Rather than a "track", think of it like the Tomorrowland Speedway with a rail going down the middle. On the Speedway, the car is free-moving and has some range of motion, but the rail keeps it from going too far to either side.
The riverboat is just like that, but with a much more limited freedom of movement left or right. I also believe there's guides under both the front and the back of the riverboat, to keep both ends within that strict envelope of travel.
The Jungle Cruise boats are essentially the same thing, too.
-Rob