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Sometimes Earth Just Goes With the Flow

      There is a river near Lake City that is rather unusual. In some places, it is over 1000 feet wide and 300 feet deep. Yet you can stand in the middle of it and not get wet. . At just four miles long, it is one of the shortest rivers around. It has a vertical drop of 600 feet per mile, yet its fastest moving sections are flowing only about eighteen feet per year. Still, it is responsible for forming the second largest natural lake in Colorado. None of this likely makes sense if you are still thinking water. The most unusual thing about this river is that it is made of flowing earth. 

The Slumgullion earth flow

      The Slumgullion earth flow, just a few miles east and south of Lake City, starts near the top of Mesa Seco and ends at Lake San Cristobal. Among earth flows, it is sort of a celebrity; its photo (accompanied, unfortunately, by only a sentence or two of explanation) appears in numerous geology textbooks. It not one continuous flow, but is actually a series of three flows built on top of each other. Because of its ease of access and unusual nature, it has been studied by many geologists, particularly over the last thirty years.

      Radiocarbon dating of trees buried in the first of three flows shows the initial flow occurred about 1200 years ago. This first flow stopped short of damming the Lake Fork of the Gunnison. A second earth flow occurred about 800 years ago damming the Lake Fork of the Gunnison and causing the formation of Lake San Cristobal. Both of those older flows are now inactive (contrary to occasional lore, cabins built on the old parts of the slide are not headed for a dunking in the lake). For the past 300 years, the active part of the slide has been flowing down from the base of Mesa Seco over the top of the older slides.     

Look out below?

      Currently, the toe (bottom) of the two-and-a-half mile long active slide is about 1/8 mile above the road where highway 149 crosses the older portion of the slide. While the fastest parts of the earth flow move eighteen feet per year, recent studies have shown the toe to be advancing about five feet per year. At this rate, the highway won't be in jeopardy for another hundred years or so. At some point though, the earth flow will likely cause some interesting problems for the highway department. At the same rate of advancement, the cabins near Lake San Cristobal in the path below will not be in danger for a thousand years. Of course, there have been three major flows in the past in about as much time, so there is little accuracy in projecting that far ahead. 

Research on the flow

      The Slumgullion earth flow is a natural laboratory for geologists and a wide variety of research projects have been conducted there in the past. Many of the studies relate to mapping the movement of the flow. The old method of using a grid of survey markers across the active and inactive parts of the flow and measuring them periodically was  time consuming and required the assistance of many people. Recent studies with GPS technologies have shown that a GPS unit placed on the slide surface and left for a few hours will record a noticeable change in position as the slide moves. (GPS is short for Global Positioning System--by detecting the exact distance to any four GPS satellites a GPS unit can calculate where on the Earth's surface it is. Unlike my retail GPS which lets me know where I am in the world within about fifty meters or so, the GPS units available to government geologists in combination with some powerful data processing computers allow detection of changes in position of less than a centimeter!)

      A current experiment is testing another method of monitoring movement of the earth flow that involves using radar in small aircraft. Radar reflectors (small, triangular-shaped aluminum pieces) have been placed on the active and inactive parts of the flow. By periodically flying over the flow with a small aircraft and a special (but fairly inexpensive) radar, it is hoped movements as little as one to two centimeters will be detected.

      A recent project used the knowledge that sound travels at different speeds through different materials to make a three-dimensional map of what is under the surface of the slide. By detonating charges at various places on the slide and listening to the echo with sophisticated recording equipment, the depth of the slide was measured. Hmmm ... detonating charges on a landslide area ... I appreciate the knowledge gained, but I'm not sorry I missed out on that one!

      Another project recently conducted used parts of the Slumgullion earth flow as a natural but miniature model of what is happening at a much larger scale along the San Andreas Fault in California. The earth flow has many places where the ground is sliding past itself. These areas may give clues to help predict or explain movements along major faults.

 

Explore the flow

      The main reason I write this column is because I think too often science is not accessible to non-scientists. Yet there are many fascinating things of scientific interest all around us. Technical scientific papers may contain interesting discoveries but be difficult or very dry to read. If the purpose of science is to explain the world around us, then why not make those explanations a little more accessible? But another reason I write this column is more selfish. Being dedicated to both a career and a family makes it difficult to find time for pursuing personal interests. Committing myself to a monthly column provides a sort of obligated opportunity to keep up with the field of science and further explore the wonders I see around me. This "obligation" has led to many rewarding experiences. The most recent was spending time with my brother exploring the Slumgullion earth flow.

      The drive down Slumgullion Pass towards Lake City on highway 149 is, in my opinion, one of the most scenic in the state. The view from the overlook at Windy Point is spectacular, and includes a glimpse of the earth flow. Nearly two miles further down the highway we found an unmarked pullover that has an easy to miss sign pointing out the earth flow. But the best view of the Slumgullion earth flow comes after rounding the hairpin curve (you can't miss it--or at least won't want to!), where the flow appears as a yellow river carrying scattered trees below the base of the Mesa Seco scarp (curved cliff) flowing right towards you.    

      Where the road crosses the slide (just look for yellow, powdery clay), a small pull off on the right offers a temporary parking spot while exploring the slide. It is an eerie and fascinating experience. Although the trees look familiar, the terrain on the slide is very different from the surrounding aspen-spruce or high meadow areas. As you climb onto the active slide, you will notice that you sink slightly with each step in the fine, yellow clay (old, weathered volcanic rock and ash that used to make up a gentler Mesa Seco), leaving tracks reminiscent of those still on the moon. Pick up some dry clay and you can funnel the powdery dust from hand to hand. But get it wet, and it instantly swells, becoming moldable enough to make a nice lump for launching at your brother, should he happen to be exploring nearby (fortunately for him, and perhaps for me, my aim has not improved with age and the unnoticed missile lands harmlessly in the powder). That this clay drastically changes properties when wet is part of the reason the earth is moving slowly beneath your feet here.

      As you continue your climb, you will notice that the trees, understandably, have a hard time growing straight up when they are rooted in a migrating foundation, and seem to swagger every which way. There is virtually no topsoil here, as the ground has not been stable long enough to support its growth. When you find a wet spot, look closely at the ground. Mineral salts have crystallized on the surface forming a pretty yet eerie crust. Looking around, you will notice that spring runoffs have carved deep channels that are now dry. You might find a young aspen tree growing out of a sucker root that is suspended like a clothes line across a deep channel carved by erosion. No doubt, the tree started growing before the channel formed.

      Spending time in an area that seems so out of place yet perfectly natural is a worthwhile experience. If you go for a drive to look at the aspen leaves this fall, why not head to Lake City? The Slumgullion earth flow is definitely worth a look.

   
1997, 2006  Dirk Oden  
 

This site was last updated 03/09/07