Storm Season Safety: Here’s What to Know During a Mudslide or Debris Flow


California is poised to face substantial rainfall and storm-related challenges due to atmospheric rivers. The California Governor’s Office of Emergency Services (Cal OES) is geared up to address any potential hazards, such as mudslides, debris flows and other incidents triggered by excessive precipitation.  

The Cal OES Watershed and Debris Flow Task Force is actively overseeing high-risk zones to ensure communities receive prompt emergency information in the event of debris flows. If rainfall is expected to meet dangerous thresholds, the hazards are communicated to local jurisdictions.  

In preparation for heavy rainfall, Cal OES works with counties and state partners to install emergency protective measures prior to high flow events. These efforts address slope instability to protect lives and infrastructure.  


  • Mudslides: a type of landslide that results from the failure of a slope, and often occurs due to the accumulation of water from prolonged rainfall and/or saturated subsurface conditions.  
  • Debris Flows: described as a “sediment-dominated slurry,” debris flows are mostly made up of soil, resulting from short-duration, high-intensity rainfall events.  
  • A mudslide is a localized slope failure, while a debris flow is a runoff event, such as a flash flood, that entrains sediment from a broad area. 
  • Debris flows and mudslides can occur many years after wildfires. Both happen fast, so heed evacuation warnings immediately. 

What to Expect during Mudslides and Debris Flows 

Mudslide and debris flow events, often triggered by heavy rainfall and subsequent soil saturation, are characterized by the rapid downhill movement of soil, rocks and other debris. Understanding the intricacies and triggers of mudslides and debris flows is essential for communities situated in regions prone to these occurrences. 

  • Rapid Onset: Mudslides and debris flows can occur with little warning. The rapid onset of these events underscores the importance of staying vigilant and listening to authorities when given evacuation orders or warnings. 
  • Debris Rush: The movement of soil, rocks and debris during a mudslide is forceful and can pose significant dangers to both individuals and property. Californians should anticipate the rush of debris knowing it has the potential to cause damage and obstruct pathways. 
  • Disrupted Infrastructure: Expect disruptions to roads, railways and utilities as mudslides and debris flows traverse the landscape. Displaced objects like trees can hinder transportation and access to essential services, emphasizing the need for Californians to plan for alternative routes and be prepared for potential sheltering-in-place. 
  • Evacuation Protocols: When dangerous conditions are present, such as heavy rainfall, authorities may issue evacuation orders for at-risk communities. Community members should be aware of these protocols, understanding the importance of promptly following official guidance to ensure their safety. 
  • Terrain Vulnerability: Steep terrain, especially in recently burned areas, is highly susceptible to mudslides and debris flows capable of causing impacts downslope. Understanding the landscape vulnerabilities specific to the region is crucial for Californians to assess and mitigate potential risks. 
  • Emergency Response: Emergency services will play a pivotal role during mudslides and debris flows. Community members should expect a coordinated response from authorities, including search and rescue operations, medical assistance and the establishment of emergency shelters.  

Health Hazards  

Health hazards associated with landslides and mudflows include: 

  • Rapidly moving water and debris that can lead to injury 
  • Broken electrical, water, gas and sewage lines that can result in injury or illness 
  • Disrupted roadways and railways that can endanger motorists and disrupt transport and access to health care 

Californians should be aware of the geographical and environmental factors contributing to these events in their region. This helps individuals make informed decisions about where to live, work and recreate, considering the potential risks posed by natural hazards. 

Where intense wildfires occur, soils can develop a layer that repels water, like rain on pavement, due to the charred remnants of organic material. Rainfall that would normally be absorbed by the soil will instead quickly run off.  

Consequently, much less precipitation is required to produce a flash flood, and the potential for mudslides and debris flows increases with the loss of organic material that holds the soil in place. Because of this, locations that are downhill and downstream from burned areas are highly vulnerable to rain that can cause mudslides and debris flows, especially in and around steep terrain. 

It’s important to know that high-flow events like mudslides and debris flows can happen in the days following rainfall. Soil can stay saturated for long periods of time.  

Early Warnings and Signs of Danger 

Swift and effective response to mudslides and debris flows hinges on understanding early warning signs. These events can accompany heavy rains or follow droughts, earthquakes or volcanic eruptions. Californians should educate themselves on indicators such as: 

  • Changes in ground texture 
  • Increased slope erosion 
  • Unusual sounds from the surrounding terrain – mudslides and debris flows can be loud 

Recognizing these warning signs empowers individuals to take prompt actions to safeguard themselves, their families and their property. 

Response and Safety 

During potential threat periods, it is crucial to heed the guidance of local authorities. Staying informed about official communications, evacuation orders and safety instructions issued by emergency services ensures a coordinated and effective community response and could ultimately save lives. 


Severe weather conditions can create power outages. Sign up for local alerts with your utility company to stay informed about possible power outages and public safety power shutoffs in your area. For those who require power for medical needs, contact your local utility providers for the medical baseline program which offers early notifications of possible shutoffs that give community members time to adjust emergency plans. Generators can also be helpful when the power goes out. Generators should be used properly, like using them outdoors and keeping them dry and protected from the elements.