Obstacles force too much water into too little space, so the water runs faster and laminar sheets break into individual ribbons of current. Then things get really complicated. If water runs into a boulder, a turbulent zone is created where the water and rock collide. The current runs faster around the boulder's edges, but behind the rock, it forms an area of backward-flowing water called an eddy. Shear zones between the eddy and the fast water can be strong enough to keep a raft from reaching the calm water.
Water crashing over a submerged ledge or rock becomes chaotic and creates a hole.
A hole creates a horizontal vortex underwater that actually rotates in an upstream direction. A rafter who falls into a hole is pushed back upstream against the ledge that created the hole, then driven down underwater. Often the only way out of a vortex is to dive to the bottom of the river, where some of the water crashing into the hole flows under the vortex. A rafter who gets into that deep current can follow it out of the hole and then resurface. In a large bucket, mix approximately 3 parts diatomaceous earth with 1 part fine sand and enough water to make a firm mixture.