Avon Calling!

Debriefing a Rapids

By Tamia Nelson
tamia@paddling.net

January 8, 2008

A Note to the Reader: Throughout this article you'll find clickable thumbnail images. To get the big picture, just hold your cursor over the thumbnail and right-click, then select "Open Link in New Window."

Most paddlers carry away memories from every river that they've ever dipped a blade in, but some rivers carve deeper channels in our consciousness than others. Something about them compels us to go back, if only in our mind's eye. Perhaps they delighted us on first acquaintance, or frightened us, or simply puzzled us. As luck would have it, this happened to one of our readers, who was inspired by "Early and Provident Fear" to describe a river trip he took some years back. He thought he knew the Shenandoah as well as any man could, yet what he saw when he rounded one bend surprised the life out of him. But why not let Dan tell the story in his own words, with a minimum of editorial tweaks?

The Waterway  The Shenandoah River between the VA Route 7 crossing east of Berrysville, Virginia, and the WV Route 9 crossing east of Charles Town, West Virginia.

The Float Plan  A two-day fishing trip with four paddlers in two canoes.

The Trip Report  The two-day adventure was everything we expected it would be, and more. Just can't say enough for all that wilderness feeling! During the second day I could tell we were nearing the take-out because what I could see downstream looked very familiar — the distant view was the uppermost area we had waded when fishing at other times.

Nowhere between where we were and where we'd waded previously looked even remotely dangerous, though we always used caution. But then we entered a long, deep, still water just above a small ledge drop with a number of rushes to shoot and a light "minefield" of rough water afterwards. I decided to slow my forward progress to get a better look. This was the time to do it because the current was weak. I found it was no problem to paddle in complete control above the drop, and we could turn upstream again at will.

As we paddled along the ledge, my eyes caught sight of large rooster tails over on the far left. I was heading over to those rooster tails to have a closer look, all the while checking my ability to turn back upstream if I needed to. I never got as close to the opposite bank as I would have liked, though, because all of a sudden a bell began clanging on a cliff high above the river. It didn't let up until I turned back — someone was sounding a warning as if the end of time was at hand!

When we got back over to the right side of the river, I could see a house on the left, overlooking the river and set a little way back from the cliff edge. The lady ringing the bell (I could see her now) must have known about those rooster tails, and she also knew that I didn't know what I was getting into.

The Question  Those rooster tails turned out to be six or seven feet high, and they extended for 100 or more feet downriver. How does a rapid like that show up in a river like this, when nowhere else in the entire 140 miles is there anything like it? The water was at normal stage, and the mighty Shenandoah has been known to crest 25 feet and more when in flood.

It's a perennial question: What's a rapid like that doing on a river like this? Or, to put it another way, what makes a benign, quick-water river suddenly turn ugly? Well, I couldn't draw on any local knowledge to answer Dan's question. I've never paddled the Shenandoah. So I fell back on general principles. That is to say, I made an educated guess. And I guessed that Dan's "rooster tails" were reaction waves. These waves can form whenever a river "stubs its toe." Maybe it's nothing more than a sudden lessening in the gradient, as a relatively steep reach of the river abruptly levels out. Or maybe the banks start closing in, narrowing the channel. Or maybe a cliff has sloughed into the river, littering the smooth bottom with big boulders. Whatever the cause, the river suddenly finds itself with less elbow room, and the water heaps up. The result? Reaction waves. They're a little bit like the folds that appear in a carpet when you shove it against the wall. In any case, Dan's letter mentioned both a cliff and a ledge, so it seemed likely that his rooster tails were in fact reaction waves (aka "haystacks" or "standing waves," the latter name serving to distinguish them from the wind-generated, traveling waves on open water).

Dan's letter also hints at the role played by water level in the formation of rapids. And as he noted, the Shenandoah was well within its banks during his trip. It was not in flood. So how come there were big waves on this otherwise placid reach of river? Good question. The answer's a bit counterintuitive, though. It turns out that floods frequently "drown out" rapids. As flood waters swell a river, it spills over its banks and the constrictions that give rise to reaction waves open up. At the same time, obstructions (like big boulders) are buried further and further below the surface. That's why rock gardens and ledges sometimes disappear when rivers are in flood. This doesn't mean that running rivers in flood is safe, however. It's not. Strainers and sweepers are encountered far more often when a river overflows its banks, and the power of the moving water rapidly becomes irresistible as the river rises. Water is very heavy, after all. And when something that heavy moves fast, watch out! But low water has its hazards, too. There's no guarantee that small ledges can be run safely when water levels drop, and weirs (low dams) are often at their deadliest during early summer. The reversal that forms at the base of the dam during low water is plenty strong enough to trap a boater, but there's not enough flow to wash the unluckily paddler out. The result? A deadly one-two punch.

This was as far as I could go with the information I had, but Dan wrote more later, and his second letter added to the picture he'd drawn in the first:

In the years since paddling that stretch of the Shenandoah, I've tried several times to examine it using MapQuest's aerial view, but it just doesn't look like what I saw from the river. There's no doubt in my mind that I found the exact stretch of river on MapQuest, but the aerial view doesn't really show the vertical rock wall well at all. It does show the house, and if you know what you're looking at you can see the rock outcrop where the lady stood to ring the bell.

This got me thinking. Dan's second letter reminded me just how useful aerial photos can be to paddlers. But it also illustrated some of their limitations. Aerial photography flattens out topography. Cliffs seem to disappear. And you can't easily tell what the water level was at the time the photo was made, either. You've got to check the date, too. Rivers are always changing, and a photo taken in the spring of last year may not show the aftermath of this year's floods (or last summer's riverfront development).

Having said that, Dan's description of the cliff and ledge posed an interesting challenge, and I decided to see what I could learn by studying the aerial photo myself, along with the relevant topographic map. The Internet made this easy. I began at TerraServer-USA — unlike many map servers, it's accessible even to folks with dial-up modems — where I quickly found Charles Town, West Virginia. Then I followed the Shenandoah upstream from the WV Route 9 crossing to see if I could identify Dan's rapids. And sure enough, I found them, on the USGS Round Hill (VA, WV) 1:50,000 quad. The reach of the river that Dan described is located along a meander called Avon Bend, near Shannondale Springs State Wildlife Management Area.

If you want to see what I saw, just right-click the thumbnail of the quad to open a larger image in a new window. The contour interval is 20 feet, so it looks like the left bank of the river rises 60-80 feet above the water near the first marked rapid, shown by blue hatch marks. The river is about 200 yards wide at this point, and it flows from west to east. A lone house sits atop the cliff, on a bluff lying west of an unnamed creek and just upstream of the first marked rapid. This might well be the home of Dan's bell-ringing guardian angel. The terrain to the west is characterized by gentle hills, while to the east the land rises abruptly. Once I was satisfied that I'd found the right quad, I clicked the tab to bring up the corresponding aerial photo, zooming in to show more detail. (The area covered by the photo is indicated by the black square superimposed over the quad above.) You can do the same. Just right-click on the thumbnail to the left. Now compare the topographic map and the photo — but remember that they don't share the same scale.

What does your inspection reveal? To begin with, note the trio of ledges stretching across the river, just where they're shown on the quad. (They look like white lines in the aerial photo.) Since these coincide with the change in terrain from rolling hills to steep upland, it's reasonable to conclude that the ledges reflect a corresponding change in the character of the underlying bedrock. I've also marked the probable site of the warning bell. But where is the cliff? Look carefully. Can you see it? The red arrows show the location, but the photo gives few hints about the steepness of the land. There's a lesson to be learned here: Unless you can get hold of a stereoscope (and the relevant stereo-pair photographs), never rely on aerial photos alone to give you a feel for unfamiliar terrain. Always consult the corresponding topographic quad. A picture may indeed be worth a thousand words, but you still need a map to read between the lines. That's a key point in understanding any discourse of rivers.

 

OK. Our story's nearly over. Thanks to Dan's good memory, the resources of the United States Geological Survey, and the helpful folks at TerraServer-USA, we've been able to explain the sudden appearance of tall rooster tails in an otherwise placid river — and we've got a pretty good idea of the underlying cause, as well. And we did it all without leaving our desk. Not a bad fifteen minutes' work, eh?

Every experienced paddler knows the value of scouting a rapids before running it. But often it's worth taking a backward glance, too. Call this debriefing a rapids, if you like. By using topographic maps and aerial photos to get a different perspective, we can almost always learn something new, even about a river that has flowed strong and deep in our mind's eye for years.

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