One of the perils of whitewater sports is getting stuck in what paddlers call a “hole” or a “hydraulic”. This river feature forms just downstream of large obstacles like rocks or low-level dams. As water pours over the obstacle and into its shadow, the flow forms a recirculating vortex-like zone. Immediately next to the obstacle, water is pulled upstream toward the obstacle and then down toward the bottom of the river. This makes the hydraulic very dangerous and hard to escape. Note in the video how the raft is held in place by the upstream motion of the water at the surface of the hydraulic. The rafters are preventing their craft from flipping over by weighing down the side experiencing the upward flow of the vortex. Escaping a hydraulic usually requires getting near its edge, where its current is weaker. If swimming, the best way to escape is to swim toward the bottom of the river and then downstream with the current of the hydraulic rather than against it at the surface.