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.
|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.
|Artesian wells near San Bernardino in the early years of settlement. Source unknown, but found at you have water mail: artesian wells in San Bernardino, California|
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).
|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.
|The Blackhawk Slide on the north side of the San Bernardino Mountains. Credit: Kerry Sieh of the U.S. Geological Survey|
|Mudflow that followed wildfires in the San Bernardino Mountains in 2004. Courtesy: U.S. Geological Survey|
|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|
The San Bernardino County Museum (sbcounty.gov).