A Discourse of Rivers
Keep Your Head Above Water! A Flash Flood Primer
By Tamia Nelson
September 10, 2013
Not long after I bought my first canoe, I took my little brother fishing. It was no ordinary fishing trip. After we left the state highway, we drove down a rutted forest track until it petered out. Then we shouldered canoe and pack — I carried the canoe — and stumbled along a recently blazed portage trail till we reached a remote Adirondack lake, a lake reputed to harbor a remnant population of native brook trout. There we set up camp on a sandspit near the mouth of a tiny stream whose headwaters lay in the col between two peaks, neither of which was high enough (or popular enough with guidebook writers) to have been given a name on the quad.
And then we went fishing. I paddled while my kid brother fished. As luck would have it, the fish weren't much interested in the spinners that were our only lures, though my brother still hooked enough brookies to make him happy. (I didn't ask the brookies how they felt about the experience, but I'm willing to bet they saw things differently.) Once the sun had dipped behind the ridge that framed the western margin of the little lake, however, it was time to call it a day. So we headed back to camp. I beached the canoe, pulling it well up on the sloping shore, then placed our paddles and life jackets inside the boat and began preparing our evening meal: cups of ramen, served with hot dogs roasted over a small fire built on the sandy margin of the tiny stream.
In the half‑light of the long summer evening, neither of us noticed the storm clouds building in the west. But they were there, just beyond the ridge, and sure enough, my sleep was broken by the crash of thunder. I woke to a world very different from the still, sultry day just ended. The tent flapped and juddered, rain hammered down on the flysheet, and lighting flickered with the maddening irregularity of a malfunctioning neon sign. My brother slept through it all, but I fretted, dozing and waking by turns until the storm moved on. Then, just as the false dawn was illuminating the landscape, I was wakened from a deep sleep by a new and ominous sound: the roar of rushing water. The tiny stream on whose bank we had pitched our tent was now a rain‑swollen torrent.
I stumbled out of the tent and was startled to find that the stream had risen at least two feet in the small hours, washing away my fire ring and taking my cookpot with it. But the biggest surprise came when I made my way to the beach where I'd left my canoe. The boat was high and dry no longer. Now it floated free, with several inches of water sloshing around in the bilge. Only the painter that I'd tied to a scraggly spruce had kept it from drifting off.
The worst was yet to come. When I returned to our camp beside the little stream, I saw that the water had risen three or four more inches in just about as many minutes. Soon our tent would be awash. Swift action was called for. I yelled to my brother to get up and get dressed, and together we ran frantically between campsite and beach, collecting our gear and throwing it higgledy‑piggledy into the canoe, which I'd emptied of most of the water by the simple expedient of turning it over and righting it again. Before many more minutes had passed, we were paddling back across the lake, on our way to the trailhead. But that was just the first leg of our journey. We still weren't out of the woods. Not by a long chalk. The portage trail was now a streambed, and once we reached my jeep, we saw that the access road — a poorly maintained dirt and gravel track that was at least a Class IV on the International Scale, even when dry — was in no better shape. In the end, my little brother walked the whole distance out to the state highway, sounding each waterlogged sag and wallow with a stout stick and testing the bottom, while I followed along behind him, creeping forward in the jeep's lowest gear.
The upshot? Our fishing trip ended almost before it had begun. But we were both safe, and that was what mattered. A rather battered cooking pot was our only casualty. And I'd learned a valuable lesson about the ways of moving water. Specifically, I'd just had a convincing demonstration of …
The Danger of Flash Floods
Moving water is always a force to be reckoned with. I've seen lazy summer streams pin a stranded canoeist to a sweeper so securely that it took three of her companions several minutes to prise her free. And I've found dead cows — or what was left of them — hanging high in the branches of tall trees along fast‑flowing rivers. A river in flood is a fearsome thing, and with enough rainfall in a short enough time, even a tiny rill can grow into a raging torrent, inundating whole swathes of lowland forest and sweeping all but the largest trees before it.
Many floods are more or less predictable, of course. In much of Canoe Country, snowmelt swells the streams and rivers in spring, and the subsequent flooding is an almost annual occurrence. This state of affairs delights gonzo boaters, but property owners who've had to abandon their possessions to the restless waters take a very different view. It's not a one‑time thing, after all. Changing weather patterns make "50‑year" and "100‑year" floods an increasingly common occurrence in some places. Cities built along big rivers now suffer spring floods with depressing regularity. If it weren't for government‑subsidized flood insurance, in fact, whole urban neighborhoods would soon be abandoned as uninhabitable, and given that the frequency of "extreme weather events" is now increasing in many of the world's most densely settled river valleys, it's anyone's guess how long such costly subsidies can continue. As T. S. Eliot observed,… [T]he river
Is a strong brown god — sullen, untamed and intractable,
Patient to some degree, [yet] …
Keeping his seasons and rages, destroyer, reminder
Of what men choose to forget.
Urban planners and waterfront developers would do well to read Eliot.
But that's not my subject here. I'm concerned with the flash floods that can follow heavy rains at any time of year, transforming cozy, chuckling creeks into cataracts overnight. As the name suggests, flash floods arise quickly, and they're no easier to forecast than the weather itself. Here's how the National Oceanic and Atmospheric Agency (NOAA) describes a flash flood:A rapid and extreme flow of high water into a normally dry area, or a rapid water level rise in a stream or creek above a predetermined flood level, beginning within six hours of the causative event (e.g., intense rainfall, dam failure, ice jam). However, the actual time threshold may vary in different parts of the country. Ongoing flooding can intensify to flash flooding in cases where intense rainfall results in a rapid surge of rising flood waters.
Well, yes. Of course. It's not Eliot, but you get the picture, I'm sure. When too much water enters a stream or river in too short a time, you get a flash flood. Unusually heavy rains are probably behind most flash floods, at least in summer, though it's not unknown for a flood to be exacerbated by the collapse of one or more beaver dams, especially in places where trapping has removed the native engineers who once maintained those dams, leaving them derelict and failure‑prone. And to make matters worse, flash floods are themselves a cause of further devastation, frequently triggering mudslides and slumps. If being caught by a rushing torrent of water is bad, getting in the way of a river of mud is even worse.
OK. We've dealt with the proximate cause of flash floods: too much water. But what about the Big Picture? What might make flash floods more likely in the years to come? The answer involves many of the usual suspects, namely …
- Road construction and paving
- Deforestation or extensive clearcutting
- Draining of wetlands
Needless to say, there's a lot of overlap between the items on this list. Road construction and paving go hand in hand with suburbanization, for instance. Of course, I've left out the Really Big One: climate change. It's the elephant in the room. Or maybe — given that the benign‑sounding "climate change" has largely supplanted the inconveniently descriptive "global warming" in public discourse — it's the shove that dares not speak its name, pushing our whole monkey‑puzzle paradise closer and closer to… To what, exactly? Uncertainties abound. Models are only as good as the data that goes into them, and acquiring that data is both time‑consuming and costly. Moreover, a lot of climate processes are imperfectly understood and therefore difficult to model. That said, the shape of things to come is already pretty clear, at least in broad outline. Ours will soon be a much warmer, wetter world. But the devilment is in the details. How warm will it get? How wet will it be? And how long can our great coastal cities hold back the rising ocean? We'll just have to wait a generation and see. Only one thing is sure: It won't be business as usual on planet earth for much longer.
Nearer term, it's a safe bet that those extreme weather events I mentioned earlier will be more common in future. Less extreme, in other words. More like normal. And it's also a pretty safe bet that flash floods will be more common, too.
Where does that leave paddlers? Well, we really have only two choices. We can stay home — assuming that home is located on a hilltop and that we have plenty of gas for the generator — and do our exploring with an Xbox or a Game Boy. Or if this doesn't appeal, we can …
Cultivate a Weather Eye
It helps to know that the risks — in most places, at any rate, and at most times — are small. But if your luck ever runs out, it's good to be prepared, and that means …
Putting Together a Toolkit. Don't worry. It won't cost much. The most important tools are always with you: your brain‑housing group, your Mark I eyeballs, and the situational awareness that paddlers develop after a few years spent contending with the caprices of moving water. But it's not enough to have the tools. You have to use them. So keep your eyes on the sky. And think twice before you make camp right next to that pretty little babbling brook when you can hear thunder in the hills.
This is the place where I ought to remind you to check the forecast before you leave for the put‑in, to see if an active front is on its way. And I've just done that, haven't I? But it's really not very useful advice. There are three reasons: (1) No forecast can be relied on for more than a day or two at most — one day is probably the outside limit in mountain country — and a lot of trips last longer than that. (2) Even short‑term forecasts have been known to be wrong, sometimes spectacularly so. And (3) while almost everyone now owns a smartphone or tablet capable of downloading realtime weather imagery and updated forecasts, these gadgets (3a) sometimes fail. Moreover, (3b) you often find yourself outside the reach of any network. That happens even in the northeastern United States. In truly remote places, network coverage is likely to be the exception, rather than the rule.
The bottom line? You'll have to emulate the watermen of old and cultivate a weather eye. (This is a good idea, anyway, of course.) Remember, too, that the stream on whose bank you've pitched your tent may have its headwaters in distant hills that are even now being pounded by one hell of a storm. The thunderheads may have spared you, but the water they've dropped is headed your way. It's just a matter of time — and not a very long time, either.
Your weather eye should also take the past into account. Flashy streams aren't hard to recognize. You'll see the …
Clues Everywhere You Look. But there's a catch: You'll need to know what to look for. Consider the photo below. See the fragment of someone's dock? And what about that uprooted tree? These weren't left behind by the propman from some visiting film crew. They're the spoor of a recent flood. The yellow bar provides scale, by the way. The planks are eight feet long.
Look for floodwrack, too. Floodwaters leave a bathtub ring of debris behind when they retreat. Multiple floods leave multiple rings, and the highest ring gives you some idea of the likely worst‑case scenario. (Until the next Big One comes along, that is.) In the second picture (below), the portable samovar provides scale, while the remnants of a great tree furnish fuel.
And here's another toppled giant:
You wouldn't have wanted to be camping on the rocky point when this fellow came to rest, would you? Me, neither. On the other hand, you, your tent, and your companions would likely have been swept away much earlier. That isn't exactly comforting, is it?
Of course, not every flood has the force to uproot mature trees. But you can get clues to a river's recent past from the condition of much lowlier vegetation. Take this grass, flattened by flood waters that have now receded, but which could easily return:
You get my drift, I'm sure. Broadening the scope of your weather eye to include the shoreline and even the ground beneath your feet pays big dividends when you're …
Choosing Your Home Away From Home. The best advice is pretty obvious: Avoid camping right at the water's edge, particularly if it's been raining hard and the water in question is a fast‑flowing stream, no matter how small. Beach camps on big lakes are pretty safe — barring the odd seiche, that is — but high ground is best, though it also pays to give slide‑prone cliff edges and scarps a wide berth. It should go without saying that making camp below a scarp is a Very Bad Idea. Those boulders and clods of earth you see around you weren't placed by the propman, after all.
The advice to avoid camping at the water's edge even applies to dry stream beds. Dry washes in the American Southwest have a reputation for morphing into torrents of turbid water whenever there's a thunderstorm in the mountains. And even in the humid, well‑watered Northeast, a sudden downpour can turn a gully into a freshet in a matter of hours. Here's an example of a good place not to camp when there's heavy rain in the forecast, whether that forecast is the official one or the product of your own weather eye:
This little trickle doesn't look like much, and during dry spells it disappears completely. But even a summer thunderstorm fills the bed to overflowing, and after several days of hard rain it becomes deep enough and fast enough to make sensible trekkers think twice before wading across. Camping anywhere in this mini‑floodplain would then be a very soggy business, indeed.
Fortunately, such traps for the unwary are easy to avoid, and you can do most of the work from (as the saying goes) the comfort of your home. Just pull out a large‑scale topographic map (1:62,500 will do, but 1:25,000 is better) and look for the dot‑dash blue lines that mark intermittent streams. Then note the places where successive contour lines make slight upslope jogs. These are draws, and whether or not they're distinguished by a dotted blue line, they're potential stream courses in heavy rain.
Lastly, learn from my early experience, as recounted in the opening paragraphs of this column: Don't assume you're out of the woods till — you guessed it — you're really out of the woods. Flash floods are as much of a concern …
During the Drive Home as they are during your search for a campsite. A surprising number of folks drown in their cars every year after misjudging the depth of water on a flooded road. If you feel you simply must drive through water, at least check the depth beforehand. And if you find that it's too deep to wade easily (or the current is too fast for you to risk venturing across), then it's too deep to drive through. Period. Better yet, follow the eminently sensible advice of the U.S. National Weather Service, whose slogan,
Turn Around, Don't Drown! …
Lays out the alternatives with admirable clarity. Turn around. Or risk drowning. I know which of these appeals most to me. What about you?
Water is the paddler's second home. But you can have too much of anything, even good things. Flash floods are rare, but they're certainly not unknown, and days of heavy rain can easily swell picturesque rills until they grow into all‑consuming cataracts. So it pays to view potential campsites with a critical eye before pitching your tent and settling in. You want to keep your head above water, don't you? Sure you do!
Related Articles From In the Same Boat
- Air, Earth, and Water — Natural History for Paddlers, a topical collection of articles, including …
- "The Dynamics of Moving Water: Theme and Variation,"
- "After the Flood: When Rivers Rearrange the Dust,"
- "When Size Doesn't Matter," and …
- "Current Affairs."
Plus some articles from elsewhere on the Web:
- "Flash Flood" and …
- "Flood" from Wikipedia, plus …
- "What Are Flash Floods?" an informative PDF from the National Weather Service, and …
- "Types of Floods and Floodplains," another helpful PDF, this one from the U.S. Federal Emergency Management Agency.
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