Floods and quakes and other geologic perils

Article by Paul Martz

Geologic Hazards – July 1997 – Colorado Central Magazine

The first article in this two-part series dealt with potential problems with a more-or-less immediate negative result: Losing control of your pickup while towing a horse trailer up Droney Gulch because of slick bentonite after a thunderstorm is pretty immediate, from my point of view.

The crust of the earth, however, usually changes slowly. Some things are rising, while new basins are sinking. Creeks and rivers are eroding their beds, or else depositing material washed down from higher up.

Since the end of World War II, man has made some pretty significant changes to the face of the planet, even to the point of altering its geologic processes. The Aswan Dam on the Nile is a case in point. Central Colorado, however, doesn’t have anything comparable (even Elephant Rock Dam wouldn’t have a similar impact).

In the late 1960s, though, Denver discovered that man could move mountains upward, literally, after the Rocky Mountain Arsenal near Denver drilled a deep disposal well right into an inactive fault. High-volume pumping lubricated the fault and a swarm of earthquakes, directly correlated with the pumping rate, resulted.

But few quakes are man-made. Most of stress induced by glacial ice has long since dissipated, but a couple of small quakes have occurred in Central Colorado in the past ten years as a result of tectonic adjustment due to erosion.

Altogether, the whole history of man is really just a snapshot of the history of the earth, and the Rockies are still rising. Those are things to think about.

Recent flooding in North Dakota and floods last winter on the Sacramento River and in Southwestern Oregon are the kinds of predictable events that always seem to prompt remarks like: “They should have known better.”

While Central Colorado doesn’t have a river system to match either those of the Dakotas, or the drainages on both sides of the Sierra Nevada, we do have water courses that can cause grief in the spring and later as a result of summer storms.

One only has to remember 1995 when we dodged the bullet from a record snow pack, by the narrow margin of two weeks of cool and cloudy weather in early June. Even so, one of my neighbors lost all of the topsoil from his lower pasture when rapidly flowing water rose up to the level of the historical floodplain and slightly beyond. The amount of debris left behind was also impressive.

Such seasonal flooding shouldn’t come as a surprise to anyone — since both the Arkansas and the Little River (South Arkansas) have easily delineated flood plains as far downstream as the Salida stockyards. Older terraces from previous stream activity are clearly evident down river, but do not represent features associated with current stream. That is a geology lesson in itself.

In the Midwest and within the Central Valley of California, flooding is a frequent, often annual, event — as it is on any major perennial river of the world. We have however, camouflaged the product and reduced the frequency of free range melt water and heavy rains with dikes and levees.

When I was in college we were taught that the Corps of Engineers had essentially made flooding along the Mississippi worse with its “reclamation” and flood control efforts. I accepted that as gospel at the time, but now I doubt it.

The flooding does seem worse, however, and it gets more expensive each time it happens, because the population continues to grow in some of the most fertile, food-producing areas of the world. Fertility, though, was actually produced as a result of the very geologic process the dikes and levees now prevent most of the time.

When one of these preventative structures fails, then there is flooding of adjacent low-lying areas. As the TV pictures showed so vividly last winter, such flooding can be impressive in its regional extent.

I doubt I’ll ever forget the sight of the casinos in Reno, Nevada, a city in the midst of a rain-shadow desert, with water flowing across their gaming floors. Similarly in January of 1974 more than 10,000 square miles of the most arid portion of Queensland in northeastern Australia were beneath flood water due to cyclonic storms in that desert. If you live by a river, even in a desert, you take your chances with Mother Nature.

But if flooding of primarily agricultural lands damages the structures put there to make food production possible, what’s our excuse here in Central Colorado for building in flood-prone areas?

The simple answer is that the floodplains of our creeks and rivers are the easiest areas to irrigate, as well as to build upon, and they usually have the most productive soils. This is also true for some, but not all, of the upper benches which were former floodplains. As a result, that’s where people ranched first, moving to progressively higher benches with the passage of time and consequent increases in population.

In one respect, the home in jeopardy mentioned in the first half of this article, is just a modern-day continuation of the trend to move higher up because there are already people lower down. Or as Ed Quillen would say, the “Stupid Zone” is expanding. But I’ll just state that the risk factors increase significantly with elevation — unless you live on the actual floodplain.

Locally, however, it is notable that there are no dwellings upon the current floodplains. Zoning keeps it that way now, and settlers had the good sense not to do so initially. That wasn’t strictly true for other parts of the state with similar geography and geology.

If you lived in Colorado during the Big Thompson Canyon disaster in 1976, the true power of a flash flood was vividly brought home by the loss of lives and structures, including a lot of occupied homes. Everything from one-room log cabins to very large houses went down river in a matter of minutes — or was destroyed on site when foundations washed away. I have seen rocks as large as full-sized Blazers carried away by water which fell miles away in that drainage. In fact, due to a lucky accident, one of my drill crews narrowly missed losing their lives with that same flow.

Flash flooding is possible in any of the drainages of the Sawatch Range, especially on the lower slopes. But it is a greater hazard in the Arkansas Hills to the east, as the Badger Basin flood of 1979 demonstrated.

On the other hand, the concrete crossing in Chalk Creek, 1½ miles from the Hot Springs, is there for a reason — as are the signs warning motorists to stay off of it when water is flowing.

Given the topography, the resulting gradients, and the soils present, any of the drainages of the Sangre De Cristo mountains from Poncha Pass all the way down to Cañon City have to be considered high risk for flash flooding.

And on the East Wall of the Sangres I’ve seen several events in the last ten years which make me wonder about the geologic wisdom of where Crestone sits.

Maybe the fact that geologists tend to deal with extremely long time spans causes us to look at potential events as hazards, because the evidence shows that such events will occur, given enough time — which translates to only a brief time in geologic terms.

It usually isn’t a case that something might someday happen, but that given a long enough time frame, it will happen. Looking at a talus cone built up over hundreds, if not thousands of lifetimes, tells us that such material has washed out in enormous volumes, and that some incredibly large rocks have been carried by volumes of water significantly greater than what is usual on a year-to-year basis today.

That’s what makes the really big geologic hazards so dangerous: the long time frame between events, together with the fact that they usually are not periodic events. You can have two “hundred-year” floods back to back if weather conditions are right. Just visualize Pueblo the year after the great flood of 1922; it could have happened again the very next year.

I live within a mile of a volcano that has been active in the last 50,000 years, the geologic equivalent of an eyeblink. I think about it every time I find my sight wandering to the hilltop that records its molten core. The springs that supply the Salida Hot Springs pool are being heated by volcanic rock at a fairly shallow depth that is probably part of the same magma source and demonstrates that a long-term hazard still exists.

If you live within sight of Fujiyama or Mt. Shasta you have a truly monumental reminder of the danger, but our little local center of activity doesn’t reveal itself so spectacularly. The much larger and non-volcanic hill of Precambrian bedrock right next to it presents no threat, yet it seems to loom over the town on cold, crisp mornings while its younger cousin is notable merely for the bulldozer scar on its flank.

So who is more foolish? The Californian who lives tens of miles away from Shasta’s peak, or a Coloradan who lives less than a rifle shot from a less ostentatious example of volcanic danger?

The answer and the question itself are examples of the conundrums that most large-scale geologic hazards represent.

In the case of a mass of magma, the passage of time with no activity may mean that the threat is diminishing and the upward motion of molten material has gone from dormant to extinct. When one looks at any of the Cascade volcanic peaks one sees very little hint of immediate danger, but as Mt. Saint Helens demonstrated in 1980, when the furnace down below cooks up, events occur within very short time spans and their effects are not predictable. If they were, the US Geological Survey wouldn’t be one employee short today.

So, do I worry about my physical safety from the little vent nearby? No, but if the ground starts rumbling with increasing frequency and growing intensity, I sure will. Magma moving toward the surface causes seismic events and these give warning that something is about to happen.

The question is “WHAT?” And that will remain, at least for the duration of my lifetime and in spite of the numbers crunchers, the continuing mystery of the earth’s crust.

There is another type of seismicity, however, that’s related to movement along faults, and it is a hazard within Central Colorado that I think only increases with the passage of time.

Most of Chaffee County lives in a bowl, surrounded by mountains that have been, and will continue to be, uplifted by movement along globally scaled faulting. Not only do we have the Rio Grande Rift, running more or less north/south, but we’re also blessed with a structure that USGS geologist Allan Hyle described as the 39th Parallel Lineament, which trends east/west.

The Rift is a system of faults with uplift occurring on the edges and subsidence, or less relative uplift, along the central axis. The Lineament is a less understood linear structure composed of demonstrable faulting, fractures with no apparent offset, ore deposits, and unusual geologic features. Like the Rift, it is over a thousand miles long and therefore of global scale. They meet right here, friends and neighbors, at the crosshairs if you will, smack in the Heart of the Rockies.

How much of a hazard do these structures and other faulting associated with mountain building represent? In my opinion a large one, but it is a hazard dissipated by time. If you go to Hollister, California, you can watch the curb and gutter on one particular street slowly slide by each other along opposite sides of the San Andreas Fault. The stress on the crust that has created the fault is slowly being relieved by nearly constant motion.

It is thought that sites where stress is accumulating, until the ground ruptures at some point in the future, are the truly dangerous places.

Our local structures are not the product of the same sort of stress field that occurs along the San Andreas. However, it is apparent that a considerable period of time, as judged by human lifetimes, has passed since the last major uplift has occurred locally.

Does this mean that with the additional passage of time the possibility of a major earthquake increases? Yes. Does it mean that the probability also increases? Not necessarily.

The evidence is that the Rocky Mountains have been going up for several million years, thus it is likely that they will continue to do so for some time into the future. If that is the case, the mechanism which has raised them to their present state will continue to operate, and faulting, sometimes with displacements in tens of feet, will also continue.

Whether such a break will occur within one lifetime, or even a hundred of them, can’t be predicted. Sometimes such events are telegraphed slightly in advance on a scale of hours or minutes; most of the time they are not.

Nor can the outcome be accurately predicted. However, a fault with ten feet of vertical offset will cause effects at the upper end of the Richter Scale. Thirty feet of displacement, which is still less than some local geologically recent faults, and the effects will probably be off the scale of any quake this century.

So are they going to say about us, “Well, they should have known better” if such an event occurs? Probably. But the big question is: Will it happen in our lifetime?

Probably not, but there is a possibility. Will one happen in the next hundred years? A stronger possibility. In the next thousand? Probably. Tomorrow? Maybe.

I won’t lose any sleep over it happening, but if it does, we’ll all be rebuilding together — assuming that we aren’t on the bottom of some new lake.

On the other hand, if you don’t like the uncertainty, you can always move to Kansas. But stay away from those rivers.

Paul Martz of Poncha Springs holds a master’s degree in geology from the University of California at Davis, and has worked as an exploration geologist for several Fortune 500 companies. Among other things, he owns and operates Headwaters Geologic Consultants.