Three basic states of flowing water exist: laminar,
turbulent, and chaotic. Laminar describes the smooth-flowing currents in an
unobstructed river. Even these currents can be complicated, for their speeds
vary. Surface water is slowed by wind, while deep currents are slowed by
friction with the riverbed. Water in the middle, a few feet below the surface,
usually runs the fastest.
Turbulence occurs when obstacles, such as rocks or a sudden
narrowing of the river channel, obstruct the current's flow. 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.
