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Paddling Articles In the Same Boat

River Rap—
The Dynamics of Moving Water

Rhythm and Tempo

By Tamia Nelson

September 25, 2001

Want to understand rivers? Just follow the water and listen to its music. We began the journey last month with "Theme and Variation," exploring the hydrologic cycle and looking at the many ways river networks carve their signature into the earth's crust, under the relentless impulse of gravity's baton. Now it's time to look (and listen) more closely.

All rivers make music, but no two rivers sing from the same musical score. Rivers are restless and willful. Only one bright thread joins them together—the quicksilver magic of water itself. But it's the shape of the land that calls the tune in each individual instance. Water flowing in a smooth and uniform channel behaves in straightforward, predictable ways, following rules laid down by the laws of physics. When water and landscape meet to form rivers, however, the result is always a complex harmony—and sometimes chaos.

You can read the score of a river on a map, or even from the air. For example, consider a river flowing down a relatively steep, wide, U-shaped valley. Chances are good that such a broad valley was gouged out by a glacier, and that the valley floor will be carpeted in glacial silts, sands, and cobbles. Each time the river floods, its waters will pluck sand and stones from the bed and set them down elsewhere. Open channels will suddenly be blocked, at the same time that new breaches are made in old barriers. Over the years, the river valley will be criss-crossed with intersecting channels. This is the hallmark of the braided stream or river. Its water is often silt-choked and cloudy. It dithers and darts, first in one direction and then in another. It's lively and quick, but also a bit hesitant and even irresolute—a bit like a J. S. Bach fugue.

Now look at a river confined to a steep, narrow V-shaped valley. There are no glacial sediments here: this valley follows a fissure in the earth's crust. It's a one-way, one-lane street in a town with tough cops. The water rushes unswervingly onward and downward, sometimes tumbling head over heels in its haste. The occasional pools don't slow it down much. It swirls and boils furiously before finding an exit. Such a river has a lot less sediment to work with than a braided stream. Its waters flow clear, but it flows fast and it keeps its channel free of most movable obstructions. It's like a Beethoven symphony, with a clearly-defined beginning, middle and end, and a resolute, unwavering theme.

Lastly, follow a river in a single meandering channel, bedded in a not-too-wide, not-to-steep valley. Cobbles armour the river bed in places, while sediments cloak the shallows whenever the gradient eases. Sometimes the river flows swiftly over smooth rock outcrops or shallow ledges, but it also lingers in deep pools. This river's been around a fair bit of time. It flows in stately procession, in a long, looping course. At each bend in the river, the swift-water channel hugs the outside, while sandbars form in the slow water on the opposite, inside bank. In the straight reaches between bends, slow-moving pools alternate with lively riffles and small, short rapids. In between riffles, this river's in no hurry. It likes to set for a spell and visit with its forested banks. Then, suddenly, as if remembering an overlooked appointment, it hurries on its way. In its varied rhythms—now slow, now sprightly—it reminds me of Telemann's Concerto in E minor.

Telemann, Beethoven, or Bach: there's a river for every taste, and paddlers who take the time to listen will find they can't help hearing the music.

Now let's take an even closer look at the score. Gravity sets the tempo. Whether a river's valley is steep or gentle, it always seeks the easiest and most direct path on its journey to the sea. But even in an unobstructed, straight reach, a river conceals hidden variations. Swim beneath the surface of a placid summer stream, and you'll find the current strongest just a little way below the surface, right in the middle of the channel. Dive even deeper, however, and you'll discover that the tug of the current almost disappears at the bottom. The same thing is true along the banks.

Now find an obstruction in the river—a mid-stream boulder, say, poking up above the surface. The river flows around it. It has no choice. The once-straight lines of the current spread apart, only to close together as soon as the obstruction has been left behind. Moreover, water now flows back upstream just behind the rock, striving to fill the "hole" in the river left by the temporary parting of the waters. If the river's current is slow, the result is a gentle eddy. But if the river is speeding along in flood, and particularly if the boulder is then completely submerged, the result is a "hole" in fact as well as name, often with a steep wave breaking upriver at the downstream edge

Mid-stream boulders aren't the only things to get in the way of a river's rush to the sea, of course. Wherever a ledge extends out into the channel from one bank, the river must either cascade over it or go around the end. When it goes around, the whole force and volume of the river is squeezed through the remaining gap, whether large or small. The river speeds up there, and the resulting tongue of water, or chute, can be both fast and turbulent. A pair of mid-river boulders can have the same effect, forcing much of a river's current through the narrow gap between them. The characteristic downstream-pointing "V" that identifies the resulting chute is one of the whitewater paddler's watermarks.

And what if a ledge extends all the way across a river, reaching right from one bank to the other? Then it has the same effect as a man-made dam. As the pool behind the dam continuously overflows, a river-wide upstream eddy—a reversal—forms below the ledge. If a reversal is powerful enough—the drop doesn't have to be very great if the volume of water flowing over it is sufficient, and the lip of the ledge is smooth—it can be deadly, holding any unlucky swimmer in a recirculating trap with no exit but a fluctuating downstream jet at the very bottom of the river.

Danger can be found even on placid rivers, however. As I mentioned earlier, a river's current is fastest on the outside of bends and loops. When the river is in flood, this current becomes a torrent, undercutting banks and toppling whole trees into the water. Then, as the water-level falls, these trees sometimes remain at the outside of bends, waiting to ensnare an unwary paddler or imprudent swimmer. Such traps, often called sweepers or snags, work with surprising efficiency even in summer's low-water flows. I've seen a paddler pinned helpless on the outside of a bend, caught in a tangle of birch in a trout-stream that moved at less than a half a mile an hour in mid-channel.

Make no mistake. Water level is very important. More than any other factor but one—the gradient, or steepness, of the bed—it determines a river's tempo. High water, fast tempo. Low water, slow tempo. A trickle that would be a lazy drift in August will be a wild and even dangerous ride in April. Easy eddies become "killer" holes, and prudent paddlers struggle to stay on the inside of bends, well away from undercut banks and deadly tangles. Runnable chutes develop powerful curling waves, or stoppers, at their downstream ends. Paradoxically, some reversals wash out completely, becoming all but invisible. Don't count on this, though! Scout every drop, especially in high water.

By now it should be obvious that a river's music follows a complex and ever-varying score, changing with the seasons and the topography of the valley. Grace-notes—channels, boulders, and sweepers—appear and disappear. Rhythm and tempo vary, from day to day and place to place, but the music is always there. Whether you prefer Beethoven or the Spice Girls, there's a river playing your music somewhere. Go and find it.

Copyright 2001 by Verloren Hoop Productions. All rights reserved.

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