True Confessions of a Debris Flow Scientist
Last January I led a mid-winter research trip with
some volunteer-for-science, die-hard, friends of mine. I study debris flows, their effects
on the river, and relations between debris fans and the canyons beaches. This trip
was designed to characterize debris fan geometries in order to betterpredict where
deposition and erosion will occur during future habitat-building floods. My
research interests are generally aimed at studying large floods, especially those
occurring in arid and semi-arid places like Grand Canyon.
On the way to the put-in someone sleeping in the back of the van suddenly
muttered, satellite map of the Pacific Ocean.... storms heading-inland in a
day or two..., then quickly rolled over and was silent. The last thing any of us
wanted to hear on the way to a January 3rd put-in was talk of winter storms. After all,
hadnt denial and fireside evocations to the High-Pressure God always worked to
ensure clear skies before? Well, almost always.
I recalled early lessons on winter weather; cut-off lows were bad,
high-pressure domes were good. True enough, if youre trying to maximize field work
during short January days.
However, if new debris flows are what you seek, then just the opposite is
hoped for. These opposing views had long caused me confusion. Of course, I always hope for
good weather on river trips, right? Wrong! Secretly, I always hope for terrible weather;
not just cold, upstream blowing wind and snow, but the kind with plenty of hard driving
rain. Why? Because I want to see a Grand Canyon debris flow for myself, of course.
Obviously, this attitude makes me unpopular on river trips, so I normally keep such
desires to myself.
The trip got off to a great start; nothing but blue skies at first. Then on
the second day, the clouds moved-in; I remembered our snoring Nostradamus. By the time we
left Buck Farm, it was snowing, but our spirits didnt waiver an inch, not even
during the blizzard that followed us all the way down to 50-Mile Canyon.
Well, whats a January trip without a little snow, I said to
my friends, as I threw on another layer.
Then the rain began.
Hey, at least it warmed-up, someone said in a muffled voice from
beneath a crumpled army surplus poncho.
These storms hardly ever last more than a couple days, its not
like were in Alaska, I said confidently, attempting to reassure the
In truth, I had never seen more ominous-looking skies in my life. The rain
kept up all day and all night, then again all the next day and night; and again the
following day, etc., etc., etc.....well, you get the picture I guess. It didnt rain
hard, just steady, but I suspected that it might turn into one of THOSE kinds of winter
trips that I have hoped for all my life; my heart gladdened. By the time all of our
scientific equipment, tents, sleeping bags, papers, maps, AND rain gear were thoroughly
soaked, I was secretly getting quite excited. The prospect of a nice, new debris flow to
study warmed the cockles of my heart.
This storm might have some real potential, I said to the group,
reminiscing back to the early days of December 1966.
Outwardly, I lamented our predicament, telling of the many
near-perfect-weather winter trips that I had been blessed with in years past.
Guess the odds finally caught up with me, I said apologetically,
as I hunkered down under our beach umbrella at lunch.
Everyone assured me they were having the time of their lives, in spite of the
monotony of the twenty-second debris fan survey, the forty-third pebble count, and the
record wet weather. After the fourth (or was it the fifth?) consecutive front passed over
us, I detected a few complaints beginning to waft through camp. A day or two of rain slows
down the surveying a little, but its usually only a minor inconvenience. However,
prolonged, wet weather like we were having affects the typical debris flow researcher in a
very different way. After a while, such a person becomes wide-eyed... mesmerized by each
rain drop, hoping for another, then another, and another... then madly another... and so
on until every pourover and rivulet in sight becomes engorged with DEBRIS FLOWS!!!
Unfortunately, I let my secret, fluvial fetish slip out one morning during breakfast; the
trip was never the same after that.
As I chewed my soggy pancakes, another rock let loose from the Redwall cliff
downstream. During prolonged, steady, winter rains, rockfall apparently becomes as common
as blowing sand during dinner in June. Id kept track of the crashes and thuds since
the rain began, and tallied thirty good rockfalls in as many hours. This was great stuff
for a young hydrologist with a catastrophic bent.
So this is how all that luscious colluvium forms, I thought to
myself wringing-out my half-eaten hotcake between bites. I was convinced that the trip
could only get better.
Colluvium is one of the main sources of sediment for Grand Canyon debris
flows; its all that loose, unconsolidated dirt and rock that blankets the Hermit
Shale and accumulates at the base of all those magnificent cliffs. It had been raining for
a while now, and the stuff was really piling up. I wondered how often this kind of storm
occurred on average. By now, I had stopped sleeping at night, choosing instead to lie
awake and count rockfalls rather than chance drowning in my miserable, leaking tent.
After swallowing another bite of rain-swollen Krusteze, I said in a fit of
unrestrained desire, I hope it really lets loose today, if its gonna rain, man
then, let it rain big!
You want it to keep raining? Mau and Lisa said simultaneously,
looking up from their cold cereal in astonishment.
Are you deranged from sleep deprivation? Everything you own is
soaked, they quickly reminded me.
I knew it wouldnt go over well as the words left my lips.
Im not crazy, I just want to see a debris flows in action... ya
know, to get a better feel for the process, I said proudly.
They both looked at me dumbstruck. Lisa stared in utter disbelief.
I thought you wanted good weather on this trip so we could get lots of work
done during these four-hour long winter days? she shot back, annoyed with my
foolhardy lack of reason.
Well, things are different now, you see, I figure weve had five
straight days of rain with average intensities of 25 to 60 mm per day, punctuated by
occasional hourly intensities of 15 to 25 mm. The rate of rockfall suggests a potential
for slope failures; Hortonian overland flow is dominating the landscape, increasing the
possibility of erosive firehose effects resulting from high antecedent
soil-moisture. Why, a debris flow could occur in this part of the canyon anytime, that is,
if it would just rain a little harder right now, I blurted out, not even trying to
suppress my enthusiasm for catastrophe.
What we need now is a thunderstorm., Jim dryly interjected.
There was a moment of silence, then another small rockfall. I grinned.
What day are we taking-out again?, Mau asked, as the
kitchen tarp emptied its reservoir onto the table where Jano and Mia were preparing our
Perhaps my friend was hoping for an early reprieve from his long-awaited
working vacation, Im not sure. I answered that wed return to Flagstaff on the
17th, provided the Diamond Creek road still existed. Otherwise, we would spend another day
on the river and take out at the lake. Mau walked away looking dejectedly at his tent
still draining over a tamarisk tree in the light drizzle.
Later that morning, as Mia and I measured rocks on the 60-Mile Creek fan
under a partially-crumpled umbrella, we heard the unmistakable crack of another chunk of
Redwall Limestone liberated from the wall by gravity.
Damn, that was a big one., I said, trying to focus through the
steady rain on the next probable impact point. The volkswagen-size clasts bounded down the
slope, catapulted off a Muav ledge on another Newtonian trajectory and ker-splashed
straight into the river.
Whoops AHaa!!! This is great! I shouted gleefully. Mia agreed
with a yell, as we jumped up and down. Apparently, she too had all the makings of true
catastrophist. After composing ourselves, we went back to the sampling task at hand...
238-Redwall, twice; 560-Supai, four times; 385-Coconino, once,
28-Unknown..., I droned on catatonically; we only had a couple hundred more to do
now. I wondered out loud why anyone faced with the choice of embracing gradualism or
catastrophism as a driving force in Earth studies would ever choose the former; we
shrugged our shoulders and returned to our measurements.
Ted, Mia said, all the numbers on the data form are
smearing; I cant keep this thing dry anymore.
I looked at what was left of the sheet and its blurred pencil etchings and
decided that we had better retreat until the rain let up a little (secretly, I hoped for
another 5 to 10 mm per hour by lunch time).
It looked like the rain might end for a while down around Dubie, in fact, it
actually stopped for nearly a whole day. I tried a new tactic to get things going again.
Instead of heading for the obvious camp at Ponchos Kitchen that evening, I decided
wed be better-off camping a foot above the rivers flood stage at the Race
Track camp. The group grumbled at my decision, but I knew what I was doing, alright. The
powers-that-be cant resist the temptation of drenching such brazen mortals. Sure
enough, that night it started raining again, and even harder than before. As we passed the
dry sands of Ponchos the next morning, I knew Id pretty much lost all
credibility with my crew.
Well, at least its raining again, I said to myself,
but theres still no sign of a debris flow, not even a little one, Damn!
A phone call to my boss from Phantom Ranch two days before revealed that
central Arizona was about to wash away. All hell was breaking loose,
hydrologically-speaking, south of the Mogollon Rim. The Verde River had reached its
highest stage in probably 1,000 years. We had left the ranch brimming with expectations of
calamity, but now the reality was settling in. It just wasnt going to debris flow,
and there wasnt a thing I could do about it.
If only these storms would move a little further north,, I said
For all I knew, by now there had been dozens of debris flows down south. I
was starting to feel a little frustrated, disappointed, and even edgy. I snapped at my
crew, Get another point up there! Hey, you missed that thalweg shot! I needed
to calm down and get a grip before I came unglued by all this constant, drizzly weather.
What would happen if things kept up like this forever? I
pondered, scratching my head as we surveyed the Fishtail Creek debris fan under a blanket
Nothing but gentle rains triggering rockfalls, that piled-up immense
mounds of colluvium without a single debris flow to move the stuff on out to the
river. I contemplated the scenario sullenly. What a grey and gradualistic world it
would be, I didnt even like thinking about it.
Why, I could lose my job if this lousy weather persisted, I
My boss and I had discussed the formation of colluvium and its relation to
debris flows, rapids and the river during many river trips; pretty heady stuff for a
couple of dirt geologists.
What would the river have been like in the absence of summer, monsoon
thunderstorms? we had wondered time and time again.
Now thunderstorms, they can practically guarantee debris flows if they occur
in the right place at the right time. Most debris flows since 1940 have been triggered by
localized thunderstorms, but only a few of those were really very significant before Glen
Canyon Dam was built. The really big winter storms, and the dissipating tropical cyclones
are best for causing the large debris flows, but those storms dont get far enough
north very often.
Suppose for just a moment, that the Canyons climate was dominated by
higher precipitation, but it occurred only as steady winter rainfall. Without intense,
summer storms and cyclones, you could run out of rapids as the river slowly wore away all
the large boulders; without debris flows its nearly impossible to get those really
big ones into the river. Grand Canyon would probably become quite a different place under
such climatic conditions; forests of ferns, moss covered boulders, grassy-green slopes
covering-up all the rocks. Yuk! Ill take aridity any day.
Im not exactly sure when our summer monsoon circulation began in
northern Arizona, but according to some experts, it probably started sometime after the
end of the last ice age. Thats about ten to twelve thousand river seasons ago;
certainly a lot of river trips, but geologically speaking, not all that long ago. I spend
a lot of time thinking about how different climates have shaped the canyon and its river,
and how things might change there again in the future. It gives me something to do while
lying awake at night in my tent, counting rockfalls, listening to the winter rain, and
waiting for the next big debris flow.