River Rap—
The Dynamics of Moving Water
Theme and Variation
By Tamia Nelson
tamia@paddling.net
August 21, 2001
Riverrun, past Eve and Adam, from
swerve of shore to bend of bay….
James Joyce got it about right. Rivers run through all of human history,
their hitherandthithering waters alternately setting limits and breaching
boundaries. Early on, we slaked our thirst at rivers, netted fish in them,
and killed animals at their fords and crossing places. Later, we learned to
grow cereals in their rich flood-plain soils. And not long after that, we
diverted their waters to quicken crops in distant fields.
Throughout the ebb and flow of history, rivers have moved our goods to
market and ferried raiding armies to the gates of great cities. Explorers
have followed rivers into the heartlands of unknown continents, while
engineers have dammed them for power and flood control, and then used them
to carry our wastes away downstream, out of sight and mind—for a little
while, at least.
None of this has changed. Rivers are still reservoirs and highways,
larders and sewers. But now we play in them, too. And while there aren't any
more rivers today than there were in our ancestors' time, there are far more
of us.
If you're a paddler, it's not likely that you'll be deaf to the music of
flowing water. I first heard its song when I was a young girl, growing up in
a village in rural New York, not far from a fetid creek known locally as
"Sewer Brook." (It has another name on the map, of course, but farm towns
didn't have waste-water treatment facilities in those days. In fact, the
town I grew up in still doesn't have one.)
Stagnant and stinking though it was, there was something mysterious and
evocative about Sewer Brook. Wads of toilet paper moved past my eyes in
stately procession, coming from places unknown and then disappearing around
a bend, headed for destinations equally secret and evocative. As a girl, I
longed to see the headwaters of this little, reeking Nile. I doubt that
Burton or Speke ever felt a more passionate curiosity about the Mountains of
the Moon than I did about the source of Sewer Brook. To me, then as now,
rivers—even tiny rills like Sewer Brook—were sources of wonder. I
longed to explore them.
And so I did, beginning with Sewer Brook. It didn't disappoint, but
tracing its course was smelly work, and its music was sadly muted. Some
years later—I was in my early teens by then—I spent time at my grandfather's
Adirondack camp. While I was there, I got to know "his" river: a
strong, brown god of a river, stained dark by tannins and swollen by
mountain torrents. Where Sewer Brook oozed over muck and trickled through
mires, my grandfather's river hammered its way through rock-clefts and
cataracts. In spring, its floodwaters heaved boulders from their places on
the bed. In summer, thunderstorm-fed freshets plowed new channels through
its many gravel bars, and rattled cobbles in plunge-pools beneath smooth,
descending tongues of water at each of a myriad of drops.
I loved spending time along (and on) my grandfather's river, listening to
its varied chorus, but I couldn't get there often. So I rode my Sears bike
to a fabled
trout stream not far from my home. Where the music of my grandfather's
river was a brooding and powerful symphony of many moods and movements, the
song of the 'Kill was a clear and lilting air—a fit companion for the
mottled-olive, pink, and yellow native brookies who called it home.
That was years ago. I'm much older now, but I still can't pass moving
water without stopping to hear the music and follow the current's weaving
dance. And I've learned something about river dynamics along the way. The
story begins with a drop of water.
Water is the soul of a river. Under the relentless tug of gravity, water
flows downslope. In flowing, it makes rivers. No water, no river. And where
does the water come from? The obvious answer—rain (or snow)—only
begs the question. Where does the rain come from? From water, of course.
Not satisfied? I'm not surprised, but such circularity is one of the
hallmarks of natural systems. Do you want to understand rivers? Then begin
at the beginning. Follow the water.
The earth has only so much water, and most of that water is salt. Seas
cover three-quarters of the earth's surface. The sun heats the seas. Water
evaporates, leaving its burden of salt behind and creating masses of humid
air. This air rises—or is pushed up— and some of the water vapor
condenses. Clouds form, and many are eventually blown over land, driven by
prevailing winds. The water droplets making up the clouds collide, absorbing
others or being absorbed themselves, until they grow so large that they
begin to fall. Much of the resulting rain seeps into the soil, recharging
underground aquifers. Some falls as snow, instead, and some of that is
trapped in ice-sheets. But the rest runs off into lakes and ponds, creating
rivulets and streams. The rain-swollen basins then spill over, and their
outlets are the headwaters of great rivers, returning water to the sea from
which it came. Meteorologists and geologists call this the "hydrologic
cycle," and it's the engine that drives all the world's flowing waters.
If you want to understand rivers, you have to follow the water.
Rivers range from rills too small to float a pack canoe to the great
Amazon and Nile. From largest to smallest, each river's flow varies from
year to year, and even from day to day. Downpours and spring melt-water send
small creeks raging over their banks and turn dry canyons into deadly
torrents. Droughts, on the other hand, slow even mighty rivers down. In
extreme cases, rivers can dry up completely. The water budget of a river is
like an investment portfolio in a roller-coaster market. Diversification is
good. A river originating in a single source is vulnerable. If that source
dries up, so does the river. But a river with multiple sources
—seasonal snowfields, swamps, and many headwater ponds, say—can
ride out the ups and downs in all but the worst years.
Swamps are particularly valuable. Think of them as nature's hedge funds.
When it rains for forty days and forty nights, swamps act as sponges,
soaking up water and reducing the likelihood of catastrophic flooding. Then,
when a drought parches the land, these same swamps slowly release their
stored water, prolonging stream flow and maintaining the web of life
dependent on it. It's too bad that swamps get in the way of human plans so
often. Every acre of swamp lost to development or "flood control"—nice
irony, that!—leaves us all a little more vulnerable to floods and
droughts, to say nothing of the loss in wildlife habitat.
OK. We have water and rain and headwater lakes. We have our river, in
short. What happens next?
It changes. Remember Heraclitus? He wasn't very optimistic about the
human prospect, but he had a keen eye. A student of his quoted him as
saying, "You can't step twice in the same river." And he was right. No
sooner does a river start to flow, than it starts to change. There's more to
a river than moving water. Rivers carry sediment—fine, and some
not-so-fine, particles of clay and sand. Fast rivers can carry more (and
larger) particles than slow rivers, but almost all rivers carry some. When a
river slows down, it drops part of its load. If this happens at its mouth,
the result is a delta. If it happens in the channel further upstream, it's a
gravel bar, beach, or sand bar, or even a dune. (Yes, river beds haves
dunes. Think of them as giant ripple marks.)
Where does the sediment come from? Some of it washes into the river. Some
of it is plucked out of the bed when the river speeds up—when it
steepens or when it's in flood. And some of it is ground out of bedrock.
Rivers are cutting engines as well as highways. All the sediment they carry
has the force of the river behind it, after all. Rivers cut their way down
into the country through which they flow. In time, rivers can grind their
way right through a mountain range. There's just such a water-gap on the
Battenkill at the New York-Vermont line. Once a mountain barrier stood
unbroken there. Today, however, a river runs through it. Looking at the
Battenkill as it winds eastward in the shadow of the green hills, it's hard
to believe that such a small, clear stream could do so much work. But it
did. Rivers are patient craftsmen. A thousand years is nothing to them.
Unlike us, they've got all the time in the world.
Let's recap. Rivers carry abrasive sediments. They pick up material in
one place and deposit it in another. When a river slows down, it drops part
of its load. And that's where dams come in. One way to slow a river down is
to build a dam across it. What happens then? Sediment collects behind the
dam. Anything else? Yes. The river below the dam is now comparatively
"clean." It looks good, to be sure—sparkling, bright, and
clear—but its saw has lost its edge. It can't cut as well as it used
to. Worse yet, it's got nothing to rebuild its bars and beaches. So while
each spring flood washes away a bit more of the scenery (and wildlife
habitat), there's nothing to replace it. When we started to dam the
Colorado, whole stretches lost the red color that gave the river its name.
That color came from the sediments it carried. Now those sediments settle
out above the dams. No sediments, no beaches. No sediments, no sand bars. In
time, what's left of this great waterway will be more like an amusement-park
flume than a river. Goodbye, Colorado. Hello, Floyd's Wonderful WaterFun
World. Welcome to the future!
That's an extreme case, of course. Most rivers still "carry and cut,"
particularly when they're in flood. And however inconvenient floods may be
for us humans, they're part of the annual cycle of any living river. The
first human civilizations grew up on the flood plains of great rivers,
nourished by soils that were renewed and replenished by each year's
inundation. Today we restore our croplands with fertilizers produced using
power generated by hydroelectric dams on those same rivers. Progress? Yes.
But at a price.
The world's rivers remain, however. They flow through both time and
space, after all, and even the biggest dam is just a momentary impediment.
Look at any atlas. Pick a river. Follow it for its entire length, tracing
the main channel and all its tributaries. Small streams feed larger streams,
and these larger streams flow together into rivers. All these tributary
rivers then join into one river. Further along, the main channel may divide.
Still further downstream, the separated channels will rejoin, and then
perhaps divide again, only to rejoin once more. The river may also scribe
lazy oxbows across a broad flood plain, emptying at last into the sea (or a
big lake). Shaped by the grain of land over which it flows, the river in
turn resculpts the land which shapes it. The process can be as gentle as a
rain of silt, or as violent as a wall of water with a volume equal to 200
times the flow of the Amazon River.
Two hundred times the flow of the Amazon? Does that sound like something
out of Jurassic Park? It's not. And it happened only
yesterday—geologically speaking—right in North America. Aerial
photos of eastern Washington state show a landscape that looks just like the
remnants of the now-dry bed of a gigantic river. Guess what? It is. At the
end of the Pleistocene, the retreat of the continental ice-sheet left a huge
ice-dammed lake near Clark Fork, on the present-day border of Idaho and
Montana. One day a small crack appeared in the crystal dam. Shortly
thereafter, the dam gave way completely. The huge lake emptied itself in
three days' time, leaving behind a landscape of pothole, channel, and
sandbar, all on a gigantic scale.
Scale. That's important. Seen in an aerial photo, this water-scoured
landscape—it's known today as the Channeled Scablands—looks
exactly like the channel left behind by a runoff stream in a cow pasture.
Apart from the scale, the remnants of the catastrophic melt-water flood and
the traces of a commonplace rivulet could be different parts of the same
river-bed. Mathematicians call this "self-similarity over scale changes,"
and it's a clue to the nature of stream flow.
Follow the water. There's no better advice for anyone who wants to
understand rivers. And in the next month's "River Rap" I'll do just that, as
I take a closer look at the dynamics of moving water on a smaller scale, in
an article titled "Rhythm and Tempo."
Copyright © 2001 by Verloren Hoop Productions. All rights
reserved.