Your #1 source for kayaking and canoeing information.               FREE Newsletter!
my Profile
Paddling Articles In the Same Boat

River Rap—
The Dynamics of Moving Water

Theme and Variation

By Tamia Nelson
tamia@paddling.net

August 21, 2001

riverrun, past Eve and Adam's, 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.

















Sponsored Ad:
NRS
Follow us on:
Free Newsletter | About Us | Site Map | Advertising Info | Contact Us

©2014 Paddling.net Inc.
Sweepstakes Banjo Shirt