It's not on a track so much as there's a narrow trench in the bottom of the river flume. Each boat has a rudder (for lack of better term, I'm sure there's a real name for it) and sets of guide wheels that rest in the trench and keeps the boat moving along the ride path. This allows the boat to have a realistic feeling of actually floating, as the trench is wide enough that the boat can bob and move slightly left and right. However, each boat operates under its own power and the skipper controls the speed, just not where the boat can go.
I could be wrong on this, but I believe that it is possible for the boats to disengage from the trench and begin to float freely if the water level gets high enough.