Monday, February 6, 2012

Rock and Road

I had a chance to explore the area of the 22 January rockslide on the Big Oak Flat Road recently. The slide is maybe a couple hundred meters west of the old diversion dam at the junction of 140 and the Big Oak Flat Road (sometimes mistakenly called Hwy. 120 here). The rockfall happened in the middle of the night, after a couple of rainy days. It originated perhaps 5-600 feet above the highway, dropping straight down a cliff face onto the talus above the road. It broke through the roadbed of the Big Oak Flat Road and NPS road crews did well to rebuild it so quickly; their patch should be repaved soon. The material that came loose totals about 500 cubic meters and is dominated by one large boulder half the size of the Yosemite Conservancy office building in El Portal. It cut a very straight swath through the mature live oaks and bay trees above and below the highway, not spreading out too much.
Surprisingly, the big rocks rolled to a stop about halfway between the Big Oak Flat Road and Highway 140, still on steep enough terrain that it seems they might have spilled to the river. One refrigerator-sized slab did make it all the way to 140 but it stopped in the storm drain ditch on the side of the road where almost no one would notice it. Some bigger rocks came within 10-20' of reaching 140.
A couple of jumbo SUV-sized boulders were stopped by a historic artifact: the old penstock foundation. This is a rock causeway built 1916-18 that supported the redwood pipe which carried water almost a mile from the diversion dam to plunge down through the turbines of the powerhouse at Cascades. It's interesting to think that the Valley used to generate its own carbon-free electricity from the Merced. Two weeks ago part of that relict operation caught a couple of big rocks (pictured) that might have smashed Hwy. 140 - an energy project that captured a rockfall's energy. (The one really big boulder stopped immediately above the causeway, slowed by the irregularities of its shape and the slope down which it tumbled.)


  1. How unusual (or not) is this event? Your description suggests there had not been a rockfall of this magnitude there since the causeway was built in 1916? (Did anyone in the park hear any sounds?)
    And it is neat to hear about yet another dam that powered Yosemite, after recently learning about the hope of restoring Hetch Hetchy. (

    I wonder if this rock fall could interest geologists into exploring re-opening this old, carbon free generating system (at Cascades)--why did it cease at all?

    Thank you for this interesting blog, which gives a newcomer a great overview of all the unusual weather events and marvels at the park over the last 2 years.

    1. The penstock support was a mix of rockwork and wooden trestles (now removed). It is likely that other rockfalls impacted the system to some scale over its 60+ years, but I don't know that history. It's my understanding that when the generators needed replacing in the early 80's NPS decided to forego that expense and just get park power from the California grid.