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Chapter 17: Streams and Floods: The Geology of Running Water
Feature Articles
The Human Angle: Whitewater Kayaking
by Stephen MarshakPerhaps, 7000 year ago, kayaks—boats with a sealed top, tapered at each end—became the principal means by which fishermen navigated the icy waters along the coast of what is now Alaska. If a kayak started to capsize, the kayaker executed a maneuver popularly called the "Eskimo roll": with carefully time sweeps of a double-ended paddle, she pivoted the boat through 360 (since their tops are sealed, kayaks can't fill with water) until it once again became upright.
Kayaking as a sport first developed just before World War I, and by the 1930s it had become popular worldwide. Sport kayakers fall into two groups: touring kayakers, who cruise long distances along coasts or down quiet rivers, and whitewater kayakers, who join rafters in challenging the rush of gravity in the rapids of a fast-moving stream. Whitewater kayaking requires not only skill and strength, but also profound knowledge or a river. Kayakers spend hours studying a challenging reach of a stream, looking for a path that steer clear of deadly boulders or falls.
Just as geologists use geology jargon to refer to parts of a river, kayakers use their own jargon when describing rapids. For example, riffles are small rapids above a shallow gravel bar; an eddy is a protected stretch in which water circles lazily; a cushion is the bulge or water that builds up over a submerged obstacle; a ledge is a row of boulders across which a small waterfall develops; a chute is a narrow channel between obstacles; and a hole is the depression on the downstream side of a boulder into which water plunges and overturns.
Depending on the violence of the flow, the size of holes, the vertical drop across ledges, and the difficulty in finding chutes, whitewater kayakers classify rapids from class I (easiest) to class V (virtually impossible to navigate). The classification depends on the size of the obstacles relative to the depth of the stream, the overall stream gradient, and the discharge. As basic strategy, river runner aim for chutes, try to avoid going over the top of a cushion (they prefer to slide off the side), and steer clear of holes. They aim for eddies whenever they need a rest.
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