Up the Creek
(1984)

Bob McGraw (Tim Matheson) and his team of slob students from Lobotomy U manage to get the win over the preppy snobs in the raft race.  Turns out that the renegade military team rigged some explosives, and that results in the river flowing in a completely different direction, allowing Bob's team to jump ahead of them and score the win.  His teammates jump out to celebrate, but Bob stays in the boat with Heather (Jennifer Runyon) for their own private celebration.  However, the celebration's short-lived, as the raft is about to head over a waterfall.  Bob's faithful mutt (Mike the Dog) can't watch.
  

Thanks Robert W.