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Overview
Regardless of whether you call them arroyos,
channels, gullies, washes or canyons, they are among the best places to find
large, well exposed outcrops. Therefore, as a geologist you'll find
yourself in them. But if you're working in one, beware of sudden and
"unexplained" flash floods -- even if the gully is large and has
clearly been "dry" for years, it can be filled to its rim in seconds.
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If you're in a dry gully and you start to see a trickle of
water like in this photo, MOVE QUICKLY for high ground. Even if it's
a clear, sunny day, this trickle of water means it rained HARD somewhere up
stream and the gully is about to flood. The consequences can be
deadly.
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Flash floods are the number one
weather-related killer in North America. A small cloud burst (a sudden
isolated rainstorm) 20 miles away upstream in the hills can send a deadly
torrent of water crashing your way an hour later even though it never rained on
you. Though distant and insignificant looking showers may not seem worth
noticing, they can pose a definite threat.
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Moving with the speed of a freight train, flash floods
typically surprise their victims, and even the strongest of swimmers can't
survive being repeatedly pummeled by tree trunks, boulders and rocky canyon
walls.
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| Debris-choked floodwater only 6
inches deep is sufficient to pull you down and wash you away.
Water only 2 feet deep can easily float and carry
off a full-size motor vehicle. |
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 Avoiding Problems
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Live map of current flooding conditions in the US. |
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Watch for signs of rain up
gradient, no matter how distant.
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Be aware that the probability
of flash flood increases after a
wildfire.
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Never camp in a wash or gully
no matter how "dry" it is.
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While in canyons, washes,
etc, keep an open ear for the sound of water. If you hear it, get
out fast.
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Seconds count. Don't
wait to see the water.
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The following is a short account of
the danger posed by flash floods told firsthand by a woman who had a direct
experience with one. It happened in the San Rafael Swell region of Utah.
(Click the image to see a larger view.)
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 On
August 8, 1992, my husband and I began a loop hike in Little Wild Horse
Canyon. We were delighted by the dazzling beauty around every
swirling curve of the canyon as we made our leisurely way toward Bell
Canyon. Out on the dirt road, we caught our first glimpse of the full sky
since we started our hike; there was a killer storm brewing. We had
just entered the mouth of Bell Canyon as the rain started. We took
shelter under a rock as the rain quickly became a downpour and just as
suddenly turned to hail. Within minutes we could not see the other
side of the canyon, the hail was so intense. |
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hail/rain continued for about 30 minutes and during that time, the flood
began. That nice dry canyon (it clearly had not had significant rain
for a long time) soon changed dramatically. Ever marvel at how tree
trunks get lodged between canyon walls 30 feet above your head? We found
out on this day. The water came down the gulch gradually, a wave at
a time; the first wave was only a few inches deep, then 5 minutes later,
another wave bigger than the first, and so on until the water coming from
our storm, from the top of the mesa, from all the canyons draining into
Bell Canyon and from the mountains combined to create a raging river that
filled the canyon to ten feet deep. Whole trees floated by us while
the walls of the canyon beneath our feet rumbles and shook as boulders
were washed down with the current. The unmistakable smell of pine
trees floated down in drafts of cold mountain air as we stared with wonder
at the spectacle before us. We could not leave the place we were in.
We watched as the water kept climbing higher, eliminating "islands" and
washing over high ground we thought was safe. Sand waves formed and
disappeared before our eves. We remained trapped for 2-3 hours
before the water level went down to where we thought we could navigate it. |
Ever marvel at how tree
trunks get lodged between canyon walls 30 feet above your head? We
found out on this day. |
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The water was 45 degrees, at the most, painful to walk in. All of those
lovely narrows in Bell Canyon became swimming holes. It was a cold,
miserable journey out of the canyon. |
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When we finally got out, our troubles were just beginning. Four 4X4
vehicles had parked in the wash at the opening to the canyons. Ours
was the only one still standing. A jeep had been washed 1/2 mile
down the canyon, an Isuzu Trooper was about 1/4 mile down, laying on its
side with an imploded windshield. A Toyota had been pushed by the water
into a large boulder where it became lodged as water rushed up the bed,
broke the glass in the cab and filled it with water, mud and debris. |
| Our car fared the best; nothing was broken,
but the water had gotten inside (about one foot deep), and the entire engine was
filled with silt. Luckily, all of our camping gear was dry in the back of our
vehicle and we retreated to high ground to spend the night. The water stopped
running completely around midnight. |
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