Question

Heavy rains in late July 2010 triggered the mass wasting that occurred in this mountain valley near Durango, Colorado. Heavy equipment is clearing away material that blocked railroad tracks and significantly narrowed the adjacent stream channel. Was this event more likely a rockfall, creep, or a debris flow? Most of us are familiar with the phrase "One thing leads to another." It certainly applies to the Earth system. Suppose the material from the masswasting event shown here had completely filled the stream. What other natural hazard might have developed?

Short answer

The event is likely a debris flow; a potential downstream flood could occur if the stream is blocked.

Step-by-step solution

Identifying the Type of Mass Wasting Event

A rockfall involves the free-fall of rock from a steep slope, which is typically rapid and involving large rock fragments. Creep is a slow, gradual downslope movement of soil. On the other hand, a debris flow is a fast-flowing slurry of water, soil, rock, and organic matter. Given the context, the event in question is likely a debris flow, as heavy rains are often associated with such fast-moving mixtures.

Analyzing the Consequences of Stream Blockage

When a stream is completely filled by material from a mass wasting event, it can lead to the formation of a natural dam. This increases the potential for water to build up behind the blockage, forming a temporary lake. The primary hazard associated with this situation is the risk of a dam break, which could lead to a sudden downstream flooding event when the accumulated water is released.

Key concepts

Debris Flow

Debris flows are a type of mass wasting event characterized by a rapid downslope movement of a slurry mixture. This slurry includes water, soil, rocks, and organic material. Imagine a river of mud and rocks, rushing down a mountain with the power to reshape the landscape. These flows are typically triggered by heavy precipitation events, such as intense storms or rapid snowmelt. Since debris flows are fast-moving and can carry large boulders, they can cause significant damage to infrastructure and pose serious safety hazards.
To identify a debris flow during a mass wasting event, look for signs such as a mixture of various debris sizes within a fluid matrix, and their path often leaves a distinct scar on the valley or slope.

Natural Hazard

Natural hazards are events that can cause damage to life and property, and often result in disruptions to human activities. They come in many forms, such as earthquakes, floods, hurricanes, and mass wasting events like debris flows and rockfalls. When discussing debris flows, it's important to understand their potential to create new natural hazards, such as stream blockages that can form unstable dams.
Addressing these hazards involves assessing the risk and implementing measures, such as barriers or warning systems, to protect people and property. Building awareness about natural hazards is crucial for communities in vulnerable areas, especially those near mountains or steep slopes prone to mass wasting.

Stream Blockage

Stream blockage occurs when debris from a mass wasting event obstructs the normal flow of a stream or river. This can lead to significant changes in the local landscape. Common causes include landslides, debris flows, and even human activity. If a stream is blocked, the water begins to pool behind the debris, potentially forming a temporary dam.
The water pressure behind these blockages can build quickly, leading to a serious risk of floods if the dam fails. To manage such risks, monitoring water levels and inspecting the stability of the debris are essential steps. Early detection can prevent catastrophic flooding and help to manage the natural changes that occur in these dynamic environments.

Rockfall

A rockfall primarily consists of the sudden free-fall or tumble of rock fragments from a steep slope or cliff. Typically, rockfalls occur without warning and can be triggered by natural processes such as weathering, earthquakes, or freeze-thaw cycles. Unlike debris flows, rockfalls are more localized, affecting a specific downslope path.
While they are less common than other mass wasting events, rockfalls can still pose serious threats to nearby infrastructure and individuals. The force and speed of falling rocks can damage buildings, roads, and other structures. To mitigate risks associated with rockfalls, understanding the geology of prone areas and maintaining solid rock structures where necessary are key preventative measures.

Creep

Creep is the slow and gradual movement of soil and rock downslope. This imperceptible mass wasting process is usually measured in millimeters to centimeters per year. Despite its slow progress, creep can significantly affect the stability of structures over time, leading to issues like tilted fences or cracking walls.
Creep is driven by factors like freeze-thaw cycles, soil moisture changes, and gravity. Detecting creep early is challenging, but maintaining proper drainage and ensuring vegetation cover can help manage its effects. As the landscape shifts over time due to creep, continuous monitoring and engineering solutions play crucial roles in protecting human developments.