River Rap—
The Dynamics of Moving Water
Rhythm and Tempo
By Tamia Nelson
tamia@paddling.net
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.