9.7: Natural Hazards of the Sierra Nevada

Introduction to Hazards

In exploring the Sierra Nevada region, it becomes evident that its breathtaking beauty is accompanied by a complex array of geologic hazards. From the rumblings of seismic activity to the cascading threat of landslides, this majestic landscape harbors a myriad of natural dangers. Nestled within its rugged terrain, earthquakes, landslides, rock falls, flooding, and volcanic activity intertwine, each presenting its own unique challenge to those who call this region home.

The Sierra Nevada's geological makeup, characterized by steep slopes and an intricate network of fault lines, sets the stage for a delicate dance of peril. Heavy precipitation, whether in the form of torrential rains or the sudden release of snow melt, serves as the catalyst for landslides, posing a significant threat to communities, vital infrastructure, and the very fabric of the landscape itself.

While the forces of nature shape this region in profound ways, it's important not to overlook the role of wildfires, though not geologic in origin, as they too wield considerable influence. Together, these hazards paint a comprehensive portrait of the challenges inherent in navigating and inhabiting the Sierra Nevada, reminding us of the delicate balance between its breathtaking allure and the ever-present specter of natural disaster.

Landslides and Mass Wasting

Mass wasting, also known as slope movement or landsliding, is the process by which soil, rock, and other debris move down a slope under the influence of gravity. This movement can vary in speed, ranging from very slow creep to rapid landslides, and is a critical geological process that reshapes landscapes over time. The key factors that contribute to mass wasting include gravity, slope steepness, water content, and the strength of the material making up the slope. When the gravitational force acting on a slope exceeds the strength of the material, mass wasting occurs.

There are several different types of mass wasting, each characterized by the material involved and the type of movement. Rockfalls occur when individual rocks or a mass of rocks suddenly break free from a steep slope or cliff and tumble down. This type of mass wasting is common in mountainous areas, particularly in places with steep, exposed rock faces. Rockslides, similar to rockfalls, involve the movement of a large mass of rock down a slope but typically occur along a defined plane of weakness, such as a bedding plane or fault line. These events can be rapid and devastating, moving large volumes of rock in a short period.

Mudslides or debris flows are another type of mass wasting where a mixture of water-saturated soil, rock, and organic material flows down a slope. These flows can be triggered by heavy rainfall, rapid snowmelt, or volcanic activity, and they can move quickly, carrying large amounts of debris over long distances. Mudslides often occur in areas with loose, unconsolidated material and steep slopes, making them common in regions with significant seasonal rainfall. Slumps are a slower form of mass wasting where a block of earth material moves down a curved surface, often creating a stepped or terraced appearance on the slope. This type of movement is typically slower than other forms of mass wasting and can be triggered by factors such as the gradual weakening of the slope material or the addition of water, which reduces the cohesion of the soil or rock. Examples of each of these mass wasting events are exhibited in Figure 9.7.1.

Figure \(\PageIndex{1}\): Examples of some types of mass wasting by the USGS is in the public domain. Access a detailed description.

The causes of mass wasting are varied but generally include natural processes such as weathering, erosion, and tectonic activity, as well as human activities like deforestation, mining, and construction, which can destabilize slopes. Water plays a crucial role in mass wasting by reducing the friction between particles, adding weight to a slope, and acting as a lubricant that can trigger or accelerate slope failure. The interaction of these factors determines the type, speed, and impact of the mass wasting event. Video 9.7.1 from the USGS further expounds upon landslide hazards.

Video 9.7.1: Landslide Hazards

Mass Wasting of the Sierra Nevada

Steep topography and elevations between 2,438 and 4,267 meters (8,000 and 14,000 feet) combine to create several different natural hazards in the Sierras. These include rockfalls, rockslides and mudslides, floods as well as droughts, avalanches and of course, earthquakes.

The steep topography and elevations of the Sierra Nevada are the result of thousands of years of earthquakes, most occurring along the eastern Sierra Nevada Fault zone, which roughly follows highway 395, running north-south along the eastern edge of the mountain range. This is a predominantly vertical normal fault where the mountain range is the footwall and the desert plateau is the headwall. The entire mountain range has lifted up along its eastern margin, creating a steep ramp of granite and volcanic rock that dips to the west. The fault zone is made of several active faults that are capable of generating large earthquakes like the Lone Pine Earthquake of 1872 which is estimated to have a moment magnitude between 7.4 and 7.9.

The high elevations of the mountains create orographic lift of the atmosphere, which causes rainfall along the western slope of the range. Over time, the rainfall has eroded the rock and carved numerous river canyons into the western slope. In the northern portion are the Pitt, Feather and American Rivers and in the south are the Mokulumne, Stanislaus, Tuolumne, Merced and Kings, among others. During the recent Ice Ages, glaciers formed in the mountains and flowed downslope, carving many of the river canyons into deep, U-shaped valleys. Along these canyons, both glaciated and not, rockfalls, rockslides and mudflows occur. Rockfalls and rockslides are created as erosion over steepens slopes and rocks give way to gravity, aided by lubrication of rain water and frost wedging from ice that forms in cracks on cold nights. Rockfalls in Yosemite Valley, such as the 1996 Happy Isles Rockfall that fell from Glacier Point receive a lot of news, however there are numerous rockfalls throughout the mountain range. Some impact roads, like the Ferguson Slide on Highway 140 (see Case Study – Ferguson Slide in this section), and some will never be observed in person, except by backpackers.

Query \(\PageIndex{1}\)

Case Study - Ferguson Slide

Introduction

In April of 2006, after two years of heavy rain in the Sierra, approximately 800,000 cubic meters of slate and phyllite slid downslope in the Merced River Canyon, burying Highway 140, between mile posts 103 and 104, and temporarily blocking the Merced River. Highway 140 was closed for 92 days and when it finally re-opened traffic was diverted around the slide and across a temporary bridge to the other side of the river. The slide was not a surprise as the rock layers tilted down toward the highway between 38 and 45 degrees and had been slipping for some time before this event.

Figure \(\PageIndex{2}\): Looking northwest of the east-facing Ferguson Rockslide, Merced River, with the talus covering Highway 140 by United States Geological Survey is licensed under public domain. Access a detailed description.

Background

The Ferguson Slide is situated in Mariposa County, California, within the rugged terrain of the Sierra Nevada Mountain range. This section of Highway 140 traverses through steep canyons and rocky slopes, making it particularly susceptible to geological hazards such as landslides. Because Highway 40 is one of the main roads into Yosemite National Park and used by many park employees to come to work, the road is exceptionally important regionally.

Event Timeline

The Ferguson Slide gained notoriety in April 2006 when heavy rainfall triggered the initial movement of the slope, causing portions of the highway to buckle and crack. Subsequent years saw intermittent periods of activity, with the slide exhibiting ongoing movement and posing challenges to road maintenance crews. In January 2019, a significant movement of the Ferguson Slide occurred, resulting in the complete closure of Highway 140 for an extended period.

Geological Factors

The geological factors contributing to the Ferguson Slide are multifaceted. They include the predominant presence of fractured and weathered metamorphic rock, prone to erosion and instability under certain conditions. The steep terrain and narrow canyon through which Highway 140 passes exacerbate the risk of landslides, as gravity acts upon loose materials. Additionally, heavy rainfall, snowmelt, and freeze-thaw cycles play pivotal roles in triggering slope movements, especially during the wetter months of the year.

Impacts and Mitigation

The closure of Highway 140 due to the Ferguson Slide has had significant ramifications for commuters, tourists, and local businesses, necessitating lengthy detours and impacting regional economies. Mitigating the impacts of the Ferguson Slide has presented substantial engineering challenges, requiring innovative solutions to stabilize the slope and ensure the long-term safety of the highway.

Because Highway 40 is one of the main roads into Yosemite National Park and used by many park employees to come to work, the road is exceptionally important regionally. In 2021, the California Department of Transportation (Caltrans) began developing a mitigation structure that many have come to know as “The Rock Shed”, which will allow the rocks to continue to move downslope but will protect the roadway underneath. As of 2023 they have stabilized much of the slide and will begin to build the rock shed structure in 2024.

Figure \(\PageIndex{3}\): 3D model of the rock shed being built to mitigate mass wasting effects in the Merced River Canyon over Highway 140 by Caltrans is licensed under public domain. Access a detailed description.

The Ferguson Slide serves as a stark reminder of the intricate interplay between geological processes and human infrastructure. As California continues to grapple with the effects of climate change and natural hazards, understanding and managing geological risks such as landslides will remain paramount for safeguarding lives and livelihoods in the region.

Avalanches

Some years the Sierra Nevada receives incredible amounts of snow during the winter. It was extreme snowfall in the winter of 1846 that trapped the Donner Party near present day Truckee and sealed their fate. Multiple snow events produce variations in the layers of snow, where some layers are light and fluffy, and others are dense and wet. It is this variation in the layers that produce avalanches when one layer slides loose on top of the layer below, sending snow crashing down a slope. Avalanches are powerful erosional agents that carry trees and rocks down slope with them, steepening the chutes they form and depositing debris where they land. Like rockfalls and slides, most avalanches occur in the back country regions of the mountains, however they do occur frequently near ski resorts and are often marked for back county skiers. Avalanches are also one of the reasons several of the tran-Sierra highways close during the winter. It is simply not worth the risk to try to keep the roads open to traffic in the winter.

Flooding

The Sierra Nevada receives substantial snowfall in the winter, which can lead to spring snowmelt and increased runoff. This runoff can cause flooding in downstream areas, particularly during warm periods when snowmelt is rapid.

The Sierra’s river canyons are also prone to floods. When atmospheric conditions are right, the western slope of the mountains are inundated with atmospheric rivers, narrow bands of subtropical moisture that are focused on western North America, generating focused and copious rainfall. The rainfall can be so intense that the water cannot soak into the ground fast enough and flows into creeks and streams and eventually the larger rivers. So much water rushes downslope that the system cannot move it fast enough, and rivers overtop their banks and flood. This process further erodes canyons, causing the slopes to steepen once again, leading to additional rockslides. In the lower elevations of the range, when moisture from rain does soak into and saturate soils, mudslides occur. Mudslides can be observed along roads and highways throughout the foothills and help to shape the topography of the region.

Drought and Wildfires

The western portion of North America is prone to prolonged droughts. A reduction in winter snowfall as well as summer rain, decreases soil moisture which stresses and kills vegetation. The dry and dead vegetation easily catches fire, and so if lighting occurs or if human activities cause a fire, the forests can burn. As climate change warms the atmosphere and creates more erratic weather patterns, droughts are more common, and consequently so are wildfires. The aftermath of wildfires cause debris flows and flooding. This is due to the loss of root structures that hold soil in place. When rain returns, the loose soil and rock are easily washed downslope across roads and into streams and rivers where they can produce local flooding.

While wildfires are not directly geological hazards, they are often exacerbated by the region's geology. The dry climate, coupled with the presence of flammable vegetation, can lead to destructive wildfires. Additionally, post-fire debris flows and erosion can follow wildfires, posing further hazards.

Seismic Hazards

The Sierra Nevada foothills are traversed by a complex network of faults that present significant seismic hazards to the region. Among the most notable of these is the Melones Fault Zone, a major fault system that runs along the western edge of the Sierra Nevada. The Melones Fault is part of a larger system of faults associated with the tectonic activity that has shaped the Sierra Nevada over millions of years. This fault zone, which extends for hundreds of kilometers, marks the boundary between the Sierra Nevada block and the Great Valley block to the west. The movement along the Melones Fault and its associated faults has contributed to the uplift of the Sierra Nevada and the deformation of the surrounding rocks. The seismic activity along this fault zone can generate earthquakes that pose risks to nearby communities, infrastructure, and natural landscapes. While Figure 9.7.4 was first seen in section 9.1 of this chapter, it is placed here to reemphasize the Sierra Nevada elevation changes influenced by faulting. Some of the major and deemed "active" faults (that is, movement having been shown in the last 11,000 years) are present.

Figure \(\PageIndex{4}\): "Relief Map of the Sierra Nevada" by Cole Heap is licensed under CC BY-NC 4.0 Constructed in a GIS using data from California Open Data Portal. View a detailed description of this image. Access a detailed description.

In addition to the Melones Fault Zone, the Sierra Nevada foothills are home to several other significant faults, such as the Bear Mountain Fault Zone and the Foothills Fault System. These faults are remnants of ancient tectonic boundaries and continue to exhibit tectonic activity. For instance, the Foothills Fault System, which includes a series of parallel faults extending from the northern to the southern Sierra Nevada foothills, has been the site of numerous small to moderate earthquakes. Although these earthquakes are generally not as powerful as those along the more famous San Andreas Fault, they still represent a serious hazard due to the potential for ground shaking, landslides, and other secondary effects.

Seismic hazards in the Sierra Nevada foothills are further complicated by the region's geology, which includes a mix of brittle metamorphic rocks and younger, more deformable sedimentary rocks. The interplay between these different rock types can amplify seismic waves, leading to more intense ground shaking during an earthquake. This makes the region particularly vulnerable to seismic hazards, even from relatively moderate-sized earthquakes. Understanding the behavior of these fault zones and the potential impacts of seismic activity is crucial for mitigating risks and protecting the communities and ecosystems within the Sierra Nevada foothills.

The risk associated with seismic hazards in the Sierra Nevada depends on factors such as the proximity to active fault lines, the geology of the area, and the vulnerability of infrastructure and communities. While the risk may be lower compared to other parts of California, it is still important for residents, businesses, and policymakers in the Sierra Nevada region to be aware of and prepared for potential seismic events.

Box 9.7.1: Tsunami! In Lake Tahoe?

A seismic event beneath Lake Tahoe, renowned for its pristine beauty nestled in the Sierra Nevada mountains, has sparked discussions among scientists about the potential for a tsunami. Recent research suggests that a massive collapse of the lake's western bank, triggered by seismic activity, could generate waves towering over 9 meters (30 feet), threatening the surrounding communities. And in some cases, it could be as simple as a landslide occurring and debris moving into the lake!

Geologists, utilizing advanced imaging techniques, have uncovered evidence of past tsunamis in the lake's geological history, indicating that such events are not unprecedented. In fact, the USGS has publications showing that landslide events may even have lowered the lake by ~10 meters (~32 feet) during one episode of slope movement! These findings have prompted calls for improved monitoring and preparedness measures to mitigate the potential impact of a future tsunami on the densely populated regions surrounding Lake Tahoe.

Figure \(\PageIndex{5}\): Lake Tahoe Bathymetry showing distribution of giant blocks of McKinney Bay Landslide by United States Geological Survey is licensed under public domain. Access a detailed description.

Hazards experts emphasize the importance of public awareness and education regarding tsunami risks, urging residents and visitors alike to familiarize themselves with evacuation procedures and warning signs. Additionally, ongoing research endeavors aim to refine predictive models and enhance early warning systems, providing vital seconds or minutes for authorities to issue alerts in the event of an impending tsunami.

While the likelihood of a catastrophic tsunami striking Lake Tahoe remains relatively low, the potential consequences underscore the imperative for proactive measures to safeguard vulnerable communities and infrastructure from such geohazards. Through collaborative efforts between scientists, policymakers, and the public, steps can be taken to mitigate risks and ensure the safety and resilience of the region in the face of potential natural disasters.

9.5.7 Mitigation and Looking Towards the Future

To mitigate these hazards, local authorities and geologists monitor seismic activity, assess landslide-prone areas, and implement measures to reduce wildfire risk. Additionally, public education and preparedness are crucial for residents and visitors to the Sierra Nevada to stay safe in the face of these geologic hazards.

Like all geohazards, they are simply part of the natural processes of uplift, erosion and weather in the Sierra Nevada. These hazards are only considered a problem if and when they impact infrastructure and human lives. As more people move to the foothills of the Sierra, and as more people use the mountains for recreation, the need to understand, plan for, and educate the public about the various geohazards of the Sierra will increase.

References

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