Friday, June 20, 2014

Research Suggests Huge Paleo-Floods On The Colorado River - What Does It Mean For Glen Canyon Dam?

The Colorado River flows along a stretch near Moab known as the "Daily"
On June 19, the LA Times ran an article about recent research done on the Colorado River near Moab, Utah. The research shows that huge paleofloods have coursed down the river within the last 2,000 years. You can access the LA Times article here. To look at the abstract of the scientific study, click here. The authors of the article inlcude Noam Greenbaum, Tessa M. Harden, Victor R. Baker, John Weisheit, Michael L. Cline, Naomi Porat, Rafi Halevi and John Dohrenwend.

Work done by Dr. Victor Baker at the University of Arizona, uncovered deposits of silt and driftwood located high up the bank along a stretch of river known as the "Moab daily," about 8 miles upstream from town. Here they found evidence for 44 large floods occurring in the last 2,000+ years. These were not merely high water year floods, but were exceptionally large floods, that likely resulted from rain on snow events. Of the 44 floods detected, 34 of them exceeded the accepted 100-year-flood level and a whopping 26 of them dwarfed 500-year-floods. Two massive paleofloods were higher than scenarios that engineers and planners now use to prepare for flood disasters. The discharge for these floods is estimated at over 300,000 cubic feet per second (cfs). To compare, the largest estimated modern flood at Lee's Ferry, Arizona was in 1884 and is estimated to have crested at 125,000 cfs.

Baker said that Glen Canyon Dam is ill prepared to handle floods such as these, should they occur today."It’s not something theoretical,” said Noam Greenbaum of the University of Haifa in Israel, the lead author of the study. “What we are actually documenting are the natural floods.

Amid placid days of drought, risks of massive floods may loom for the Colorado River
All of this research dances around the question of the purpose, sensibility and utility of building dams on wildly fluctuating river systems like the Colorado. The dams were originally conceived as a way to protect people from devastating floods and to save water for times of need during droughts. In this respect they have worked marvelously. However, others have pointed out that there are some negatives to this manner of river management. It is obvious that when the dams were conceived and constructed, all of the negatives were minimized and the positives were accentuated. As time has moved on since the era of dam building, some of the negatives are popping up like an unwanted house guest.

As a geologist, I naturally tend to look at both sides of an argument before leaning one way or the other. I understand for example, that these dams have allowed our southwestern culture to thrive. Yet, is it a good thing that 35 million people are dependent on a river that fluctuates between scorching drought and (now) humungous floods? The dams have clearly done what we wanted them to do. But at the same time, our knowledge of the rivers' inherent nature was virtually non-existent at the time the dams were conceived and built. Do we need to rethink dams on the Colorado River?

One observation I cannot escape is that the dams are piling up sediment for which there is no obvious, easy or inexpensive solution. In the long-term, dams on sediment rich rivers is a really bad idea. I don't know how you can continue to have 35 million people (and increasing), dependent on a system of water delivery that was conceived in the hydrologic 'dark ages.' To me, it doesn't make sense and I know that the detriments of the dams will only come into clearer focus as our science and technology reveal the wild character of the river.

“Nature is variable,” Baker said in the LA Times article. "The Southwest has suffered in the wake of recent droughts, but the long-term history of the region suggests that can change quickly. Ignoring Mother Nature is not too smart,” Baker concluded.

Greenbaum points to the uncertainty of the future and hopes to seek out more long-term flood records along the river. “During the last 2,000 years, the climate changed a lot. It was maybe sometimes wetter, different periods were dryer,” he said. None of this was really known in 1922 when the Colorado River Compact was ceremoniously signed in Santa Fe.

Sharlot Hall, forever the First Lady of Arizona said it best in 1906 in the last stanza of her incredibly prescient poem, "Song of the Colorado":

O ye that would hedge and bind me — 
remembering whence I came! 
I, that was, and was mighty, 
ere your race had breath or name!  
Play with your dreams in the sunshine — 
delve and toil and plot — 
Yet I keep the way of my will to the sea, 
when ye and your race  are not!   

Sharlot Hall, 1906

How did she know? As they say, may you live in interesting times!


  1. Awesome commentary, Wayne!

  2. Hi Wayne: The event that created Grand Falls on the LCR is a small-scale example of what could happen with one of the dams on the CR. Once the reservoir behind the lava dam at Grand Falls completely silted in, water found it s way around the dam and voila, we have Grand Falls. Of course if something like this were to happen on the CR at Glen Canyon Dam creating a falls around the dam might eventually result in destruction of the dam. What a mess that would be!!


  3. This is definitely food for thought. Our beloved Colorado River continually reminds us that we are not in control. Excellent post Wayne.

  4. When I was a guide at Hoover Dam in the mid 90's all of what you write about was both well known and scarcely acknowledged. Glen Canyon has long been thought the more fragile of the two "mightiest" dams on the river, but if one went down from a megaflood, there would be a chain reaction downstream all the way to the Gulf of California. People will say in defense that there is ample capacity in this drought behind both dams, a drought that may be with us a long time. But megafloods are not inconsistent with droughts. As you point out, an atmospheric river dumping on a snowpack will do the job. Rivers move sediment with a force proportional to the cube of their velocity, so as the water rises in a reservoir a good portion of the volume beneath it will be sediment, flowing in without the impediment of higher water reaching further upstream. For Hoover Dam, the level at which sediment would clog the intakes is pretty low. Hoover is built to last a lot longer as a monument to shortsightedness than as a functional installation. There are no lessons learned, there are only generations that die off so that new generations can make their own mistakes.

  5. Topping showed the 1884 flood at Lees ferry was likely closer to 250,000 cfs and 1921 was 170,000.

    But what does that mean for the dam. with so many upstream diversions probably not much? Theoretically the dam can bypass 250,000 cfs although as we know 90,000 cfs didn't play out too well in 1983.

  6. oops, I meant 210,000 cfs for 1884. A significant revision from what O'Connor and others published for the deposits in axe handle cove.


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