This is ultimately a story about what may be the steepest mountain in the world (although I cannot yet confirm this). But the story involves a little bit of a journey down memory lane if you can bear with me a bit!
When I was a kid in Ontario, California in the 1960s, we possessed one of those wonderful things that kids don't have enough of today: a big backyard. There was room enough for a big lawn for ball games, large hedges and trees, and climbable walls around the lot. And enough bare ground that a kid could dig nice deep holes, looking for fossils or buried treasures. But what I found when digging those holes was a lot of rocks. Big rocks, cobbles really, of granite and gneiss and schist, although I didn't know those terms at the time. But I did wonder where the rocks came from.
Earth science wasn't much of a thing in my primary education in the 1960s, but I knew enough to think the somewhat rounded rocks came from a river. But there were no rivers to speak of in the Inland Empire east of Los Angeles. I got an education about that in 1969 when the biggest floods in nearly two generations hit the valley. Streets turned into rivers, and numerous houses and buildings were destroyed by mudflows coming out of the nearby San Gabriel Mountains. Nearby Day Canyon recorded an outflow corresponding to 33 inches of precipitation across its small drainage basin in 24 hours on February 25, a state record.
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| The perspective of this photo may deceptive; all the road you see here is sloping downhill for the entire 14 miles |
And then, in the 1970s, it was high school and the cross-country team. A favorite training route was to run up Euclid Avenue in Ontario and Upland (AKA State Route 83). It still is one of the prettiest city roads in the state, with a wide median planted in Pepper Trees and numerous architecturally distinctive homes dating from the early 1900s. It runs for 14 miles in a straight line from San Antonio Heights to the Chino Hills.
On the easy days we needed only to run a four-mile out-and-back practice to Foothill Avenue, but when the coaches were bearing down, we needed to run all the way up to Baseline or further (6-8 miles). The thing is, the farther one ran up the hill, the steeper it got. Thus was my introduction to the geometry of alluvial fans. During the mudflows and flashfloods that produce the fans, the coarser debris drops out first, and finer-grained materials get carried further out into the plains below. A fan has a concave profile, becoming steepest at the top.
Some days, the coaches would drive us up into the barrens at the top of the fan, and our runs included a series of breathtaking terraces (and I mean this in the literal sense, as we were breathless by the time we climbed them). I had no idea at the time why they were there. It seemed like alluvial fans should have a smooth profile, not a terraced one.
In the late 1970s I was in college, and my education about alluvial fans and earthquake faults became a bit more complete. These alluvial fans that I had exhausted myself on during cross-country practice were textbook examples of alluvial processes, and maps of them were indeed a part of my laboratory exercises. I also learned that the terraces were actually fault scarps, produced in the last few thousand years by titanic earthquakes that have been lifting the San Gabriel Mountains. They've been uplifted so rapidly that the mountain ridge that includes Ontario and Cucamonga Peaks may be one of the steepest mountain slopes in the world (I heard this statistic at a conference, but I have not been able to locate the reference). The mountains are so steep that mass wasting is a far more dominant form of erosion than river flow. And the mountains are indeed massive, rising 7,000 feet from their base to the highest peaks.
Thus it was that when I left the region in the middle 1980s, the cities below were growing, but the alluvial fans above them had defeated attempts at agricultural development (the lower slopes were ideal for vineyards and citrus orchards). The surfaces were ignored, or used for garbage dumping, shooting, and off-road vehicle travel. They were considered wastelands. Someone had had the bright idea of putting Chaffey College up there, miles away from the main population centers in the valley (it's visible in the lower left corner of the map above), but the college stood apart, surrounded by scrublands.
Sometimes, the lands that seem so barren do in fact have value, and the more they disappear, the more precious the remainder becomes. So maybe it is a good time to ask: what are alluvial fans good for anyway? Here are my thoughts in no particular order:
Alluvial fans are a vast sponge that could hardly be designed better to capture water and store it underground, safe from evaporation. The Inland Empire became an agricultural powerhouse in the last century on the basis of the citrus fruit industry. It was a desert climate that very rarely froze, and yet had a wealth of water underground. Sometimes at the distal end of fans, artesian springs produced fountains of water that could be easily utilized in the vineyards and orchards (artesian springs and wells are those that flow due to underlying pressure and don't have to be pumped to bring water to the surface).
Urbanization robs much of the fan surface of the ability to absorb water, given that pavement and buildings tend to shunt floodwaters into the concrete flood channels. They in turn are designed to carry water downstream without damaging buildings. If they have enough capacity, that is.
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| The south slope of Cucamonga Peak, Can anyone see a viable climbing route? I don't think chocks and pitons would work in the rotten rock, but I suppose you could anchor to the trees. That's how I climbed a similar (but shorter) canyon in my youth. |
Alluvial fans are a buffer from huge mass wasting events. The mountains above the Inland Empire are, as pointed out previously, among the steepest mountains on the planet. In addition, the rocks that make up the steep cliffs are badly fractured and jointed from the intense faulting and pressure resulting from their uplift. I can't find many records of people climbing the mountain from the south other than up a ridge after a wildfire had cleared the brush. These slopes are exceedingly unstable, and landslides and slope failures are a constant hazard.
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| The Blackhawk Slide on the north side of the San Bernardino Mountains. Credit: Kerry Sieh of the U.S. Geological Survey |
It may be an extreme example, but the Blackhawk Slide on the north side of the nearby San Bernardino Mountains is a gigantic debris avalanche that traveled 5.6 miles across the surface of the alluvial fan about 17,400 years ago. It was probably set off by a large earthquake, and traveled on a cushion of compressed air. Such huge events are extremely rare, but not out of the realm of possibility.
Mudflows are also considered a form of mass wasting, and the upper parts of alluvial fans are the danger zone for the flows containing the largest boulders (which can be ten feet or more across in extreme instances).
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| Mudflow that followed wildfires in the San Bernardino Mountains in 2004. Courtesy: U.S. Geological Survey |
Wildland-urban interfaces are a rising concern as urbanization spreads into once wild landscapes. Among the greatest concerns are the incidences of wildfires spreading into cities because of their proximity to chaparral-covered slopes. I'm not speaking as an expert here, but it seems to me that alluvial fan surfaces are a more defensible surface than rugged slopes. Housing developments that butt up against the hillsides would seem to be in the greatest danger in our new normal of drought, rising temperatures and increasing wildfires.
Finally, alluvial fans are a unique and rapidly disappearing ecosystem. The alluvial fans hosted a wide variety of shrubs, grasses, and wildflowers, along with excellent habitat for all manner of mammals, birds, reptiles, and amphibians. Some of the Southern California species are found nowhere else in the world. This rare ecosystem has a name, the Riversidian Alluvial Fan Sage Scrub (RAFSS). It is limited to the alluvial fans along the southern exposures of the Transverse Ranges, including the San Gabriel, San Bernardino, and San Jacinto Mountains. Those fans are as much as 90% urbanized now. There is very little of the original landscape left.
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| Satellite image of the alluvial fans north of Interstate 210 at Upland and Rancho Cucamonga. The blue marker shows the location of the North Etiwanda Preserve |
And so we come to the present day. I returned to the landscape of my youth on a trip last week, and the changes were astounding. I knew that urban development had been creeping up onto the fans, but I never had a good look at the magnitude of the changes. Housing developments and shopping centers have swept up like a tsunami onto the upper reaches of the alluvial fans. Chaffey College has been engulfed by the wave of development and is surrounded by housing tracts.
Despite my dismay at the magnitude of urban development, I found out that a significant portion of the RAFSS has been preserved as the
North Etiwanda Preserve. The relatively recent extension of the 210 Pasadena Freeway into San Bernardino had destroyed a significant part of the RAFSS, and as mitigation, 762 acres were given to San Bernardino County for preservation in 1998. Other land acquisitions brought the size of the preserve to 1,176 acres (nearly two square miles). Even better, the lands preserved were contiguous with the slopes leading up Cucamonga Peak, providing an intact ecosystem connected to the nearby trees and chaparral of San Bernardino National Forest.
A three mile long trail winds through the preserve, with numerous interpretive signs detailing the geology, biology, archaeology and recent human history of the region. I didn't have time to walk the entire route, but I was able to see (to my tectonic delight) a perfectly pristine fault scarp running across the preserve. You can see the terrace in the pictures above and below. The last major earthquake probably happened one or two thousand years ago, and may have ranged as high as magnitude 7.5.
Cucamonga Peak is a southern California treasure, a fact not always appreciated by those who live on the alluvial fans below. It is a dangerous neighbor as well, with earthquakes, fires, and floods a serious concern. The North Etiwanda Preserve is a wonderful resource for learning about this unique landscape, and is one of the few places where one can get a sense of the landscape that existed before urbanization swallowed it up.
On a final note, when I was at Chaffey College in 1976, the geology department got a call from a gravel quarry just east of where the preserve is today. They had found a bone of some sort. It turned out to be a fragment of a tusk from a Columbian Mammoth, one of the many megafauna species that wandered these alluvial fans during the last ice age (although no glaciers came close to this place). One could almost imagine the Columbian Mammoths, Dire Wolves, Sabertooth Cats, Giant Ground Sloths, Short-faced Bears, Horses, and Camels that once roamed across Southern California while strolling the trail at the preserve.
For more information about the natural history of the North Etiwanda Preserve, check out this website at the
The San Bernardino County Museum (sbcounty.gov).
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