. marks, and the size, of course. Where was it and how big do you think it was? They are generally found in the Mississippi River drainage system, which covers a lot of territory, and some other "Southern" rivers.