The Colorado River Basin has experienced incredibly large flood events in the past. Present-day river deposits can be used to understand the size and timing of floods that have happened within the region. Here, Science Moab speaks with Dr. Vic Baker, who studies what he calls “Paleoflood Hydrology,” and discusses the ways he and his team determine how big flood events were and how we can understand the possibility of floods into the future.
Science Moab: A lot of your work has looked at paleoflood hydrology. Can you explain what this is?
Baker: The basic idea is that we look to the evidence that nature has presented about the biggest floods that have happened. Nature happens to be a very good recorder of the most extreme floods and the evidence is preserved for a long period of time. We found that when you have a narrow river canyon, the floodwater doesn't spread out widely. Instead, the depth goes up greatly with the increasing flow. The water is moving at much higher velocities, so it carries very coarse sediment with it. When the velocity drops rapidly, like at the mouth of a tributary valley, all that coarse sediment drops out. That sediment is a record of that extreme flood.
We now have very sophisticated methods that allow us to determine the age of the flood and also the magnitude of the flood, the discharge, the velocity, all of these factors. In fact, it is better than the conventional approaches to studying floods, because the modern record of flood measurement is so short that it doesn't include rare events. So if extreme floods are important, we have to use paleoflood hydrology to understand them.
Science Moab: What does 100,000 CFS of water look like for the river system and for areas along the river? And why does it matter, specifically in Moab?
Baker: In Moab, that water would be generally confined to the channel and would likely come up to the base of the Colorado River bridge, but wouldn't get into Moab proper.
Years ago, there was a concern being voiced about the uranium tailings pile here in Moab, so engineers calculated what a much larger flood would do. That’s called the “Probable Maximum Flood,” which is not something they thought ever happened. That flood was calculated at 300,000 CFS, which is about three times as big as a typical Colorado River annual flood and would actually get into the boundaries of Moab. A flood that size would also have impinged against the uranium tailings pile, which would be a concern but when that problem was being addressed, it was thought that the probable maximum flood was so rare that it was discounted as a hazard.
Science Moab: So can you tell me about what is being discovered out in nature? What was flooding like in the history that's still present on the landscape?
Baker: The general thing we find is that floods tend to cluster in time and big floods seem to occur in certain preferential conditions. We’ve found that floods, in many cases, have been much larger than what theoretical studies have shown. The flood event hypothesized in the uranium tailings example assumed a probable maximum flood of about 300,000 CFS. We did a paleoflood study upstream of Moab and produced a 2,000-year record of natural flooding. In that 2,000-year record, we found 44 large paleofloods and nearly all of them exceeded the level that's supposedly represented by the hundred-year flood. Two of these exceeded the probable maximum flood. So this probable maximum flood was definitely not a probable maximum flood. That is a big concern because the flood would have spilled all around the nuclear waste [in the Moab tailings pile]. The paleoflood studies show that that’s a much more common flood than was being hypothesized.
This interview has been edited for length and clarity. To learn more and listen to the rest of Vic Baker’s interview, visit www.sciencemoab.org/paleoflood-hydrology.