The term “landslide” refers to a wide range of mass wasting events that result in rock, soil, or fill moving downhill under the influence of gravity (Figure 9.1). These events occur when friction between the earth material (i.e., rock and soil) and the slope is overcome, allowing the earth material to fail and move downslope. Landslides may be triggered by high rainfall, earthquakes, erosion, deforestation, groundwater pumping, or volcanic eruptions. They range in size from the simple raveling of a stream embankment to the collapse of an entire mountainside that involves tens of thousands of cubic meters (yards) of material. In the Rocky Mountains, every year at least one road will be temporarily closed as the result of an avalanche, earth movement, or rockfall event. Mass wasting events can also dam streams and rivers, creating lakes. If such dams fail, a flood will result somewhere downstream.

Landslides are common in mountainous parts of the Southwest thanks to a combination of steep terrain, poorly consolidated sediments, and melting snowpack that leads to soil saturation (Figure 9.2). They often occur in high valleys with little vegetative cover. In years that are particularly wet or rainy, landslide incidence increases as unstable soils on saturated slopes break free of the rock. Some very fast landslides can reach speeds exceeding 32 kilometers per hour (20 miles per hour). Although many slides in the Rockies are small, or take place in remote and inaccessible locations, people and property are impacted each year by these events. In the winter, many of the same mountainous areas that are prone to landslides during the year are subject to avalanches—rapid flows of snow, ice, and rock. Avalanches occur when the strength of the snow is overcome, or when a weak layer in the snow fails. These snow failures can result from storms, warming weather, sunny slopes, earthquakes, and people moving over the snow. Hundreds of avalanches occur every winter in the mountains of Colorado and Utah.

Figure 9.1: Common types of landslides.

Figure 9.2: Landslide incidence and risk in the Southwestern US.

Utah has seen some of the largest landslides in US history. In April 1983, a massive landslide dammed the Spanish Fork River, destroying roads and flooding the town of Thistle with more than 80 million cubic meters of water that backed up behind the naturally formed dam (Figure 9.3). This dam eventually created a lake 60 meters (200 feet) deep and 5 kilometers (3 miles) long. Thistle was almost completely destroyed, and the nearby railroad and highways had to be rebuilt on higher ground. While these transportation routes were closed, communities in eastern and southeastern Utah were completely cut off from the rest of the state for up to eight months. Direct and indirect costs of the Thistle landslide have been estimated to be as high as $950 million (adjusted for inflation); the state of Utah and the United States Geological Survey have categorized this landslide as the costliest in the nation. More recently, in April 2013, a massive landslide at Utah’s Bingham Canyon Mine (also known as the Kennecott Copper Mine) displaced almost 70 million cubic meters (2.5 billion cubic feet) of dirt and rock from the side of the pit. This was the largest nonvolcanic landslide in the history of North America; luckily, thanks to an early warning system, no injuries occurred. Two years later, the mine is still cleaning up debris from the slide. Massive landslides in Utah aren’t just restricted to recent history, either. In 2014, scientists in Dixie National Park discovered the remnants of the largest known landslide anywhere on earth. This major prehistoric slide occurred 21 million years ago and stretched over 2700 meters (1700 miles)—an area the size of Rhode Island. Geologists studying the site have concluded that it originated when a volcanic field collapsed, and took place over an extremely short period of time, during which the friction of moving blocks pulverized and even melted the surrounding rocks.

Mudflows or earthflows are fluid, surging flows of debris that have been fully or partially liquefied by the addition of water. They can be triggered by heavy rainfall, snowmelt, or high levels of ground water flowing through cracked bedrock. Higher temperatures, thick melting snowpack, and an increase in spring rainstorms are thought to have generated the 2012 mudflow in Mesa County, Colorado, in which a slide five kilometers (three miles) long and 1.2 kilometers (¾ of a mile) wide claimed the lives of three men as well as triggered a small earthquake (Figure 9.4). The Grand Mesa area, where the slide occurred, is prone to landslides due to a soft underlying layer of claystone that erodes easily from runoff and snowmelt.

Figure 9.3: In spring 1983, a major landslide near Thistle, Utah created this dam and the resulting “Lake Thistle,” inundating the town.

Figure 9.4: The catastrophic 2012 mudslide in Mesa County, Colorado, triggered by melting snow and unusually heavy rainfall, rushed down a mountain near the town of Collbran into the West Salt Creek Valley.

Debris flows are a dangerous mixture of water, mud, rocks, trees, and other debris that moves quickly down valleys. The flows can result from sudden rainstorms or snowmelt that creates flash floods. In Chalk Cliffs, Colorado, one or more small debris flows occur every year after periods of intense rainfall. Though less hazardous than debris flows that occur in populated areas, these deposits have blocked roads and diverted streams (Figure 9.5). Debris flows can also occur in otherwise stable landscapes after the occurrence of large wildfires, which can destabilize the ground due to the removal of vegetation and desiccation of the soil. Heavy rainfall following the fire can then cause the burned slopes to fail. The Sandia and Manzano Mountain areas in central New Mexico have been studied extensively regarding their susceptibility to postwildfire debris flows.

Figure 9.5: Remnants of a 2002 debris flow near Buena Vista, Colorado, which blocked Chaffee County Road 306 in 11 places and trapped several motorists.

In the Rocky Mountains, where the bedrock contains many discontinuities (folded bedding planes, faults, joints, and cleavage) resulting from several episodes of mountain building (the Antler, Laramide, and Sevier orogenies), rock slides and rockfalls are common, especially along transportation routes running through the mountains. US Highway 6 in Colorado, State Route 9 (the Zion-Mount Carmel Highway) in Utah, and I-70 in west-central Colorado are often impacted by rockfalls, leading to frequent road closures (Figure 9.6). Stretches of highway can remain closed for periods of several months. Rockfalls can also have fatal consequences in populated areas where buildings have been constructed in high-hazard zones (Figure 9.7)

Not all mass wasting events are rapid—slow land movement, known as soil creep, is generally not hazardous, but can impact structures over a long period of time. Slumps and creep are common problems in parts of the Southwest with a wetter climate and/or the presence of unstable slopes, especially in the Great Plains and on the Colorado Plateau. Many areas in the Southwest contain expansive soils generated from clay-rich parent materials, especially volcanic ash or debris. Certain clay minerals can absorb water and swell up to twice their original volume. The pressures exerted through expansion of the minerals in the soil can easily exceed 22 metric tons per square meter (5 tons per square foot)—a force capable of causing significant damage to highways and buildings. An estimated $9 billion of damage to infrastructure built on expansive clays occurs each year in the United States, making swelling soils one of the costliest hazards. In addition, when the clay dries and contracts, the particles settle slightly in the downhill direction. This process can result in soil creep, a slow movement of land that causes fences and telephone poles to lean downhill, while trees adjust by bending uphill (Figure 9.8). Human development can exacerbate this process when homes are built along steep embankments, disturbing vegetation that would otherwise stabilize the slope or adding water to the land in the form of yard irrigation or septic systems.

Figure 9.6: Boulders weighing as much as 60 metric tons (66 tons) are blasted and removed from I-70 at Glenwood Canyon, Colorado, after a major rockfall closed the highway. The rocks punched holes in elevated sections of the roadway; luckily, no one was injured.

Figure 9.7: This house in Rockville, Utah, was demolished in a 2013 rockfall that destroyed the house, garage, and car and killed two residents. A motorist who witnessed the event estimated that it lasted only 10 seconds. Multiple nearby houses are also located in this high-hazard zone.

Expansive soils can be found all over the US, and every state in the Southwest has bedrock units or soil layers that are possible sources (Figure 9.9). Clay minerals that expand and contract when hydrated and dehydrated due to their layered molecular structure are generically referred to as smectite; soils that tend to form deep cracks during drought are often indicative of the presence of smectite. The Colorado Plateau and Great Plains regions have the highest risk of damage caused by swelling soil. Here, clays are typically composed of montmorillinite or bentonite, which have a very high shrink/swell potential. In the Basin and Range, the clay-rich beds of the Pantano Formation are prone to expansion, as are old alluvial fan surfaces along river terraces.

Figure 9.8: Some influences of soil creep on surface topography.

Figure 9.9: Approximate distribution of expansive soils in the Southwestern US. This map is based on the distribution of types of bedrock, which are the origin of soils produced in place. (Where substantial fractions of the soil have been transported by wind, water, or ice, the map will not be as accurate.)

Significant or repeated changes in moisture, which can occur from human use or in concert with other geologic hazards such as earthquakes, floods, or landslides, greatly increase the hazard potential of expansive soils. Because precipitation is infrequent in much of the Southwest, low-moisture soils also have a high potential for hydrocompaction, where dry silt and clay particles lose their cohesion upon wetting. This process causes the soil to collapse, settling lower. If hydrocompaction occurs over deeper layers that have been severely dried due to prolonged drought or receding groundwater levels, the settling topsoil may fall into and expose giant underground fissures, called desiccation cracks (Figure 9.10). These fissures can be up to a meter (3 feet) wide, 3 meters (9 feet) deep, and as much as 300 meters (1000 feet) long.

Figure 9.10: This giant desiccation crack appeared in a road in Graham County, southeastern Arizona. Such cracks typically occur as a network of polygonal fissures.

Slumping occurs when expansive minerals are present on steeper slopes, and involves the downward movement of a larger block of material along a surface that fails when the weight of the saturated soils can no longer be supported (Figure 9.11). Slumping is common near roads and highways, thanks to the presence of steeper hills, roadcuts, and construction. On steep, high slopes, slumping often precedes earthflows and mudflows that develop farther downslope as water is added to the slump while it mixes the moving material.

The key to reducing expansive soil hazards is to keep the water content of the soil constant—in the dry Southwest, the best option is to utilize proper drainage methods, prevent the infiltration of surface water, and use moisture protection barriers around houses and other structures. There are also chemical stabilizers, including lime, potassium, and ionic agents, that can increase the clay’s structural stability. Damage to life and property from larger mass-wasting events can be reduced by avoiding landslide hazard areas or by restricting access to known landslide zones. Hazard reduction is possible by avoiding construction on steep slopes or by stabilizing the slopes. There are two main ways to accomplish stabilization: 1) preventing water from entering the landslide zone through runoff, flooding, or irrigation and 2) stabilizing the slope by placing natural or manmade materials at the toe (bottom) of the landslide zone or by removing mass from the top of the slope.

Figure 9.11: A hillslide slump in Rocky Flats National Wildlife Refuge, Colorado. The slump occurred on a steep, manmade embankment after a severe storm saturated the soil.