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Whitewater Rafting

 

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Overview
Few sights reveal nature's power as clearly as rapids in a fast-flowing river. Water pounds against rocks, sprays into the sky, and boils into white froth. The thought of rafting through such turbulence is scary-unless you are an experienced river runner. Rapids look chaotic, but they are predictable. The volume of water, the steepness (or gradient) of the river, the width of the channel, and the obstacles in the water all have understandable effects on the rapids. Experts can read a stretch of rapids, spotting the hazards and seeing the safest way through. Knowing the amount of water flowing in a river is important because the river's speed increases as more water flows through it. Double the water means double the speed, so a mild rapid becomes a dangerous one during the rainy season. Rafters also must know the flow because water is heavy, weighing 1,000 kilograms per cubic meter (62 pounds per cubic foot), and in rapids it exerts tremendous pressure on a raft. 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.

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